NOMINAL SLING STRENGTH
Is based upon the nominal (catalog) rope strength of the wire rope used in the sling and other factors which affect the overall strength of the sling. These other factors include splicing efficiency, num- ber of parts of rope in the sling, type of hitch (e.g. straight pull, choker hitch, basket hitch, etc.), diameter around which the body of the sling is bent (D/d) and the diameter of pin used in the eye of the sling (Figure 1).
is the angle measured betweena horizontal line and the sling leg of the body. This angle is very important and can have a dramatic effect on the rated capacity of the sling. As illustrated above, when this principle applies whether one sling is used to pull at an angle, in a basket hitch or for multi-legged bridle slings. Sling angles of less than 30 degrees are not recommended.
configurations affect the rated capacity of a sling. This is because the sling leg or body is passed around the load, through one end attachment or eye. The contact of the sling body with the end attachment or eye causes a loss of sling strength at this point. If a load is hanging free, the normal choke angle is approximately 135 degrees. When the angle is less than 135 degrees an adjustment in the sling rated capacity must be made (Figure 3). As can be seen, the decrease in rated capacity is dramatic. Choker hitches at angels greater than 135 degrees are not recommended
since they areunstable.
Extreme care should be taken to determine the angle of choke as accurately as possible.
NOMINAL SPLICE EFFICIENCY
is the efficiency of the sling splice. Any time wire rope is disturbed such as in splicing an eye, the strength of the rope is reduced. This reduction must be taken into account when determining the nominal sling strength and in calculating the rated the capacity. Each type of splice has a different efficiency, thus the difference in rated capacities for different types if slings. Nominal splice efficiencies have been established after many hundreds of tests over years of testing.
D/d ratio is the ratio of the diameter around which the sling is bent divided by the body diameter of the sling (Figure 4). This ratio has an effect on the rated capacity of the sling only when the sling is used in a basket hitch. Tests have shown that whenever wire rope is bent around a diameter the strength of the rope is decreased. Figure 5 illustrates the percentage of decrease to beexpected.
This D/d ratio is applied to wire rope slings to assure that the strength in the body of the sling is at least equal to the splice efficiency. When D/d ratios smaller than those listed in the rated capacity tables are necessary, the rated capacity of the sling must be decreased.
is the maximum static load a sling is designed to lift. The tables give rated capacities in tons of 2000 pounds. Rated capacities contained in all the tables were calculated by computer. Each value was calculated starting with the nominal component rope strength and working up from there. Due to computer rounding of numeric values, rated capacity values for 2, 3 or 4 leg slings may not be even multiples of single leg values and may differ by a small amount. This represents the state- of-the-art technology and tables found in other publications which differ by this small amount should not be construed to be in error. The difference is generally no more than one unit for any slingdiameter.
When a wire rope is bent around any sheave or other cir- cular object, there is a loss of strength due to this bending action. As the D/d ratio becomes smaller this loss of strength becomes greater and the rope becomes less effi- cient. This curve, derived from actual test data, relates the efficiency of a rope to different D/d ratios. This curve is based on static loads only and applies to 6X19 and 6X37 class ropes.
is a specific load applied o a sling or assembly in a non-destructive test to verify the workman- ship of the sling. All swaged socket or poured socket assemblies should be proof loaded. The proof load is gen- erally two (2) times the vertical rated capacity for mechani- cal splice slings. The maximum proof load for hand tucked slings is 1.25 times the vertical rated capacity. Care should be taken to assure that sling eyes are not damaged during the proofload.
are generally eight (8) sling body diameters wide by sixteen (16) body diameters long. Whenever possible thimbles are recommended to protect the rope in the sling eye. Eye dimensions for thimbles are contained in table 2. Table 2 contains only dimensions for thimbles used in standard single part slings. Other spe- cialized thimbles are available. Consult your sling manu- facturer for details.
should not be any greater than the natural width of the sling eye. For any sized eye and type of sling body, the maximum allowable pin diameter may be calcu- lated as follows.
Maximum pin diameter = (2L + W) x 0.2 WhereL = length of eye W = width of eye
The minimum pin diameter should never be smaller than the nominal sling diameter.
GRADE & CONSTRUCTION
of wire rope for slings is gen- erally accepted to be bright Improved Plow Steel or Extra Improved Plow Steel grade 6×19 or 6×37 classification reg- ular lay. IWRC rope has a higher rated capacity than Fiber Core rope for mechanically spliced slings, but the same rated capacity for hand tucked slings. This is because when making a hand tucked splice, the core (IWRC) of the rope is cut in the splice area and doesn’t add to the overall strength of the sling. Rated capacities of slings using gal- vanized rope depend on the method of galvanizing. The sling manufacturer should be consulted regarding rated capacities for these types of slings.
