Health and safety requirements state that limits need to be applied to the weight of loads to provide a safe working load. This is designed to stipulate the safe load bearing capacity on each wheel of a loaded vehicle, for instance, or the rigging of a crane. The safe working load (SWL) for a cable is usually marked on the cable and should not be exceeded. It allows for a more even distribution of cargo, and hence a more even distribution of weight. The greater the number of lifting points, the less stress on each point. In the case of a safe working load for a crane, a number of factors need to be taken into account in order to calculate the stress put on the individual lift. Calculations need to take into account not only the number of lifting points and the total load, but also the angle that the load has to be lifted from. The greater the angle from the vertical, the more stress is put on the crane cable.
Determine the total load to be carried. This may appear on a cargo manifest or may have to be calculate individually and totalled. Once the total weight has been calculated, divide the total by the number of lifting points. This will give the load at each point. As an example, we will use a weight of 5,000 kg for each lifting point.
Check your angles
Determine the angle of each load to the horizontal. An experienced worker may be able to determine the angle by eye, but if in doubt, use a protractor or measuring device. We need to use the formula Fe1/sin(A). The angle A is the angle of the cable to the horizontal. Use a scientific calculator. For example, if the angle is 60 degrees to the horizontal, we arrive at 1.55. We will call this the angle factor. Multiply the weight of our load by the angle factor. 5000 kg multiplied by 1.55 equals 7750.
The safety angle
The result obtained gives us the tension at each point of the load. As an extra safety factor, multiply this figure by 0.8. In our example, 0.8 multiplied by 7750 equals 6200.
Check and double check
Compare this result with the safe working load stipulated on the cable. In our example, if the resultant weight of our load is 6200, and the recommended safe working load recommended for the cable is 5500, we would have to reduce the tension on the cable by reducing the load or increasing the number of lifting points.