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.