MINIMUM SLING BODY LENGTH
is the minimum length of wire rope between splices, sleeves or fittings. Generally the minimum body length is equal to ten (10) times the sling body diameter. This allows approximately one and one half (1 1/2) rope lays between splices. For multi-part slings the minimum body length between splices is equal to forty (40) times the component ropediameter.
is generally plus or minus two (2) body diameters, or plus or minus 0.5% of the sling length, whichever is greater. The legs of bridle slings, or matched slings are normally held to within one (1) body diameter.
Tolerances on poured or swaged socket assemblies are generally much closer. Tolerances should always be specified to the sling manufacturer before the order is placed. This eliminates a lot of frustration and confusion later.
HAND TUCKED SPLICE
A HAND TUCKED splice is made by passing the wire rope around a thimble or forming an eye and splicing the dead end (short end) into the live and (long end) of the rope.
Normally, each dead end strand is given one forming tuck and three full tucks around the same strand in the body of the rope. One additional full tuck is made when splicing more pliable wire ropes such as 6X37 classification. A “forming tuck” is made by prying two adjacent strands apart, inserting a dead end strand into the opening and passing the strand under one, two, or three adjacent strands in the body of the rope. The dead end strand is set or locked tightly.
A “full tuck” is made by inserting a dead end strand under and rotating in one full 360 degrees turn around and strand in the body of the wire rope. The tucked strand is set or locked tightly. Each subsequent full turn of the dead end strand around the live end strand constitutes an addi- tional fulltuck. “Setting” or “locking” of a dead end strand is accomplished by pulling the strand end in under considerable force. A marlin spike is inverted in the same opening in the body of the rope ahead of the tucked strand and is rotated about the axis of the rope back to the start of the tuck. Certain end useages may indicate the desirability of special splices such as the
Navy Admiralty Splice or logging splice. Splices made by these special methods may also attain the efficiencies used in calculating the rated capacity tables where the rope quality and number of tucks are equivalent to that outlined above. Development of such efficiencies should be confirmed by the sling fabricators making suchsplices.
Serving or wrapping of wire rope sling splices does not affect the splicing efficiencies nor rated capacities. Such servings are optional, although unserved splices are pre- ferred because they permit visual inspection of the spliced area.
NOMINAL SPLICE EFFICIENCIES
are measured in terms of efficiency (where efficiency = actual breaking strength of spliced termination divided by actual breaking strength of the rope). The efficiency will change from splice to splice because of the many variable factors involved in producing the splice. Splice efficiencies given in table 3 were estab- lished so that these normal variations are accommodated. The design factor used in establishing the rated capacities further assures that the sling will lift the load even in those rare instances when the splice efficiency falls slightly below the values given in the tables. Rated capacities shown in this manual have met with the most exacting test, that of the test of time and use in over fifty years of actual fieldexperience.
for hand tucked slings are about the same as for any other type of sling. The use of a swivel on a single leg lifts as well as free hanging loads which may rotate is not recommended. A tag line should always be used to prevent rotation of the slingbody.
When the sling body of a hand tucked splice is allowed to rotate, the splice could unlay and pull out, thus causing the load to drop.
MECHANICAL SPLICE slings come in two basic types. One being the Returned Loop and the other the Flemish Eye or farmers splice. In either case, the splice is com- pleted by pressing (swaging) one or more metal sleeves over the rope juncture.
The returned loop is fabricated by forming a loop at the end of the rope, sliding one or more metal sleeves over the short end of the loop eye and pressing these sleeves to secure the end of the rope to the sling body. This makes an economical sling and in most cases one that will give satisfactory service. A drawback to this type of sling is that the lifting capacity of the sling depends 100% upon the integrity of the pressed or swaged joint. Should the metal sleeves(s) fail, the entire eye will alsofail.
