The myth of the motte and bailey castle in Scotland
An assessment of medieval earthwork fortifications in Scotland and their relationship to traditional Anglo-Norman motte and bailey castles, and earlier Scottish sites....
Following a visit to The Verne Citadel, Portland, by Geoffrey Salter and Peter Davies (H.C.C. Architect) it has been possible to examine the surviving pieces of a Guthrie rolling bridge.
The rolling bridge consist of two I section girders, each 15ft. long. At the outer and inner ends are axles fitted into bushes bolted to the sides of the girders.Each of these axles has two wheels that are free to rotate. These are held in place by collars bolted to the axles.
Forward of the centre point of the girders is a third axle. This one passes through the bushes and projects outside the girder in order for it to engage in the cups of the lifting arms of the bridge raising mechanism. Two 3/4" diameter tie bars, one at each end, provide strength. All six wheels are flanged on their inner sides.When the bridge is in the 'run in' position these flanges run on the iron rails along the inner sides of the trays fitted to the courtyard flagstones.
This lines the bridge up accurately with the lifting mechanism and ensures that the two inner sets of wheels run down the incline of the granite sill.
Author: David Moore
Article Source: http://www.victorianforts.co.uk/
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