Project Leader: Author(s): Science Group · 2.2 PENDULUM FRICTION RIG Preparation of the pendulum...
Transcript of Project Leader: Author(s): Science Group · 2.2 PENDULUM FRICTION RIG Preparation of the pendulum...
Harpur Hill, Buxton Derbyshire, SK17 9JN T: +44 (0)1298 218000 F: +44 (0)1298 218590 W: www.hsl.gov.uk
Dry Contaminants Scoping Study
HSL/2006/23
Project Leader: Dr. Marianne Loo-Morrey Phd., Msc., BSc.
Author(s): Dr. Marianne Loo-Morrey Phd., Msc., BSc.
Science Group: Human Factors
© Crown copyright (2006)
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ACKNOWLEDGEMENTS The work reported here was carried out at the request of Mr. Stephen Taylor, Construction Division Technology Unit in collaboration with the Tribology Research Lab at the University of Sheffield.
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CONTENTS
1 INTRODUCTION ...................................................................................................1
2 EXPERIMENTAL ..................................................................................................2 2.1 Low-speed friction rig ...........................................................................................2 2.2 Pendulum friction rig.............................................................................................3 2.3 Application of Contaminants.................................................................................3 2.4 Test Specimens & Contaminants...........................................................................4
3 KEY FINDINGS......................................................................................................6 3.1 Dry Contaminant Lubrication Mechanisms ..........................................................6 3.2 Effect of Surface Roughness .................................................................................7 3.3 Reproducibility of Data and Transport of Contamination.....................................9 3.4 Particle cohesiveness and its importance ............................................................10 3.5 Slip Mechanism Map...........................................................................................13
4 CONCLUSIONS....................................................................................................15
5 FUTURE WORK...................................................................................................16
6 REFERENCES ......................................................................................................17
7 APPENDICES........................................................................................................18 Appendix 1 – University of Sheffiled Report “ The Mechanisms of Pedestrian Slips Caused by Solid Particle Contamination.........................................................................18
8 GLOSSARY ...........................................................................................................70
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EXECUTIVE SUMMARY Pedestrian slipping in the presence of a fluid contaminant can be understood in terms of squeeze film theory. The understanding of this theory provides a rigorous scientific underpinning as to why the pendulum test is suitable for assessing the slip resistance of wet floors, and a means by which HSE/HSL can give informed advice to duty holders on flooring requirements based on the viscosity of likely contaminants.
There is currently no such fundamental understanding of the mechanisms involved in pedestrian slipping in the presence of dry contaminants.
Objectives
The aims of the scoping study were:
• To investigate the effects of particulate contaminants on coefficient of friction. • To generate simple mechanisms to describe the observed behaviour. • To generate an idea of the quantities of contaminant required to increased slip potential. • To provide a framework for interpreting slip resistance data. • To highlight potential routes for further research.
Main Findings
Adhesive friction is significantly affected by the presence of solid contaminants, but hysteretic friction is not.
Smooth floor (Rz of the order of a few m) are highly susceptible to the effects of dry contaminants. Even small quantities of dry contaminant can significantly reduce available friction and result in a high slip risk.
Three distinct dry contaminant mechanisms have been identified with the contact during slipping: Sliding: This mechanism occurs when the limiting shear stress at the particle-
shoe interface is greater than the particle-floor interface, but less than the shear stress required to shear the particles within the layer. The particles than all slide with the shoe.
Shearing: This mechanism occurs when the layer of particles itself shears.
Tumbling/Rolling: This mechanism occurs for larger single particles layers. The particles tend to roll or tumble through the contact.
Results indicate that the sliding and rolling mechanisms on smooth floors pose the most significant risk to pedestrians.
Dry contaminants with a particle diameter of approximately 50 m or less (e.g. Flour,Talc, Coco, and Baking Powder) have higher levels of cohesion and therefore are likely to clump and promote the sliding mechanism in the contact.
Pendulum tests showed that rougher surfaces break down the sliding particle layer and promote shearing of the particle layer.Floor surface micro-roughness (Rz) may be a means of controlling the slipping hazard posed by dry contaminants.
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Recommendations
The current scoping has highlighted a number of potential avenues for future research including:
• To identify the physical characteristics of flooring and footwear which promote good hysteretic friction to better inform advice to duty holders regarding controlling the hazards posed by dry contaminants
• To enlarge the database of laboratory data on slip resistance in particle contaminated conditions using the pendulum and sled tester.
• To further understand the mechanisms by which solid particles cause reduction in friction between the shoe and the floor.
• To reconsider the current suggested site investigation test procedure in cases where there is dry particulate contamination to ensure it is the best approach to take.
• To define simplified criteria and standards for advice on flooring and footwear requirements to deal with dry contaminants
• Examine the possibility of adapting current testing methods for assessing the anti-slip performance of footwear for use with dry contamination
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1 INTRODUCTION
The work reported here was carried out at the request of Mr. Stephen Taylor, Construction Division Technology Unit in collaboration with the Tribology Research Lab at the University of Sheffield.
Mr. Taylor received an unsolicited project proposal for the development of a slip resistance test method for use in dry contaminated conditions. The author was requested to review the proposal and give an opinion as to it’s suitability for further consideration by HSE. While there were numerous technical and financial reasons for not pursuing the project proposal as received by Mr. Taylor, it did highlight the limited work conducted into dry contaminants by HSL / HSE in the past.
Previous dry contaminant research [Lemon et al.] has been largely empirically based, and focused on establishing the suitability of the pendulum test method for use in dry contaminated situations. This work clearly demonstrated that the pendulum test could distinguish between clean dry floors and floors with dry contaminants. The pendulum test data indicated that the presence of dry contaminants could reduce the level of available fiction to the point where pedestrian slipping incidents could reasonably be expected to occur.
Consideration of the research proposal received by Mr. Taylor highlighted a significant knowledge gap in HSE’s understanding of the pedestrian slipping problem. Pedestrian slipping in the presence of a fluid contaminant can be understood in terms of squeeze film theory [Richardson and Griffiths, Lemon and Griffiths]. The understanding of this theory provides a rigorous scientific underpinning as to why the pendulum test is suitable for assessing the slip resistance of wet floors, and a means by which HSE/HSL can give informed advice to duty holders on flooring requirements based on the viscosity of likely contaminants.
There is currently no such fundamental understanding of the mechanisms involved in pedestrian slipping in the presence of dry contaminants.
It was therefore decided to conduct a small scoping study to determine the feasibility of developing a model for pedestrian slipping in the presence of dry contaminants. The aims of the scoping study were:
• To investigate the effects of particulate contaminants on coefficient of friction. • To generate simple mechanisms to describe the observed behaviour. • To generate an idea of the quantities of contaminant required to increased slip potential. • To provide a framework for interpreting slip resistance data. • To highlight potential routes for further research.
The development of a model for dry contaminant slipping would require a thorough understanding of contact mechanics and an understanding of how to model solid debris within a contact. It was therefore decided to conduct this work in conjunction with the Tribology Research Lab at the University of Sheffield as they have an established track record in researching contact mechanics and modelling contaminants within a contact. The aim of this report is to summarise the scoping study, which is reported in full in the University of Sheffield report, which is reproduced in Appendix 1.
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2 EXPERIMENTAL
Dry contaminant slip resistance tests were conducted using two different test methods, the pendulum test and the University of Sheffield low speed friction rig.
2.1 LOW-SPEED FRICTION RIG
Four-S samples were cut from standard pendulum slider samples (see Figure 2.6; 76 x 6.35 x 25.4mm) with the dimensions of 25 x 25mm (±1mm) and mounted on 1 inch diameter steel buttons using double sided tape (proved in preliminary tests to generate sufficient shearing resistance). The floor tile samples were cut to dimensions of 50 x 150mm and also fixed with double-sided tape after cleaning with acetone.
Figure 2.1: Low speed friction rig apparatus photo
The required load was positioned on the pivoting arm of the LSFR (see Figures 2.1 and 2.2) and Four-S ‘button’ was placed in its housing and aligned 10mm from the edge of the counter-face. Zeroing of the force transducer was performed and test initiated with a sample capture rate of 20Hz. Three runs were performed for each configuration of the apparatus.
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LOADSLIDER
SPECIMEN
COUNTERBALANCE FORCE
TRANSDUCER
BI-AXIALPIVOT
(a) Lateral view
(b) Plan view
FRICTIONALLOAD
MEASUREDFORCE
Figure 2.2: Sliding mechanisms observed on LSFR and pendulum tests
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2.2 PENDULUM FRICTION RIG
Preparation of the pendulum apparatus, shown in Figure 2.3, was conducted in accordance with the United Kingdom Slip Resistance Group (UKSRG) Guidelines [UKSRG], which are typically used by HSL for forensic site investigations.
Particles were applied by two methods, one equivalent to that used for the LSFR and the other using a paintbrush or finger to apply a very fine layer to the slider only. Two series of eight tests were performed for each configuration. The first set involved re-application of contaminant after every stroke whilst in the second set contaminant was applied prior to the first stroke only.
2.3 APPLICATION OF CONTAMINANTS Contaminants were applied to the floor tile and forced into depths of 500, 1000, 1500 and 2000µm by the use of specially made ‘combs’, see Figure 2.5. The resulting particle tracks had a width of 40mm allowing a 30% overlap each side of the slider. Problems were encountered with the smaller particles caused by high cohesiveness with sweeping occurring at all depths, preventing the generation of even layers. Subsequently a degree of compaction was required though the pressure applied was always less than that applied by the pendulum or low speed friction rig (LSFR).
High speed camera
Pendulum tester
Glass counter-face and powder
Slider
Figure 2.4: High speed video apparatus for Pendulum Tester
PENDULUM ARM
INITIAL POSITION
FOUR-S RUBBERSLIDER
SRV SCA LE SRV INDICATOR
126mm (±1mm) CONTACT STROKE
FLOOR SPECIMEN
Lateral view
PENDULUM ARM
INITIAL POSITION
FOUR-S RUBBERSLIDER
SRV SCA LE SRV INDICATOR
126mm (±1mm) CONTACT STROKE
FLOOR SPECIMEN
Lateral view
Figure 2.3: Stanley Pendulum Tester schematic
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Figure 2.5: ‘Combs’ used for generating specific particle thickness’
2.4 TEST SPECIMENS & CONTAMINANTS In both test configurations standard Four-S rubber specimens have been used (IRHD hardness 94° – 98°), as shown in Figure 2.6.
Figure 2.6: LSFR Four_S samples
Preparation of the Four-S sample followed a similar protocol to that adopted for pendulum tester where by the contact surface was initially conditioned with P400 abrasive paper followed by a series of unidirectional runs along wet 3µm pink lapping paper. It is suggested that this produces a similar surface to that seen in ‘bedded-in’ shoes. Table 2.1 shows the test contaminant particles used. These were chosen to be typical of particles found in factory situations and have been associated with slip risks. The particles sizes were measured using a laser scattering analyser. The full report in Appendix 1 gives photographs of the particles and the particle size distributions.
