Rating of Lifting Gear

8/11/2019 Rating of Lifting Gear

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Lifting Equipment Engineers Association 2013 - training\2-4u 1

UNIT NO. 2.4

RATING OF LIFTING GEAR FOR GENERAL PURPOSES

This unit considers the rating of lifting equipment, that is to say how we establish themaximum load that an item may lift in any particular circumstance. Depending onyour daily duties you will already be familiar with various types of lifting equipment.The principles for rating the various items are much the same, however we will lookmore closely at slings as they require a more detailed explanation.

Initially it is necessary to understand some basic terms. Much of the testers andexaminers work is related to the law and to standards. The terms used in thelegislation, the standards, found in manufacturers literature and in various codes ofpractice are in everyday use. It is surprising then that many testers and examinersdo not understand their correct meaning. Let us consider these terms as a

preliminary to our studies in this unit.

1. Lifting Equipment This term is used in two different ways in LOLER. It is a generic term used tocover all lifting accessories and appliances, but also has a more specificmeaning covering lifting appliances and their anchorages and fixings.

2. Lifting Accessories Any device such as a sling, shackle, eyebolt, clamp, spreader beam etc usedto connect the load to a lifting appliance but which is not itself part of the loador the appliance. At one time lifting accessories were referred to as liftinggear or lifting tackle. Although both these terms are still in common use,lifting accessories is the only term used in the Supply of Machinery (Safety)Regulations and LOLER. This course, Lifting Gear General, is concerned withlifting accessories and we will often use the term lifting gear or simply gearin the text.

3. Lifting Machine/Lifting Appli ance A device or mechanism, such as a crane, crab, winch, pulley block, gin wheel,chain block, which does the work in lifting the load or provides the means ofmovement, or the supporting structure and anchoring devices for such amechanism, eg runway, gantry etc, which may also permit a suspended loadto be moved in the horizontal plane. These items are covered by the termlifting equipment in LOLER.

4. Working Load L imit (WLL sometimes called maximum SWL) The maximum load or mass that an item of lifting equipment is designed tosustain, ie raise, lower or suspend. This is the load required to be marked onan item by the product standards.

5. Safe Working Load (SWL) The maximum load or mass (as certified by a competent person) that an item

of lifting equipment may raise, lower or suspend under particular service


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conditions. It is the SWL which is required to be marked on the item byLOLER and which appears on any report of thorough examination.

6. The Minimum Breaking (or f ailure) Load (MBL) The minimum breaking load is the calculated load at which a sample of the

item will break or fail. From this value is derived the WLL etc. In order toensure that this value has been achieved for some products, eg wire rope, asample may be tested to destruction and the actual breaking load recorded, orthe wire from which the rope is made is tested and the aggregate breakingload is calculated. Other items are designed so as to sustain the WLL plus aminimum factor of safety, eg a roundsling, however they may fail if this isincreased by only a tiny amount. For practical purposes we can considerthese tests to be the same. For a new product a sample must not fail undertest at a lesser amount than the minimum specified in the relevant standard.The MBL should be expressed in the SI unit of force (Newtons) or Mass(tonnes, kilograms), as required by the relevant standard.

7. Factor of Safety (FOS), Coefficient of Util isation, Working Coeffi cient These terms all have much the same meaning, but perhaps Factor of Safetydescribes the function better than the more recent terms, which are replacingit in standards and legislation. It is a factor which is applied to the MBL todetermine the WLL. It varies with the product to take account of thesusceptibility to damage and considers the type of stresses the item will meetin normal use.

Where the conditions of use are more severe than those considered by theproduct standard, eg in a chemical environment, the user will apply anincreased FOS, so reducing the value of the SWL from that of the WLL.

8. Mode Factor A factor applied by the user (slinger or rigger) that takes into account thegeometry of a sling assembly to obtain the maximum load he may lift for aparticular mode of use. When rating and marking slings, an assumption ismade that the sling leg will be in straight pull, if this condition is varied, egchoke hitch, the load in the sling leg will also vary. The mode factor takesaccount of this, as the marked SWL is the maximum load that may be appliedto the leg, its correct use ensures that the leg will not be overloaded.

Note : Some confusion exists as to the WLL and SWL. The WLL is determined bythe designer/manufacturer and is based on the mechanical properties of the item. Acompetent person specifies the SWL, based on the use to which the item will be putand will be dependant on:

(i) Whether the load is dead or alive(ii) The consequences of failure, eg when carrying radioactivematerial the risk is high and a greater factor of safety is called for

(iii) How the load is applied, eg slowly or suddenly(iv) The degree of possible misjudgement of weights or angles(v) The duty cycle and working environment

(vi) Any other factors which affect the safe working life of the item or thesafety of the lifting operation.


