{HEAD ~ The tooth shape} at around the pitch circle of the outer tooth gear of {TAIL ~ the pinion} to be connected to the prime mover or driven shaft has a tooth surface which is in axial direction at around the central portion of the tooth width and which contacts at around the tooth width end at the allowable displacement angle and the backlash is made small and the outer tooth gear of the pinion is crowned in to an exponential function curve 
{ HEAD ~ The tooth shape } at around the pitch circle of the outer tooth gear of { TAIL ~ the pinion } to be connected to the prime mover or driven shaft has a tooth surface which is in axial direction at around the central portion of the tooth width and which contacts at around the tooth width end at the allowable displacement angle and the backlash is made small and the outer tooth gear of the pinion is crowned in to an exponential function curve
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