To what value could the gear hardness be reduced in Problem 15.31, without making the gear teeth weaker than the pinion teeth based on surface fatigue?
Problem 15.31
A pair of mating gears have 20° full-depth teeth with a diametral pitch of 8. Both gear and pinion are made of steel heat-treated to 350 Bhn, and both have a face width of 1.0 in. The teeth are cut with a top-quality hobbing operation. The pinion has 20 teeth and rotates 1100 rpm. It is mounted outboard on the shaft of an electric motor, and drives a 40-tooth gear that is positioned inboard on an accurately mounted blower shaft. The design life corresponds to 5 years of 60 hours/week, 50 weeks/year operation. Using a reliability of 99% and a safety factor of 1.5, estimate the horsepower that can be transmitted, based only on bending fatigue.