# Why are Whitworth threads angled at 55°?

You're probably thinking, "Well.. Why not? It's gotta be something.." Right?

But 55° is kind of an odd angle if you think about it.. and it does seem like a pretty specific choice considering a perfect equilateral/equiangular triangle consists of 60° angles. Wouldn't 60° be the default/simple/obvious choice for a triangular profile? Wouldn't it have been easier to manufacture? It seems like 60° would at least be the most logical starting point (like starting at zero) during the design phase. Or any other common, suitable, trigonometric angle; i.e. 30°, 45°, even 90°.

Also, Whitworth strikes me as a pretty meticulous, character; detail oriented, with a careful consideration for all aspects of his designs. Plus, when it comes to mechanical engineering, he has a history of favouring experimental & unorthodox geometry.

In 1841 Sir Joseph Whitworth produced a paper on a universal system of screw threads. He then collected a variety of screws and proposed a universal thread using their average pitch and depth.

The result was the 'Whitworth thread' with the depth and pitch of constant proportion, giving the 'V' thread a mean angle of 55 degrees and the number of threads per inch was specified for various diameters. The thread was first introduced in his own workshop and was in universal use by 1858.

(ref: The Whitworth Society whitworthsociety.org/history.php?page=2 )

The angle is derived from the amount of force it would take to pull in apart. IE: If the angle was 70% is would take less force to pull it apart.

• surely the strength is also related to the pitch - very coarse threads are stronger... – Solar Mike Jan 8 '18 at 22:09
• The strength is also related to the pitch but I don't think course threads are stronger. If you had a bolt that was screwed in 20mm a course thread would only have say 5 threads holding it but if you used a smaller pitch it would have say 8 threads holding it. That I believe means it would take more force to pull it apart because there is more metal in contact with metal holding it. – Zenzs Jan 8 '18 at 22:18
• But you also increase the friction with more threads per inch... not necessarily the strength. – Solar Mike Jan 8 '18 at 22:25
• Yes because of more contact there is more friction when screwing it in and out but if you were to hang a weight off it, it is that more contact which would make it harder to pull it out. Theres more contact holding it in there. – Zenzs Jan 8 '18 at 22:31
• In 1841 Sir Joseph Whitworth produced a paper on a universal system of screw threads. He then collected a variety of screws and proposed a universal thread using their average pitch and depth. The result was the 'Whitworth thread' with the depth and pitch of constant proportion, giving the 'V' thread a mean angle of 55 degrees and the number of threads per inch was specified for various diameters. The thread was first introduced in his own workshop and was in universal use by 1858. (ref: The Whitworth Society whitworthsociety.org/history.php?page=2 ) – Donald Gibson Jan 9 '18 at 0:26