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Contaminant Ref.
No. Mean particle size (µm)
[Approximate] Uni-axial particle
compressive strain (at 125.6 kPa)
Talcum Powder 1 10 0.4 Cocoa Powder 2 15 0.2
Flour 3 25 0.25 Baking Powder 4 30 0.15
Bicarbonate of Soda 5 50 0.04 Table Salt 6 500 0.01
Ballotini (glass spheres) 7 500 0.01 Sand 8 1000 0.01
Table 2.1: Particle contaminant data
Floor materials were selected to achieve a range of surface roughness (shown in Table 2.2). In these tests, the roughness parameter Rz (mean peak to valley height) has been used. This is the parameter that HSL/HSE routinely records during site investigations to determine the roughness of a flooring material. The full report in Appendix 1 shows some photographs of the tile specimens used.
Tile Ref. Code Mean Rz (µm) Perlina polished marble PED/05/156 0.51
Polished Granite PED/05/158 0.85 Limestone PED/05/157 8.69 Olympus PED/05/155 12.32
Riven slate PED/05/159 34.18 Table 2.2: Tile counter-face data
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3 KEY FINDINGS
3.1 DRY CONTAMINANT LUBRICATION MECHANISMS Detailed observations of the LSFR and pendulum tests have identified three mechanisms for dry contaminant behaviour during a slip 1) sliding, 2) shearing and 3) tumbling and rolling which are illustrated in Figure 3.1.
Sliding: This mechanism occurs when the shear stress at the particle-shoe
interface is greater than the particle-floor interface, but less than the shear stress required to shear the particles within the layer.
Shearing: This mechanism occurs when the shear stress at the particle-flooring interface is greater than the shear stress required to shear the particles in the layer, but less than the shear stress at the particle-shoe interface.
Slider
Floor counter-face
Shear plane
Floor counter-face
Slider
Floor counter-face
SliderPowder
Slider
Floor counter-face
Shear plane
Floor counter-face
Slider
Floor counter-face
SliderPowder
Slider
Floor counter-face
Shear planeSlider
Floor counter-face
Shear plane
Floor counter-face
Slider
Floor counter-face
Slider
Floor counter-face
SliderPowder
Figure 3.1: Sliding mechanisms observed on LSFR and pendulum tests, where τps is the
shear stress at the particle-shoe interface, τpf is the shear stress at the particle-floor interface and τcrit is the shear stress required to shear the particles within the layer
SLIDING τps > τpf < τcrit : Entire powder layer has velocity of slider with extremely small surface layer of fine particles.
SHEARING τpf > τcrit < τps : Shear failure of particle layer occurs resulting in contact thinning and material seen in wake of slider.
ROLLING / TUMBLING For larger single particle layers, with counter face roughnesses to allow rolling at shear stresses < τpf, particles will roll and tumble rather than slide
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Tumbling/Rolling: This mechanism occurs for larger single particles layers, where the floor surface roughness is small compared to the particle diameter, such that rolling occurs at shear stresses less than the shear stress at the particle floor interface.
Slip resistance tests indicated that of the three dry contaminant mechanisms identified, sliding was generally associated with the greatest reduction in slip resistance, followed by rolling/tumbling and finally shearing which resulted in the smallest reduction in available friction.
These results imply that when attempting to control the slip risk posed by dry contaminants measures should be taken to select flooring and footwear, which will promote a shearing type mechanism for the particles in the shoe/floor contact.
3.2 EFFECT OF SURFACE ROUGHNESS
3.2.1 Components of Friction The friction in sliding contacts involving an elastomer (any rubber like material) is composed of two parts; adhesive friction (also known as adhesion, see glossary) and hysteretic friction (also known as hysteresis, see glossary) [Moore]. The total sliding friction is given by the sum of these two components:
HystereticAdhesiont fff += (1) Adhesive Friction When an elastomer, such as the pendulum slider or a shoe sole, comes into contact with a counter-face (a flooring material), temporary adhesive bonds are formed between them at the molecular level. When subjected to sufficient shear stress, the temporary bonds will stretch, break and reform in a cyclic slip-stick fashion. Adhesive friction is a measure of how difficult it is to break the temporary bonds between the elastomer and the counter-face. The more difficult it is to break the bonds the higher the level of adhesive friction. The magnitude of the adhesive force depends on the area of the elastomer in contact with the counter-face, which is directly related to the surface roughness of the counter-face, see Figure 3.2.
Figure 3.2: Schematic of effect of roughness on contact area
From Figure 3.2 it can be seen that increasing the surface roughness of the counter face reduces the area of contact between the elastomer and the counter-face. The highest levels of adhesive friction can therefore be reasonably be expected for the smoothest surfaces.
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Hysteretic Friction
The hysteresis component of friction is a measure of the way energy is absorbed internally by the elastomer. It arises mainly from the way in which the elastomer deforms around the asperities (peaks in the surface, see Appendix 1) on the counter-face surface. As the surface roughness of the counter face increases, the size of the asperities on the surface increases and the deformation of a elastomer in contact with it is increased. It would therefore be reasonable to expect the level hysteretic friction to be higher for rougher surfaces.
If an ideal symmetrical asperity is considered, see Figure 3.3a, the pressure profile developed will be symmetrical in the static case. However, if there is relative motion between the elastomer and asperity, as during the pendulum swing or when a pedestrian walks, the viscoelastic nature of the elastomer causes a non-symmetric pressure profile biased towards the direction of movement to be developed, see Figure 3.3b.
The asymmetric nature of the pressure profile generated when there is relative motion between the elastomer and the counter-face results in a net retardation force, which is responsible for the hysteretic component of friction.
The total friction measured in a sliding contact between an elastomer and a counter-face is combination of these two components, adhesive friction and hysteretic friction.
3.2.2 Pendulum Results
The Rz surface roughness values for the tiles used in this project are given in Table 3.1.
Tile Ref. code Mean Rz (µm) Perlina polished marble PED/05/156 0.51
Polished Granite PED/05/158 0.85 Limestone PED/05/157 8.69 Olympus PED/05/155 12.32
Riven slate PED/05/159 34.18 Table 3.1: Tile counter-face data
AsperityElastomerElastomer
Symmetric pressure profile(no relative motion)
Retarding bias (relativemotion)
a) Static case b) Relative motion between elastomer and counter-face.
Figure 3.3: Schematic of hysteretic deformation of an elastomer around and asperity
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Pendulum data for the tiles in the clean and dry condition and when contaminated with baking powder are shown in Figure 3.4.
Figure 3.4 Graph comparing PTV and coefficient of friction pendulum data for the flooring samples in the clean dry condition and in the contaminated condition. Rz surface roughness of the flooring samples increases from left to right.
In the clean dry condition the highest pendulum values and therefore the highest levels of total friction were achieved for the smoothest tiles. As the surface roughness increases the PTV value decreases, as might be reasonably expected from the reduced effect of adhesive friction. When the dry contaminant was introduced (baking powder) the PTV values measured were lower for all the floors studied. However the reduction in PTV observed decreased as the surface roughness of the flooring increased.
This finding suggests that the presence of a dry contaminant significantly reduces the effect of adhesive friction, which is the dominant friction component for smooth floor, but that the hysteretic friction, which is the dominant friction mechanism for rougher floors is relatively unaffected by the presence of a dry contaminant.
The pendulum test results suggest that surface roughness may have a significant role to play in controlling the slip risk posed by dry contaminants.
3.3 REPRODUCIBILITY OF DATA AND TRANSPORT OF CONTAMINATION
During tests with both the pendulum test and the LSFR apparatus dry contaminant particles tended to adhere to the slider surface. The adhesive nature of the sliders tends to cause particles to adhere to it. Even if the Four-S specimen was well cleaned (with a dry cloth), the PTV was lower than the water-cleaned sample by up to 10 - 20 PTV, particularly for the finer, more cohesive powders.
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The action of the pendulum tends to sweep contaminant particles away from the floor test area. Repeated test swings without the application of fresh contaminant generated similar results to those in which the contaminant was reapplied between each swing of the pendulum. Tests on the Perlina tile showed that on average the PTV for the different contaminants increased by approximately 3 points over a series for eight test swings of the pendulum without the application of fresh contaminant. Such smooth floors result in particle sliding for almost all cases (outlined in detail in the full report Appendix 1) with very small volumes of contaminant required to significantly reduce the friction measured by the pendulum (up to approximately 70 points). The pendulum results indicate that only a very small amount of dry contaminant particles are needed to appreciably reduce friction. The practical consequences of these results are that reductions in friction will be sustained for a number of steps on a smooth floor. This is analogous to the effects of liquids where a wetted slider will exhibit low friction coefficients on a smooth dry floor for a number of swings, but to a lesser degree.
This observation has serious implications for real workplace situations. It suggests that there is a high probability of pedestrians transporting dry contaminants such as dust over considerable distances into a building on their footwear. It again highlights the importance of entrance matting systems as a first line of defence for preventing the ingress of contamination in to building and further supports HSE/HSL’s position that the minute levels of contamination carried on a pedestrian’s footwear can be sufficient for certain types of flooring to pose a high potential for slip.
3.4 PARTICLE COHESIVENESS AND ITS IMPORTANCE
It was observed in both test methods that smaller particles form a compacted layer and a clean, ‘swept’ region behind the slider. Particles ahead of the slider do not get entrained and are instead forced along or displaced around the edges of the slider. This sliding mechanism causes low friction and warrants further investigation. For this condition to be observed, the following criteria must be satisfied:
rppf ττ < and critpf ττ < Where τpf is the shear stress at the particle-floor interface τrp is the shear stress at the rubber- particle interface τcrit is the required shear stress to shear the particles within the layer.
Thus the shear occurs at the particle floor interface, i.e. the dry contaminant particles and the pedestrian’s shoe stick together and slide.
All particles below approximately 50 m diameter, tended to clump together. This can be attributed to cohesive forces. Cohesiveness is a measure of the mechanical shear strength of a powder. A coulomb powder will generally follow a stress-strain curve as shown in Figure 3.5a, [Orband and Geldart]. When the shear strength of the particles is reached, failure occurs causing relative particle movement. There is a critical particle diameter below which, the shear strength rapidly increases (represented in Figure 3.5b). This critical particle diameter was found to be between 50 and 60µm.
