BS 721-2:1983

Foreword
Committees responsible
Specification
1 Scope
2 Definitions
3 Symbols
4 Form of worm threads and wormwheel teeth
5 Information on worm gear elements
6 Formulae for calculation of dimensions
7 Accuracy of manufacture
8 Normal backlash
9 Load capacity
10 Efficiency and lubrication
11 Irreversibility
12 Information to be given on drawings
Appendices
A Typical wormwheel rim sections
B Examples of calculations
C Design procedure based on axial pitch of worm
Tables
1 Worm pitch tolerances
2 Worm profile tolerance
3 Worm thread thickness tolerance
4 Wormwheel pitch tolerance
5 Normal backlash
6 Worm gear zone factor Z
7 Basic stress factors for worm gears
8 Application factor KA
Figures
1 Normal section of basic rack (dimensioned in terms
    of axial module)
2 Entry gap in wormwheel teeth
3 Determination of q for maximum efficiency for any
    worm speed
4 Determination of q for worm speeds up to 300 r/min
5 Maximum value of addendum modification coefficient
    (X2,max)
6 Minimum value of addendum modification coefficient
    (X2,min)
7 Speed factor for worm gears (wear), Xc
8 Speed factor for worm gears (strength), Xb
9 Total equivalent running time and related life
    factors for wear and strength
10 Coefficient of friction for worm gears
11 Typical drawing of a worm
12 Typical drawing of a wormwheel
13 Wormwheel rim section
14 Minimum thickness of wormwheel rim section
15 Wormwheel rim section: alternative design of centre

Applies to tooth form, dimensions of gearing, tolerances for four classes of gears according to function and accuracy, calculation of load capacity and information to be given on drawings. Provides examples of load calculations.

This Part of BS 721 specifies the requirements for worm gearing based on axial modules. Four classes of gear are specified, which are related to function and accuracy.

This Part applies to worm gearing comprising cylindrical involute helicoid worms and wormwheels conjugate thereto. It does not apply to pairs of cylindrical gears connecting non-parallel axes known as crossed helical gears.

Appendix A gives some typical wheel rim sections together with methods of attachment. Appendix B gives examples of calculations using the formulae given in the standard. Provision is made in appendix C for a method of application of existing hobs designated in terms of axial pitch.

NOTE 1. Worm gearing designed to this standard is not to be regarded as irreversible.

NOTE 2. The titles of the publications referred to in this standard are listed on the inside back cover.