Machinery's Handbook, 31st Edition
Spur Gearing 2215 having very narrow point widths, and higher pressure angles require closer control of cen ter distance when backlash requirements are critical. In those cases where consideration of angular position or backlash is critical and both pinion and gear contain relatively large numbers of teeth, a 14 1 ∕ 2 -degree pressure angle may be desirable. In general, pressure angles less than 20 degrees require greater amounts of tooth modification to avoid undercutting problems and are limited to larger total numbers of teeth in pinion and gear when operating at a standard center distance. Information Sheet B in the Standard provides tooth proportions for both 14 1 ∕ 2 - and 25-degree pressure angle fine-pitch gears. Table 4 provides tooth proportions for fine- pitch spur and helical gears with 14 1 ∕ 2 -, 20-, and 25-degree pressure angles, and Table 5 provides tooth parts. Diametral Pitches: Diametral pitches preferred are: 20, 24, 32, 40, 48, 64, 72, 80, 96, and 120. Table 5. American National Standard Fine Pitch Standard Gear Tooth Parts— 14 1 ∕ 2 -, 20-, and 25-Degree Pressure Angles Diametral Pitch Circular Pitch Circular Thickness Standard Addend. Standard Dedend. Special Dedend. a
P
p
t
a
b
b
20 24 32 40 48 64 72 80 96
0.1571 0.1309 0.0982 0.0785 0.0654 0.0491 0.0436 0.0393 0.0327 0.0262
0.0785 0.0654 0.0491 0.0393 0.0327 0.0245 0.0218 0.0196 0.0164 0.0131
0.0500 0.0417 0.0313 0.0250 0.0208 0.0156 0.0139 0.0125 0.0104 0.0083
0.0620 0.0520 0.0395 0.0320 0.0270 0.0208 0.0187 0.0170 0.0145 0.0120
0.0695 0.0582 0.0442 0.0358 0.0301 0.0231 0.0208 0.0189 0.0161 0.0132
120
a Based upon clearance for shaved or ground teeth. The working depth is equal to twice the addendum. The whole depth is equal to the addendum plus the dedendum. For minimum number of teeth see page 2233 . Other American Spur Gear Standards.— An appended information sheet in the Ameri can National Standard ANSI B6.1-1968, R1974 provides tooth proportion information for three spur gear forms with the notice that they are “not recommended for new de- signs.” These forms are therefore considered to be obsolescent but the information is given on their proportions because they have been used widely in the past. These forms are the 14 1 ∕ 2 -degree full depth form, the 20-degree stub involute form and the 14 1 ∕ 2 -degree composite form which were covered in the former American Standard (ASA B6.1-1932). The basic rack for the 14 1 ∕ 2 -degree full depth form is shown on page 2211 ; basic formulas for these proportions are given in Table 6.
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