Electric single-girder cranes and double girder electric cranes difference

Electric single girder name implies, is only one beam,  chainb lock  directly beneath the beam upside down on the track;
Advantages: simple structure, easy to use;
Disadvantages: lifting height is restricted.
Electric double beam, there are two parallel beams,  manual hoist  mounted on a small car, the  trolley  placed in the beam just above the track;
Advantages: easy to use, lifting height relative single beam, the same height of the  hoist equipment , lifting height to be higher than a height of the beam cross-section;
Disadvantages: the structure is relatively complex, multi-set of running gear with a small frame, maintenance points relative to single beam more.