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The marked SWL normally has the same value as the WLL , but it may be less. Forinstance, in coal mining and nuclear power stations the WLL is often significantlyreduced to obtain the SWL, as there are particular dangers associated with theseactivities and these call for higher safety margins than in, say, an engineering works.

For new equipment, the WLL/SWL should be expressed in SI units of mass, ietonnes and kilograms. Older items may still be found in service which are marked inthe imperial units of tons and hundredweights.

Manufacturers of standard or series produced items, who do not know theapplication in which the item will be used, specify the WLL, leaving the individualusers to decide whether or not the factor of safety is sufficient for their particularapplication. Where an item is being specially manufactured for a specific applicationthe designer is able to take all of the relevant matters into account and can use theapplicable factors, thus he is able to specify the SWL which is to be marked on theitem.

RATING OF LIFTING GEAR LOLER requires that the SWL is clearly marked on all items of lifting equipment,however both standards and the Supply of Machinery (Safety) Regulations requireall newly manufactured lifting equipment to be marked with the WLL. This is notreally a contradiction, as they will usually have the same value. As we noted above,it is only when conditions of use require additional safety margins that the user willapply a factor greater than that normally associated with the item. This may thenrequire him to re-mark the item with the SWL.

Whilst old, pre-LOLER, legislation permitted certain slings to be unmarked and atable of appropriate SWLs to be displayed no such items should now exist. BritishStandards have, for many years, recommend that all gear be marked with itsmaximum SWL or WLL because of the difficulties often found in reading tables inindustrial situations and the possibility of incorrect material or grade identification.The adoption of the uniform load method of rating slings has also rendered the useof charts obsolete. LOLER now requires all items to be marked with the SWL.

With most items of lifting equipment, the marked SWL is arrived at by making asimple mathematical calculation applying a factor of safety to the designed minimum-breaking load. This is then the marked WLL and is the maximum load the item may

sustain. With slings the situation is more complex. Whilst the WLL of the sling instraight pull can be established this way, if the sling is to be rigged in any other way,eg choke hitch, further calculation by the slinger or rigger is necessary to find themaximum load that may be lifted. Sling users must therefore apply the correct modefactor for the particular type of sling and its configuration of use.


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RATING OF MULTI-LEG SLINGSThe rating of multi-leg slings can be very complex and calls for the designer to makecertain assumptions about the use of the sling. As the angle that the sling leg makesto the vertical affects the stress in the leg (see LEEA Code of Practice for the SafeUse of Lifting Equipment Appendix 1.5 paragraph 1A5.3.3.1 and Figure 1A5.8), this

is taken into account and it is a matter of geometry to establish the maximum loadthat can be lifted.

There are two methods of rating multi-leg slings: the uniform load method and thetrigonometrical method. The calculation to establish the WLL is however based onthe same principle for both methods. From our Part 1 studies we know that if a forceis applied to a sling leg, which is disposed at an angle, then the resultant load in theleg will be greater than the force applied. In fact it will increase by a factor equal tothe cosine of the angle that the sling leg makes to the vertical. With the uniform loadmethod of rating we limit the calculation to only two angles, 45 to the vertical forgeneral use and 60 to the vertical for additional use, whilst with the trigonometricmethod the calculation is made for the specific angle to the vertical that the sling legwill be used at.

(i) Uniform Load Method This method was first recommended in a British Standard Publication PD 6464(1972) and subsequently specified in BS 6166: Part 1 1986. It is the preferredmethod for rating all general purpose slings and is used internationally. It should beused for rating ALL general purpose slings and is the only method detailed inHarmonised Standards that support the European Machinery Directive.

By the uniform load method, a multi-leg sling is rated with a maximum load for use atany angle within the permitted range. This has many advantages for both the userand the sling manufacturer. As the WLL is calculated based on an angle of 45 to thevertical and limited to this WLL when the angle is reduced, (a) users do not have to

judge different angles and calculate the actual load that may then be lifted and (b)manufacturers can use master links which are capable of carrying the WLL at thatangle only and can therefore use lighter construction links than would be necessaryif the WLL was allowed to increase as the angle decreased.

Traditionally in the UK the rating was expressed in terms of the included angle, ie theangle between the opposite sling legs, of 0-90 and in special cases additionally at

an included angle between 90-120. This method of expressing the rating forthree-leg slings then calls for the user to double the angle of the leg to the vertical toobtain the included angle, as there is no opposite leg. Although this method ofmarking is not now used, many existing slings will be found in use marked this way.

