Design of Self Compensating Soft Loading Hood

Design of Self Compensating Soft Loading Hood

William McBride*, Dusan Ilic**(Received: 8 December 2006; in revised form: 13 June 2007; accepted: 25 June 2007)

DOI: 10.1002/ppsc.200601124

1 Introduction

Belt conveying systems form an integral part of almostall materials handling installation and are the dominantform of transport for most bulk materials. There hasbeen significant development in this field as presentedby Roberts [1], Harrison [2] and Wheeler [3] and it con-tinues to be an active research area with increasing per-formance pressures being applied to belt conveying sys-tems from numerous directions. It is not uncommon atpresent to discuss conveyor belt velocities of 8 m/s andwith these high speed conveyors increasing importanceis placed onto; structural design for fatigue loading, opti-misation of idler spacing for energy reductions, idler sealdesign for life extension and drive control algorithms forboth starting and stopping. With increasing belt speed

the ancillary infrastructure is also required to performduties to a new level. At the loading point of a highspeed conveyor is the problem of supplying the bulk ma-terial with a suitable velocity to reduce the wear on thebelt cover, and provide maximum belt carry capacity. Atthe discharge end of the conveyor the difficulty is to cap-ture and control the energy inherent in the bulk mate-rial. At 5000 t/hr and 8 m/s the energy contained in thestream is approximately 45 kW. If this energy content isnot appropriately controlled, then wear and plant da-mage are inevitable.Work on transfer chute design has been progressing ontwo separate fronts. Computer simulations work by Nor-dell [4] and more recently Sinnot [5] have shown the le-vel of detail that can be achieved using computationalmethods. However advanced these tools have become,to date these computation tools remain fundamentallyvalidation tools, as distinct from design tools. In indus-trial application research, work by McBride [6, 7] andWheeler et al. [8] present the current best practice ontransfer chute design with Roberts [9, 10] and Wensrich[11] presenting more theoretical analysis’.

370 Part. Part. Syst. Charact. 24 (2007) 370–374

* Dr. W. McBride (corresponding author), School of Engineer-ing, University of Newcastle, University Drive Callaghan2308 NSW (Australia).E-mail: [email protected]

** Mr. D. Ilic, Tunra Bulk Solids, University of Newcastle, Uni-versity Drive, Callaghan 2308 NSW (Australia).

Abstract

This paper presents a design, and a design method, suffi-cient to engineer a passive solution to the problem ofdischarge bias resulting from tonnage fluctuation withsoft loading transfer chutes. This is achieved by consid-ering the momentum change inherent in the bulk mate-rial stream through the hood section of a soft loadingtransfer point. This momentum change is utilised tomove the hood in a predefined path to ensure the dis-charge centroid remains consistent.The majority of soft loading transfer points are betweenconveyor belts that include a plan view change in the

material direction which immediately impacts on the de-sign of the transfer point. This impact is, the designermust optimise the transfer for a narrow bandwidth ofthroughput to achieve optimal outcomes or else acceptthe potential for mis tracking on the receiving belt. Thisis due to variations in throughput tonnage altering thelocation of the discharged materials centroid from afixed hood section, resulting in a tendency for the re-ceiving belt to mistrack due to biased loading.

Keywords: belt conveying, hood and spoon, materials handling, soft loading, transfer chute

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Part. Part. Syst. Charact. 24 (2007) 370–374 371

The design of a typical soft loading transfer chute can bebroken into several distinct steps. The first step is to cap-ture the incoming stream of bulk material in a way thatpreserves the incoming momentum of the material andminimises the wear on the chute surface. In our opinionafter the point of capture the material needs to be re-di-rected to a purely vertical stream, typically achieved inthe hood section. After the hood section, the stream canbe directed along the receiving belt feed direction usinga loading spoon. If this goal is achieved then the streamof bulk material will contain no transverse momentumrelative to the receiving belt, and will have its centroidof mass aligned with the centre of the belt. Failure ofeither criterion can result in belt mistracking. For readerreference, Figure 1 illustrates the main components of ahood and spoon transfer point.At present, the majority of transfers use a fixed positionhood mainly due to simplicity. A number of transfer sta-tions exist with a shuttle hood allowing feed to one of anumber of conveyors, however the shuttle systems areonly used for gross translations not fine positioning asdealt with in this paper. The shuttle system also does notalter the height of the hood as a function of mass flowrate thus allowing a poor incidence angle into the hoodwith variable throughput.When considering a 90 degree plan view directionchange with a fixed hood, tonnage variations can causesignificant problems. As the throughput decreases fromthe design value, the centroid of the discharge will movetowards the back plane (far side) of the hood. With in-creasing tonnage the opposite occurs, additionally bothvariances naturally cause an alteration in the incidenceangle of the bulk material onto the hood as well. If ton-nage fluctuation is low this has little consequence but

