Guthrie Rolling Bridge - Running The Bridge Out

Running The Bridge Out

Running the bridge out is effected by pushing it along its rails until the outer two wheels are at the edge of the granite sill of the ditch. When the bridge is pushed beyond this point the two outer wheels do not descend the slopes of the granite slabs but instead the bridge remains in a horizontal position. This is due to the centre axle being offset from the centre of the girders causing the bridge to run on its inner two wheels. The middle two wheels follow along the trays. When the bridge is just less than halfway out, the ends of the centre axle then engage in the cups of the lifting arms. The middle two wheels partially descend the slopes of the granite sill. The bridge continues to move out, its outer end dipping below the horizontal, its inner wheels still running along the trays of the courtyard. The weight of the bridge, at the point where the centre of gravity of the lifting arms is beyond the pivot point of the arms and the stays, causes the bridge mechanism to descend, its wheels running up the vertical racers set into the wall of the pit. At a point where the inner wheels are at the top of the slope down the granite sill, the outer ends of the girders and the outer wheels hit curved sprung metal guides.These guide the outer end of the bridge upwards so that the ends of the girders come to rest on the lip of the granite slab, the inner wheels descending the slope of the inner granite slab. The inner end of each girder is angled to form a close fit with the granite slab.