Double girder overhead cranes for picking and placing within factories and work places are of a twin beam design. As you can see on the photographs below, the hoist sits astride the cranes 2 main beams. The means the hoist itself is often called a “crab unit or hoist crab”.
Double girder cranes are usually used when larger safe working loads are required, and are much more common upwards of 10 tonnes or 15 tonnes SWL’s, or in much wider overhead cranes.
It is commonly thought is that double girder cranes obtain better lifting heights, or hook height. This is because the hook can be lifted high up between the twin beams slightly. However as the hoist sits on the top, the crane has to be lower to avoid the roof, so the hook to floor height often results in being the same.
In addition when an industrial unit has a pitched roof, the hoist when sitting on top of the cranes beam’s can clash on the factory’s eaves. This can cause the hoist to have very limited left and right travel depending on the pitch of the factory roof. end stops are used to limit the hoists left and right travel.
One way to overcome the lack of hoist left and right travel is to under notch the cranes twin beams below the level of the gantry rails. Click here for more details on under notched cranes beams.
If roof height, hook coverage, or price is an issue, you can always consider a single girder crane.
Learn more about overhead cranes in general.