Machinery's Handbook, 31st Edition
2252 Standards for Spur and Helical Gears cylinder, grade of gear, drawing number of mating gear, working center distance, and backlash. For single helical gears, the above data should be supplemented with hand and lead of the tooth helix; and for double helical gears, with the hand in relation to a specific part of the face width and the lead of tooth helix. Inspection instructions should be included, care being taken to avoid conflicting require ments for accuracy of individual elements, and single- and dual-flank testing. Supplemen tary data covering specific design, manufacturing and inspection requirements or limitations may be needed, together with other dimensions and tolerances, material, heat treatment, hardness, case depth, surface texture, protective finishes, and drawing scale. Addendum Modification to Involute Spur and Helical Gears.— The British Standards In- stitute guide PD 6457:1970 contains certain design recommendations aimed at making it possible to use standard cutting tools for some sizes of gears. Essentially, the guide covers addendum modification and includes formulas for both English and metric units. Addendum Modification is an enlargement or reduction of gear tooth dimensions that results from displacement of the reference plane of the generating rack from its normal position. The displacement is represented by the coefficient X, X1, or X2, where X is the equivalent dimension for gears of unit module or diametral pitch. The addendum modification establishes a datum tooth thickness at the reference circle of the gear but does not necessarily establish the height of either the reference addendum or the working addendum. In any pair of gears, the datum tooth thicknesses are those that always give zero backlash at the meshing center distance. Normal practice requires allowances for backlash for all unmodified gears. Taking full advantage of the adaptability of the involute system allows various tooth design features to be obtained. Addendum modification has the following applications: avoiding undercut tooth profiles; achieving optimum tooth proportions and control of the proportion of receding to approaching contact; adapting a gear pair to a predetermined center distance without recourse to non-standard pitches; and permitting use of a range of working pressure angles using standard geometry tools. BS 436, Part 3:1986 “Spur and Helical Gears”.— This part provides methods for calcu lating contact and root bending stresses for metal involute gears, and is somewhat similar to the ANSI/AGMA Standard for calculating stresses in pairs of involute spur or helical gears. Stress factors covered in the British Standard include the following: Tangential Force is the nominal force for contact and bending stresses. Zone Factor accounts for the influence of tooth flank curvature at the pitch point on Hertzian stress. Contact Ratio Factor takes account of the load-sharing influence of the transverse con tact ratio and the overlap ratio on the specific loading. Elasticity Factor takes into account the influence of the modulus of elasticity of the material and of Poisson’s ratio on the Hertzian stress. Basic Endurance Limit for contact makes allowance for the surface hardness. Material Quality covers the quality of the material used. Lubricant Influence, Roughness, and Speed The lubricant viscosity, surface roughness and pitch line speed affect the lubricant film thickness, which in turn, affects the Hertzian stresses. Work Hardening Factor accounts for the increase in surface durability due to the mesh ing action. Size Factor covers the possible influences of size on the material quality and its response to manufacturing processes. Life Factor accounts for the increase in permissible stresses when the number of stress cycles is less than the endurance life.
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