Load Lifting Slings
Load Lifting Slings
Slings are accessories used for lifting and transferring of loads with the help of cranes, telphers, or hoists. Slings enable in the activities related to create anchors, attach to loads, lift loads, pull loads, and lower loads. The dominant characteristics of a sling are determined by the components from which it is made. As an example, the strengths and weaknesses of a sling made of steel wire rope are essentially the same as the strengths and weaknesses of the wire rope from which it is made.
Slings are generally one of six types namely (i) chain, (ii) wire rope, (iii) metal mesh, (iv) natural fiber rope, (v) synthetic fiber rope, or (vi) synthetic web. In general, use and inspection procedures tend to place these slings into three groups (i) chain, (ii) wire rope and mesh, and (iii) fiber rope web. Each type has its own particular advantages and disadvantages. Factors which are required to be taken into consideration when choosing the best sling for the job include the size, weight, shape, temperature, and sensitivity of the material to be moved, as well as the environmental conditions under which the sling is to be used. The slings used in steel industry are normally made of either steel chain or steel wire rope.
Slings have either single leg or multiple legs. In case of multiple leg slings, number of legs can be two, three, or four. The slings need to have certain minimum information which is required to be permanently and legibly marked on it. This minimum information consist of (i) an identification mark, and (ii) the working load limit (WLL) at given angles.
Various components of the sling are to match. All the components such as hooks, links, or shackles etc. used with the sling are of suitable material and strength to provide adequate safety protection. The attachments are to be properly installed and are to have a WLL at least equal to that of the sling with which they are used.
Slings are not only made of various materials but they also come in various configurations for different purposes. Hitch is a sling configuration whereby the sling is fastened to an object or load, either directly to it or around it. Correct application of slings is essential since improper application can be dangerous and can cause serious accidents.
Different configurations of the slings
Single vertical hitch – It supports a load by a single vertical part or leg of the sling. The total weight of the load is carried by a single leg, the sling angle is 90 degrees (sling angle is measured from the horizontal) and the weight of the load can equal the WLL of the sling and fittings. End fittings can vary but thimbles are to be used in the eyes. The single vertical hitch is not to be used for lifting loose material, lengthy material or anything difficult to balance. This hitch provides absolutely no control over the load since it permits rotation. The single vertical hitch is used on items equipped with lifting eyebolts or shackles.
Bridle hitch – Two, three or four single hitches can be used together to form a bridle hitch for hoisting an object with the necessary lifting lugs or attachments. Bridle hitch, which is used with a wide assortment of end fittings, provides outstanding load stability when the load is distributed equally among the legs, the hook is directly over the load’s centre of gravity and the load is raised level. For the distribution of the load equally, it is necessary to adjust the leg lengths with turnbuckles. Proper use of a bridle hitch requires that sling angles be carefully measured to ensure that individual legs are not overloaded. When a four leg sling lifts a rigid load, it is to be assumed that the load is carried by two of the legs only and the ‘rate’ of the four-leg sling is that of a two leg sling. This is because the load may not be distributed evenly.
Single basket hitch – This configuration is used to support a load by attaching one end of the sling to the hook, then passing the other end under the load and attaching it to the hook. In this hitch, it is to be ensured that the load does not turn or slide along the rope during a lift.
Double basket hitch – It consists of two single basket hitches placed under the load. On smooth surfaces, the legs tend to draw together as the load is lifted. For countering this, the hitch is braced against a change in contour, or other reliable means, to prevent the slings from slipping. It is also necessary to keep the legs far enough apart to provide balance, but not so far apart that they create angles of less than 60 degrees with the horizontal.
Double wrap basket hitch – It is a basket hitch wrapped completely around the load and compressing it rather than merely supporting it, as it is done in the ordinary basket hitch. The double wrap basket hitch can be used in pairs like the double basket hitch. This method is excellent for handling loose material, pipe, rod or smooth cylindrical loads because the sling is in full 360 degree contact with the load and tends to draw it together. On smooth surfaces, this type of hitch is a better choice.
Single choker hitch – It forms a noose in the sling. It does not provide full 360 degree contact with the load. Hence it is not to be used to lift loads difficult to balance or loosely bundled. Choker hitches are useful for turning loads and for resisting a load which wants to turn. Use of the choker hitch with two legs provides stability for longer loads. Like the single choker, this configuration does not completely grip the load. The load is to be lifted horizontally with slings of even length to prevent the load from sliding out.
