Mass of the load
It is essential that the mass of the load to be lifted is known.
It is assumed that the attachment point of the hook is directly above the centre of gravity of the load. To lift the load the following conditions should be met:
a) For single-leg and single endless wire rope slings the attachment point should be vertically above the centre of gravity.
b) For 2-leg wire rope slings the attachment points should be either side of and above the centre of gravity.
c) for 3- and 4-leg wire rope slings the attachment points should be distributed in plan around the centre of gravity. It is preferable that the distribution should be equal and that the attachment points are above the centre of gravity.
If the attachment points using a) or b) are at or below the centre of gravity, other lifting arrangements should be used.
When using 2-, 3- and 4-leg wire rope slings the attachment points and sling configuration should be selected to achieve angles between the sling legs and the vertical within the range marked on the sling. Preferably all angles to the vertical (angle â) should be equal. Angles to the vertical of less than 15° should be avoided if possible as they present a significantly greater risk of load imbalance.
All multi-leg slings exert a horizontal component of force (see figure) which increases as the angle between the sling legs is increased. Care should always be taken to ensure that the load to be moved is able to resist the horizontal component of force, without being damaged.
Method of connection
A wire rope sling is usually attached to the load and the lifting machine by means of terminal fittings. Sling legs should not be twisted or knotted.
The lifting point should be seated well down in a hook, never on the point or wedged in the opening. The sling hook should be free to incline in any direction so as to avoid bending. For the same reason, the terminal fitting should be free to incline in any direction on the hook to which it is fitted.
The rope may be passed under or through the load to form a choke hitch or basket hitch. When using basket hitch method and where it is necessary, due to the danger of the load tilting, to use more than one sling, this should preferably be done in conjunction with a lifting beam.
When a wire rope sling is used in a choke hitch, the rope should be allowed to assume its natural angle and should not be hammered down.
When attaching the sling to the lifting hook, ensure that there is adequate clearance to permit articulation and to prevent damage to the sling. Never force, hammer or wedge a sling into position. If there is insufficient clearance, fit a shackle between the sling and the hook.
To prevent the formation of kinks and subsequent weakening of the rope of slings having soft eye terminations, ensure that the effective diameter of the shackle pin / hook is at least twice the diameter of the rope.
In the case of a mutli-leg sling the tip of a sling hook should be directed outwards. No rope should be wrapped around a crane hook.
The steel wire rope sling may be attached to the load in several ways:
In this case lower terminals are connected to the attachment points. Selection of hooks and attachment points should be such that the load is carried in the seat of the hook and tip loading of the hook is avoided.
In this case sling legs are passed through or under the load and the lower terminal back hooked are reeved onto the rope. This method can, therefore, be used where no suitable attachment points are available and has the additional advantage that the wire rope sling legs tend to bind the load together.
Where choke hitch is employed the working load limit (WLL) of the sling should be no more than 80 % of that marked.
If two or more wire rope sling legs are used in a choke hitch or a double choke hitch care should be taken:
1) if it is important, to avoid imparting a torque to the load, to align the chokes;
2) if it is important, to avoid the load rolling or moving laterally when first lifted, to ensure that (at least) one leg passes either side of the load.
When endless slings are used they should be so placed such that any joining ferrules or splices are in the free length of the sling.
There are two methods of forming a basket hitch; passing a single sling through a load or wrapping two slings around the load. The second method is not suitable where the slings are able to move towards each other when the load is lifted or when lifting loads which are not held together such as loose bundles; a choke hitch is preferred.
Symmetry of loading
Working load limits are given for wire rope slings in a range of sizes and for different configurations. These WLL values have been determined on the basis that the loading of the wire rope sling is symmetrical. This means that when the load is lifted the wire rope sling legs are symmetrically arranged in plan and subtend the same angles to the vertical.