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ε
σs
D
σs
Increasingconsolidation
pressure
Critical diameter Figure 3.5: (a) Generic stress-strain curve and (b) variation of critical stress with particle
diameter for powders Industrial experience suggests that particles below around 55µm cause increased handling problems as a result of this effect. With many food and industrial contaminants falling within this size range (e.g. Cocoa ~ 15µm, Baking Powder ~ 30 µm and Talc ~ 10 µm) the effects of cohesiveness are significant. In real workplace situations, the depth of the contaminant layer is likely to be small. The weight of the particles is therefore almost negligible and all layers of the powder can be thought of as uncompressed. In this situation the effects of cohesiveness generate macroscopic particulate structures that support the weight of the higher particle layers. The effect of these structures is to reduce the bulk density of the particles and decrease the macroscopic powder stiffness. Volumetric strain under compression can be a method for comparing the cohesiveness of particles and predicting their effect on CoF. Uni-axial compression was used to compare the particles and was performed using a syringe of known volume. Subjected to a pressure approximately equivalent to that used in the pendulum and sliding rig, the particles behaved as shown in Figure 3.6. It can be seen that a marked increase in the compressive strain occurs as the particle diameter decreases. The transition point occurs within the shaded band between approximately 45 and 60µm, which corresponds well the results obtained by Orband and Geldart [Orband and Geldart].
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Figure 3.6 Graph of powder compressive strain against particle diameter. This finding indicates that the powders of small particle size will agglomerate. This will promote the sliding mechanism, leading to a situation of high slip potential.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 100 200 300 400 500 600 700 800 900 1000
Particle diameter
Com
pres
sive
str
ain
Sand
BallotiniBicarbonate of Soda
Baking Powder
Flour
Cocoa
Talcum Powder
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3.5 SLIP MECHANISM MAP
A simple map showing the regions in which the different mechanisms occur is shown in Figure 3.7. This is based purely on particle diameter and surface roughness.
The map shows the following features:
(i) Sliding Occurs on Low Roughness Floors. It has been observed that sliding occurs when the flooring has a low roughness (in the order of a few microns) or when d/Rz is sufficiently large so as to make the flooring roughness negligible. From current studies, a potential transition occurs (between sliding and shearing) at a floor Rz surface roughness of approximately 15 – 20 µm for the tested contaminants. Floors with a roughness of less than this value do not allow sufficient mechanical interlocking between the asperities and contaminant to exceed τcrit and sliding occur, potentially causing a low CoF. Essentially a rough floor cuts through the layer of dry contaminant particles and stops it sliding as one block with the shoe surface.
(ii) The Cohesive Boundary. Particles tend to stick together when their size is below
approximately 55 m. This is approximately the size of bicarbonate of soda particles (and has been termed the ‘cohesive boundary’). Current observations suggest that the floor surface roughness for which the sliding mechanism occurs, decreases with particle size until the cohesive boundary is reached. This is likely to be due to the reduction of the cohesive forces, tending to prevent the agglomeration of particles. The critical level of surface roughness required to prevent the sliding mechanism increases as particle size increases.
(iii) Particle Layer Shearing Takes Place On Rough Floors. Inter-particle shearing
occurs when the surface roughness is large but particle diameter relatively small. Asperities on the floor surface prevent the particle layer sliding in a bulk form. As shearing takes place, the layer between slider (or a pedestrian’s shoe) and floor reduces in thickness as particles are lost. In the case of smaller cohesive particles, the uncompressed
Figure 3.7: Map of particle size, d and tile floor roughness,
Rz, showing regimes of slip mechanisms.
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material ahead of the contact is not entrained meaning that removal and thinning of material in the contact occurs. Increasing the particle diameter such that cohesive effects are negligible causes a reduction in the separation of shoe and floor often followed by particle tumbling and rolling.
(iv) Particle Rolling/Tumbling Occurs for the Larger Particles. Particle tumbling and
rolling occurs when there is sufficient traction to preferentially allow the particles to tumble and roll rather than slide against the floor. This occurs for particles of greater diameter than the cohesive boundary (~ 50 microns). Shape factor (see full report Appendix 1) greatly influences the mechanism, with particles of increased roundness rolling at lower levels of floor roughness.
(v) The Effect of Particle Shape. The map shows the variation of dry contaminant
mechanism with shape factor (see full report Appendix 1). A straight vertical shift of the boundaries has been predicted both below and above the cohesive boundary. This is shown as the red dotted lines on Figure 3.7. Further work is required as it is likely that shape factor has less of an effect below the cohesive boundary with inter-particle bonds being comparable to forces generated by mechanical interlocking.
(vi) Depth of the Particle Layer. The current map does not incorporate any information
about the depth of the particle layer. In the case of small cohesive powders, a range of particle depths will end in the same sliding mechanism. However significant depths may result in a transient progression from shearing to sliding (through depth loss from the shearing process). Shearing to rolling/tumbling will also occur for particles with a tendency to roll. This is discussed in detail in the University of Sheffield Report in Appendix 1 for ballotini particles. Further work is required to incorporate a quantitative depth / surface density function.
Region ‘A’ represents a high-risk situation where adhesive friction dominates. Very little contaminant is required to cause a significant reduction in coefficient of friction. Contamination can originate from areas external to the considered floor and be transported considerable distances by sticking to the sole for pedestrian’s footwear. Additionally, the small size and low volume of contaminants required to pose a slipping hazards on smooth floors, means sighting by pedestrians is unlikely. In a similar manner to liquid contaminants, polished floors in areas such as entrance halls present significant risks without sufficient measures to remove contamination from the shoes and surface in general. In addition, the sliding mechanism means powder is only required at the impact point with no relative motion between slider and powder layer. The rolling or tumbling and (to less of an extent) shearing mechanisms however, require a constant input of contaminant to maintain the effective lubrication and are therefore more dependant on a larger area of contamination.
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4 CONCLUSIONS
Adhesive friction is significantly affected by the presence of solid contaminants, but hysteretic friction is not. Smooth floors ( Rz of the order of a few m) are therefore more susceptible to contamination (up to a 70 PTV point reduction). Even very small quantities of dry contaminant particles can reduce the shoe-floor adhesion and cause a high slip risk for pedestrians. Small particles are able to adhere to the soles of shoes allowing transference of the contamination over appreciable distances and continued reduced friction coefficient for many steps. Three significant slipping mechanisms were observed within the contact; sliding, shearing and rolling/tumbling. Sliding and rolling on smooth floors pose the most significant risk. Sliding: This mechanism occurs when the shear stress at the particle-shoe
interface is greater than the particle-floor interface, but less than the shear stress required to shear the particles within the layer. Sliding contacts require contaminant only at the impact site with no further contaminant required to feed the contact.
Shearing: This mechanism occurs when the shear stress at the particle-flooring interface is greater than the shear stress required to shear the particles in the layer, but less than the shear stress at the particle-shoe interface. Leading the particle layer itself to shear.
Tumbling/Rolling: This mechanism occurs for larger single particles layers. Tumbling rolling contacts require a constant supply of contaminant throughout the slip (though only at a depth of a single particle).
Results indicate that the sliding and rolling mechanisms on smooth floors pose the most significant risk to pedestrians. Dry contaminants with a particle diameter of approximately 50
m or less have higher levels of cohesion and therefore are likely to promote the sliding mechanism in the contact.
Pendulum test results indicate that floor surface micro-roughness (Rz) may be a means of controlling the slipping hazard posed by dry contaminants.
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5 FUTURE WORK
The current scoping has highlighted a number of potential avenues for future research including:
• To identify the physical characteristics of flooring and footwear which promote good hysteretic friction to better inform advice to duty holders regarding controlling the hazards posed by dry contaminants
• To enlarge the database of laboratory data on slip resistance in particle contaminated conditions using the pendulum test.
• To further understand the mechanisms by which solid particles cause reduction in friction between the shoe and the floor.
• To reconsider the current suggested site investigation test procedure in cases where there is dry particulate contamination to ensure it is the best approach to take.
• To define simplified criteria and standards for advice on flooring and footwear requirements to deal with dry contaminants
• Examine the possibility of adapting current testing methods for assessing the anti-slip performance of footwear for use with dry contamination
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6 REFERENCES
P. Lemon and S. Griffiths, “Further Application of Squeeze Film Theory to Pedestrian Slipping.”, HSL report, IR/L/PE/97/9, 1997.
P. W Lemon, S. C Thorpe, S. L Jefferies, C. Sexton and M. Hawkins, “Pedestrian Slipping: Dry Contaminants.”, HSL Report, PE/01/15, Health & Safety Laboratory, 2001
D. F. Moore, ‘The Friction and Lubrication of Elastomers’ Pergamon Press, Oxford, Chapter 2, 1972.
J. L. R. Orband and D. Geldart ‘Direct measurement of powder cohesion using a torsional device’, Powder Technology 92, 25b – 33, (1997).
M. T. Richardson and R. S. Griffiths, “The Application of Squeeze Film Theory to Pedestrian Slipping Research.” HSL report, IR/L/PE/96/4, 1996.
UKSRG, United Kingdom Slip Resistance Group, “The Measurement of Floor Slip Resistance - Guidelines Recommended by the UK Slip Resistance Group”, Issue 3, October 2005.
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7 APPENDICES
APPENDIX 1 – UNIVERSITY OF SHEFFILED REPORT “ THE MECHANISMS OF PEDESTRIAN SLIPS CAUSED BY SOLID PARTICLE CONTAMINATION
The Mechanisms of Pedestrian Slips Caused by Solid Particle Contamination.
R. S. Mills R. S. Dwyer-Joyce
The Tribology Research Lab at the University of Sheffield The University of Sheffield
Department of Mechanical Engineering
Mappin Street
Sheffield S1 3DJ
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i Summary
An investigation has been undertaken to consider the effects of solid contaminant particles on
slipping risks to pedestrians in the workplace. Experiments were carried out using a sled-type
friction tester and a pendulum tester. In both cases emphasis was placed on observations of
the mechanisms of particle behaviour and slip. Results show that adhesive friction is
significantly affected whilst hysteretic friction is not. Smooth floors (in the order of a few
microns) can therefore be highly susceptible with a reduction in Pendulum Test Value (PTV)
of up to 70 points. Small particles can adhere to the soles of shoes allowing transference of
contaminant between different places and reducing the friction experienced by a pedestrian
for many steps. Very small amounts of particles were shown to lead to large reductions in
PTV.
Three slipping mechanisms were shown to occur; sliding, shearing and rolling/tumbling.
Sliding and rolling on smooth floors pose the most significant risk (medium to high on the HSL
slip risk classification scale). A simplified map of the regimes (of particle size and roughness)
where these mechanisms occur has been created.
The layer of particles required to cause increased risk depends on the mechanism. Sliding
requires particles at the impact point only, whilst rolling/tumbling requires a layer of fresh
particles for the duration of the slide. Further research into the relationship of different
roughness parameters and the use of a customised rig for the pendulum tester are suggested
to progress the current findings.
