A hoist is used for lifting items up and down. When the hoist is fitted into a trolley, it can also travel horizontally and move its load to another location. The trolley is mounted on a “bridge”, which is a beam with wheels on each end (called end trucks). The bridge rides on the rails of an elevated runway, which enables the bridge crane to move loads of substantial weight back and forth. Key advantages of using a bridge crane include:
The trolley is a conveyor that consists of a continuous loop of chains or wire rope cable. The trolley is suspended from a track, which is supported by two end trucks. The trolley carries the hoist across the bridge girder (or girders) for the required distance.
A combined unit of hoist and trolley frame. Multiple hoists can be set up on a single trolley or on separate trolleys.
The hoist is mounted to the bottom of the trolley, from which it lifts or lowers items with a hook or other lifting attachment. The “hoist” can be a Wire Rope Hoist (which provides durable, long-term, reliable usage) or a Chain Hoist (which is for light-duty applications and lower weight capacities). When cost is a factor in the purchase decision, chain hoists are usually chosen over wire rope hoists.
A common early example of a "single fixed pulley" is the rope pulley which was pulled over a wheel to lift a bucket of water out of a well. This simple pulley reversed the direction of the "lifting force". When you add more ropes and wheels, you can reduce the amount of "lifting force" needed to lift the weight. Because more ropes are used, they need to be pulled further - even though the lifting force is reduced by adding the pulleys.
The chain hoist is a closed loop lifting mechanism. Because the chains are easier to grab by hand, chain hoists are generally selected for moving lighter objects, which are appropriate for manual lifting.
Pendants gives the overhead crane operator precision control over the movements of the bridge crane. Pendant configurations vary per the necessary equipment control functions. Each button will be assigned a specific operating function. Some pendants are suspended from the overhead’s trolley hoist, thus the operator must either walk along beside the load or on an independent load track system.
Some pendants are enabled with VFD (a “Variable Frequency Drive”). The controls on a VFD provide a higher level of customization, which can vary the frequency of the motors to achieve smoother motion acceleration and deceleration. One button on the VFD pendant controls the current spend while the second button is used for acceleration.