Load capacity is one of the fundamental indicators of crane performance, and the load capacity directly determines the performance level of crane. The load capacity of a crane is determined by the following parts:
Lever. A balanced crane contains a beam that revolves around the pivot. According to the lever principle, the huge load on a shorter end can be balanced at the longer end with relatively small force, and the load capacity of the crane is thus determined. And the ratio of the load to the applied force is equal to the length ratio of the longer end to the shorter end of the lever, which is a mechanical advantage.
Pulley block. A jib type crane has an inclined support ("boom") to support a fixed pulley block, which is twined by a rope with a set of movable pulley blocks connecting the heavy object. When the hoister pulls the free end of the rope, the pulley system will provide a force equaling to the total force of the applied force multiplied by the number of strands between the pulley blocks. It's also a mechanical advantage.
Hydraulic cylinder. It can be used to directly increase the load or indirectly transfer the crane boom or beam carrying another lifting device. Like all machines, the crane also follows the law of energy conservation. That is, the energy output to the crane will not exceed the energy input to the machine. For example, if a pulley system provides 10 times of the applied force, the load action is only 1/10 of the force applied. Because the energy is proportional to the product of force and the distance, output energy is approximately equal to the input energy (The energy loss caused by friction may be smaller in practice).
For stability, the total torque of each part of the lifting system must be close to zero to ensure that the crane is not tipped over. In practice, the maximum allowable load (rated load) must be smaller than the load that causes the overturn, thus providing a safety margin.
According to American modern crane standard, the rated load of crawler crane is 75% of tipping load, and the rated load of mobile crane with leg is 85% of tipping load. The crane design requirements and some other aspects related to safety is regulated by the American Society of Mechanical Engineers in the standard ASME B30.5-2014 “Mobile and Locomotive Cranes”.
The standard for cranes installed on ships or offshore platforms is more stringent due to the dynamic loads generated by the ship's movements. In addition, the stability of a ship or platform must also be taken into account.
For a crane with fixed base or main girder, the torque generated by the boom, arm and load is offset by the base or main girder. The stress in the base must be less than the yield stress of the material or the crane being destroyed.