Double wrap choker hitch – It is formed by wrapping the sling completely around the load and hooking it into the vertical part of the sling. This hitch is in full 360 degrees contact with the load and tends to draw it tightly together. It can be used either singly on short, easily balanced loads or in pairs on longer loads. The loosely-bundled loads are usually lifted with this hitch.
Endless slings or Grommet slings – These are useful for a variety of applications. Endless chain slings are manufactured by attaching the ends of a length of chain with a welded or mechanical link. An endless wire rope sling is made from one continuous strand wrapped onto it to form a six-strand rope with a strand core. The end is tucked into the body at the point where the strand is first laid onto itself. These slings can be used in a number of configurations, as vertical hitches, basket hitches, choker hitches and combinations of these basic arrangements. They are very flexible but tend to wear more rapidly than other slings because they are not normally equipped with fittings and thus are deformed when bent over hooks or choked.
Braided slings – These are generally fabricated from six to eight small diameter ropes braided together to form a single rope which provides a large bearing surface, very high strength, and flexibility in every direction. They are easy to handle and almost impossible to kink. The braided sling can be used in all the standard configurations and combinations but is especially useful for basket hitches where low bearing pressure is desirable or where the bend is extremely sharp.
For the selection and use of the slings, it is required that intelligence, care, and common sense are exercised. Slings are to be selected in accordance with their intended use, based upon the size and type of load and the environmental conditions of the workplace. All slings are required to be visually inspected before use to ensure that there is no obvious damage.
While describing the type of the steel load lifting slings there are standard symbols used. In case of first symbol, ‘S’ denotes single sling, ‘C’ denotes single choker sling with a standard end link on each end, no hooks, ‘D’ denotes double branch sling, ‘T’ denotes triple branch sling, and ‘Q’ denotes quadruple branch sling. In case of second symbol ‘O’ denotes oblong master link of standard dimensions, and ‘P’ denotes pear shaped master link. In case of third symbol, ‘S’ denotes sling hook, ‘G’ denotes grab hook, ‘F’ denotes foundry hook, and ‘L’ denotes latch lock. Sling tags are coded with numerals 1 through 4 to reflect number of branches in sling. Additional coding is defined as ‘AS’ denotes adjustable single, ‘SB’ denotes single basket, ‘ES’ denotes endless single, ‘ED’ denotes endless double, ‘SAL denotes single adjustable, ‘DAL’ denotes double adjustable loop, ‘AD’ denotes adjustable double.
Steel chain slings
Steel chain slings are made from steel chains (Fig 1). Chains are generally used because of their strength and ability to adapt to the shape of the load. However, care is required to be taken, using alloy steel chain slings since these slings are subject to damage by sudden shocks. Misuse of steel chain slings can damage the sling, resulting in sling failure and possible accident.
Fig 1 Steel chain slings
Steel chain slings are best suited for lifting hot materials. The chain can be heated to temperatures of upto 500 deg C. However, when these slings are consistently exposed to service temperatures in excess of 300 deg C, it is necessary to reduce the WLL as per the recommendations of the sling manufacturer.
All types of steel chain slings are to be visually inspected prior to use. While inspecting the steel chain slings, special attention is to be paid for any stretching, wear in excess of the allowances made by the sling manufacturer, and nicks and gouges. These are all indications that the sling can be unsafe and is to be removed from service.
The material used for steel chain sling can be C (carbon) steel, or alloy steel. In case of C steel the steel is to have the composition with C – 0.35 % maximum, P (phosphorous) -0.040 % maximum and S (sulphur) – 0.050 % maximum. In case of alloy steel, the steel is to have the composition with C – 0.35 % maximum, P – 0.035 % maximum, S – 0.040 % maximum, Ni (nickel) – 0.40 % minimum, and at least one of the other alloying elements present in an alloying amount Cr (chromium) – 0.40 % minimum, or Mo (molybdenum) – 0.15 % minimum. In case of stainless steel chain slings, the material is to be a 300 series austenitic stainless steel.
Steel chain slings are to be made by electric welding or gas welding process. Welded steel chain slings are to have permanently affixed durable identification stating size, grade, rated capacity, and sling manufacturer. Hooks, rings, oblong links, pear shaped links, welded or mechanical coupling links, or other attachments, when used with steel chain slings are to have a rated capacity at least equal to that of the steel chain. Job or shop hooks and links, or makeshift fasteners, formed from bolts, and rods etc., or other such attachments, are not to be used. Whenever wear at any point of any chain link exceeds the allowable limits, the sling is to be removed from service.
