Table 9c. Tooth Proportions Recommended for Enlarging Fine-Pitch Pinions of 20-Degree Pressure Angle— 20 Diametral Pitch and Finer ANSI B6.7-1977
Standard Center Distance (Long and Short Addendums) Reduced Gear Dimensions
Enlarged C.D. System Pinion Mating with Standard Gear
Enlarged Pinion Dimensions
Contact Ratio with a 24-Tooth Gear
Contact Ratio n Mating with N
Basic Tooth Thickness, t P 2.14114 2.09854 2.05594
Dedendum Based on 20 Pitch, b b P
Contact Ratio Two Equal Pinions
Basic Tooth Thickness, t G 1.00045 1.04305 1.08565
Dedendum Based on 20 Pitch, b b G
Recommended Minimum No. of Teeth, N
Number of Teeth, a n
Outside Diameter, D oP 10.0102 11.0250 12.0305 13.0279 14.0304 15.0296 15.9448 16.8560 17.7671 18.6782 19.5894 20.5006 21.4116 22.3228 23.2340 24.1450 25.0561 26.0000
Addendum, a P
Addendum, a G
7 8 9
1.5051 1.5125 1.5152 1.5140 1.5152 1.5148 1.4724 1.4280 1.3836 1.3391 1.2947 1.2503 1.2058 1.1614 1.1170 1.0725 1.0281 1.0000
0.4565 0.5150 0.5735 0.6321 0.6787 0.7232 0.7676 0.8120 0.8564 0.9009 0.9453 0.9897 1.0342 1.0786 1.1230 1.1675 1.2119 1.2400
0.697 0.792 0.893 0.982 1.068 1.151 1.193 1.232 1.270 1.323 1.347 1.385 1.423 1.461 1.498 1.536 1.574 1.602
1.003 1.075 1.152 1.211 1.268 1.322 1.353 1.381 1.408 1.434 1.458 1.482 1.505 1.527 1.548 1.568 1.588 1.602
0.2165 0.2750 0.3335 0.3921 0.4387 0.4832 0.5276 0.5720 0.6164 0.6609 0.7053 0.7497 0.7942 0.8386 0.8830 0.9275 0.9719 1.0000
2.0235 1.9650 1.9065 1.8479 1.8013 1.7568 1.7124 1.6680 1.6236 1.5791 1.5347 1.4903 1.4458 1.4014 1.3570 1.3125 1.2681 1.2400
42 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24
1.079 1.162 1.251 1.312 1.371 1.427 1.457 1.483 1.507 1.528 1.546 1.561 1.574 1.584 1.592 1.598 1.601
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
2.01355 1.97937 1.94703 1.91469 1.88235 1.85001 1.81766 1.78532 1.75298 1.72064 1.68839 1.65595 1.62361 1.59127 1.57080
1.12824 1.16222 1.19456 1.22690 1.25924 1.29158 1.32393 1.35627 1.38861 1.42095 1.45320 1.48564 1.51798 1.55032 1.57080
1.602 a Caution should be exercised in the use of pinions above the horizontal lines. They should be checked for suitability, particularly in the areas of contact ratio (less than 1.2 is not recommended), center distance, clearance, and tooth strength. b The actual dedendum is calculated by dividing the values in this column by the desired diametral pitch and then adding to the result an amount Δ found in Table 9a. As an example, a 20-degree pressure angle 7-tooth pinion meshing with a 42-tooth gear would have, for 24 diametral pitch, a dedendum of 0.4565 ÷ 24 + 0.0004 = 0.0194. The 42-tooth gear would have a dedendum of 2.0235 ÷ 24 + 0.004 = 0.0847 inch. All dimensions are given in inches.
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