British Standard Whitworth

British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard.

History

The Whitworth thread was the world's first national screw thread standard,^[1] 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 (BSW). An example of the use of the Whitworth thread are 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.^[2]

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). From ​¹⁄₄ in up to ​1 ¹⁄₂ in, thread pitch is the same in both systems except that the thread pitch for the ​¹⁄₂ in bolt is 12 threads per inch (tpi) in BSW versus 13 tpi in the UNC.

Thread form

Whitworth thread form

The form of a Whitworth thread is based on a fundamental triangle with an angle of 55° at each peak and valley. The sides are at a flank angle of Θ = 27.5° perpendicular to the axis. Thus, if the thread pitch is p, the height of the fundamental triangle is H = p/(2 tan Θ) = 0.96049106 p. However, the top and bottom ​¹⁄₆ of each of these triangles is cut off, so the actual depth of thread (the difference between major and minor diameters) is ​²⁄₃ of that value, or h = p/(3 tan Θ) = 0.64032738 p. The peaks are further reduced by rounding them with a 2×(90° − Θ) = 180° − 55° = 125° circular arc. This arc has a height of e = H sin Θ/6 = 0.073917569 p (leaving a straight flank depth of h − 2e = 0.49249224 p) and a radius of r = e/(1 − sin Θ) = 0.13732908 p.

Comparison of standards

Two spanners, both nominal size ​⁵⁄₈ in, with a diagram superimposed to show the logic that allows them both to be nominal size ​⁵⁄₈ in when their actual sizes are clearly different (across-flats distance vs screw diameter). The across-flats definition is the common standard today, and has been for many decades. The larger spanner in this photo is from the 1920s or earlier. Its face was polished to allow the size stamp to show well in the photograph. This example is American, but it illustrates the way that spanners for Whitworth fasteners were typically labelled.

The British Standard Fine (BSF) standard has the same thread angle as the BSW, but has a finer thread pitch and smaller thread depth. This is more like the modern "mechanical" screw and was used for fine machinery and for steel bolts.

The British Standard Cycle (BSC) standard which replaced the Cycle Engineers' Institute (CEI) standard was used on British bicycles and motorcycles. It uses a thread angle of 60° compared to the Whitworth 55° and very fine thread pitches.

(To simplify matters, the term hexagon will be used in this paragraph to denote either bolt head or nut.) Whitworth (spanner) markings refer to the bolt diameter rather than the distance across the flats of the hexagon (A/F) as in other standards. Confusion also arises because BSF hexagon sizes can be one size smaller than the corresponding Whitworth hexagon. This leads to instances where a spanner (wrench) marked ​⁷⁄₁₆ BSF is the same size as one marked ​³⁄₈ W. In both cases the spanner jaw width of 0.710 in, the width across the hexagon flat, is the same. However, in World War II the size of the Whitworth hexagon was reduced to the same size as the equivalent BSF hexagon purely to save metal during the war and they never went back to the old sizes afterwards. Thus it is today uncommon to encounter a Whitworth hexagon which takes the nominally correct spanner. Spanners in this case may be marked ​⁷⁄₁₆ BS to indicate that they have a jaw size of 0.710 in and are designed to take either the (later) ​⁷⁄₁₆ BSW or ​⁷⁄₁₆ BSF hexagon.^[6][7][8]

The British Association screw thread (BA) standard is sometimes classed with the Whitworth standard fasteners because it is often found in the same machinery as the Whitworth standard. However it is actually a metric based standard that uses a 47.5° thread angle and has its own set of head sizes. BA threads have diameters of 6 mm (0BA) and smaller, and were and still are particularly used in precision machinery.

The Whitworth 55° angle remains commonly used today worldwide in form of the 15 British standard pipe threads defined in ISO 7, which are commonly used in water supply, cooling, pneumatics, and hydraulic systems. These threads are designated by a number between 1/16 and 6 that originates from the nominal internal diameter (i/d) in inches of a steel pipe for which these threads were designed. These pipe thread designations do not refer to any thread diameter.

Other threads that used the Whitworth 55° angle include Brass Threads, British Standard Conduit (BSCon), Model Engineers (ME), and British Standard Copper (BSCopper).

Current usage

The widely used (except in the U.S.) British Standard Pipe thread, as defined by the ISO 228 standard (formerly BS-2779), uses Whitworth standard threads. Even in the United States, personal computer liquid cooling components use the G​¹⁄₄ thread from this series.

The Leica Thread-Mount used on rangefinder cameras and on many enlarging lenses is ​1 ¹⁷⁄₃₂ in by 26 turns-per-inch Whitworth, an artefact of this having been developed by a German company specializing in microscopes and thus equipped with tooling capable of handling threads in inches and in Whitworth.

The ​⁵⁄₃₂ in Whitworth threads have been the standard Meccano thread for many years and it is still the thread in use by the French Meccano Company.

Stage lighting suspension bolts are most commonly ​³⁄₈ in and ​¹⁄₂ in BSW. Companies that initially converted to metric threads have converted back, after complaints that the finer metric threads increased the time and difficulty of setup, which often takes place at the top of a ladder or scaffold.

Fixings for garden gates traditionally used Whitworth carriage bolts, and these are still the standard supplied in UK and Australia.

Historical misuse

British Morris and MG engines from 1923 to 1955 were built using metric threads but with bolt heads and nuts dimensioned for Whitworth spanners and sockets.^[9] In 1919 Morris Motors took over the French Hotchkiss engine works which had moved to Coventry during the First World War. The Hotchkiss machine tools were of metric thread but metric spanners were not readily available in Britain at the time, so fasteners were made with metric thread but Whitworth heads. ^[10]

In the 2011 movie Cars 2 by Disney / Pixar, the vital clue to the discovery of the villain, Sir Miles Axlerod, is that he uses Whitworth bolts. Although Axlerod does not precisely resemble any real car (whereas numerous other characters are closely modelled on real cars), he seems most closely to match the original Range Rover Classic. In reality, early model Range Rovers used parts with imperial dimensions, although the photograph of the villain's engine is virtually identical to the later 3.5 litre single plenum Rover V8 (A design purchased from GM's Buick).

See also

Other thread standards:

• British Association screw threads (BA)
• British standard brass thread
• British Standard Cycle
• British standard fine thread (BSF)
• British standard pipe thread (BSP)
• ISO metric screw thread
• Unified Thread Standard (UTS, including UNC, UNF, UNS and UNEF)

References

Bibliography

• Oberg, E., Jones, F.D., Hussain, M., McCauley, C.J., Ryffel, H.H. and Heald, R.M. (2008) Machinery's handbook : a reference book for the mechanical engineer, designer, manufacturing engineer, draftsman, toolmaker, and machinist, 28th Ed., New York : Industrial Press, ISBN 978-0-8311-2800-5, p. 1858–1860