LOADING A CRANE
The lifting capacity of a crane is usually expressed in metric tons. This unit indicates the greatest permitted moment of a crane. For example, a crane of 200 metric tons is capable of lifting a load weighing 10 tonnes to a distance of 20 metres from the boom. A metric ton is not an exact unit, as the calculation of the greatest permitted moment must include the effect of the weight of the jib (which depends on the length of the jib) as well as the weight of the hoisting ropes, trolley and hook.
Another key figure of lifting capacity is the maximum load, which depends on the dimensioning of the lifting equipment. Usually, the lifting capacity is demonstrated by using a load chart. The chart indicates the largest permitted load at a given distance. It is easy to see from load charts that the lifting capacity is slightly greater when a shorter jib is used than with a long jib.
The height of the crane also affects the lifting capacity. When lifting is performed extremely high up, the weight of the hoisting rope increases significantly and reduces the useful load.