The main points considered while designing them is the capability to lift the load, the stability offered by it and last but not the least the tolerance power. The science behind each of the components is portrayed here. The lever: The principle of the lever allows a heavy load attached to the shorter end of the beam to be lifted by a smaller force applied in the opposite direction to the longer end of the beam. The ratio of the load's weight to the applied force is equal to the ratio of the lengths of the longer arm and the shorter arm, and is called the mechanical advantage. The pulley: It is the part that actually lifts the loads. Cables are wrapped multiple times round the fixed block and round another block attached to the load. When the free end of the cable is pulled by hand or by a winding machine, the pulley system delivers a force to the load that is equal to the applied force multiplied by the number of lengths of cable passing between the two blocks. The hydraulic cylinder: This can be used directly to lift the load or indirectly to move the jib or beam that carries another lifting device. Like all other machines, they also obey the principle of conservation of energy. This means that the energy delivered to the load cannot exceed the energy put into the machine. For example, if a pulley system multiplies the applied force by ten, then the load moves only one tenth as far as the applied force. Since energy is proportional to force multiplied by distance, the output energy is kept roughly equal to the input energy. For stability, the sum of all moments about any point such as the base must equate to zero. In practice, the magnitude of load that is permitted to be lifted (called the 'rated load' in the US) is some value less than the load that will cause it to tip (providing a safety margin). Standards for the ones mounted on ships or offshore platforms are somewhat stricter because of the dynamic load on the crane due to vessel motion. Additionally, the stability of the vessel or platform must be considered. The users are actually not aware of the principles that are applied behind each of these crane parts but it is really important to know about them.