Committees responsible for this British Standard

The preparation of this British Standard was entrusted by the Mechanical Engineering Standards Committee (MEE/-) to Technical Committee MEE/17 upon which the following bodies were represented:

Association of Consulting Engineers
British Clock and Watch Manufacturers' Association
British Gear Manufacturers' Association
British Horological Institute
British Railways Board
Engineering Equipment Users' Association
Federation of Manufacturers of Construction Equipment and Cranes
Gauge and Tool Makers' Association
Institution of Mechanical Engineers
Institution of Production Engineers
Machine Tool Industry Research Association
Milling Cutter Association
Ministry of Defence
National Coal Board
North East Coast Institution of Engineers and Shipbuilders
Society of Motor Manufacturers and Traders Limited

Foreword

This Part of this British Standard has been prepared under the direction of the Mechanical Engineering Standards Committee and specifies worm gearing in metric units.

Part 1 of this standard specifies worm gearing in imperial units.

Compliance with a British Standard does not of itself confer immunity from legal obligations.

1 Scope

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.

2 Definitions

3 Symbols

4 Form of worm threads and wormwheel teeth

4.1 Standard worm thread surfaces

4.2 Basic rack

4.3 Thread thickness

4.4 Wormwheel teeth and entry gaps

4.5 Wormwheel tooth fillet

4.6 Face width of wormwheel

5 Information on worm gear elements

5.1 Worm gear designations

5.2 Determination of z₁ and z₂

5.3 Diameter factor q

5.4 Lead angle of worm thread

6 Formulae for calculation of dimensions

6.1 Limiting values of centre distance for given values of z₁, z₂, q and m

6.2 Limiting values of axial module for given values of a, z₁, z₂ and q.

6.3 Formulae for detail dimensions of worm and wheel

7 Accuracy of manufacture

7.1 Tolerances applicable to worms

7.2 Wormwheel pitch tolerances

8 Normal backlash

9 Load capacity

9.1 Determination of load capacity

9.2 Steady load conditions

9.3 Determination of equivalent running time for loading conditions other than a steady load

9.4 Momentary overload capacity

9.5 Vehicle axle transmission gears

10 Efficiency and lubrication

11 Irreversibility

12 Information to be given on drawings

Publications referred to

BS 721-2:1983

Specification for worm gearing - Metric units

Current, Under Review

Published:

30 Sep 1983