with large variances such as for blending systems, wherethroughputs can range from 5% of the design value up-wards the problems can be significant, with offset load-ing of the receiving belt leading to tracking issues.In more general terms, to aid in the control of the bulkmaterial through a transfer point, we suggest from ex-perience, a winged cross section be employed through-out, Figure 2 is indicative of the cross section and illus-trates a typical constriction in flow area to generate athickened bed of bulk material. The constriction towardsthe discharge end of the hood reduces the area of the load-ing spoon subject to impact, and more importantly allowsthe capture of the stream from the hood section, higher upin the loading spoon. With gravity pulling the bulk materialaway from the hood the constriction can be achieved quitequickly without significantly impacting on the stream velo-city. In contrast, constrictions in the loading spoon area sig-nificantly impact on stream velocity leading to a muchhigher tendency of blockage.The curvature of both hood and spoon sections chutescan be single or compound radii, though single is mostcommon for ease of manufacture. The radii used will im-pact directly on the wear life of the transfer chute as thesmaller the radii the higher the centripetal accelerationand thus generally, the wear rate.The summation of all of the design aims leads to a hoodwith as large a radius that will still ensure the bulk mate-rial has sufficient normal pressure to allow for streamconcentration as needed.As the tonnage rate is increased or decreased, the cen-troid of the discharged material from the hood willmove. If the hood incorporates significant lateral con-traction, then the movement of the discharge centroid ismagnified compared to the thickness change on the feedconveyor. Additional problems with variable tonnagerates occur as the impact angle of the material onto thehood will vary depending on the bed depth of the con-veyor. As the impact angle increases, often so does the

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.ppsc-journal.com

Fig. 1: Main items in a hood and spoon chute.

Fig. 2: Typical view of a constricting hood used for transfer chutes.


wear rate leading to a potential for premature failures inthe hood section.To combat the problems of belt mistracking due to vari-able tonnage, a number of strategies could be employed.It is plausible that an active system could be implemen-ted to control the location of the hood section to ensurethat the centroid of the discharge was always optimal,but this would require a belt weighing system, PLC con-trol and actuators to control the motion in a way suitedto industrial sites.The approach presented here utilises the momentum ofthe incoming stream to act against the mass of a pre ba-lanced hood allowing the hood to passively react tochanges in material flow rate. The theory for the designis presented in the following text.

2 Design

The control force utilised in the passive hood system isderived by the change in momentum between the in-coming stream and the outgoing stream. This reactionforce can be used to move the hood providing it is suita-bly supported on control links. Figure 3 illustrates thetest rig used for validation. The upper link provides allof the torsion stability of the chute with the lower linksproviding location for the lower edge of the chute. Theupper and lower links are set parallel to ensure the thereis no rotation during elevation, though under some cir-cumstances this could be useful. The declination angleof the links needs to be calculated for each installationand is dependent on the contraction ratio of the hood

and the variation in tonnage rate expected through thetransfer point.Visual inspection of numerous chute flows indicate thatthe bulk material does not form a flat free surface in thetransfer chute but does retain a conic section to a smalldegree. However, to simplify computation a flat profileis assumed. The total area of any section through a chuteof the shape shown in Figure 4 is given by Eq. (1).

Area = BZ + Z2 tan (a) (1)

Centroid � Z3

3B � 4Z tan �a�2B � 2Z tan �a�

� �(2)

Using the flow rate of solids through the chute and hav-ing selected the contraction of the hood based on nor-mal hood and spoon chute design criterion; McBride [7],we can apply Eq. (1) and Eq. (2) at the discharge andentry of the hood to determine the net direction ofmovement required. It should be noted that in Eq. (2)the predicted centroid is measured from the backplaneof the transfer chute as shown in Figure 4.

The parallel links are installed orthogonal to the re-quired movement direction as determined using proce-dure outlined below. To determine the point of contactof the bulk material with the hood graphical means orthe equations presented by Roberts [10] can be used.

3 Example System

Conveyor system – 600 mm belt, 3.6 m/s conveyingspeed, 350 t/hr max throughput;Chute – Constant radius, constant back plane (B) widthContact angle – top of stream = 13° below

horizontalbottom of stream = 21° below

horizontal(theoreticalcontact angle)


Fig. 3: Complete hood arrangement. In the test arrangement useda set of tension springs were installed from point arrowed to sup-port point on the conveyor structure above this pivot. Locationand style (tension/compression) will likely be determined byinstallation restrictions.

Centroid height

Height Z

B

True bulk material surface

Assumed bulk material surface

Chute Walls

Fig. 4: Cross section through chute, showing section of flowingmaterial. The reader may consider reviewing Figure 3 to bettervisualise this cross section.

Part. Part. Syst. Charact. 24 (2007) 370–374 373

3.1 Force Analysis from Momentum Change

For analysis of impact damage, the incremental momen-tum change can be analysed, however for this case, onlythe gross change in momentum needs consideration. Inthis case the velocities are as follows,V1 ave = 3.6 m/s at 17° below horizontal (average contactangle)Post impact V = 2.94 m/s at 47° below horizontalV2 = 3.68 m/s at 90° below horizontal (vertically down)(velocities presented are based on Roberts [10])Vector subtraction V1–V2 = 4.27 at 38° above horizontalWith Q = 350 t/hr = 97 kg/sec then the reaction forceFr = Q × DV = 97 * (V1–V2) = 415 N @ 38° above hori-zontal.Lower tonnage rates provide correspondingly less reac-tion force to drive the hood into position.

