Rails upon which the travelling engine or traveller can move are laid on top of the runners, and are turned up at the ends of the platform to serve as buffers to the engine platform.
End ElevationSide Elevation
Fig. 10.—Elevation of Travelling Gantry
The engine platform consists of two trussed beams of timber about 3 feet apart, connected at their ends with short pieces of the same scantling, and fitted with grooved wheels to move upon the rails. Rails are also laid upon each beam and serve for the traversing motion of the crab. Movement of the traveller is obtained from the crab, which is worked either by manual or steam power, and acts through a system of shafting and geared wheels. Movement in three directions is necessary from the crab: vertically for lifting, and horizontally in two directions, transversely and longitudinally. Travellers are made up to 50 feet wide and any required length.
Another method of building travellers is shown in [fig. 10].
In this case, the rails upon which the traveller moves in a longitudinal direction are fixed on sleepers on the ground level, and the standards and runners of the first example are not required. In their place is constructed a triangulated system of balk timber framing. The platform is fixed to the head pieces, and is braced as shown. Less timber is used in their construction, but owing to the greater weight a steam winch is required to impart motion.
Gantries which serve as a base for lighter forms of scaffolding.—These erections are in reality elevated platforms, and allow of a clear way for a footpath where required. They are constructed of two frames, placed apart according to the width of the path over which the platform stands ([fig. 11]).
The method of erection, so far as the side frames are concerned, is the same as for the first example of travelling gantries. Stability is, however, gained by cross-bracing as shown in figure, thus making strutting unnecessary. The platform can be laid by placing short boards 9 in. by 3 in. across the runners when the platform is narrow. It is more usual, however, to place joists 10 in. by 2 in. across, and on these to lay the boards longitudinally. The joists average 2 to 3 feet apart, the braces are about 2 in. by 7 in. On the outside of the scaffold, parallel to the sills, balk timbers are placed forming a ‘fender’ to prevent the vehicular traffic from injuring or disturbing the erection.
Front Elevation