The accessory drive unit of a large aircraft engine involves anidentical pair of 20° full-depth spur gears. The gears have 50teeth and rotate together at 5000 rpm. The diametral pitch is 12and the face width 1.5 in. The gears are made of alloy steel,casehardened to 62Rc (680 Bhn). Although the case extendsthroughout the fillet, it is conservatively decided to use the corehardness of 500 Bhn for calculating bending fatigue strength. Thetooth profiles are finished with a fine-grinding operation thatgives a degree of precision justifying the use of curve A of Figure15.24 and the assumption that teeth share the load. The loadinginvolves only very mild shock, justifying the use of Ko = 1.1.
(a). Estimate the horsepower that can be transmitted with 99%reliability, based on tooth bending fatigue.
(b). Estimate the horsepower that can be transmitted for 109cycles with 90% reliability, based on surface fatigue.