A thorough periodic inspection of steel chain slings in use is needed on a regular basis which is to be determined on the basis of (i) frequency of sling use, (ii) severity of service conditions, (iii) nature of lifts being made and (iv) experience gained on the service life of slings used in similar circumstances. Such inspections shall be at least once every 12 months.
Steel chain slings are to be cleaned prior to each inspection, as dirt or oil can hide damage. The inspector is to be certain to inspect the total length of the sling, periodically looking for stretching, binding, wear, or nicks and gouges. If a sling has stretched so that it is now more than 3 % longer than it was when new, it is unsafe to use the sling and it is to be discarded.
Binding is the term used to describe the condition which exists when the sling has become deformed to the extent that its individual links cannot move within each other freely. It is also an indication that the sling is unsafe. Generally, wear occurs on the load bearing inside ends of the links. Pushing links together so that the inside surface becomes clearly visible is the best way to check for this type of wear. However, wear can also occur on the outside of links when the chain sling is dragged along abrasive surfaces or pulled out from under heavy loads. Either type of wear weakens slings and more likely to cause accidents.
Heavy nicks and/or gouges are needed to be filed smooth, measured with calipers, then compared with the manufacturer’s minimum allowable safe dimensions. When in doubt, or in borderline situations, the sling is to be discarded. In addition, repair of the welded components on a sling is not to be tried.
Steel wire rope slings
These slings are made of steel wire rope (Fig 2). Wire rope is composed of individual wires which have been twisted to form strands. The strands are then twisted to form a wire rope. When wire rope has a fiber core, it is usually more flexible but is less resistant to environmental damage. On the other hand, a core which is made of a wire rope strand has higher strength and is more resistant to heat damage. Wire rope slings with steel core are normally used in steel plants.
Fig 2 Wire rope slings
Steel wire ropes are defined by the ‘lay’. The lay of a wire rope can mean any of three things. The first thing consists of one complete wrap of a strand around the core which means one rope lay is one complete wrap of a strand around the core. The second thing consists of the direction the strands are wound around the core. Wire rope is referred to as right lay or left lay. A right lay rope is the one in which the strands are wound in a right-hand direction like a conventional screw thread while a left lay rope is just the opposite. The third thing consists of the direction the wires are wound in the strands in relation to the direction of the strands around the core. In regular lay rope, the wires in the strands are laid in one direction while the strands in the rope are laid in the opposite direction. In lang lay rope, the wires are twisted in the same direction as the strands.
In regular lay ropes, the wires in the strands are laid in one direction, while the strands in the rope are laid in the opposite direction. The result is that the wire crown runs almost parallel to the longitudinal axis of the rope. These ropes have good resistance to kinking and twisting and are easy to handle. They are also able to withstand considerable crushing and distortion due to the short length of exposed wires. This type of rope has the widest range of applications. Lang lay rope is recommended for many excavating, construction, and mining applications, including draglines, hoist lines, dredgelines, and other similar lines. Lang lay ropes are more flexible and have greater wearing surface per wire than regular lay ropes. In addition, since the outside wires in lang lay ropes lie at an angle to the rope axis, internal stress due to bending over sheaves and drums is reduced causing lang lay ropes to be more resistant to bending fatigue.
A left lay rope is one in which the strands form a left-hand helix similar to the threads of a left-hand screw thread. Left lay rope has its greatest usage in oil fields on rod and tubing lines, blast hole rigs, and spudders where rotation of right lay loosens couplings. The rotation of a left lay rope tightens a standard coupling.
The number of strands and the standard construction determine the classification of a wire rope. A strand consists of a ‘centre’ which supports a specified number of wires around it in one or more layers. The strands provide all the tensile strength. Physical characteristics, such as fatigue resistance and the ability to resist abrasion are directly affected by the design of the strands. In most strands with two or more layers of wires the inner layers support the outer layers in such a manner that all wires can slide and adjust freely when the strand flexes. Normally a strand made up of a small number of large wires is more abrasion resistant and less fatigue resistant than a strand of the same size made up of many smaller wires.
The life of the steel wire rope sling is affected by the several operating conditions such as (i) bending, (ii) stresses, (iii) loading conditions, (iv) jerking (speed of load application), (v) abrasion, (vi) corrosion, (vii) sling design, (viii) materials handled, (ix) environmental conditions, and (x) earlier usage history etc. Further, the weight, size, and shape of the loads to be handled also affect the service life of a wire rope sling.
