Gantry crane selection - single and double girder cranes

We buy from the  hoist gantry crane , it should be how to choose it?
1 single girder and double girder gantry crane gantry crane selection
Under normal circumstances, from the weight of 50t or less, in the span of less than 35m, no special requirements, should use a single main beam. If you require a large gate leg width, high working speed, or frequently lifting heavy pieces, grew up pieces, you should choose double girder gantry crane.
(2) the length of the span and cantilever
Span gantry crane crane itself is affecting the quality of the important factors. Selection, to meet the conditions of use and equipment conform span series of standards under the premise of the span should be minimized.
3 to determine the principles of Tread
(A) along a  bridge gantry crane  to meet stability requirements of the track direction;
(B) the dimensions of the goods to be able to pass the legs flat steel;
(C) pay attention to the track B and the span S proportional relationships, and generally track B = (1/4-1/6) S.
4 Pitch size determination - gantry crane

At work, gantry cranes and yard external dimensions of goods and transport vehicles between channels should leave some space dimensions, in order to facilitate loading and unloading operations. General transport vehicles in the span of loading and unloading, should be kept above the door legs with 0.7m spacing. Spreader should not work with the transport vehicles with more than 0.5m spacing, cargo interlude legs, there should be more than 0.5m spacing.

5 Electrical equipment selection - overhead gantry crane
It should meet the common gantry crane GB/T14406-1993 the relevant provisions.