20
ii Contents The Mechanisms of Pedestrian Slips Caused by Solid Particle Contamination. .................... 19 i Summary .......................................................................................................................... 20 ii Contents ........................................................................................................................... 21 1.0 Nomenclature ............................................................................................................... 23 2.0 Introduction ................................................................................................................... 24
2.1 Aims:...................................................................................................................... 24 3.0 Background................................................................................................................... 25
3.1 Current status of work performed by the HSL....................................................... 25 3.2 Pendulum friction tester......................................................................................... 25 3.3 Low speed friction rig (LSFR): Sled-type tester .................................................... 26 3.4 Forces and velocity in human Gait ........................................................................ 27 3.5 Potential problems with test methods.................................................................... 28
4.0 Experimental technique and apparatus ........................................................................ 30 4.1 Low-speed friction rig ............................................................................................ 30 4.2 Pendulum friction rig.............................................................................................. 32 4.3 Application of Contaminants.................................................................................. 32 4.4 Test Specimens & Contaminants .......................................................................... 33
5.0 Results and Discussion: ............................................................................................... 35 5.1 Particulate lubrication mechanisms in the shoe-floor contact ............................... 35 5.2 Effect of particles on coefficient of friction ............................................................. 40 5.3 Repeat testing (single application of contaminant to slider).................................. 45 5.4 Particle cohesiveness and its importance ............................................................. 46 5.5 Map of Particle Slip Mechanisms .......................................................................... 48 5.6 Effect of applied pressure on traction.................................................................... 51 5.7 Effect of shape factor on traction........................................................................... 52
6.0 Conclusions .................................................................................................................. 54 7.0 Implications of findings .......................................................................................... 55 7.1. Implications for the HSE:........................................................................................... 56
Forensic and site investigations........................................................................................... 56 8.0 Future Work .................................................................................................................. 58 9.0 References.................................................................................................................... 60 10.0 Glossary of terms ...................................................................................................... 61 Appendix 1: Particle photos................................................................................................ 63 Appendix 2: Tile photos ........................................................................................................... 64 Appendix 3............................................................................................................................... 64 Appendix 3............................................................................................................................... 65 Appendix 4............................................................................................................................... 66 Appendix 5: Description of Shape factor (F) ...................................................................... 67
21
Appendix 6: Selected test data:............................................................................................... 68 6.1 LSFR test data....................................................................................................... 68 6.2 Perlina tile test results (Pendulum method)........................................................... 68 6.3 Clean tile tests (Pendulum method) ...................................................................... 69
22
1.0 Nomenclature
Rz
µS
µd
θ
P
R
p
A
E*
δ
D
hx
V
F
τx
Mean peak–valley height roughness (µm)
Static coefficient of friction
Dynamic coefficient of friction
Slider angle of attack (degrees)
Applied normal load (N)
Radius of curvature (m)
Particle perimeter (m)
Particle or contact area (m2)
Modified contact stiffness (Nm-2)
Contact deformation (m)
Particle diameter (m)
Contact separation at position x (m)
Relative contact sliding velocity (ms-1)
Shape factor
Shear stress at interface ‘x’
23
2.0 Introduction
Slips, trips and falls are a significant cause of accidents in the workplace causing as many as
one in three major workplace accidents [1]. Reduction in the coefficient-of-friction (CoF) due
to the presence of a liquid is well understood and models are available (Reynolds equation
[2]) to predict the behaviour of a contact subjected to such contamination. There are also
readily available simple criteria for use by flooring manufacturers, factory operators, and
inspectors for liquid slip resistance.
Solid particulate contaminants pose more of a problem. The particles come in many forms;
they are discrete in nature (they are not a continuum like liquids) and their size can be large
relative to the floor roughness. This leads to many mechanical properties such as stiffness,
fracture toughness and hardness being possible factors determining the CoF when particles
are present. Inter-particle forces which are fundamental for much of the continuum behaviour
of liquids are very much smaller for particles and vary substantially with particle size.
This report is intended to highlight the key parameters involved in slipping on solid particle
contamination, develop simple models for the mechanisms involved and suggest possible
avenues for future research. This study is not exhaustive, it is intended to generate a
framework for interpreting data, accidents, and developing criteria for slip resistance in
particle contamination cases.
2.1 Aims:
• To investigate the effects of particulate contaminants on coefficient of friction.
• To generate simple mechanisms to describe the observed behaviour.
• To generate an idea of the quantities of contaminant required to cause increased slip
potential.
• To provide a framework for interpreting slip resistance data.
• To highlight potential routes for further research.
24
3.0 Background
3.1 Current status of work performed by the HSL
To date, the HSL have only undertaken relatively small projects to investigate the risks
associated with solid particle contaminants [3] and [4] and have concentrated on empirical
data obtained from the Stanley Pendulum Tester and ‘Sled-Type’ testers. The results
illustrated the effects of solid particulate contamination clearly. But the work was not aimed at
mechanistically describing the processes involved. This project has been undertaken to
progress the work and apply simple models to the observed phenomena.
An important finding proposed in [3] was that the roughness of the flooring material would not
play a fundamental role unless it was comparable to the dimensions of the contaminant. The
work of this report suggests that roughness can be important at all scales and controls the
effect of the contaminants on the different friction components (adhesion and hysteretic). It is
acknowledged however that further testing is required to construct a comprehensive
knowledge base.
The current investigation concentrates on friction measurements from high speed impacts
using the Stanley Pendulum Tester and low speed continuous force measurement using a
custom built sled-type rig.
3.2 Pendulum friction tester
The preferred method used by the HSL to determine the CoF in the field is the Stanley
Pendulum Tester [1]. This apparatus consists of a weighted slider fixed to the end of a
pivoting arm. The arm is released from an initially horizontal position and contact with the
flooring material occurs at the bottom of the stroke for a distance of 126 mm. As the slider is
drawn across the flooring material, a portion of the kinetic energy is lost due to the frictional
retarding force, resulting in a lower final arm position. The output scale of ‘Pendulum Test
Value’ (PTV) can be converted to CoF using equation 1 (from BSI 96/104915 [B/208]).
1
31110 −
⎟⎠⎞
⎜⎝⎛ −=
PTVµ (1)
Based on observed PTV results, floor condition is given a grade corresponding to an
expected slipping risk as shown in Table [1].
25
Pendulum PTV Associated ‘slip potential’ 0 - 24 High 25 - 35 Moderate
36+ Low Table [1]: Slip risk classification, based on pendulum test values [1]
For clean dry and wet conditions, the pendulum tester has been shown to be effective for
measuring the friction between the rubber test slider and floor material [1]. However, solid
particle contaminants are more problematic. For all pedestrian-slipping applications, a fluid
can be considered a continuum with the surface roughness being far greater than the size of
the fluid particles. Intermolecular bonding and very short range equalisation (pressure etc.)
means that the properties such as particle stiffness and geometry are far less significant.
Instead the shearing parameters such as viscosity take precedence. Particles on the other
hand, act in a far more discrete manner with stiffness, strength, geometry and size potentially
being significant. This makes the modelling procedure a much more complex task.
3.3 Low speed friction rig (LSFR): Sled-type tester
The low speed friction rig is similar in action to the sled type testers held by the HSL. The
dynamics for such devices are very different from the pendulum tester (the velocity between
slider and tile are around 3 orders of magnitude different). However similar particle movement
mechanisms were observed in both apparatus. Additionally, a transient loading profile can be
obtained which can be used to observe effects such as particle layer thinning.
The basic operation of the device is to apply a constant normal load to the slider specimen.
The tile is then drawn past the slider at constant velocity, generating a retarding force that is
measured by a compression load cell. The transient CoF can then be determined. This could
potentially be of much more use than the pendulum PTV, as it gives information that is
dependent on time and therefore stroke position. Since the critical cause of a slide is likely to
occur near the impact site, the resolved data is useful as it highlights low CoF periods. It
should be mentioned that the future work section outlines a customised rig that could be
incorporated into the pendulum laboratory tests of tile specimens. This would enable
pendulum dynamics (and standard PTV results) to be coupled with transient CoF
measurements that could be resolved in both time and stroke position.
26
3.4 Forces and velocity in human Gait
As the heel of the foot is brought into contact with the ground (heel contact - HC), the typical
impact velocity is 0.2 ms-1 [5] for unrestricted, level walking. If the pedestrian is transporting a
load this value can increase (though Redfern and Rhoades [6] found that for loads of up to
13.5 kg the HC velocity varied between 0.14 and 0.24 ms-1). The stated values are the mean
from a number of subjects but it is important to note that subject build, mobility and age are all
factors that will affect the leg dynamics. The velocity of the pendulum is therefore around one
order of magnitude (ten times) larger than actual heel velocity. The angle of attack of the
slider at contact is 26º which is comparable to the 23.5º [5] seen at HC.
The loading at HC in a human is highly variable and depends upon parameters such as body
mass and gait with some subjects arresting the movement of their foot prior to contact and
others using the ground (Whittle [7]). Redfern et al [5] suggest that for walking on a horizontal
surface, the peak load (expressed as impact force per unit body weight) would be
approximately 10 Nkg-1 (or roughly equal to body weight) occurring at approximately 25% into
the stance phase. However the foot reaches the ‘foot-flat’ (FF) position approximately 15%
into the stance phase at which point the reaction force is around 8.5 NKg-1, the point of slip
initiation is likely to occur before FF whilst the shoe makes a non-zero angle with the floor.
The pressure generated will depend upon the area of shoe in contact with the ground of
which little literature is available (highly shoe and gait dependant). The pendulum generates a
contact pressure of 126 kPa when the chamfered contact edge has a width of 2.33 mm and is
equivalent to the low speed friction rig (LSFR – also used in this work) applying 8 kg to the
rubber specimen. For a human weighing 80 kg, the equivalent area of contact required
immediately prior to FF would be up to 5400 mm2 (approximate are of rear raised portion of
shoe). The larger the angle the foot makes with the floor (i.e. closer to 23º) the smaller the
area of contact, resulting in an increase in contact pressure.
Though the maximum shear force of approximately 1.5 Nkg-1 occurs at a position 19% into
the stance (Redfern et al [5]), the critical position in the stance where the shear to normal load
ratio is largest and contact area smallest is likely to occur within the blue shaded region (A) of
figure [1] (before FF), when the shear stress ranges from 0 to around 1.25 Nkg-1.
27
Figure [1]: Sketch of relative variation of normal and shear reactions over a single step (based
on figure (1) from [3]), HC – Heel Contact, FF – Floor Flat, TO – Toe Off.
Human Pendulum LSFR
Slider Velocity (m/s) 0.2 3.2 0.006
Angle (degrees) 23.5 26 0
Applied pressure (KPa) Variable ≈ 126 126 126
Table [2]: Approximate conditions of the different contacts
3.5 Potential problems with test methods
The Pendulum Test The pendulum test is a convenient and realistic method for determining slip resistance. The
dynamics are closer to the real mechanisms of human slip (i.e. the heel striking the floor).
than conventional sliding friction testers (where one surface slides over another). However,
the impact velocity of the pendulum when it strikes the floor is approximately 3.2ms-1.
