BS 978-1:1968

Co-operating organizations
Foreword
Specification
1 General
1.1 Scope
1.2 Terminology and notation
1.3 Preferred range of normal diametral pitches
1.4 Basic rack tooth profile
2 Design details
2.1 Outside diameter and centre distance
2.2 Modification of addendum and centre distance
3 Accuracy of gear blanks and gears
3.1 General
3.2 Dual flank composite testing
3.3 Limits of tolerance on blanks and gear elements
4 Limits and tolerances of assembly
4.1 Centre distance
4.2 Depth of engagement
5 Information to be given on drawings
Appendices
A Centre distance and addendum modification
B Examples of calculations
C Comparable diametral pitches and metric modules
D Single flank composite testing: limits of tolerance
E Calculation of measuring pin dimensions for
     external and internal spur gears

Gears of pitch range greater than 20, up to 200 diametral pitch. Standard pitches; two basic racks (one with backlash allowance); modification of addendum and centre distance to suit various conditions. Four grades of accuracy with elemental and dual flank composite test tolerances. Appendices: single flank testing, calculation of measuring pin dimensions, calculations for various design conditions.

This British Standard relates to involute spur, helical and crossed helical gears, having diametral pitches finer than 20, in four accuracy grades. Examples of application are:

Class A. Astronomical apparatus and scientific instruments; colour printing machinery; control systems (when multiple trains of gears are involved this grade would generally be used for low speed components where the effect of positional accuracy is important).

Class B. Navigational instruments; high speed components of control systems where quietness and smooth running are essential; machine tool speed control; photographic instruments.

Class C. General purpose instruments; accounting and calculating machinery; counters; clockwork mechanisms.

Class D. Textile machinery; switch operating gear; clockwork mechanisms where accuracy is not the primary consideration.

It is recommended that all spur, helical and crossed helical gears for use in horology should be produced to this British Standard with the possible exception of gears for weight or spring-driven mechanisms in which the pinion is the driven member and has 12 or fewer teeth. In such cases it may be preferable to use gears conforming to BS 978-2²⁾.