The Harmonised European Standards for various slings take a different approachexpressing the rating at the angle of the legs to the vertical. This is a more logicalapproach as this is the angle that is actually used in establishing the WLL, itemphasises that the angle of each leg affects the share of the load it will carry and,in the case of three leg slings is more easily understood by the user. The workingload limits are shown at 0-45 and in special cases 45-60. So that users will not be

confused, it is recommended that a tag or label with a pictogram is used similar tothat shown in figure 1.


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Figure 1

Although we will consider the assumptions made in rating multi-leg slings later in thisunit, we should note here that, in the case of four leg slings, the uniform load methodof rating assumes that only three of the legs will effectively carry the load and thatthe fourth leg will only serve to balance the load. Therefore the rating for three andfour leg slings is the same.

The following chart shows the design factors that should be applied to the WLL of asingle leg to establish the WLL of multi-leg sling assemblies or where a number ofsingle slings are being used in combination .

0-45(0-90)

45-60(90-120))

Two leg sling 1.4 1.0

Three and Four leg sling 2.1 1.5

Design Factor s

Figure 2 shows how the angle is measured for two, three, and four leg slings. is thesymbol used to indicate the angle to the vertical and is used to indicate thetraditional included angle. = 2

(a) Two leg sling

(b) Three leg sling


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(c) Four leg sling

Figure 2

The master and intermediate links of slings designed strictly in accordance with theuniform load method are capable of supporting the maximum rated load marked onthe sling at any angle between those specified. They may therefore be weaker thanthose that would be used in the case of trigonometrically rated slings and should not be used to support greater loads at reduced angles.

(ii) Trigonometri cal Method This method was traditionally used in the UK to calculate the WLL of slings atvarious angles of the legs to the vertical. Even after the publication of BS 6166 in1986, many manufacturers continued to use this method until the adoption of theHarmonised European Standards. It provided the means of preparing the tablesrequired by the older lifting equipment regulations. The trigonometric method is thenon-preferred method of rating. If used now it should be restricted to slings designedfor a single purpose, eg the top sling of a lifting frame, where the angle of use cannotnot be altered. Although still permitted by the Supply of Machinery (Safety)Regulations and LOLER, its use has almost ceased since sling manufacturers haveadopted Harmonised Standards.

The WLL is calculated using the design factors as follows:

For a two-leg sling (Fig 2a)WLL = 2 x WLL of a single leg x cos

For a three-leg sling (Fig 2b)WLL = 3 x WLL of a single leg x cos

For a four-leg sling (Fig 2c)WLL = 4 x WLL of single leg x cos

Example: A two-leg chain sling is used to lift a load with an included angle between the

legs of 60. If the WLL of a single leg is 2 tonnes, calculate the WLL of thesling for this lift.


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WLL of the sling = 2 x WLL of a single leg x cos

Figure 3

From the diagram, = 30 Cosine 30 = 0.866 (from trigonometrical tables)

. .. WLL of the sling = 2 x WLL of a single leg x Cos 30

Hence WLL of the sling = 3.46 tonnes

In this case the master and intermediate links must be designed to be capable ofcarrying the maximum load that can be lifted by the sling for any calculated angle ofuse. They are therefore larger than those used for the uniform load method. Caremust also be taken in the case of existing four leg slings rated this way, as they willusually have been assembled assuming that all four legs carry an equal share of theload, but in more recent times some manufacturers may have adopted the ratingassumption that only three legs will carry the load. It is therefore vital that testers andexaminers establish how the sling is assembled and rated.

Assumpti ons for the Rating of Mul ti -leg Slings We said that, in calculating the WLL of multi-leg slings, some fundamentalassumptions have to be made by the designer (or standard writers), these are:(a) The sling legs are identical in all respects except that the lower terminal

fittings may vary. (The leg lengths may vary in cases where the points ofattachment to the load are not in the same horizontal plane provided that theloading is equal, see f).

(b) The terminal fittings are connected to the lifting appliance and the loadattachment points in such a manner that the legs are not bent across or

around the load, choked, back hooked or otherwise prevented from taking upa straight line under load.(c) All legs are at the same angle to the vertical.(d) The angle or range of angles between the sling legs at which the sling is rated

is not exceeded.(e) The sling legs are symmetrically disposed in plan, ie for three-leg slings all

angles between legs in plan view are equal and for four-leg slings, theopposite angles between adjacent legs in plan are equal.

(f) For four-leg slings the length of each leg exactly matches the position ofattachment points. If this is not achieved most or all of the weight will becarried by only two legs of the sling.


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MARKINGBefore being used, all lifting gear must be marked, either directly or by means of atag attached to it, with the following data:

(a) Identification data(b) Quality mark or material identification(c) SWL; and for new equipment(d) Any other marks required by the standard being worked to or by legislation,

eg CE mark.