It is possible that contact pressures generated in human movement maybe to be larger than
those modelled using the pendulum or low speed friction rig (LSFR) as a result of the small
contact area at and for a short time after impact, meaning that care must be taken when
analysing recorded data.
The larger impact velocity or preceding air ‘wave’ ahead of the slider may act to scatter the
particles to a greater degree than the heel of a shoe, generating a less accurate
representation of impact conditions. It will be demonstrated in the report this did not prove to
be a major issue.
An inherent limitation of the pendulum is its inability to measure the transient properties of a
slide. All the ‘information’ within the slide is fused into a single PTV measurement. It is not
possible to extract the effects of local phenomena within the slide other than by visualisation.
28
Sled-Type Friction Testers It has been suggested that sled type testers do not accurately mimic the conditions of impact
for a floor that has been contaminated with a liquid. This may lead to results that are not
representative of an actual slipping situation. Though some work has been performed with
respect to particle contaminants [3] further work is required to consider its suitability for such
an application.
The relative velocity between slider and flooring is much lower than that seen in the pendulum
and human gait (the device used in this study had a velocity around 3 orders of magnitude
smaller than that of the pendulum).
The angle generated between floor and slider is 0º which is greatly different to the 23.5º
observed in humans at the point of impact. The effects of a converging contact are therefore
not simulated.
29
4.0 Experimental technique and apparatus
4.1 Low-speed friction rig
Four-S samples were cut from standard pendulum slider samples (76 x 6.35 x 25.4mm) with
the dimensions of 25 x 25mm (±1mm) and mounted on 1 inch diameter steel buttons using
double sided tape (proved in preliminary tests to generate sufficient shearing resistance). The
floor tile samples were cut to dimensions of 50 x 150mm and also fixed with double-sided
tape after cleaning with acetone.
Figure 2: Low speed friction rig apparatus photo
95
380
45
LOADSLIDER
SPECIMEN
COUNTERBALANCE FORCE
TRANSDUCER
BI-AXIALPIVOT
(a) Lateral view
(b) Plan view
FRICTIONALLOAD
MEASUREDFORCE
Figure 3: Low speed friction rig schematic of operation
30
The required load was positioned on the pivoting arm (figure [3]) of the LSFR (figure [2]) and
Four-S ‘button’ was placed in its housing and aligned 10mm from the edge of the counter-
face. Zeroing of the force transducer was performed and test initiated with a sample capture
rate of 20Hz. Three runs were performed for each configuration of the apparatus.
31
4.2 Pendulum friction rig
Preparation of the pendulum apparatus (figure [4]) followed exactly, the standard HSL
procedure [12].
PENDULUM ARM
INITIAL POSITION
FOUR-S RUBBERSLIDER
SRV SCA LE SRV INDICATOR
126mm (±1mm) CONTACT STROKE
FLOOR SPECIMEN
Lateral view
PENDULUM ARM
INITIAL POSITION
FOUR-S RUBBERSLIDER
SRV SCA LE SRV INDICATOR
126mm (±1mm) CONTACT STROKE
FLOOR SPECIMEN
Lateral view
Figure 4: Stanley Pendulum Tester schematic
Particles were applied by two methods, one equivale
other using a paintbrush or finger to apply a very fine la
Two series of eight tests were performed for each c
application of contaminant after every stroke whilst in th
prior to the first stroke only.
4.3 Application of Contaminants
Contaminants were applied to the floor tile and force
2000µm by the use of specially made ‘combs’ (figure
width of 40mm allowing a 30% overlap each side of t
with the smaller particles caused by high cohesiveness
preventing the generation of even layers. Subsequentl
though the pressure applied was always less than that
friction rig (LSFR).
32
High speed camera
Pendulum tester
Glass counter-face and powder
Slider
Figure 5: High speed video apparatus for
Pendulum Tester
nt to that used for the LSFR and the
yer to the slider only.
onfiguration. The first set involved re-
e second set contaminant was applied
d into depths of 500, 1000, 1500 and
[6]). The resulting particle tracks had a
he slider. Problems were encountered
with sweeping occurring at all depths,
y a degree of compaction was required
applied by the pendulum or low speed
Figure [6]: ‘Combs’ used for generating specific particle thickness’
4.4 Test Specimens & Contaminants
In both test configurations standard Four-S rubber specimens have been used (IRHD
hardness 94° – 98°), as shown in Figure [7].
Figure [7]: LSFR Four_S samples
Preparation of the Four-S sample followed a similar protocol to that adopted for pendulum
tester where by the contact surface was initially conditioned with P400 abrasive paper
followed by a series of unidirectional runs along wet 3µm pink lapping paper. It is suggested
that this produces a similar surface to that seen in ‘bedded-in’ shoes.
Table [3] shows the test contaminant particles used. These were chosen to be typical of
particles found in factory situations and have been associated with slip risks. The particles
sizes were measured using a laser scattering analyser. Appendix 1 gives photographs of the
particles.
Contaminant Ref. No.
Mean particle size (µm) [Approximate]
Uni-axial particle compressive strain
(at 125.6 kPa) Talcum Powder 1 10 0.4 Cocoa Powder 2 15 0.2
Flour 3 25 0.25 Baking Powder 4 30 0.15
33
Bicarbonate of Soda 5 50 0.04 Table Salt 6 500 0.01
Ballotini (glass spheres) 7 500 0.01 Sand 8 1000 0.01
Table [3]: Particle contaminant data
Floor materials were selected to achieve a range of surface roughness (shown in Table [4]).
In these tests, the roughness parameter Rz (mean peak to valley height) has been used. This
is the parameter that HSL records in determining the roughness of a flooring material.
Appendix 2 shows some photographs of the tile specimens used.
Tile Ref. code Mean Rz (µm) Perlina polished marble PED/05/156 0.51
Polished Granite PED/05/158 0.85 Limestone PED/05/157 8.69 Olympus PED/05/155 12.32
Riven slate PED/05/159 34.18 Table [4]: Tile counter-face data
34
5.0 Results and Discussion: Results are divided into two parts. The first tests describe experiments performed to visualise
the mechanisms of particle motion in both the pendulum and the sled-type LSFR tester. This
is followed by data obtained using the pendulum tester to investigate the effects of particle
type and floor surface.
5.1 Particulate lubrication mechanisms in the shoe-floor contact
Observations of the LSFR and pendulum tests have highlighted three mechanisms for particle
movement during a slip. These have been termed sliding, shearing and rolling/tumbling. A
schematic for the mechanisms is given in figure [8].
Slider
Floor counter-face
Shear plane
Floor counter-face
Slider
Floor counter-face
SliderPowder
Slider
Floor counter-face
Shear plane
Floor counter-face
Slider
Floor counter-face
SliderPowder
Slider
Floor counter-face
Shear planeSlider
Floor counter-face
Shear plane
Floor counter-face
Slider
Floor counter-face
Slider
Floor counter-face
SliderPowder
SLIDING τps > τpf < τcrit : Entire powder layer hasvelocity of slider with extremely smallsurface layer of fine particles.
SHEARING τpf > τcrit < τps : Shear failure of particlelayer occurs resulting in contact thinningand material seen in wake of slider.
Figure 8: Sliding mechanisms observed on LSFR and pendulum tests
ROLLING / TUMBLING For larger single particle layers, withcounter face roughnesses to allow rollingat shear stresses < τpf, particles will rolland tumble rather than slide
35
Sliding generally results in the lowest friction, then rolling/tumbling, followed by shearing. In
an ideal situation the conditions should be designed to promote shear of the particle layer.
This is achieved by rougher surfaces (compared to the particle size) that cuts through the
layer, or by particles which to not adhere into a clump.
Which of these mechanisms occurs can be deduced by observing the wake and under-side of
the slider at the end of the slip. If no material is present in the wake and a compacted layer is
observed on the slider, the particles must slide against the flooring material and remain as a
bulk layer. A gradually thinning layer suggests shearing is occurring. If the wake contains
particles and the slider has striations there must be relative motion between the particles and
both the slider and the counter-face. This suggests that rolling and tumbling of the particles
must be occurring.
High speed camera photos of pendulum pad Visualisation of particle movement during the pendulum test was obtained using high-speed
video. Images were recorded up through a sheet of toughened float glass as the slider
passed over. Video was recorded at a frame rate of approximately 2500 frames per second
(fps). This enabled the different mechanisms to be viewed directly. Three cases were studied
to demonstrate the mechanisms of sliding, shearing, and rolling/tumbling.
Case 1: Talcum powder
Figure [9] shows the results of talcum powder. The talcum powder layer adheres to the shoe
and slides across the glass.
The path swept by the pad is almost completely devoid of particles as a result of the sliding
layer. This suggests that the volume of material required in maintaining the slip is very small.
The sliding layer of contaminant that slides with the pad can be seen in the magnified view of
figure [9b].
This also means that only particles are needed at the actual point of heel strike for slip to
occur (as they are effectively carried with the shoe). There is no need for continual
replenishment.
36
Figure [9a and b]: High speed images of talcum powder contaminant in pendulum contact
It is interesting to note that immediately after impact; the pad oscillates vertically slightly and
results in the deposition of a small amount of material. This is only seen in the case of the
smallest and most cohesive powders though evidence of this effect (through deposition of pad
material) can be seen on both the P400 abrasive and pink lapping papers.
Case 2: Bicarbonate of Soda
The experiments using bicarbonate of soda showed a transitional situation. Initially shearing
(figure [10a]) occurs immediately after impact. The particles can be seen passing through the
contact and remaining on the glass surface. This occurs whilst the layer of particles is thick.
The layer then shears down to a thin layer. Then sliding takes place (figure [10b]). It is likely
that the initial shearing reduces the depth of the particle layer to a critical height when the
required critical shear force (τCrit) exceeds the critical sliding stress at the particle-floor
37
interface (τpf). Again, in this case, material is only required at the impact as no new material
will be entrained into the contact.
Figure [10a and b]: High speed images of Bicarbonate of soda contaminant in pendulum
contact (a. immediately following impact, b. Close to end of stroke)
Case 3: Ballotini
Ballotini are glass micro-spheres, very close to spherical in shape. In the case of experiments
using these particles (figure [11a and b]) rolling of the particles is observed. The particles
pass through the contact and remain in the swept path. In this case, relative motion of the
particles with the contact results in their ejection at the rear of the pad. This means that a
constant contaminant supply ahead of the pad is required in order to maintain the reduced
CoF.
