GEAR TRAINS 439 to 0.0007 in for precision gears.The increase in angular backlash as a function of error in center distance is approximately 6B=43200(4Can minutes of arc (9.3) where pressure angle,AC error in center distance,and d pitch diameter of the gear on the shaft where the backlash is measured. In servomechanisms,where motors are driving,for example,the control surfac- es on an aircraft,backlash can cause potentially destructive "hunting"in which the control system tries in vain to correct positional errors due to backlash"slop"in the mechanical drive system.Such applications need antibacklash gears which are really two gears back-to-back on the same shaft which can be rotated slightly at as- sembly with respect to one another,and then fixed so as to take up the backlash.In less critical applications,such as the propeller drive on a boat,backlash on reversal will not even be noticed. Shift in center distance New pitch point Pitch New,larger Base circle pitch radius radius of is unchanged pinion Pitch point of pinion shifts position Velocity at New pitch 9 pitch point circles Base circle is unchanged Pressure New angle Pitch pressure radius angle New,larger 0=20° of gear 0=23° pitch radius of gear Line of action (common normal) is tangent to both base circles (o)Correct center distance (b)Increased center distance FIGURE 9-8 Changing center distance of involute gears changes the pressure angle and pitch diameters