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When the center distance is increased by a relatively small amount, DC, a backlash space develops
between mating teeth, as in Figure 1.21. The relationship between center distance increase and linear
backlash, B_LA, along the line of action, is:

B_LA = 2(DC)sin f                                                                                    (21)

            This measure along the line-of-action is useful when inserting a feeler gage between teeth to measure backlash.
            The equivalent linear backlash measured along the pitch circle is given by:

B = 2(DC) tan f                                                                                     (22a)

where:
DC = change in center distance
f = pressure angle

Hence, an approximate relationship between center distance change and change in backlash is:

DC= 1.933 DB for 14½° pressure-angle gears                                             (22b)
DC= 1.374 DB for 20° pressure-angle gears                                                (22c)

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