The flemish eye splice is fabricated by opening or unlaying the rope body into two parts, one having three strands and the other having the remaining three strands and the core. The rope is unlayed far enough back to allow the loop or eye to be formed by looping one part in one direction and the Flemish eye spliceother part in the other direction and lay- ing the rope back together. The strands are rolled back around the rope body. A metal sleeve is then slipped over the ends of the splice and pressed (swaged) to secure the ends to the body of the sling. Nominal splice efficiencies expressed in
table 4 and in the rated capacity tables are based on this splicing method. Splice efficiencies for other splicing methods should be confirmed by the sling manufacturer. Notice that the splice efficiency factor plays no role in the calculation of the Choker Hitch rated capacity. This is because as the rope passes through the eye of the sling in a choke, the weakest part of the sling is in the body of the sling at the choke point. Thus the splice being higher in efficiency, has no effect on the rated capacity, because the efficiency factors are not additive. Rated capacities for single part, choker and basket hitches are calculated exactly the same as for hand tucked slings except for the nominal splice efficiencies. The rated capacities adjustment table 1 for choker hitches also applies for mechanical spliced slings. Minimum D/d ratio for basket hitches is 25. This larger D/d ratio is required because the Nominal Splice Efficiency ishigher.
Are no different from other slings except care should be taken not to deform or damage the sleeve. Stainless Steel slings which have sleeves made of a differ- ent grade or type metal than the rope body may experience accelerated deterioration due to an electro chemical reaction between the two metals. This is particularly evident in salt water or brackish conditions.
ZINC OR RESIN POURED SOCKET TYPE TERMINATIONS
While some people may debate whether zinc or resin poured sockets are truly slings, they are generally included in the sling category. This type of termination has tradi- tionally been the method for determining the rope’s actual breaking strength. All other types of end terminations have been compared to poured sockets. Their efficiency is therefore established to be 100% for all grades and constructions ofrope.
Choker hitches are not used as much with poured sockets as which the other more general types of slings. When such slings are used in a choker hitch, the rated capacity adjustment table 1 applies.
Rope assemblies with poured attachments are generally used as a straight tension member where the rope body does not contact the load and is otherwise kept free from distortion or physical abuse. In such cases the minimum recommended design factor is 3.0. If the assembly is used as a sling then a design factor of 5.0 should be used to calculate the rated capacity. Rated capacities for these slings used in basket hitches are the same as mechanical spliced slings and use the same D/d ratiofactors.
Length tolerances or poured attachments can be somewhat more stringent than other types of slings. The manufacturer should be contacted and agreement reached before the order is placed. Tolerance as small as plus or minus 1/8” is not out of the ordinary for this type of assem- bly. Specifications such as type of fitting, pin orientation, whether zinc or resin is to be used and type of application should also be supplied to the manufacturer when ordering these types of assemblies. Those inexperienced in the socketing process should not try to fabricate assemblies without first getting expert training. It is far better to leave fabrication of this type of assembly to the experts. The fol- lowing socketing methods are general in nature and have withstood the test of time. Slight variations to these methods will produce equal results.
The two procedures, while achieving the same end result, differ significantly. It is highly recommended that all poured sockets whether they be zinc or resin, be proof loaded.
CABLE-LAID WIRE ROPE SLINGS
Cable-laid slings are fabricated from a machine made
rope compromised of seven small wire ropes. The cablelaid body is typically 7x7x7, 7x7x19, 7x6x19, 7X6X36 Classification IWRC. This construction makes for a pliable rope and sling. These slings are used where flexibility and resistance to kinking and setting are more important than resistance to abrasion. Since the rope is made up of many smaller wire ropes, the slings can bend around smaller diameters without taking a permanent set or a kink. The many small wires are susceptible toabrasion.
The rated capacity adjustment Table 1 for choker hitches applies to cable-laid slings as well. Note the difference in the efficiency factor for calculating vertical choker hitch rated capacities.
Rated capacity for a basket hitch is based on a D/d ratio of 10, where “d” is the diameter of the cable-laid fabric.
Tolerances and minimum sling lengths are also figured using the cable-laid fabric diameter.
BRAIDED MULTI-PART SLINGS
Multi-part braided slings or Multi-parts as they are known, are generally hand fabricated slings which are “braided” from 2,3,4 and up to as many as 48 pieces or parts of rope. Generally 4,6,8&9 parts are the more common.
They can be either flat or round and offer the ultimate in flexibility and versatility. These are truly the heavy weights of the lifting industry. This book covers only theround
type slings. They snug up tightly to the load in a choker hitch and resist kinking and setting. Loads in excess of 4000 tons have been lifted with multi-part slings.
Nominal Splice Efficiency for multi-part slings is 0.70 for component rope 3/32” through 2” diameters. For larger component rope slings, consult the sling manufacturer for splice efficiencies.
Because of the multi-rope component construction, multipart slings react differently than standard wire rope slings in a choker hitch therefore the nominal splice efficiency is present in the equation. The adjustment Table 1 applies to multi-part slings also.
