E:\Bill\BEI\~ME311\2014\HW5Handout.doc ME311 Homework 5 Problem 7.34 This is really a fluctuating stress condition (the lift screw never gets compressed), which Hamrock incorrectly solves as a fully reversing condition. I’m just pointing that out and asking you to solve it as if it were fully reversing. Actual Loading: Assume This: Also assume: • All k’s = 1 • Loading is axial Time → σmax 0 -σmax σmaxE:\Bill\BEI\~ME311\2014\HW5Handout.doc Fully Reversing Fatigue Problem A solid steel shaft (AISI 1020, Q&T 870°C) with 1/2” diameter is used to turn a grinding wheel that hangs out from a bearing. It is loaded by a bending force of 60 Lbs at the wheel during a production grinding operation. A. Assuming that there is only a radial force on the wheel (no friction at the contact where the grinding takes place), determine the maximum stress on the shaft right where it leaves the rotating bearing (near point A). B. As the shaft rotates, point A alternately sees tension and compression. Given that the surface of the shaft is ground and we want only a 0.1% failure rate, how many rotations could this shaft endure? C. Draw the S-N diagram for this case, and show the operating point. D. If each grinding operation contacts the wheel for 5 seconds, and the shaft is turning at 900 RPM, how many parts could be ground before the shaft might fail? E. What should the grinding force be reduced to for the shaft to have infinite life? 60 Lb A Rotating Bearing 6 in Grinding
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