Machinery's Handbook, 31st Edition
2214
Spur Gearing Table 4. Tooth Proportions for Fine-Pitch Involute Spur and Helical Gears of 14 1 ∕ 2 -, 20-, and 25-Degree Pressure Angle ANSI B6.7-1977 Item Spur Helical Addendum, a . P 1000 . P 1000 n Dedendum, b . . (min.) P 1200 + 0002 . . (min.) P 1200 0002 n + Working Depth, h k . P 2000 . P 2000 n Whole Depth, h t . . (min.) P 2200 + 0002 . . (min.) P 2200 0002 n + Clearance, c (Standard) . . (min.) P 0200 + 0002 . . (min.) P 0200 0002 n + (Shaved or Ground Teeth) . . (min.) P 0350 + 0002 . . (min.) P 0350 0002 n + Tooth Thickness, t At Pitch Diameter . t P = 15708 . t P 15708 n n = Circular Pitch, p p N D n d P or or π π π = p P n n π = Pitch Diameter Pinion, d P n cos P n n ψ Gear, D P N cos P N n ψ Outside Diameter Pinion, d o P n + 2 cos P n 1 2 n ψ + a k Gear, D o P N + 2 cos P N 1 2 n ψ + c m Center Distance, C P N n 2 + cos P N n 2 n ψ +
All dimensions are in inches. P = Transverse Diametral Pitch P n = Normal Diametral Pitch t n = Normal Tooth Thickness at Pitch Diameter p n = Normal Circular Pitch
ψ = Helix Angle n = Number of pinion teeth N = Number of gear teeth
American National Standard Tooth Proportions for Fine-Pitch Involute Spur and Helical Gears.— The proportions of spur gears in this Standard (ANSI B6.7-1977) follow closely ANSI B6.1-1968, R1974, “Tooth Proportions for Coarse-Pitch Involute Spur Gears.” The main difference between fine-pitch and coarse-pitch gears is the greater clearance speci- fied for fine-pitch gears. The increased clearance provides for any foreign material that may tend to accumulate at the bottoms of the teeth and also the relatively larger fillet radius resulting from proportionately greater wear on the tips of fine-pitch cutting tools. Pressure Angle: The standard pressure angle for fine-pitch gears is 20 degrees and is rec ommended for most applications. For helical gears this pressure angle applies in the nor mal plane. In certain cases, notably sintered or molded gears, or in gearing where greatest strength and wear resistance are desired, a 25-degree pressure angle may be required. However, pressure angles greater than 20 degrees tend to require use of generating tools
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