Cranes, defined as mounted mechanical instruments used to transfer small and medium sized loads over a short distance, come in a variety of designs including the bridge crane and the gantry crane. Cranes whether fixed or mobile are driven manually or by power. The bridge mounted crane is one of the few types of cranes that cannot be modelled as an industrial vehicle, the other is the gantry crane (also referred to as overhead travelling bridge cranes). The bridge crane is 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.
The dimensions of a generic bridge crane could include the following:
Trolley designed for maximum cross slope of 1%, End trucks designed for maximum longitudinal slope of 3%, NEMA 4 weatherproofing, Cold weather protection, emergency brakes that will stop the crane in 12′ and act as a hold down (brake sufficient for a 50 MPH wind velocity), completely bolted construction to facilitate erection as well as disassembly for easy relocation, radio remote controls and a full length catwalk with access ladder on 1 leg.
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.
Gantry cranes, or overhead bridge cranes, are versions of the crane which lift objects by a hoist which is fitted in a trolley and can move horizontally on a rail or pair of rails fitted under a beam. Overhead travelling cranes and gantry cranes are particularly suited to lifting very heavy objects and huge gantry cranes have been used for shipbuilding where the crane straddles the ship allowing massive objects like ships’ engines to be lifted and moved over the ship. In essence, overhead or bridge cranes refer to a crane with a movable bridge carrying a movable or fixed hoisting mechanism and travelling on an overhead fixed runway structure.
Overhead bridge cranes, also known as bridge cranes or as suspended cranes, have the ends of the supporting beam resting on wheels running on rails at high level, usually on the parallel side walls of a factory or similar large industrial building, so that the whole crane can move the length of the building while the hoist can be moved to and fro across the width of the building.