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Unformatted text preview: MECH 325 Machine Elements
Tutorial 3 – Gear Stress Analysis and Power Screws 1. Adapted from Shigley, Problem 14‐19 (8 and 9th Eds.) A commercial enclosed gear drive consists of a 20° pinion having 16 teeth driving a 48‐ tooth gear. The pinion speed is 300 rev/min, the face width is 2 in, and the diametral pitch is 6 teeth/in. The gears are grade 1 steel, through‐hardened at 200 Brinell, made to No. 6 quality standards, uncrowned, and are to be accurately and rigidly mounted. Assume a pinion life of 108 cycles and a reliability of 0.90. The power to be transmitted is 5 hp. Part 1: To be demonstrated by the TA a. Determine the AGMA bending stress in the pinion b. Determine the corresponding safety factor for the pinion bending stress Part 2: To be done by the class c. Determine the AGMA contact stress in the pinion d. Determine the corresponding safety factor for the pinion contact stress Part 3: Extra practice (to be done at home) e. Repeat the above bending and contact stress analysis for the gear and show that the stresses are 9.59 ksi and 102.1 ksi, respectively, and that the safety factors in bending and contact are 3.29 and 1.02, respectively. 2. Adapted from Shigley, Problem 8‐4 (8th and 9th Eds.) A single‐threaded 25 mm power screw is 25 mm in diameter with a pitch of 5 mm. A vertical load on the screw reaches a maximum of 6 kN. The coefficients of friction are 0.05 for the collar and 0.08 for the threads. The frictional diameter of the collar is 40 mm. Part 1: To be demonstrated by the TA a. Find the torque to raise the load Part 2: To be done by the class b. Find the torque to lower the load c. Find the overall efficiency Part 3: Extra practice (to be done at home) d. If the square threads are replaced by ACME threads (thread angle = 29°), show the new torque to raise the load is 16.41 N∙m e. Show that if the collar friction coefficient is reduced to 0.02, the minimum thread friction coefficient for which the new ACME thread power screw will be self‐locking will be f = 0.0340. th ...
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- Fall '10