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GOOD SLINGING PRACTICE BASIC RULES ... 4 4.5. SAFE WORKING LOADS - FOUR-LEG SLINGS. You will have noticed that the Riggers Chart treats 2, 3 and 4 leg slings as having the same SWL for various configurations. With a 4-leg sling, unless a load-sharing device exists in the system, the load will ALWAYS be taken by TWO slings only. The other two will only serve to balance the load. Angle to vertical Included angle is between two opposite legs. The reason for this situation is that it is virtually impossible for the length of the slings to be exactly equal, and the placement of the slings to load attachment points to be such that the distance from their underside to the crane hook is exactly equal. If the load is flexible, some sharing of the load will take place as the load twists. The degree of sharing is dependent on the stiffness of the load. If the effect of this stiffness is unknown, we MUST assume it to be perfectly stiff, and NO load sharing takes place. Sling calculations are therefore the same as for two-leg slings (see Sheet 4.3). Of special concern with precast slung from four points, is the capacity of the pin anchors. Let's see, now. Ten tons to pick up - 4 x 2.5 tonne pins = 10 tonnes. 10mm chains ought to do. Sweet, eh! OH, REALLY? Remember that one leg at least will be slack, and the load will then be carried by only two slings. Therefore, the included angle must be 90°. Sling angle = 90°. Sling load must be 7.0 tonnes. Therefore, 4 x 10-tonne pins are required, and 16mm chains. Some difference! 2.5 pin anchors 10 TONNES 8.5m across diagonals CRANEAGE - SLINGING: BASICS PAGE 6 6m 10mm chain slings
GOOD SLINGING PRACTICE BASIC RULES ... 4 4.6 LOAD SHARING DEVICES These are examples of load sharing devices which distribute a load equally among sling legs. One side slung with fixed slings, the other side using a delta plate to ensure all slings are equally loaded. SWL 6.0 T Use a lifting beam with two sets of two leg slings, to ensure all legs are equally loaded. Alternative - use a snatch block. Use short two-leg slings and a further set of two-legs, to ensure equal load sharing. CRANEAGE - SLINGING BASICS PAGE 7
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