Markings should be readily visible, permanent and legible and if on the lifting gearthey should not affect its strength, eg cause notch effect, chemical reaction, etc.The size of characters used should be chosen so as not to impair the mechanicalproperties of the item. Where a tag is used, the identification data should (wherepossible) be marked directly on the item so that if the tag is lost the documentationcan be traced and the information retrieved. In the case of webbing slings androundslings this should be on a part of the label enclosed by a stitched portion of thesling or cover.

Marking of Multi -leg Slings (1) Slings rated by the uniform load method should be permanently made up and

marked with SWL in the following manner:

For included angles 0 to 90:- eg SWL 6.3t 0 - 45,For included angles 90 to 120 :-eg SWL 4t 45 - 60,

Where users have a mix of slings marked in terms of the included angle, theyshould be encouraged to consider re-marking them with the angle expressedto the vertical so as to avoid confusion in use.

(2) Slings rated by the trigonometrical method should be permanently made upand marked with the SWL at the specific angle in the following manner:

SWL 6.3t at 45

Note For three-leg slings the included angle range 0-45 (or for older slings 0 to

90)

for uniform load method, or 45 (or for older slings 90) fortrigonometrical method, refers to a condition when any leg does not exceed45 to the vertical .

You are also advised to read appendix 1.5 LEEA Code of Practice for the Safe Useof Lifting Equipment where a fuller explanation is given. Some guidance will also befound in the text of the standard being worked to.

MODE FACTORS APPLIED BY THE USER (CHOKE HITCH/BASKET HITCH) If a choke hitch is used in the slinging arrangement, then the marked SWL will bederated by the use of a mode factor to establish the maximum load the slinging

arrangement may be used to lift. The amount by which the sling is derated depends


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on the material and the appropriate mode factors are specified in the various productstandards.

Similarly if a sling is used in a basket hitch, the actual load that may be lifted will begreater than the marked SWL. Again this will vary with the material, the angle of the

sling parts and other factors. Details of the mode factors are given in the relevantproduct standards.

By way of revision, you are advised to read those sections of the Code of Practicefor the Safe Use of Lifting Equipment that give details of slings as the design andmode factors, including those to be applied due to the rigging arrangement, are fullydetailed and explained.

PRACTICAL CONVERSIONS FOR USE IN RATING LIFTING EQUIPMENT As equipment manufactured to imperial standards may still be found in usealongside equipment to metric standards, and as it may be desirable to have all ofthe equipment in a factory marked with its safe working load in the same units toavoid operative error, standard conversions are used. The British Standards Instituteoffers the following guidance with regard to marking SWL when converting fromimperial to metric units:

"Safe Working Loads of less than 1000kg should be marked in kilograms to thenearest whole kilogram. SWL's of 1000kg or more should be marked in tonnes. Onlyone place of decimals should be used except for 1.25t; for integral values of SWL the0 after the decimal point should be omitted."

This system has certain advantages when converting from imperial to metric units orwhere a mix of metric and imperial rated equipment exists side by side as it avoidsthe possibility of metric markings being confused with tons and hundredweights. If,by accident, the metric figure after the decimal place is read as cwts, the mistake willalways be on the safe side since, in fact, the single decimal figure is always half theequivalent cwts.

This system of marking can also be used when an item is produced which is not to astandard, eg a spreader or lifting frame. Items produced in compliance withHarmonised Standards should be marked in accordance with the standard.

The following table gives examples of the conversions; others can then be calculatedon the same basis.


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Examples of the Conversions:

1 cwt = 50kg 1 Ton = 1 t 1 Ton 11 cwt = 1.5 t

1 Ton 1 cwt = 1 t 1 Ton 12 cwt = 1.6 t2 cwt = 100kg 1 Ton 2 cwt = 1.1 t 1 Ton 13 cwt = 1.6 t

1 Ton 3 cwt = 1.1 t 1 Ton 14 cwt = 1.7 t

5 cwt = 250kg 1 Ton 4 cwt = 1.2 t 1 Ton 15 cwt = 1.7 t

1 Ton 5 cwt = 1.25 t 1 Ton 16 cwt = 1.8 t


1 Ton 7 cwt = 1.3 t 1 Ton 18 cwt = 1.9 t


12 cwt = 625kg 1 Ton 9 cwt = 1.4 t 2 Ton = 2 t

15 cwt = 750kg 1 Ton 10 cwt = 1.5 t

It will be noted from the table that in the case of, say, 1 ton 3 cwt, the exactconversion would be 1.15t, but as the second decimal place is disregarded, thisbecomes 1.1 t.

Examples

(i) ton = 750 kg.(ii) 1 ton 5 cwt = 1.25 t.(iii) 4 ton 17 cwt = 4.8 t.

Rating of Lifting Gear

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