38
Figure [11a and b]: High speed images of ballotini contaminant in pendulum contact
39
5.2 Effect of particles on coefficient of friction
Particles on a Smooth Tile Pendulum tests have shown that for smoother tiles (in the order of a few microns), a small
volume of contaminant at the heel of the footwear can result in a substantial reduction in
friction. Figure [12] shows the variation of the CoF with conditions for the Perlina
(PED/05/156). All contaminants cause a large drop in frictional resistance with one
constituting a high slip potential, two a moderate slip potential and only one a low slip
potential. This generates similar implications, though less severe, to those seen in fluids,
where a thin film of liquid forms in the converging channel between rubber and counter-face.
It is important to note that a degree of variation occurs in the clean stroke. This is likely to
result from minute contamination causing disruption to the adhesive friction component.
Figure [12]: Effect of particles on coefficient of friction, pendulum test data on Perlina tile.
Adhesive and Hysteretic Friction
The friction in sliding contacts involving an elastomer has two parts; adhesion and hysteresis
[9]. The total sliding friction is given by:
HystereticAdhesiont fff += (2)
40
Adhesive bonds on a molecular level occur between the elastomer and counter-face which
when subjected to a sufficient shear stress, stretch, break and reform in a cyclic slip-stick
fashion. The magnitude of the adhesive force depends on the area of the elastomer in contact
with the counter-face, which is directly related to the surface roughness of the counter-face
(figure [13]).
Figure [13]: Schematic of effect of roughness on contact area
Increasing the roughness reduces the area of contact. Therefore high total friction occurs for
the smoothest dry surfaces such as float glass and polished marble. This initially reduces as
roughness increases. However as roughness increases, deformation of the elastomer around
the asperities increases and leads to the hysteresis component of friction increasing (this is
the energy absorbed internally by the elastomer).
If a symmetrical asperity is considered, the pressure profile developed will be symmetrical in
the static case. However if the relative motion between the elastomer and asperity is not zero,
the viscoelastic nature of the elastomer causes a non-symmetric pressure profile biased
towards the direction of movement and generates a net retardation force in addition to any
adhesive retardation, as shown in figure [14].
The EffeTable 1
and con
Tile roug
AsperityElastomerElastomer
Symmetric pressure profile(no relative motion)
Retarding bias (relativemotion)
Figure [14]: Schematic of hysteretic deformation
ct of Rough Tiles shows the tiles used in these tests along with their Rz value. In figure [15] the clean
taminated (with baking powder) PTVs for the different flooring materials are shown.
hness increases from left to right.
41
The highest friction occurs for the clean smooth tiles. As roughness increases this drops. In
all cases the presence of particles causes a reduction in friction. This is because the particles
block the adhesion between shoe and floor. However, this reduction is greatest for the
smooth floor cases. For these tiles the adhesion mechanisms of friction is dominant (and this
has been removed by the particles). For the rougher tiles the hysteretic friction is more
important, this is not affected by the presence of particles.
Figure [15]: Effect of flooring roughness on coefficient of friction
The difference between clean and contaminated friction is shown on the plot. This clearly
decreases with an increase in surface roughness. This may suggest that roughness reduces
the effect a contaminant will have on available friction.
Further testing is required with additional contaminants to generate a comprehensive
understanding though it is likely that all will disrupt the adhesive friction component in a
similar manner.
Tile Roughness Profiles – Roughness on Different Scales Figure [16] shows the measured surface profiles for a sample of each of the floorings. Current
experimental work concentrates only on mean peak to trough height (Rz). However, further
work is required to consider whether parameters such as the local micro roughness and
asperity pitch affect traction.
42
Figure [16]: Surface profiles for selected tiles
A difference in micro-roughness can clearly be seen between the Slate (PED/05/159) and
Olympus (PED/05/155) tiles. It is plausible that the Olympus tile would give a (relatively) lower
friction than the Slate for a given particle size as a result of an increased tendency to slide on
the smoother micro-roughness surface. The micro-rough slate tile would break-up
contaminant layers and cause more shearing.
Particles Adhering to the Shoe Specimen During the tests particles tend to adhere to the shoe surface. The adhesive nature of the
elastomer tends to cause particles to adhere to it. Even if the Four-S specimen was well
cleaned (with a dry cloth), the PTV was lower than the water-cleaned sample by up to 10 - 20
PTV, particularly for the finer, more cohesive powders. It is essential that all specimens were
thoroughly water cleaned between tests.
This phenomenon was also observed with the LSFR where visually clean specimens (cloth
wiped) showed reduced traction after having been in contact with the contaminant (figure
[17]).
43
Figure [17]: Visually clean elastomer specimens tested on steel (having been in contact with
table salt) – water cleaned specimen (highest CoF) in pink
44
5.3 Repeat testing (single application of contaminant to slider)
Repeated sliding without the application of fresh contaminant to the slider generated similar
results to those in which the contaminant was reapplied after each swing of the pendulum
(figure [18]). The action of the pendulum sweeps particles away from the floor. However, this
does not have a big effect on friction. This indicates that only a very small amount of particles
are needed to reduce friction.
The average increase in PTV for the different contaminants was 2.6 for eight passes of the
slider. Such smooth floors result in particle sliding for almost all cases with very small
volumes (in the order of 2 – 3mm3) of contaminant required to significantly reduce the friction
seen by the pendulum (up to approximately 70 points).
Figure [18]: Variation of slip potential for various contaminants against Perlina tile following a
single application of contaminant to elastomer pad.
The practical consequences of these results are that reductions in friction will be sustained for
a number of steps on a smooth floor. This is analogous to the effects of liquids where a
wetted elastomer will exhibit low friction coefficients on a smooth dry floor for a number of
swings.
45
5.4 Particle cohesiveness and its importance
It has been observed in both tests that smaller particles form a compacted layer and a clean,
‘swept’ region behind the pad. Particles ahead of the slider do not get entrained and are
instead forced along or displaced around the edges of the slider. This sliding mechanism
causes a low friction and warrant further investigation. For this condition to be observed, the
following criteria must be satisfied:
rppf ττ < and critpf ττ <
Where τpf is the shear stress at the particle-floor interface
τrp is the shear stress at the rubber- particle interface
τcrit is the required shear stress to shear the particles within the layer.
Thus the shear occurs at the particle floor interface. The particles and the shoe stick together
and slide.
All particles below approximately 50 microns, tended to clump together. This can be attributed
to cohesive forces. Cohesiveness is a measure of the mechanical shear strength of a powder.
A coulomb powder will generally follow a stress-strain curve as shown in figure [19a] [10].
When the shear strength of the particles is reached, failure occurs causing relative particle
movement. There is a critical particle diameter below which the shear strength rapidly
increases (represented in figure [19b]). This critical particle diameter was found to be between
50 and 60µm.
i
ii
iii
ε
σs
D
σs
Increasingconsolidation
pressure
Critical diameter Figure[19]: (a) Generic stress-strain curve and (b) variation of critical stress with particle
diameter for powders
Industrial experience suggests that particles below around 55µm cause increased handling
problems as a result of this effect. With many food and industrial contaminants falling within
this size range (e.g. Cocoa ~ 15µm, Baking Powder ~ 30 µm and Talc ~ 10 µm) the effects of
cohesiveness are significant.
46
In the case of spillages and contaminant situations of potential risk to humans, the depth of
the contaminant layer is small. The weight of the particles is therefore almost negligible and
all layers of the powder can be thought of as uncompressed. In this situation the effects of
cohesiveness generate macroscopic particulate structures that support the weight of the
higher particle layers. The effect of these structures is to reduce the bulk density of the
particles and decrease the macroscopic powder stiffness.
Volumetric strain under compression can be a method for comparing the cohesiveness of
particles and predicting their effect on CoF. Uni-axial compression was used to compare the
particles and was performed using a modified syringe of known volume. Subjected to a
pressure approximately equivalent to that used in the pendulum and sliding rig, the particles
behaved as in figure [20]. It can be seen that a marked increase in the compressive strain
occurs as the particle diameter decreases. The transition point occurs within the shaded band
between approximately 45 and 60µm, this corresponds well the results obtained by [10].
Th
tha
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 100 200 300 400 500 600 700 800 900 1000
Particle diameter
Com
pres
sive
str
ain
Sand
BallotiniBicarbonate of Soda
Baking Powder
Flour
Cocoa
Talcum Powder
Figure 20: Graph of powder compressive strain against particle diameter
e consequences of this finding are that the smaller powders will agglomerate. This means
t sliding will occur, leading to a situation of high slipping potential.
47
5.5 Map of Particle Slip Mechanisms
A simple map showing the regions in which the different mechanisms occur is shown in figure
[21]. This is based purely on particle diameter and surface roughness.
The ma
(i) Sw
s
a
a
th
a
E
b
(ii) Ta
h
ro
dcrit ~ 55µm
Rz crit ~ 15-20 µm
Rolling &Tumbling
Shearing
Sliding
Rz
Surf
ace
roug
hnes
s
Particle diameter
Decreasing FIncreasing F
Figure [21]: Map of particle size, D and tile floor roughness, Rz, showing regimes
of slip mechanisms.
p shows the following features:
liding Occurs on Low Roughness Floors. It has been observed that sliding occurs
hen the flooring has a low roughness (in the order of a few microns) or when d/Rz is
ufficiently large so as to make the flooring roughness negligible. From current studies,
potential transition occurs (between sliding and shearing) at a floor roughness of
pproximately 15 – 20 µm for the tested contaminants. Floors with a roughness of less
an this value do not allow sufficient mechanical interlocking between the asperities
nd contaminant to exceed τcrit and sliding occur, potentially causing a low CoF.
ssentially a rough floor cuts through the layer of particles and stops it sliding as one
lock with the shoe surface.
he Cohesive Boundary. Particles tend to stick together when their size is below
pproximately 55 microns. This is around the size of bicarbonate of soda particles (and
as been termed the ‘cohesive boundary’). Current observations suggest that the floor
ughness for the sliding mechanism to occur, drops with particle size until the
48
cohesive boundary is reached. This is likely to be due to the reduction of the cohesive
forces, tending to prevent the agglomeration of particles. Bigger particles need a larger
roughness to stop them sliding as a result of the d/Rz ratio.
(iii) Particle Layer Shearing Takes Place On Rough Floors. Inter-particle shearing
occurs when the roughness is large but particle diameter relatively small. The
asperities prevent the particle layer sliding in a bulk form. As shearing takes place, the
layer between shoe and floor reduces in thickness as particles are lost. In the case of
smaller cohesive particles, the uncompressed material ahead of the contact is not
entrained meaning that removal and thinning of the contact occurs. Increasing the
particle diameter such that cohesive effects are negligible causes a reduction in the
separation of shoe and floor often followed by particle tumbling and rolling.
(iv) Particle Rolling/Tumbling Occurs for the Larger Particles. Particle tumbling and
rolling occurs when there is sufficient traction to preferentially allow the particles to
tumble and roll rather than slide against the floor. This occurs for particles of greater
diameter than the cohesive boundary (~ 50 microns). Shape factor (see Appendix 5)
greatly influences the mechanism, with particles of increased roundness rolling at lower
floor roughness.
