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29_ch 17 Mechanical Design budynas_SM_ch17

# 29_ch 17 Mechanical Design budynas_SM_ch17 - wires...

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448 Solutions Manual • Instructor’s Solution Manual to Accompany Mechanical Engineering Design Fatigue without bending : n f = F f F t = 24 . 2 11 . 76 = 2 . 06 Ans . Fatigue, with bending : For a life of 0.1(10 6 ) cycles, from Fig. 17-21 ( p / S u ) = 4 / 1000 = 0 . 004 F f = 0 . 004 ( 240 )( 2 )( 72 ) 2 = 69 . 1 kip Eq. (17-50): n f = 69 . 1 39 11 . 76 = 2 . 56 Ans . If we were to use the endurance strength at 10 6 cycles ( F f = 24 . 2 kip) the factor of safety would be less than 1 indicating 10 6 cycle life impossible. Comments: • There are a number of factors of safety used in wire rope analysis. They are differ- ent, with different meanings. There is no substitute for knowing exactly which fac- tor of safety is written or spoken. • Static performance of a rope in tension is impressive. • In this problem, at the drum, we have a ﬁnite life. • The remedy for fatigue is the use of smaller diameter ropes, with multiple ropes supporting the load. See Ex. 17-6 for the effectiveness of this approach. It will also be used in Prob. 17-30. • Remind students that wire ropes do not fail suddenly due to fatigue. The outer
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Unformatted text preview: wires gradually show wear and breaks; such ropes should be retired. Periodic in-spections prevent fatigue failures by parting of the rope. 17-30 Since this is a design task, a decision set is useful. A priori decisions • Function: load, height, acceleration, velocity, life goal • Design Factor: n d • Material: IPS, PS, MPS or other • Rope: Lay, number of strands, number of wires per strand Decision variables: • Nominal wire size: d • Number of load-supporting wires: m From experience with Prob. 17-29, a 1-in diameter rope is not likely to have much of a life, so approach the problem with the d and m decisions open. Function : 5000 lbf load, 90 foot lift, acceleration = 4 ft/s 2 , velocity = 2 ft/s, life goal = 10 5 cycles Design Factor : n d = 2 Material : IPS Rope : Regular lay, 1-in plow-steel 6 × 19 hoisting...
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