Flexibility is also a factor in case of wire ropes. Usually, wire ropes with higher flexibility are selected when smaller radius bending is needed. Wire ropes with lesser flexibility are used when the rope is to move through or over abrasive materials.
Wire rope slings are to be visually inspected before the use. The slings are to be checked for the twists or lay of the sling. If randomly distributed ten number of wires in one lay are broken, or five number of wires in one strand of a rope lay are damaged, the sling is not to be used. However, it is not enough to check only the condition of the wire rope. End fittings and other components are also required to be inspected for any damage which can make the sling unsafe. It is essential to keep a close watch on the slings being used.
Although every steel wire rope sling is lubricated during its manufacture, yet for lengthening the useful service life of the sling, it is also be lubricated during its use. There is no set rule on how much or how often the lubrication of the wire rope is to be done. It depends on the conditions under which the sling is used. More frequent lubrication of the wire rope is needed if the loads are heavier, the numbers of bends are more, or the conditions under which the sling operates are more adverse.
Steel wire rope slings are required to be stored in a good ventilated, dry building or shed. The slings are not to be stored on the ground or are allowed to be continuously exposed to the elements since this makes the slings susceptible to corrosion and rust. If it is needed to store the wire rope slings outside, then it is to make sure that the slings are set off the ground and protected. Using of the sling several times a week, even at a light load, is a good practice. Normally, the slings which are used frequently or continuously, give useful service far longer than those which are kept idle.
Wire rope slings can provide a margin of safety by showing early signs of failure. Factors which indicates that a steel wire rope sling is to be discarded include (i) severe corrosion, (ii) localized wear (shiny worn spots) on the outside, (iii) a one-third reduction in outer wire diameter, (iv) damage or displacement of end fittings such as hooks, rings, links, or collars due to overload or misapplication, (v) distortion, kinking, and bird caging, (vi) excessive broken wires, or (vii) other evidence of damage to the wire rope structure. These parameters are determined by inspection of the wire rope sling.
In the method used for the determination of the wire rope sling to be removed from service or not, the outside individual wires are not separated from the wire rope to make them available for measuring. For measuring the wear or scraping of one-third the original diameter, the measurement is to be made with a micrometer at the worn or scraped area and compared to the original diameter of whole wire rope. If the difference of this measurement is equal to, or more than, one-third the original diameter of an individual outside wire, the wire rope sling is needed to be removed from the service. A wire rope sling is allowed to be left in service with respect to a pass/fail gauge measurement if the difference between the original diameter of the whole wire rope and a pass/fail gauge outer diameter failed measurement is less than one-third the original diameter of the outside individual wire.
There are four characteristics which are considered while selecting a wire rope sling for providing the best service. These are (i) strength, (ii) ability to bend without distortion, (iii) ability to withstand abrasive wear, and (iv) ability to withstand abuse.
The strength of a wire rope is a function of its size, grade, and construction. It needs to be sufficient to accommodate the maximum load which is to be applied. The maximum load limit is determined by means of a suitable multiplier. This multiplier is the number by which the ultimate strength of a wire rope is divided to determine the WLL. Thus a wire rope sling has a design factor (multiplier). New wire rope slings have a design factor of 5. As a sling suffers from the rigours of continued service, however, both the design factor and the sling’s ultimate strength are proportionately reduced. If a sling is loaded beyond its ultimate strength, it will fail.
The wire rope is required to have the ability to withstand repeated bending without the failure of the wires from fatigue. Fatigue failure of the wires in a wire rope happens due to the development of small cracks under repeated applications of bending loads. The failure takes place when ropes make small radius bends. For the prevention of fatigue failure, the wire rope sling is to be used with blocking or padding for increasing the radius of the bend.
The ability of a wire rope sling to withstand abrasion is determined by the size, number of wires, and construction of the rope. Smaller wires bend more readily and therefore offer greater flexibility but are less able to withstand abrasive wear. On the other hand, the larger wires of less flexible ropes are better able to withstand abrasion than smaller wires of the more flexible ropes.
The abuse of the wire rope sling causes the sling to become unsafe long before any other factor results into the sling damage. Abuse of the wire rope sling causes serious structural damage to the wire rope, such as kinking or bird caging which reduces the strength of the wire rope. (In bird caging, the wire rope strands are forcibly untwisted and become spread outward).