(v) The Effect of Particle Shape. The map shows the variation of mechanism with shape
factor (section 5.7). A straight vertical shift of the boundaries has been predicted both
below and above the cohesive boundary. This is shown as the red dotted lines on
figure 21. Further work is required as it is likely that shape factor has less of an effect
below the cohesive boundary with inter-particle bonds being comparable to forces
generated by mechanical interlocking.
(vi) Depth of the Particle Layer. The current map does not incorporate any information
about the depth of the particle layer. In the case of small cohesive powders, a range of
particle depths will end in the same sliding mechanism. However significant depths
may result in a transient progression from shearing to sliding (through depth loss from
the shearing process). Shearing to rolling/tumbling will also occur for particles with a
tendency to roll. This can be seen clearly in appendix 3 for ballotini particles. Further
work is required to incorporate a quantitative depth / surface density function.
The red region represents a high-risk situation where adhesive friction dominates. Very little
contaminant is required to cause a significant reduction in traction. Contaminant can originate
from areas external to the considered floor and be transported by sticking to the sole.
Additionally, the small size and low slipping volume means sighting by pedestrians is unlikely.
In a similar manner to liquid contaminants, polished floors in areas such as entrance halls
49
present significant risk without sufficient measures to remove contaminant from the shoes and
surface in general.
In addition, the sliding mechanism means powder is only required at the impact point with no
relative motion between slider and powder layer. The rolling or tumbling and (to less of an
extent) shearing mechanisms however, require a constant input of contaminant to maintain
the effective lubrication and therefore more dependant on a larger area of contamination.
50
5.6 Effect of applied pressure on traction
The pendulum tester is designed such that a pre-defined load is applied to the contact region
for the entire stroke. The applied pressure (neglecting contact area changes due to pad wear)
is therefore constant. This required that the LSFR had to be used for these tests. Appendix 4
gives results for ballotini particles subjected to increasing loads. It can be seen that in all
cases that transition from shearing to rolling occurs as the initial depth of the particles was
greater than their diameter. Following the transition, increasing load only slightly increases the
coefficient of friction. This is likely to be due to the increased depression depth of the particles
within the slider.
Though no data is presented, the effect of increasing load is likely to have the following
effects:
Small particles (below ~ 50 microns):
Increasing contact pressure will increase mechanical interlocking which will generate higher
critical inter-particle shear stresses (τcrit). This means that sliding (as opposed to shearing) will
occur at larger particle depths.
Large particles (above ~ 50 microns):
If particles of high shape factor are considered (i.e. rounder particles) little effect other than
added rolling resistance (due to increased slider deformation) will be observed. However
particles of lower shape factor will tend to want to slide under higher loads, as a result of the
shear requirements to permit their tumbling (τr) being greater than the sliding shear stress
(τpf).
Both of these scenarios (like a decrease in shape factor) will tend to want to push the
transition line in the mechanism map upwards, allowing sliding to be sustained at higher floor
roughness.
51
5.7 Effect of shape factor on traction
Particle shape is an important factor (Appendix 5) in determining how a particle will behave in
a constrained contact. In the case of larger particles, those with circular profiles will have a
tendency to roll while those with large aspect ratios will tumble and slide. As particle size
decreases the shape factor may become important in determining how mechanical
interlocking will affect the internal and interface shear stresses in a layer many particles thick.
Current studies have shown that cohesion is important in particles below the ‘cohesive
boundary’ of approximately 50 microns. At the level of the smallest particles, it is technically
challenging to determine the effect of shape whilst keeping the effective diameter constant.
Analysis of this type may form the focus of future study.
The larger particles have shown shape factor dependency in determining whether particles
will roll, tumble or slide. In the case of the ballotini, the distribution function for the shape
factor is fairly tight about the peak as a result of the manufacturing method. However sand
and realistic contaminants are likely to have much looser distributions. Potentially, if two or
more particle movement mechanisms exist within this distribution then all of these
mechanisms have the potential to occur.
Figure [22]: Effect of particle distribution on preferential mechanism
52
In the case of the larger particles, it has been shown that two possible mechanisms can
occur, namely rolling or tumbling and sliding. Considering these two mechanisms, it is
possible to envisage a number of scenarios. The first, that all particles tumble and a constant
distribution of particles is seen in the wake of the slider. The second is that the particles
tumble and stick depending on the roughness at a particular point on the counter-face
(assuming uniform roughness on the slider). The third is that the particles only slide and in
this case no new particles can become entrained in the contact. For a distribution of shape
factors this means that last possibility may create a ‘wall’ where, after sufficient distance,
sliding becomes the dominant mechanism irrespective of whether there are other particles
favouring an alternative mechanism. Figure [22] outlines this affect though assumes an
unrealistic, instantaneous transition from one mechanism to the other.
Though no quantitative data has been obtained this phenomenon can be seen in the particle
tracks of the LSFR (figure [23]) where a sliding mechanism has given way to rolling in the
ballotini.
Figure [23]: Image of transition from sliding to rolling on LSFR
53
6.0 Conclusions
• Adhesive friction is significantly affected by the presence of solid contaminant, hysteretic
friction is not. Smooth floors (in the order of a few microns) are therefore more susceptible
to contamination (up to a 70 PTV point reduction).
• Even very small quantities of particles can reduce the shoe-floor adhesion and cause a
high slip risk.
• Small particles are able to adhere to the soles of shoes allowing transference of the
contaminant and continued reduced friction coefficient for many steps.
• Three significant slipping mechanisms were observed within the contact; sliding, shearing
and rolling/tumbling. Sliding and rolling on smooth floors pose the most significant risk.
• Sliding only requires contaminant at the impact site with no further contaminant required to
feed contact.
• Rolling/tumbling contacts require a constant supply of contaminant throughout the slip
(though only at a depth of a single particle).
• Load has little effect on larger, rolling particles but has the potential to increase the
roughness at which powders below 50 microns slide and larger particles with smaller
shape factors tumble.
• Larger depths of particles can cause transitions in mechanisms, commonly shearing to
sliding and shearing to rolling/tumbling.
54
7.0 Implications of findings
The work has shown that though solid contaminants can have different mechanisms to
generate a reduction in traction, the effects of the high risk situations occur in a similar
manner to liquid contamination.
Key finding: Particles have a significant effect on adhesive friction but small effect on
hysteretic friction. In cases of low hysteretic friction (i.e. low roughness floors) the traction is
determined by the CoF between the floor and particles.
Two Main High Risk situations:
Case 1: Sliding – High to Medium risk for most particles
Flooring: Smooth (less than approximately 10 microns) and low adhesive properties
e.g. Polished stone, vitreous tiles and polished laminate wood flooring.
Contaminant: Favours sliding and has a low CoF when in contact with flooring. Additional
contaminant transmittance risk or prolonged low traction conditions if particles adhere to sole
of shoes.
Case 2: Rolling / Tumbling – Potentially high risk for particles of large shape factor
Flooring: Risk will reduce as roughness increases
Contaminant: Favours rolling or tumbling (e.g. hard, large). Larger shape factors (i.e.
rounder particles) will provide less rolling resistance and reduce CoF.
The loss of adhesive friction is by far the biggest problem. Since hysteretic friction requires
sufficient floor roughness, it is this parameter that has the strongest control, offering increased
resistance in both cases 1 and 2.
55
7.1. Implications for the HSE:
The work performed will affect the way in which the HSL and HSE conduct investigations
involving solid particulate contaminants. However the education of employers on the matters
of solid contaminants and advice to duty holders regarding flooring and shoe material will also
fall into the responsibility of the HSE. A brief outline of these implications for the HSE is
outlined.
Forensic and site investigations
The following outlines a potential test procedure. Further work needs to be performed to
finalise acceptance/failure criteria.
1. Suspect conditions outlined in cases 1 and 2
2. Follow an investigation procedure similar to the following:
(i) Measure Rz roughness of flooring (in critical orientation)
(ii) Provide a qualitative indication of particle size.
(iii) Rate the visibility of particles (less visible – higher risk)
(iv) Perform Pendulum tests on a clean region with contaminant on slider
only, with repeat swings to consider rate of regain of sufficient friction.
(v) Perform pendulum tests on as-found contaminated areas.
(vi) If any of the tests above generate a high risk condition, remedial action
must be imposed (e.g. rougher flooring, stringent cleaning policies and
transmittance prevention methods).
3. Continue research into effects of specific roughness parameters and constructing
data bank for friction generated by different particle types on different floorings.
Education road-shows and guidance to employers
1. Understanding of why particulate contamination creates risk
2. How to take measures to prevent contamination occurring.
3. What sort of measures should be used to remove contaminant following spillage.
4. What sort of measures should be provided to remove contaminants adhered to soling
(e.g. bristle and sacrificial adhesive mats).
5. Understanding of principles for why rough flooring should be used in areas known to
suffer from contamination (e.g. workshops and kitchens)
56
Currently, some of the concepts obtained from these investigations have been used to make
an educational video (which is being used in road-shows) demonstrating visually the effects of
a small quantity of contaminant on a smooth, hard floor.
Advice to duty holders
Footwear. Hysteretic friction has been shown to be effected less by the presence of solid
particle contamination than adhesive friction. It is therefore suggested that HSL conduct
further investigations and/or develop procedures to test prospective soling materials such that
advice can be given (to footwear manufacturers) regarding the desired characteristics for
appropriate footwear.
Flooring. Further investigation into the effects that different roughness parameters have and
which are most significant for minimizing the effects of contaminants (may be environment
dependent e.g. the flooring required for a kitchen area could be different to that required for a
sand yard). Advice can then be given to those manufacturing and installing floors. All floorings
currently considered have been hard in nature and generally develop low particle-floor
interface shear stresses. Further investigation (by the HSL) into using other types such as
resilient flooring should be carried out.
57
8.0 Future Work
As has been stated, one of the key limitations of the Stanley Pendulum Tester is the inability
to extract localised kinetic information from within the sliding contact. All frictional loads are
fused to generate a single PTV. This cannot show transient variations in normal and shear
loads that could aid the understanding of the slider-particle interactions. Since it is not
possible to visualise these interactions directly for anything other than very smooth surfaces
(a roughness of only a few microns), the only available information is the single PTV and
resulting track.
In this study, a custom built low speed friction rig capable of measuring the shear and normal
forces was used. Though this aids understanding, it possesses very different dynamics to the
pendulum, with velocity alone being almost three orders of magnitude less than at pad
impact.