3.2 Linkage Angles

In the test conveyor, using a hood with no convergencethe support links are required to be installed at the aver-age angle between material in and material out. In thiscase the average incoming incidence angle is 17°, with a90° discharge angle. This results in the links on our testfacility being installed at 36.5 degrees below horizontal.If the test conveyors chute had a 2:1 contraction thenthe links would need to be installed at a much steeperangle as the variance in the discharge thickness will beapproaching 2 times that of the feed stream thickness.Each installation using this style of system will requireindividual evaluation to optimise the results.

3.3 Balancing Springs

A freebody diagram and the vector addition is presentedas Figure 5 which illustrates the method of calculationfor the required balancing springs. But in essence themass of the hood section complete with all parasiticmasses such as linkages forms a deadweight load that isacted upon by the force resulting from the change inmomentum. The linkage system forms a reaction direc-tion and the difference between the intersection of thedeadweight and this linkage direction ‘vector’ is themagnitude of the spring force required to balance thehood. It is important to conduct the full evaluation ofmomentum change through to spring force require-ments for a range of conditions to optimise the springingsystem.

4 Results

The results to date have been qualitative only, however theresults of the test program indicated that a consistent dis-charge centriod is achievable though at this stage friction inthe hinges has hampered accurate control. This is expectedto be a function of the small scale of this test rig andflexure straps are being designed for installation to elim-inate the friction issue in this installation.Of more importance was that in the testing undertaken,it was found that no damper unit was required. Duringthe design stage some concern was raised about dynamicinstability of the system to a step change in the through-put. With the limited testing to date this has not been anissue though the friction present in the pivot pins mayhave limited unwanted movement. If this technique ofmaterial flow control were to be used on larger facilities,particularly those fed by a scraper chain reclaimerwhere a definite periodic pulsing of the feed stream isexperienced, dampers may be required to prevent har-monic oscillations from occurring, simple dynamic ana-lysis should clearly identify the majority of potential is-sues for most installations. For due diligence, resonantcoupling of the feed pulse period and the natural fre-quency of the hood should be evaluated.

5 Conclusion

This paper has shown a new method of dealing with theinherent load bias associated with the use of hood andspoon type transfer chutes handling a highly variable

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.ppsc-journal.com

Link direction

Reaction Force

Deadweight

Spring forcerequired

Reaction Force

Link

Deadweight

Spring force

Fig. 5: Free body diagram of chute forces and vector addition todetermine spring requirements.


throughput. A design method has been presented to al-low the design of this type of system for any size con-veyor system. For installations that form parts of blend-ing systems this design may significantly reduce theissues associated with belt mistracking when operatingaway from the optimal design tonnage.

6 References

[1] A. W. Roberts, Recent Developments in Belt Conveying– Bulk Solids and Conveyor Belt Interactions, in:A. Levy, A. Kalman (Editors), Handbook of Conveyingand Handling of Particulate Solids, Elsevier Science B.V., 2001, pp. 225–233.

[2] A. Harrison, Dynamic analysis and measurement of steelcord conveyor belts, PhD thesis, 1984, University of New-castle, Australia.

[3] C. A. Wheeler, Calculating the Flexure Resistance ofBulk Solids Transported on Belt Conveyors. Part. Part.Syst. Charact. 2004, 21, 340–347.

[4] L. Nordell, Particle flow modelling: Transfer chutes andother applications. Proc. BELTCON 9 Int. Mat. Hand-ling Conf., Johannesburg, South Africa, 1997.

[5] M. Sinnot, P. Cleary, W. McBride, Prediction of ParticleFlows and Blockage Problems in Realistic 3D TransferChutes, Proc. 7th World Congr. Chem. Eng., Glasgow,Scotland, July 2005.

[6] W. McBride, Efficient Transfer Chutes – A Case StudyProc. BELTCON 9 Int. Mat. Handling Conf., Johannes-burg, South Africa, 1997.

[7] B. McBride, Curved Transfer Chute – Results of SiteTrials and Optimisation, From Powder to Bulk IMechEConf., London 2000.

[8] C. A. Wheeler. T Krull, A. W. Roberts, S. J. Wiche,Reducing Dust Emission from Grain Handling ShipLoaders, AIChE 2006 Spring Nat. Meet., 40th AnnualLoss Prevention Symposium, Orlando, Florida, April 24–26, 2006.

[9] A. W. Roberts, Chute Design Considerations for Feedingand Transfer, Proc. BELTCON 11 International Materi-als Handling Conference, Johannesburg, South Africa,2001.

[10] A. W. Roberts, Chute Performance and Design ForRapid Flow Conditions. Chem. Eng. Techn. 2003, 26,163–170.

[11] C. M. Wensrich, C. A. Wheeler, Optimisation in LoadingChute Design, Proc. 8th Int. Conf. Bulk Mat. Storage,Handling Transport., University of Wollongong, Austra-lia, July 5–8, 2004.


Design of Self Compensating Soft Loading Hood

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