In the case of 3-leg wire rope slings, if the legs are not symmetrically arranged in plan the greatest tension will be in the leg where the sum of the plan angles to the adjacent legs is greatest. The same effect will occur in 4-leg wire rope slings except that the rigidity of the load should also be taken into account. With a rigid load the majority of the mass must be assumed to be taken by only 3- or even 2-legs with the remaining leg or legs serving only to balance the load.
In the case of 2-, 3- and 4-leg wire rope slings, if the legs subtend different angles to the vertical the greatest tension will be in the leg with the smallest angle to the vertical. In the extreme case, if one leg is vertical, it will carry all the load.
If there is a lack of symmetry in plan and unequal angles to the vertical, the two effects will combine and may either be cumulative or tend to negate each other; but if the following is satisfied, the loading can be assumed to be symmetric providing the load to be lifted does not exceed 80% of marked WLL:
a) wire rope sling leg angles to the vertical are all not less than 15°; and
b) wire rope sling leg angles to the vertical are all within 15° to each other; and
c) in the case of 3- and 4-leg wire rope slings, the sum of the plan angles to the adjacent legs is within15° of each other.
If not all of the above parameters are satisfied then the loading should be considered as asymmetric and the lift referred to a competent person to establish the safe working load for the wire rope sling. Alternatively, in the case of asymmetric loading, the wire rope sling should be rated at half the marked WLL.
If during a test lift the load is unstable it should be lowered and the slinging arrangement changed. This can be done by moving the attachment points or by using suitable shortening device in one ore more parts. Those shortening device shall be used according to CERTEX recommendations.
Safety of lift
Hands and other parts of the body should be kept away from the sling to prevent injury as the slack is taken up. When ready to lift, the slack should be taken up until the rope is taut. The load should be raised slightly and a check made that it is secure and assumes the position intended. Persons undertaking the lift should be aware of the potential hazards associated with the load tilting or swaying. This is especially important with basket or other loose hitches where friction retains the load.
Multi-leg wire rope slings with less than the full number of legs in use
As a general principle, wire rope slings should be used only for the purpose for which they have been designed. In practice, however, occasions may arise when a lift needs to be made using a smaller number of legs than the number of legs in the sling. In such cases the WLL should be reduced from that marked on the sling by applying the relevant factor given in table below.
Legs that are not in use should be hooked back to reduce the risk of such legs swinging freely, or snagging when the load is moved.
Types of sling
|Number of legs used||Factor to apply to marked WLL|
|3- and 4-leg||2||2/3|
|3- and 4-leg||1||1/3|
Working Load Limit (WLL)
Taking into consideration the recommendations and the cumulative effects of de-rating, the method of slinging should be decided and a suitable wire rope sling or wire rope slings selected so that the mass to be lifted does not exceed the WLL.
Landing the load
The landing site should be prepared. It should be ensured that the ground or floor is of adequate strength to take the load taking account of any voids, ducts, pipes etc. which may be damaged or collapse. It should also be ensured that there is adequate access to the site and that it is clear of any unnecessary obstacles and people. It is preferable to use timber bearers or similar material to avoid trapping the sling or to protect the floor or load or to ensure the stability of the load when landed.
The load should be landed carefully ensuring that hands and feet are kept clear. Care should be taken to avoid trapping the wire rope sling beneath the load as this may damage it. Before allowing the rope to become slack, the load should be checked to ensure that it is properly supported and stable. This is especially important when several loose objects are in basket hitch and choke hitch.
When the load is safely landed the wire rope sling should be carefully removed to avoid damage or snagging or cause the load to topple over. The load should not be rolled off the sling as this may damage the sling.
Storage of wire rope slings
When not in use wire rope slings should normally be kept on a properly designed rack. They should not be left lying on the ground where they may be damaged.
If the wire rope slings are to be left suspended from a crane hook, the sling hooks should be engaged in an upper link to reduce the risk of sling legs swinging freely or snagging.
If it is likely that wire rope slings will be out of use for some time they should be cleaned, dried and protected from corrosion, e.g. lightly oiled.