Figure [24]: Schematic and image of suggested pendulum force measurement device
58
A custom rig for the Stanley Pendulum Tester is suggested to enable measurement of normal
and shear loads directly for the duration of the contact. A schematic of the device is shown in
figures [24a] and [24b]. The basic principle would involve some data-logging software to
record normal and shear loads. A high acquisition rate would be required to obtain sufficient
fidelity in time and therefore it is envisaged that two optical triggers would be used to initiate
and terminate the data capture program. Structurally it is proposed that a simple ‘S’ type
tension/compression load cell be used to measure the compressive normal load. For shear
load measurement a co-axial cell is suggested, with the specimen being supported by thin,
laterally compliant cantilever supports which would enable the transmission of the normal load
but minimise lateral elastic resistance and so allow a better voltage/force ratio.
Though this rig would be laboratory based and require the machining of specific test
specimens, it has the potential to be used for dry, liquid contaminated and particle
contaminated conditions. The ability to analyse the transient impact forces directly, together
with the PTV and pad track data would give a much greater insight into the conditions in the
contact and aid the development of models to describe contact behaviours.
59
9.0 References
[1] ‘The assessment of pedestrian slip risk – The HSE approach’, Health and safety
document, (2004).
[2] Williams, A. J. ‘Engineering Tribology’, Oxford University Press, 232 – 237.
[3] Lemon, P. W., Thorpe, S. C., Jefferies, S. L., Sexton, C. and Hawkins, M.,
‘Pedestrian Slipping: Dry Contaminants’ PE/01/15. Human Factors Group, Health and Safety
Laboratory (2001)
[4] Lemon, P. W., Loo-Morrey, M. and Brown, E., ‘Assesment of Floor Surface
Slipperiness – Woodman Furnature Ltd.’ PS/03/19. Human Factors Group, Health and Safety
Laboratory (2003)
[5] Redfern, M. S., Cham, R., Gielo-Perczak, K., Grönqvist, R., Hirvonen, M.,
Lanshammar, H., Marpet, M. and Pai, C. Y., ‘Biomechanics of slips’ Measuring Slipperiness:
human locomotion and surface factors. Taylor and Francis. 37 – 65.
[6] Redfern, M. S. and Rhoades, T. P., ‘Fall prevention in industry using slip resistance
testing’ in Bhattacharya, A. and McGlothlin J. D. (eds) Occupational Ergonomics, Theory and
Applications (New York / Basle / Hong Kong: Marcel Dekker), 463 – 476, (1996).
[7] Whittle, M. W. ‘Generation and attenuation of transient impulsive forces beneath the
foot: A review’, Gait and Posture 10, 264 – 275, (1999).
[8] BS 7976-2:2002 ‘Pendulum Testers – Part 2: Method of Operation’ British Standards
[9] Moore, D. F., ‘The Friction and Lubrication of Elastomers’ Pergamon Press, Oxford,
Chapter 2, (1972)
[10] Orband, J. L. R. and Geldart, D. ‘Direct measurement of powder cohesion using a
torsional device’, Powder Technology 92, 25b – 33, (1997).
[11] Johnson, K. L., ‘Contact Mechanics’. Cambridge University Press, (2004)
60
10.0 Glossary of terms
Adhesion: Bonding of surfaces in contact by short range molecular
forces (van der Waals) the strength of which depends on the
contact area between shoe and flooring.
Agglomeration: Collecting / massing of particles
Asperity: A smooth surface is never completely flat and if viewed at
sufficient magnification, will show peaks and valleys. These
peaks are called asperities. An analogy can be drawn with a
saw blade in which the tips of the cutting teeth are the
asperities.
Continuum: Material acts as a continuous substance. E.g. a puddle of
water (which is a continuum) is described as a single body of
water because the size of the water molecules is very, very
much smaller than the floor roughness. However a heap of
sand is formed out of many discrete particles where the size
of these particles is comparable to the floor roughness. This
means that each particle must be considered separately and
so the heap cannot be considered a continuum.
Coulomb powder: An ideal powder with a degree of shear resistance. Below the
critical shear strength the particles will move relative to each
other elastically, but by no more than a fraction of the particle
diameter (if the force was removed, they would return to their
starting positions). Above the shear strength, the particles
will move past each other such that if the shear force was
removed, they would be in a different place to their starting
position.
Elastomer: Rubber-like material
Hysteresis: Energy dissipation by the deformation of the shoe material
physically realised as a frictional force.
Uniaxial compression: Compression of a material in a single direction whilst being
constrained in the other two directions
61
Van de Waals forces: A force felt between atoms and molecules at a molecular
level
Visco-elastic: Exhibiting both elastic (spring like) and viscous (think treacle
but very much thicker) properties. This means that if a visco-
elastic material is deformed it takes a little while for it to
regain its original shape. Visualisation of this effect can be
performed with foam, which if impressed by a finger, will take
a few seconds to regain its shape once the finger is
removed.
62
Appendix 1: Particle photos
Baking powder
Bicarbonate of Soda
Cocoa Powder
Flour
Table Salt
600 - 1200 micron sand
Talcum Powder
Ballotini
63
Appendix 2: Tile photos
64
Tiles (in order)
Perlina polished marble (PED/05/156)
Polished granite (PED/05/158)
Limestone (PED/05/157)
Olympus (PED/05/155)
Riven Slate (PED/05/159)
Appendix 3
Experimental runs using 500 micron, glass Ballotini particles (shape factor > 0.95) on steel (Rz ~10 microns) and varying particle depth
0
0.05
0.1
0.15
0.2
0.25
0 2 0 0 4 0 0 6 0 0 8 0 0 10 0 0 12 0 0
Dyn
amic
CoF
Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 1.0 mm
0
0.02
0.04
0.06
0.08
0.1
0 2 0 0 4 0 0 6 0 0 8 0 0 10 0 0 12 0 0
Dyn
amic
CoF
Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 0.5 mm
0
0.05
0.1
0.15
0.2
0.25
0 200 400 600 800 1000 1200 1400
Dyn
amic
CoF
Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm
0
0.04
0.08
0.12
0.16
0.2
0 2 0 0 4 0 0 6 0 0 8 0 0 10 0 0 12 0 0 14 0 0
Dyn
amic
CoF
Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 2.0 mm
.05 Dynamic rolling CoF: 0 65
Appendix 4
0
0.04
0.08
0.12
0.16
0.2
0
Dyn
amic
CoF
0
0.04
0.08
0.12
0.16
0.2
0
Dyn
amic
CoF
0
0.05
0.1
0.15
0.2
0.25
0
Dyn
amic
CoF
0.00
0.04
0.08
0.12
0.16
0.20
0
Dyn
amic
CoF
Experimental runs using 500 micron glass Ballotini particles (shape factor > 0.95) on steel (Rz ~10 microns) and varying normal load
Load: 2 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm
200 400 600 800 1000 1200 1400
Load: 4 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm
2 0 0 4 0 0 6 0 0 8 0 0 10 0 0 12 0 0 14 0 0
Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm
200 400 600 800 1000 1200 1400
Load: 20 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm
200 400 600 800 1000 1200
66
Appendix 5: Description of Shape factor (F)
67
Appendix 6: Selected test data:
6.1 LSFR test data
Powder
talc cocoa flourBaking powder
Bicarb of soda salt sand CLEAN
Tile
characteristic dimension (microns) 10 15 25 30 50 500 900
Perlina 0.51 0.14 0.1 0.14 0.14 0.13 0.1 0.1 1.14 Limestone 8.69 0.1 0.34 0.17 0.26 0.49 0.14 0.18 0.6 Olympus 12.32 0.17 0.21 0.11 0.24 0.32 0.25 0.14 0.62
Slate 34.18 0.42 0.55 0.35 0.71 0.61 0.22 0.43 0.65
6.2 Perlina tile test results (Pendulum method)
Pendulum Test Values for (A) Contaminant application to slider for every swing, (B) single
application of contaminant to slider followed by un-cleaned swings.
SWING Diameter Powder CLEAN 1 2 3 4 5 6 7 8 MEAN
15 Cocoa A 96 30 29 28 28 30 29 29 29 29.0
B 101 29 31 30 30 30 31 31 31
25 Flour A 89 36 36 36 36 36 36 37 37 36.3 B 99 37 38 37 37 36 36 36 36
30 Baking powder
A 100 25 25 25 25 25 25 26 25 25.1
B 98 26 27 29 30 30 29 28 28
50 bicarbonate
of soda A
86 21 22 22 22 22 21.8 B 92 21 22 22 22 23
500 Table salt A 102 36 38 40 34 36 24 46 25 34.9 B 106 25 27 30 31 33 33 33 33
68
6.3 Clean tile tests (Pendulum method)
Tile Rz Direction PTV Mean
Perlina 0.51 a 84 84 84 84 84 83.33 b 84 84 83 84 83 c 81 82 82 83 84
Polished granite 0.85 a 100 98 100 99 98 89.87
b 90 90 91 91 91 c 84 81 79 76 80
Limestone 8.69 a 60 60 60 60 60 60.73 b 62 63 63 64 64 c 59 59 59 59 59
Olympus 12.32 a 60 60 60 60 60 59.93 b 60 60 61 61 61 c 58 59 59 60 60
Riven slate 34.18 a 60 60 60 60 60 60.00 b 72 73 72 72 72 72.20 c 66 66 66 67 67 66.40
69
70
8 GLOSSARY
Adhesion: Bonding of surfaces in contact by short range molecular
forces (van der Waals) the strength of which depends on
the contact area between shoe and flooring.
Agglomeration: Collecting / massing of particles
Asperity: A smooth surface is never completely flat and if viewed at
sufficient magnification, will show peaks and valleys.
These peaks are called asperities. An analogy can be
drawn with a saw blade in which the tips of the cutting
teeth are the asperities.
Continuum: Material acts as a continuous substance. E.g. a puddle of
water (which is a continuum) is described as a single body
of water because the size of the water molecules is very,
very much smaller than the floor roughness. However a
heap of sand is formed out of many discrete particles
where the size of these particles is comparable to the floor
roughness. This means that each particle must be
considered separately and so the heap cannot be
considered a continuum.
Coulomb powder: An ideal powder with a degree of shear resistance. Below
the critical shear strength the particles will move relative
to each other elastically, but by no more than a fraction of
the particle diameter (if the force was removed, they
would return to their starting positions). Above the shear
strength, the particles will move past each other such that
if the shear force was removed, they would be in a
different place to their starting position.
71
Elastomer: Rubber-like material
Hysteresis: Energy dissipation by the deformation of the shoe
material physically realised as a frictional force.
Uniaxial compression: Compression of a material in a single direction whilst
being constrained in the other two directions
Van de Waals forces: A force felt between atoms and molecules at a molecular
level
Visco-elastic: Exhibiting both elastic (spring like) and viscous (think
treacle but very much thicker) properties. This means that
if a visco-elastic material is deformed it takes a little
while for it to regain its original shape. Visualisation of
this effect can be performed with foam, which if
impressed by a finger, will take a few seconds to regain
its shape one the finger is removed.