running wheels which enable the end carriages to travel on the longitudinal gantry girders or runway, and the crab or jenny, which carries the hoisting mechanism, and moves across the span on FIG.
Over the river these are lattice girders, with transverse girders 12 ft.
apart, and longitudinal and subsidiary transverse girders dividing but usually the time taken is one and a half minutes.
Where the rail-gauge is narrow and great weight is not desired, blocking girders are provided across the under side of the truck; these are arranged so that, by means of wedges or screws, they can be made to increase the base.
The principal component parts of a traveller are the main cross girders forming the revolving bridge, the two end carriages on which the bridge rests, the cranes.