14.5.
The screw threads combination is very much complex. There is no necessity of grading fits as in case of plain shaft and hole because there is a complicated interplay between pitch, thread*angle and effective diameter. Although in an ideal case, a single class of fit would suffice for most of the purposes, the variables and conditions are such that three grades of accuracy are found desirable.
14.5.1.
Effective diameter.
The effective diameter tolerance must take into consideration the three variables (i) the major diameter; («) the length of thread engagement; (Hi) the pitch. Furthermore the variables are related to the tolerance as indicated by following expression :
where D = major diameter, L = length of engagement, p = pitch, c, b, c are constants.
The value of these constants will depend upon prevailing accuracy of tools and machines used to produce the screw threads. Increase in accuracy will reduce the value of constants. The
value of these constants according to British Standard Specifications for Withworth threads are
The specifications provide for three grades of fits, (a) close, (b) medium, (c) free.
The basic formula gives effective diameter tolerance for medium fit. Close fit tolerances are two-thirds of free fit, and free fit tolerance is one and a half times that of medium fit. This results in tolerance increasing by 50% each time from close-medium-free fit.
Close fit. This applies to screw requiring a fine snug fit.
Medium fit. Suitable for better class of ordinary interchangeable screw threads. Free fit. Suitable for bulk of screw threads of ordinary commercial quality. Major Diameter. Its tolerance is given by expression :
Effective Dia. tolerance
This is applicable for all grades of fits (for external threads) For internal threads no such tolerance is laid down. Minor Diameters. Tolerance (external threads)
The wide tolerance on the core diameter of internal threads permits of a tapping drill being used of ample size to prevent binding at the root of the tap. This follows a general practice and results in a flat crest to the root of the thread.
14.5.2.
Example Determine the limiting dimensions for a bolt and nut 3/4 inch B.S.F. (Medium fit).
(a) Basic sizes : Full diameter = 0.75″,
Core diameter = 0.75″ – 2 x Depth = 0.75″ – 2 x (0.64 x 1/12) = 0.6434 inch Effective diameter = 0.75 – 0.64p. = 0.75 – 0.64 x 1/12 = 0.75 – 0.053 = 0.6967 inch.
Hence limiting size for Bolt and Nut would be
Effective Dia. | Major Dia. | Minor Dia. | |||||||
Max. | Min. | Tol. | Max. | Min. | Tol. | Max. | Min. | Tol. | |
BOLT | 0.6909 | 0.0058 | 0.750 | 0.7413 | 0.0087 | 0.6434 | 0.6318 | 0.0116 | |
NUT | 0.7025 | 0.6967 | 0.0058 | — | — | — | 0.667 | 0.6434 | 0.0236 |