Wire rope slings, like chain slings, are to be cleaned prior to each inspection since the slings are also subject to hidden damage due to the dirt or oil. Also, the slings are to be lubricated at regular intervals. Lubrication prevents or reduces corrosion and wear due to friction and abrasion. Before applying any lubricant, however, the sling is to be dry since application of lubricant to wet or damp sling traps moisture against the metal and hastens corrosion. Corrosion deteriorates wire rope. It may be indicated by pitting, but it is sometimes hard to detect.
The eyes of the wire rope sling are to be designed to provide what amount to ‘small inverted slings’ at the ends of the sling body. Hence, the width of the eye opening is affected by the same general forces which apply to legs of a sling. A sling eye is not to be used over a hook or pin with a body diameter larger that the natural width of the eye. The eye of the sling is not to be forced onto a hook. On the other hand, the eye is always to be used on a hook or pin with at least the nominal diameter of the rope.
Important aspects of load lifting slings
There are 4 main factors which are to be taken into consideration when safely lifting a load. These are (i) the size, weight, and center of gravity of the load, (ii) the number of legs and the angle the sling makes with the horizontal line, (iii) the rated capacity of the sling, and (iv) the history of the care and usage of the sling.
Determination of the weight of the load to be lifted is the most important step. The capacity of the sling is never to be exceeded. Also, the stability of the load is a critical step. A stable load is one in which the centre of gravity of the load is directly below or in line with the main hook. The centre point of an object is that point at which the object is balanced. The entire weight is required to be considered as concentrated at this point.
As the angle formed by the sling leg and the horizontal line decreases, the rated capacity of the sling also decreases. Hence, the smaller the angle between the sling leg and the horizontal, the greater the stress on the sling leg and the smaller (lighter) the load the sling can safely support. Larger (heavier) loads can be safely moved if the weight of the load is distributed among more sling legs.
The rated capacity of a sling varies depending upon the type of sling, the size of the sling, and the type of hitch. The capacity of the sling is to be known to the user of the sling. Charts or tables which contain this information generally are available from sling manufacturers. The values given are for new slings. Older slings are to be used with additional caution. Under no circumstances the rated capacity of the sling is to be exceeded.
History of the sling uses is needed to be known to the sling user. Mishandling and misuse of slings are the primary causes of accidents involving their use. The majority of injuries and accidents, however, can be avoided by becoming familiar with the essentials of proper sling care and usage.
A key factor in determining sling stress is sling angles. The rated capacity of any sling depends on its size, its configuration, and the angles formed by its legs with the horizontal. For instance, a two leg sling used to lift 1000 kilograms (kgs) has a 500 kgs of load on each leg at a sling angle of 90 degrees. The load on each leg goes up as the angle goes down. At 30 degrees angle the load is 1000 kgs on each leg. It is advisable to keep sling angles higher than 45 degrees whenever possible. The use of any sling at an angle lower than 30 degrees is extremely hazardous. This is especially true when an error of only 5 degrees in estimating the sling angle can be very dangerous. Low sling angles also create large, compressive forces on the load which can cause buckling, especially in longer flexible loads.
It is necessary that the WLL rated capacity of the sling is not exceeded. The WLL is the maximum load which is ever to be applied to the sling, even when the sling is new and when the load is uniformly applied which means straight line pull only. The side loading of the sling is to be avoided. Further, the catalog ratings are based upon usual environmental conditions, and consideration is to be given to unusual conditions such as extreme high or low temperatures, chemical solutions or vapours, and prolonged immersion in salt water etc. Such conditions or high-risk applications necessitate reducing the WLL.
Proper care and usage are essential for maximum service and safety. Slings are to be protected from sharp bends and cutting edges by means of cover saddles, burlap padding, or wood blocking, as well as from unsafe lifting procedures such as overloading.
When selecting a sling to handle a load, the sling-to-load angle and the tension that is applied to the sling, as a result of the angle is to be considered. Slings with adequate WLL to handle the ‘scale’ weight of an article have disastrously failed because of an inadequate consideration of the sling angle and the resultant tension. Sling failure leads to accidents. All the loads to be handled by the sling is to always to consider the angle, the resultant tension, and the actual strength requirements of the sling.
Before making a lift, there is a need of a check for being certain that the sling is properly secured around the load and that the weight and balance of the load have been accurately determined. If the load is on the ground, the load to drag along the ground is not allowed. This can damage the sling. If the load is already resting on the sling, then it is to be ensured that there is no sling damage prior to making the lift.
Before the lift is made, it is also to be ensured that the position the hook is directly over the load and the sling is seated squarely within the hook bowl. This provides the maximum lifting efficiency without bending the hook or overstressing the sling.