Harper Bridges

Introduction


Typical Harper Cast Fence Post
The Harpers were involved in the construction of light wire suspension bridges from about 1865 until 1924. Initially they designed and built and latterly designed.

John Harper left his parents’ farm near Turriff in Aberdeenshire around the age of 9 and found employment as a market gardener before  moving down to Edinburgh with his brother Hugh, where they became wire fencers. This continued in Glasgow and in 1856 they returned to Aberdeen to set up their own business as fencers and gate manufacturers. Before long they had established a foundry. In 1863, aged 30, John Harper registered a patent ‘device for straining wire’, which was crucial to the development of bridge building.

Illustrated is a corner fence post from the disused Speyside line of the Great Northern Scottish Railway (GNSR). This post has within it four wire tensioning devices visible, each able to rotate round the axis at right angles to the wire that passes through the outer casing, to be wound onto the rotating spindle.

The wire was wound up within the post using a crank tool rather like an ‘Allen’ key. Once the desired tension was generated, the spindle was locked by the insertion of a peg into the opposing notches, one on the spindle and the other on the casing. This was similar to the ‘key’ required to fix a wheel to its axle after the creation of slots in both components – a technique central to Harpers main business of supplying power drive machinery to Aberdeen’s mills. It is interesting that the notches on the spindle are at 90 degree intervals while those on the casing are at 120 degree intervals. This means that a rotation of only 30 degrees will allow another opposing pair to accept the locking key or peg.
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