Rated capacity for a basket hitch is based on a minimum D/d ratio of 25, where “d”=component rope diameter.
Length tolerances for component ropes of 3/8” diameter and smaller are plus or minus 10 component rope diame-
ters, or plus or minus 1.5% of the sling length whichever is greater. The legs of matched slings shall be within 5 component rope diameters of each other. For component rope diameters 7/16” and larger, the tolerance is plus or minus 6 component rope diameters, or plus or minus 1% of the sling length whichever is the greater. Legs of matches slings shall be within 3 component rope diameters of each other. Minimum Sling Length betweenloops, sockets of sleeves is recommended to be 40 times the component rope diameter of the braided body.
Grommets are a unique type of sling. They form a complete circle and automatically double the number of lifting legs. Several types are available, such as strand laid hand tucked and cable laid mechanical. Grommets work well in basket and choker hitches and general applications will find them used in this manner. Another unique advantage to grommets is that the load contact points may be rotated or moved around the sling to even out the wear points. The only area that should not come into contact with the load is the splice area. The sling manufacturer will usually mark the area of hand tucked grommets with paint to help the user more easily identify the splicearea.
Tolerances for grommets are generally plus or minus
1% of the circumferential length or 6 body diameters whichever is the greater.
A minimum circumference of 96 body diameters is recommended. This measurement is normally an inside circum ferential measurement. The requirement for a minimum circumference of 96 times the body diameter for grommets and endless slings was based on the requirement to have at least three free rope lays on either side of the tuck of a hand spliced endless grommet prior to being bent around a hook or pin five times the body diameter. To eliminate the possibility of confusion, this requirement was adopted for mechanically spliced endless grommets as well. consult the sling manufacturer for smallercircumferences.
The same general precautions apply to grommets as apply to all other types of slings. However, it should be noted that since a grommet is a continuous circle, the noted D/d ratio becomes a very important consideration.
The D/d ratio must be applied to the lifting pins as well as the load. normally the lifting pins will be the smallest diameter in the system other than the diameter of the grommet.
No loads should be handled on the D/d smaller than the 5 times the sling body diameter. If they must, consult the sling manu- facturer. Rated capacities covered in the section are based on a D/d ratio of5.
STRAND LAID HAND TUCKED GROMMET
A Strand Laid Hand Tucked Grommet is made from one continuous length of strand. No sleeves are used to make the joint.
This results in a very smooth circular sling. Because of the sling body construction, grommet slings react differently than standard wire rope slings in a choker type hitch therefore the presence of the nominal splice effi- ciency factor in the equation. Rated capacity adjustment table 1 applies.
STRAND LAID MECHANICALSPLICE GROMMET
Strand Laid Mechanical Splice grommets are made from one continuous length of wire rope joined by pressing or swaging one or more sleeves over the rope juncture.
This type of grommet is not as smooth as the hand tucked, but offers economy and ease of manufacture. An
advantage is that the swaged sleeves give clear indication of the splice area.
CABLE LAID HAND TUCKED GROMMET
Cable Laid Hand Tucked Grommets are fabricated in the same manner as strand laid hand tucked grommets except one continuous length of wire rope is used. This makes for a flexible smooth sling. the body diameters are
some- what odd sized because the grommet body is built up from a standard diameter componentrope.
CABLE LAID MECHANICAL SPLICE
(See Rated Capacity Tables Section) Cable Laid Mechanical Splice Grommets are fabricated from one continuous length of cable laid wire rope fabric with the ends joined by one or more mechanical sleeves.
WIRE ROPE SLING INSPECTION AND REMOVAL CRITERIA
BASIC INSPECTION CRITERIA FOR WIRE ROPE SLINGS
The goal of a sling inspection is to evaluate remaining strength in a sling which has been used previously to determine if it is suitable for continued use.
The specific inspection intervals and procedures are required by the Occupation Safety and Health Act (OSHA) and by ANSI B30.9 Regulations, and the responsibility for performance of inspections is placed squarely upon the sling user by Federal Legislation.
As a starting point, the same work practices which apply to all “working” wire ropes apply to wire rope which has been fabricated into a sling. Therefore, a good working knowl- edge of wire rope design and construction will be not only useful but essential in conducting a wire rope
Because wire rope is a rather complex machine, no precise rules can be given to determine exactly when a wire rope sling should be replaced. There are many variables, and all must be considered.
