Machinery's Handbook, 31st Edition
2300
CHECKING GEAR SIZES CHECKING GEAR SIZES Checking Gear Size by Measurement Over Wires or Pins
The wire or pin method of checking gear sizes is accurate, easily applied, and especially useful in shops with limited inspection equipment. Two cylindrical wires or pins of prede termined diameter are placed in diametrically opposite tooth spaces (see diagram). If the gear has an odd number of teeth, the wires are located as nearly opposite as possible, as shown by the diagram at the right. The overall measurement M is checked by using any sufficiently accurate method of measurement. The value of measurement M when the pitch diameter is correct can be determined easily and quickly by means of the calculated values in the accompanying tables. Measurements for Checking External Spur Gears when Wire Diameter Equals 1.728 Di- vided by Diametral Pitch.— Table 1 and Table 2 give measurements M , in inches, for check- ing the pitch diameters of external spur gears of 1 diametral pitch. For any other diametral pitch, divide the measurement given in the table by whatever diametral pitch is required. The result shows what measurement M should be when the pitch diameter is correct and there is no allowance for backlash. The procedure for obtaining a given amount of back- lash will be explained later. Table 1 through Table 4 inclusive are based on wire sizes con forming to the Van Keuren standard. For external spur gears, the wire size equals 1.728 divided by the diametral pitch. The wire diameters for various diametral pitches will be found in the left-hand section of Table 5.
M
M
Even Number of Teeth: Table 1 is for even numbers of teeth. To illustrate the use of the table, assume that a spur gear has 32 teeth of 4 diametral pitch and a pressure angle of 20 degrees. Table 1 shows that the measurement for 1 diametral pitch is 34.4130; hence, for 4 diametral pitch, the measurement equals 34.4130 ÷ 4 = 8.6032 inches. This dimension is the measurement over the wires when the pitch diameter is correct, provided there is no allowance for backlash. The wire diameter here equals 1.728 ÷ 4 = 0.432 inch (Table 5). Measurement for even numbers of teeth above 170 and not in Table 1 may be determined as shown by the following example: Assume that number of teeth = 240 and pressure angle = 14 1 ∕ 2 degrees; then, for 1 diametral pitch, figure at left of decimal point = given No. of teeth + 2 = 240 + 2 = 242. Figure at right of decimal point lies between decimal values given in table for 200 teeth and 300 teeth and is obtained by interpolation. Thus, 240 − 200 = 40 (change to 0.40); 0.5395 − 0.5321 = 0.0074 = difference between decimal values for 300 and 200 teeth; hence, decimal required = 0.5321 + (0.40 × 0.0074) = 0.53506. Total dimension = 242.53506 divided by the diametral pitch required. Odd Number of Teeth: Table 2 is for odd numbers of teeth. Measurement for odd num bers above 171 and not in Table 2 may be determined as shown by the following example: Assume that number of teeth = 335 and pressure angle = 20 degrees; then, for 1 diametral
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