BS 721-1:1963

Co-operating organizations
Foreword
Specification
1 Scope
2 General information on worm gear elements
 a Module
 b Axial pitch and normal pitch designations
 c Diameter factor q
 d Worm gear designations
3 Notation
4 Form of threads and teeth
 a Standard worm thread surfaces
 b Basic rack
 c Undercutting of worm threads
 d Thread thickness
 e Wormwheel teeth and entry gaps
 f Wormwheel tooth fillet
 g Face width of wormwheel
5 Formulae for calculation of dimensions
 a Limiting values of centre distance for given values
    of t, T, q and m
 b Limiting values of m for given values of C, t, T
    and q
 c Formulae for detail dimensions of worm and wheel
 d Rim section
6 The general system of worm gear design
 a Obtaining suitable values of t, T, q and m
 b Standard axial modules
 c Standard diameter factors (q)
 d Determinations of t and T
 e Guidance for determination of q
 f Procedure for selecting standard worms
7 Tables and dimensions of standard worms and wheels
 a Tables of worm gear designations
    (i) Standard centre distances for industrial worm
          gears
    (ii) Standard centre distances for axle
          transmission gears
 b Examples of worm gear designations
 c Detail dimensions of worms and wheels
8 Tolerances
 a Worms:
    (i) runout of tip diameter
    (ii) pitch tolerances
    (iii) profile tolerances
    (iv) thread thickness tolerances
 b Wormwheels: pitch tolerances
9 Normal backlash
10 Working temperatures
11 Efficiency and lubrication
12 Irreversibility
13 Information to be given on drawings by the purchaser
Tables
1 Examples of designations t/T/q/m for industrial worm
    gears for all worm speeds
2 Examples of designations t/T/q/m for industrial worm
    gears for speeds up to 300 rev/min
3 Recommended numbers of worm threads t and wheel
    teeth T for axle transmission worm gears
4 Dimensions of standard worms of unit axial module
5 Worm gear zone factors
6 Stress factors for worm gears
7 Duty factor
Appendices
A Load capacity
A1 Determination of load capacity
A2 Determination of equivalent running time for loading
    conditions other than a steady load
A3 Momentary overload capacity
A4 Axle transmission gears
B Examples of calculations
C Design procedure based on axial pitch of worm
D Procedure for the design of worms to suit standard
    diametral pitch cutting tools corresponding to the
    normal pitch of the worm
Charts
1 Maximum positive value of addendum modification
    coefficient for 1 module (kW1)
2 Maximum negative value of addendum modification
    coefficient for 1 module (kW2)
3 Determination of q for worm speeds up to 300 rev/min
4 Determination of q for maximum efficiency for any
    worm speed
5 Coefficient of friction of worm gears
6 Speed factors for worm gears (for wear) (Xc)
7 Speed factors for worm gears (for strength) (Xb)
8 Expected life factors for strength and wear

Cylindrical involute helicoid worms with a normal pressure angle of 20° in a range of axial modules from 0.052 in to 1 in. Design, formulae for calculating worm and wormwheel dimensions; four gear classes, with tolerances for pitch, profile and worm thread thickness, and five grades of backlash for assembly. Drawings show information to be given when ordering. Appendices: determination of load capacity, design calculations for typical gears, design procedure based on axial pitch of the worm, design of worms to suit standard diametral pitch cutting tools corresponding to the normal pitch of the worm.

This British Standard applies to worm gearing and comprises cylindrical involute helicoid worms and worm wheels conjugate thereto, the dimensions of which are expressed in inches, Four classes of gears are provided which are related to function and precision. The range of modules is given in convenient steps from 0.052 to 1.

This standard does not refer to pairs of cylindrical gears connecting non-parallel axes known as crossed helical gears.