Table 9d. Tooth Proportions Recommended for Enlarging Fine-Pitch Pinions of 25-Degree Pressure Angle— 20 Diametral Pitch and Finer ANSI B6.7-1977, Information Sheet B
Standard Center Distance (Long and Short Addendums) Reduced Gear Dimensions
Enlarged C.D. System Pinion Mating with Standard Gear
Enlarged Pinion Dimensions
Dedendum Based on 20 Pitch, Dedendum Based on 20 Pitch, b b P
Contact Ratio with a 15-Tooth Gear
Basic Tooth Thickness, t P 2.18362 2.10029 2.01701
Contact Ratio Two Equal Pinions
Basic Tooth Thickness, t G 0.95797 1.04130 1.12459
Dedendum Based on 20 Pitch, b b G
Recommended Minimum No. of Teeth, N
Contact Ratio n Mating with N
Number of Teeth, a n
Outside Diameter, D oP 8.7645 9.7253 10.6735 11.6203 12.5691 13.5039 14.3588 15.2138 16.0686 17.0000
Addendum, a P
Addendum, a G
6 7 8 9
1.3822 1.3626 1.3368 1.3102 1.2846 1.2520 1.1794 1.1069 1.0343 1.0000
0.5829 0.6722 0.7616 0.8427 0.9155 0.9880 1.0606 1.1331 1.2057 1.2400
0.696 0.800 0.904 1.003 1.095 1.183 1.231 1.279 1.328 1.358
0.954 1.026 1.094 1.156 1.211 1.261 1.290 1.317 1.343 1.358
0.3429 0.4322 0.5216 0.6029 0.6755 0.7480 0.8206 0.8931 0.9657 1.0000
1.8971 1.8078 1.7184 1.6371 1.5645 1.4920 1.4194 1.3469 1.2743 1.2400
24 23 22 20 19 18 17 16 15 15
1.030 1.108 1.177 1.234 1.282 1.322 1.337 1.347 1.352
1.94110 1.87345 1.80579 1.73813 1.67047 1.60281 1.57030
1.20048 1.26814 1.33581 1.40346 1.47112 1.53878 1.57080
10 11 12 13 14 15
1.358 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. All values are for 1 diametral pitch. For any other sizes of teeth, all linear dimensions should be divided by the diametral pitch. Note: The tables in the ANSI B6.7-1977 standard also specify Form Diameter, Roll Angle to Form Diameter, and Top Land. These are not shown here. The top land is in no case less than 0.275/ P . The form diameters and the roll angles to form diameter shown in the Standard are the values which should be met with a standard hob when generating the tooth thicknesses shown in the tables. These form diameters provide more than enough length of involute profile for any mating gear smaller than a rack. However, since these form diameters are based on gear tooth generation using standard hobs, they should impose little or no hardship on manufacture except in cases of the most critical quality levels. In such cases, form diameter specifications and master gear design should be based upon actual mating conditions.
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