Gearing Standard
Page 45
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rAA NEW British Standard, " Worm Gearing" (B.S. 721) has been prepared to take account of current production techniques and differs from the earlier edition in several respects.
The method of design for cylindrical involute helicoid worms and wheels is explained and the form of basic rack of 20° pressure angle is specified. With the aim of minimizing the number of tools required, a fixed number of preferred centre distances, ratios, and diameter factors is recommended. Formulle for calculating the dimensions of worms and wheels are given, together with dimen sional tables for standard worms of unit axial module.
Four classes of gear are specified with tolerances for pitch, profile and worm thread thickness in each class. Five grades of backlash for assembly are also given. The permissible load capacity is now greater than in the previous edition.
Copies of B.S. 721 can be obtained from the B.S.I. Sales Branch, 2 Park Street, London, W.I. The price is 15s. each plus postage to non-subscribers.