A crane is a lifting machine equipped with a winder, wire ropes or chains and sheaves that can be used both to lift and lower materials and to move them horizontally. It uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal potential of a human.
In order for a crane to be stable, the sum of all moments about any point such as the base of the crane must equate to zero. In practice, the magnitude of load that is permitted to be lifted (called the "rated load") is some value less than the load that will cause the crane to tip.
The moment created by the boom, jib, and load is resisted by the pedestal base or kingpost. Stress within the base must be less than the yields stress of the material otherwise the crane will fail.
Cranes, like all machines, obey the principle of conservation of energy. This means that the energy transferred 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. In practice output energy is slightly less, because some energy is lost to friction and other inefficiencies.
Picture by: Ahsan Iqbal, 08/02/2009
In order for a crane to be stable, the sum of all moments about any point such as the base of the crane must equate to zero. In practice, the magnitude of load that is permitted to be lifted (called the "rated load") is some value less than the load that will cause the crane to tip.
The moment created by the boom, jib, and load is resisted by the pedestal base or kingpost. Stress within the base must be less than the yields stress of the material otherwise the crane will fail.
Cranes, like all machines, obey the principle of conservation of energy. This means that the energy transferred 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. In practice output energy is slightly less, because some energy is lost to friction and other inefficiencies.
Picture by: Ahsan Iqbal, 08/02/2009
This is a very good example.Just wanted to add some reasons a crane could fail.When lifting up loads from very muddy ground (e.g clay soil),the soil adds an extra force by holding the load to the ground thesame thing that happens if you stick a shovel in the ground as the shovel will be needed to be shaken to get it out.This extra force due to the soil creates an extra force for the crane and some crane controlers tend to move the jib from side to side trying to shake the load from the soil.This causes some extra moments that the crane was not designed for and could lead to the collapse of the crane.
ReplyDeleteBy Chukwudi Okenwa