Machinery's Handbook, 31st Edition
Standards for Spur and Helical Gears
2251
British Standard for Spur and Helical Gears BS 436: Part 1: 1967: Spur and Helical Gears, Basic Rack Form, Pitches and Accuracy for Diametral Pitch Series, now has sections concerned with basic requirements for general tooth form, standard pitches, accuracy and accuracy testing procedures, and the showing of this information on engineering drawings to make sure that the gear manufacturer receives the required data. The latest form of the standard complies with ISO agreements. The standard pitches are in accordance with ISO R54, and the basic rack form and its modifications are in accordance with the ISO R53 “Basic Rack of Cylindrical Gears for General Engineering and for Heavy Engineering Standard”. Five grades of gear accuracy in previous versions are replaced by grades 3 to 12 of the draft ISO Standard. Grades 1 and 2 cover master gears that are not dealt with here. BS 436: Part 1: 1967 is a companion to the following British Standards: BS 235 “Gears for Traction” BS 545 “Bevel Gears (Machine Cut)” BS 721 “Worm Gearing” BS 821 “Iron Castings for Gears and Gear Blanks (Ordinary, Medium and High Grade)” BS 978 “Fine Pitch Gears” Part 1, “Involute, Spur and Helical Gears”; Part 2, “Cycloidal Gears” (with addendum 1, PD 3376: “Double Circular Arc Type Gears”; Part 3, “Bevel Gears” BS 1807 “Gears for Turbines and Similar Drives” Part 1, “Accuracy” Part 2, “Tooth Form and Pitches” BS 2519 “Glossary of Terms for Toothed Gearing” BS 3027 “Dimensions for Worm Gear Units” BS 3696 “Master Gears” Part 1 of BS 436 applies to external and internal involute spur and helical gears on paral lel shafts and having normal diametral pitch of 20 or coarser. The basic rack and tooth form are specified, also first and second preference standard pitches and fundamental tolerances that determine the grades of gear accuracy, and requirements for terminology and notation. These requirements include: center distance a ; reference circle diameter d , for pinion d 1 and wheel d 2 ; tip diameter d a for pinion d a1 and wheel d a2 ; center distance modifica- tion coefficient γ ; face width b for pinion b 1 and wheel b 2 ; addendum modification coef- ficient x , for pinion x 1 and wheel x 2 ; length of arc l ; diametral pitch P t ; normal diametral pitch p n ; transverse pitch p t ; number of teeth z , for pinion z 1 and wheel z 2 ; helix angle at reference cylinder β ; pressure angle at reference cylinder α ; normal pressure angle at reference cylinder α n ; transverse pressure angle at reference cylinder α t ; and transverse pressure angle, working,α tw . The basic rack tooth profile has a pressure angle of 20. The Standard permits the total tooth depth to be varied within 2.25 to 2.40, so that the root clearance can be increased within the limits of 0.25 to 0.040 to allow for variations in manufacturing processes; and the root radius can be varied within the limits of 0.25 to 0.39. Tip relief can be varied within the limits shown at the right in the illustration. Standard normal diametral pitches P n , BS 436 Part 1:1967, are in accordance with ISO R54. The preferred series, rather than the second choice, should be used where possible. Preferred normal diametral pitches for spur and helical gears (second choices in parenthe ses) are: 20 (18), 16 (14), 12 (11), 10 (9), 8 (7), 6 (5.5), 5 (4.5), 4 (3.5), 3 (2.75), 2.5 (2.25), 2 (1.75), 1.5, 1.25, and 1. Information to be Given on Drawings: British Standard BS 308, “Engineering Drawing Practice”, specifies data to be included on drawings of spur and helical gears. For all gears the data should include: number of teeth, normal diametral pitch, basic rack tooth form, axial pitch, tooth profile modifications, blank diameter, reference circle diameter, and helix angle at reference cylinder (0 for straight spur gears), tooth thickness at reference
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