OSHA specifies that a wire rope sling shall be removed from service immediately if ANY of the following conditions are present:
- Broken Wires: For sing-part slings, 10 randomly distributed broken wires in one rope lay, or five broken wires in one strand of one rope lay. For multi-part slings these same criteria apply to each of the component ropes. For this inspection, a broken wire shall only be counted once; that is, each break should have twoends.
- Metal Loss: Wear or scraping of one third the original
diameter of outside individual wires. This is quite difficult to determine on slings an experience should be gained by the inspector by taking apart old slings and actually meas- uring wirediameters.
Distortion: Kinking, crushing, birdcaging or other damage which distorts the rope structure. The main thing to look for is wires or strands that are pushed out of their original positions in the rope. Slight bends in a rope where wires or strands are still relatively in their original positions would not be considered serious damage. But good judg- ment isindicated.
heat Damage: Any metallic discoloration or loss of internal lubricant caused by exposure to heat.
Bad End Attachments: Cracked, bent or broken end fittings caused by abuse, wear oraccident.
Bent hooks: No more than 15 percent over the normal throat openings, measured at the narrowest point, or twist- ing more than 10 degrees ispermissible.
Metal Corrosion: Severe Corrosion of the rope or end attachments which has caused pitting or binding of wires should be cause for replacing the sling. Light rusting usually does not affect strength of a sling, however. In addition to these seven conditions specified by OSHA, the following are also important:
Pulled Eye Splices: Any evidence that they eye splices have slipped, tucked strands have moved, or pressed sleeves show serious damage may have sufficient cause to reject asling.
Unbalance: A very common cause of damage is the kink which results from pulling through a loop while using a sling, thus causing wires and strands to be deformed and pushed out of their original position. This unbalances the sling, reducing it’sstrength.
Disposition of Retired Slings: the best inspection program available is of no value if slings which are worn out an have been retired are not disposed of properly. when it is determined by the inspector that a sling is worn out or damaged beyond use, it should be tagged immediately DO NOTUSE.
This sling should then be destroyed as soon as possible by cutting the eye an fittings from the rope with a torch. This will help assure that an employee will not mistakenly use a sling which has been retired from service.
It should also be obvious that a good inspection program will not only provide safer lifting conditions, but will also extend the life of the slings and thereby reduce lifting costs.
FEDERAL WORK RULES REQUIRE SPECIFIC INSPECTION INTERVALS
Government regulations are also specific on WHEN to inspect.
Both ANSI Standard B30.9 and OSHA require that wire rope slings receive two types of inspections: a DAILY visu- al inspection, and additional inspections where service conditions warrant.
Daily visual inspections are intended to detect serious damage or deterioration which would weaken the sling. This inspection is usually performed by the person using the sling in a day-to-day job. He should look for obvious things, such as broken wires, kinks, crushing, broken attachments, severe corrosion, etc.
Additional inspections should be performed at regular intervals based on, (1) frequency of sling use, (2) severity of service conditions, (3) nature of lifts, and (4) prior expe- rience based on service life of slings used in similar circumstances.
It is required that these additional inspections be carried out by a designated person who much have good knowledge of wire rope. An accurate WRITTEN and dated record of all conditions observed should be kept. Any deterioration of the sling which could result in appreciable loss of original strength should be carefully noted, and determination made on whether further use would consti- tute a safety hazard.
HOW TO INSPECT
Precisely how to make proper, adequate inspections is not detailed by OSHA- yet it is in the HOW of inspection that the big difference between a good inspection and some- thing less become apparent.
- First, it is necessary that all parts of the sling arereadily visible. The sling should be laid out so every part is accessible.
- Next, the sling should be sufficiently cleaned of dirt.
and grease so wires and fittings are easily seen. This can usually be accomplished with wire brush orrags.
- The sling should then be given a thorough, systematic examination throughout it’s entire length, paying particular attention to sections showing the mostwear.
- Special attention should also be paid to fittings and end attachments and areas of the sling adjacent to these fittings.
- When the worst section of a sling has been located, this area should then be carefully checked against the OSHAcriteria.
- Label or identify slings that areinspected.
- Keep records of inspections that include dates and corresponding conditions ofslings.
- Dispose immediately of slings that arerejected.
A knowledgeable inspector will also insist on proper storage for out of use slings to make his job easier if not for the good of the slings. Inspections are much easier and probably more thorough when slings are available for inspection in an orderly arrangement, out of the weather, away from heat and dirt.
WORKING LOAD LIMITS MECHANICAL SPLICE
The information provided on Working Load Limits in the Slings section were obtained from the Wire Rope Technical
Board out of the Wire Rope Sling Users Manual.