British Standard Whitworth - History

History

See also Screw thread > History of standardization

The Whitworth thread was the world's first national screw thread standard, devised and specified by Joseph Whitworth in 1841. Until then, the only standardization was what little had been done by individual people and companies, with some companies' in-house standards spreading a bit within their industries. Whitworth's new standard specified a 55° thread angle and a thread depth of 0.640327p and a radius of 0.137329p, where p is the pitch. The thread pitch increases with diameter in steps specified on a chart. The Whitworth thread system was later to be adopted as a British Standard to become British Standard Whitworth. An example of the use of the Whitworth thread is the Royal Navy's Crimean War gunboats. These are the first instance of "mass-production" techniques being applied to marine engineering as the following quotation from the obituary from The Times of 24 January 1887 to Sir Joseph Whitworth (1803–1887) shows:

The Crimean War began, and Sir Charles Napier demanded of the Admiralty 120 gunboats, each with engines of 60 horsepower, for the campaign of 1855 in the Baltic. There were just ninety days in which to meet this requisition, and, short as the time was, the building of the gunboats presented no difficulty. It was otherwise however with the engines, and the Admiralty were in despair. Suddenly, by a flash of the mechanical genius which was inherent in him, the late Mr John Penn solved the difficulty, and solved it quite easily. He had a pair of engines on hand of the exact size. He took them to pieces and he distributed the parts among the best machine shops in the country, telling each to make ninety sets exactly in all respects to the sample. The orders were executed with unfailing regularity, and he actually completed ninety sets of engines of 60 horsepower in ninety days – a feat which made the great Continental Powers stare with wonder, and which was possible only because the Whitworth standards of measurement and of accuracy and finish were by that time thoroughly recognised and established throughout the country.

An original example of the gunboat type engine was raised from the wreck of the SS Xantho by the Western Australian Museum. On disassembly, all its threads were shown to be of the Whitworth type.

With the adoption of BSW by British railway lines, many of which had previously used their own standard both for threads and for bolt head and nut profiles, and improving manufacturing techniques, it came to dominate British manufacturing.

In the USA, BSW was replaced when steel bolts replaced iron, but was still being used for some aluminium parts as late as the 1960s and 1970s when metric based standards replaced the Imperial ones.

American Unified Coarse was originally based on almost the same imperial fractions. The Unified thread angle is 60° and has flattened crests (Whitworth crests are rounded). Thread pitch is the same in both systems except that the thread pitch for the 0.5 in bolt is 12 threads per inch (tpi) in BSW versus 13 tpi in the UNC.