Choosing The Right Bridge Crane Option
Bridge cranes are composed of a beam that bridges a bay (wall to wall) that moves on two tracks mounted on either wall. A hoist and trolley, which traverses across the bridge, provides a total of three axes of motion. The hoist moves the load up and down, the trolley moves the load right and left, and the bridge of the crane moves the load forward and backward.
New and used bridge cranes (which are becoming increasingly popular as a cost effective option) represent a noteworthy class of cranes which function with a pulley system suspended from a trolley that travels on tracks that run along either one or two beams that lie horizontal to one another. The beams are known as the bridge and they are supported at both ends. More often than not the bridge is mobile along the pair of horizontal rails, allowing the crane to serve a large rectangular area. In the case of a circular space, a rotary can be utilised. When space is a problem a jib crane is usually considered.
There are two distinct advantages of a bridge crane. The first is greater flexibility for hook coverage and increased control over the load being handled. The second advantage is that it offers the lowest possible number of obstructions on the factory floor. Bridge and gantry cranes are highly flexible, responding in an oscillatory manner to external disturbances and motion of the bridge and trolley. Payload oscillation has adverse consequences. Swinging of the hook makes positioning difficult and inefficient. When the payload or surrounding obstacles are of a hazardous or fragile nature, the oscillations present a safety hazard as well.
Two examples of bridge cranes are the top running and the under hung crane. The overhead traveling variant is formed by adding a pair of single flange wheels to the design. This enables the crane to run directly on top of the runway beam, saving cost on the expensive crane rail. The under hung crane is different due to the end trucks riding on the bottom flange of the runway beam. This is typically only available on a capacity of 10t or less.
Which to choose between the top running and the under running crane is determined primarily by two factors. The first being capacity and the second being support structure. The under running crane can support up to 25tons, although the practical limit is usually closer to 15 tons. The top running crane can support considerably more. The second factor is support structure. Under running cranes (for lighter cranes) can be suspended directly from the overhead steel, whereas top running cranes require a bracket attached to the building or independent columns for support. The bracket attachment can weigh up to 10 tons.
The top running configuration of bridge cranes is best used in cases where the end user has issues with headroom. The most space efficient configuration is the double girder, top running crane system. When headroom is not an issue a top or under running configuration can be used with either the double or single girder bridge crane.
