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Backlash is the distance (spacing) between two “mating” gears measured at the back of the driver on the pitch circle. Backlash, which is purposely built in, is very important because it helps prevent noise, abnormal wear and excessive heat while providing space for lubrication of the gears. (See Figure 2.6)

Center Distance is the distance between the center of the shaft of one spur gear to the center of the shaft of the other spur gear. In a spur gear drive having two gears, center distance is equal to one-half the pitch diameter of the pinion (which, you will remember from Chapter 1 is the smaller of two spur gears) plus one-half the pitch diameter of the gear. Or, better still, simply add the sum of the two pitch diameters and divide by two. (See Figure 2.7)

The center distance of a 4-inch pitch diameter gear running with a 2-inch pitch diameter pinion is 3 inches. 4" + 2" ÷ 2 = 3" CD