Cranes
There are several types of
cranes that are used in the construction industry, in the lifting of heavy
consignments, and in the manufacturing industry for lifting and movement of
heavy loads. Safety precautions should be observed while lifting and moving loads
to ensure crane stability.
What Is a Crane
A crane is normally fitted
with a drum that can wind rope on it and chains or wire ropes for holding the
materials. Sheaves are installed that are used for the lifting and lowering of
the objects, and also for their horizontal movement. Cranes create mechanical
advantage by a suitable configuration mechanism. Thus, loads can be moved and
lifted that otherwise would not be within human capability.
Stability of Cranes
Stability of cranes is of
utmost significance to ensure the safety of humans and materials. Though
adequate safety features are incorporated in cranes by the manufacturer, the
user must be aware of the safety requirements that must be fulfilled to ensure
their safety. Stability of cranes can be achieved by ensuring that the total of
moments at the base should be zero. In actual usage, the permissible load to be
lifted, that is called the rated load, is less than the tipping crane load.
This provides a margin of safety for the crane stability. Generally, the crane
safety standards recommend the rated loads to be 75% to 85% of the tipping
loads. These safety features are determined keeping in view the design features
of the crane, the soil conditions, the uniformity of ground level, and other
factors. Cranes used on ships have more stringent safety margins due to the
dynamic load conditions. When cranes are used on a fixed platform, the moment
produced by the jib, boom, and load is opposed by the platform base. The stress
in the base should be lower compared to the material yield stress.
Tower Crane
Tower cranes are one type of
modern crane. Tower cranes are secured with the ground, or fixed with the walls
of structures. Tower cranes normally offer the most suitable arrangement of the
lifting capability and the height, and are most appropriate for the tall
buildings construction. The boom and the counter-jib are located on the
turntable. During the suspension of loads by the jib, the counter-jib hold
concrete blocks as counterweights. The crane is operated from a cabin located
at the tower top, or can be directed by remote control from the ground.
Electric motors are used to move the wire cables through sheaves. The operator
hooks or unhooks the loads by working in combination with the rigger. The
operator and the rigger use radio contact or hand signals, for the crane
operation. Normally, during construction of tall buildings, the height of the
tower cranes is considerable, due to which a small crane or a derrick is used
for the disassembly of the tower crane.