TENSION:

Tensioners and fence post patent

The tensioners were developed from the fence post patent and found on bridges from 1870-1893.

 

There were two sizes of tensioner, the smaller being ‘No 1’, found at Tanarmouth. The original bridge is likely to have been built around 1870.

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The smaller tensioner had 4 notches on the spindle (as had the fence post) and 5 on the casing.  The minimal increment of tension was thus (90°-72°) or 18°. The larger tensioner soon followed in Upper Glentanar which had 6 notches on the spindle and 8 on the casing.

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The minimal increment of tension was therefore (60°-45°) or 15°. As well as being slightly higher geared for larger loads, it was also more secure since being symmetrical, two keys were used.

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The tensioners were attached to the top and bottom of the masts (and in two 1871 bridges at Waterside of Aboyne and Shocklach extra ones in the middle).  For the tops (and middles) the tensioners were back to back, each cable passing through the post to gain the opening on the back of the tensioner and be threaded onto the spindle.  It’s likely that some form of secondary tension facilitated threading the cables.

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TENSIONING LATER BRIDGES

 

The last bridge known to have the cable tensioners described above was the one built at Feugh Lodge which is still in use.  There the platform or deck cables were tensioned in this way.  By this time Louis was setting his stamp on the design of the bridges and moved to the usual pattern of a bank to bank main cable over a saddle at the masthead. The cables were cast into a block that was threaded onto a staple screw.  This in turn connected with a cable loop from the anchorage.  Generally, both main and deck cables connected in a common anchorage.

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THE FENCEPOST PATENT

The Harper strainer fencepost was patented in 1863. The central spindle is protected from the elements and has 4 notches; the surrounding casing has 3. This means that for every 30° of rotation, there is an opportunity somewhere on the circumference to drive in the key once the desired tension is applied.

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I was fortunate to acquire a strainer post and five ‘runners’ over the years and placed them along the boundary of my property. The posts were 7 feet 3 inches long (2.21m), including about 3 feet below ground.  The post was supported at the bottom by a foot plate (to resist sinking) through which the hollow post was drained and there were two fins higher up to resist rotation.  The posts were laid in hardcore.  I assisted fencer John Hay of Huntly.

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