Machinery's Handbook, 31st Edition
2318
CHECKING GEAR SIZES
. M 25 096815 25 02618 k Checking Enlarged Pinions by Measuring Over Pins or Wires.— When the teeth of small spur gears or pinions would be undercut if generated by an unmodified straight-sided rack cutter or hob, it is common practice to make the outside diameter larger than standard. The amount of increase in outside diameter varies with the pressure angle and number of teeth, as shown by Table 7 on page 2225 . The teeth are always cut to standard depth on a generating type of machine such as a gear hobber or gear shaper; and because the number of teeth and pitch are not changed, the pitch diameter also remains unchanged. The tooth thickness on the pitch circle, however, is increased and wire sizes suitable for standard gears are not large enough to extend above the tops of these enlarged gears or pinions; hence, the Van Keuren wire size recommended for these enlarged pinions equals 1.92 ÷ diametral pitch. Table 12 gives measurements over wires of this size, for checking full- depth involute gears of 1 diametral pitch. For any other pitch, merely divide the measure ment given in the table by the diametral pitch. Table 12 applies to pinions that have been enlarged by the same amounts as given in Table 7 and Table 8, starting on page 2225 . These enlarged pinions will mesh with standard gears; but if the standard center distance is to be maintained, reduce the gear diameter below the standard size by as much as the pinion diameter is increased. . . . . 30 6 2832 2 0 01109 1 2941 inches . . # # # = + + = a Table 12. Checking Enlarged Spur Pinions by Measurement Over Wires Measurements over wires are given in table for 1 diametral pitch. For any other diametral pitch, divide measurement in table by given pitch. Wire size equals 1.92 ÷ diametral pitch. Number of Teeth Outside or Major Diameter (Note 1) Circular Tooth Thickness (Note 2) Measure ment Over Wires Number of Teeth Outside or Major Diameter (Note 1) Circular Tooth Thickness (Note 2) Measure ment Over Wires
14 1 ∕
2 -degree full-depth involute teeth: 13.3731 1.9259 13.6186 14.3104 1.9097 14.4966 15.2477 1.8935 15.6290 16.1850 1.8773 16.5211 17.1223 1.8611 17.6244 18.0597 1.8449 18.5260 18.9970 1.8286 19.6075 19.9343 1.8124 20.5156 20.8716 1.7962 21.5806 21.8089 1.7800 22.4934 22.7462 1.7638 23.5451 23.6835 1.7476 24.4611 24.6208 1.7314 25.5018 25.5581 1.7151 26.4201 26.4954 1.6989 27.4515 27.4328 1.6827 28.3718 28.3701 1.6665 29.3952 29.3074 1.6503 30.3168 30.2447 1.6341 31.3333 31.1820 1.6179 32.2558 32.1193 1.6017 33.2661 33.0566 1.5854 34.1889
20-degree full-depth involute teeth:
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
10 11 12 13 14 15 16 17
12.936 13.818 14.702 15.584 16.468 17.350 18.234 19.116
1.912 1.868 1.826 1.783 1.741 1.698 1.656 1.613
13.5039 14.3299 15.4086 16.2473 17.2933 18.1383 19.1596 20.0080
Note 1: These enlargements, which are to improve the tooth form and avoid undercut, conform to those given in Table 7 and Table 8, starting on page 2225 where data will be found on the minimum number of teeth in the mating gear. Note 2: The circular or arc thickness is at the standard pitch diameter. The corresponding chordal thickness may be found as follows: Multiply arc thickness by 90 and then divide product by 3.1416 × pitch radius; find sine of angle thus obtained and multiply it by pitch diameter.
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