Capítulo 01 introdução
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This document provides two examples of analyzing the forces and critical angle required for impending motion of a pin on a surface. It considers the forces and reactions for motion to the left and right. The critical angle is determined by resolving the applied force into components related to mass and acceleration and friction. The pin will accelerate if the angle is less than critical and will not move if the angle is greater than critical. The role of pin diameter is also briefly mentioned.
Capítulo 01 introdução
- 1. Chapter 1 D B G F Facc A E f f 1 1 cr C Impending motion to left Fcr Consider force F at G, reactions at B and D. Extend lines of action for fully-developed fric- tion DE and BE to find the point of concurrency at E for impending motion to the left. The critical angle is θcr. Resolve force F into components Facc and Fcr. Facc is related to mass and acceleration. Pin accelerates to left for any angle 0 < θ < θcr. When θ > θcr, no magnitude of F will move the pin. D B G FЈ FЈacc A EЈ иE f f 1 1 C d Impending motion to right Ј FcrЈ crЈ Consider force F at G, reactions at A and C. Extend lines of action for fully-developed fric- tion AE and C E to find the point of concurrency at E for impending motion to the left. The critical angle is θcr. Resolve force F into components Facc and Fcr. Facc is related to mass and acceleration. Pin accelerates to right for any angle 0 < θ < θcr. When θ > θcr, no mag- nitude of F will move the pin. The intent of the question is to get the student to draw and understand the free body in order to recognize what it teaches. The graphic approach accomplishes this quickly. It is im- portant to point out that this understanding enables a mathematical model to be constructed, and that there are two of them. This is the simplest problem in mechanical engineering. Using it is a good way to begin a course. What is the role of pin diameter d? Yes, changing the sense of F changes the response. Problems 1-1 through 1-4 are for student research. 1-5 shi20396_ch01.qxd 6/5/03 12:11 PM Page 1
- 2. 2 Solutions Manual • Instructor’s Solution Manual to Accompany Mechanical Engineering Design 1-6 (a) Fy = −F − f N cos θ + N sin θ = 0 (1) Fx = f N sin θ + N cos θ − T r = 0 F = N(sin θ − f cos θ) Ans. T = Nr( f sin θ + cos θ) Combining T = Fr 1 + f tan θ tan θ − f = KFr Ans. (2) (b) If T → ∞ detent self-locking tan θ − f = 0 ∴ θcr = tan−1 f Ans. (Friction is fully developed.) Check: If F = 10 lbf, f = 0.20, θ = 45◦, r = 2 in N = 10 −0.20 cos 45◦ + sin 45◦ = 17.68 lbf T r = 17.28(0.20 sin 45◦ + cos 45◦ ) = 15 lbf f N = 0.20(17.28) = 3.54 lbf θcr = tan−1 f = tan−1 (0.20) = 11.31◦ 11.31° < θ < 90° 1-7 (a) F = F0 + k(0) = F0 T1 = F0r Ans. (b) When teeth are about to clear F = F0 + kx2 From Prob. 1-6 T2 = Fr f tan θ + 1 tan θ − f T2 = r (F0 + kx2)( f tan θ + 1) tan θ − f Ans. 1-8 Given, F = 10 + 2.5x lbf, r = 2 in, h = 0.2 in, θ = 60◦ , f = 0.25, xi = 0, xf = 0.2 Fi = 10 lbf; Ff = 10 + 2.5(0.2) = 10.5 lbf Ans. x y F fN N T r shi20396_ch01.qxd 6/5/03 12:11 PM Page 2
- 3. Chapter 1 3 From Eq. (1) of Prob. 1-6 N = F − f cos θ + sin θ Ni = 10 −0.25 cos 60◦ + sin 60◦ = 13.49 lbf Ans. Nf = 10.5 10 13.49 = 14.17 lbf Ans. From Eq. (2) of Prob. 1-6 K = 1 + f tan θ tan θ − f = 1 + 0.25 tan 60◦ tan 60◦ − 0.25 = 0.967 Ans. Ti = 0.967(10)(2) = 19.33 lbf · in Tf = 0.967(10.5)(2) = 20.31 lbf · in 1-9 (a) Point vehicles Q = cars hour = v x = 42.1v − v2 0.324 Seek stationary point maximum dQ dv = 0 = 42.1 − 2v 0.324 ∴ v* = 21.05 mph Q* = 42.1(21.05) − 21.052 0.324 = 1367.6 cars/h Ans. (b) Q = v x + l = 0.324 v(42.1) − v2 + l v −1 Maximize Q with l = 10/5280 mi v Q 22.18 1221.431 22.19 1221.433 22.20 1221.435 ← 22.21 1221.435 22.22 1221.434 % loss of throughput 1368 − 1221 1221 = 12% Ans. xl 2 l 2 v x v shi20396_ch01.qxd 6/5/03 12:11 PM Page 3
- 4. 4 Solutions Manual • Instructor’s Solution Manual to Accompany Mechanical Engineering Design (c) % increase in speed 22.2 − 21.05 21.05 = 5.5% Modest change in optimal speed Ans. 1-10 This and the following problem may be the student’s first experience with a figure of merit. • Formulate fom to reflect larger figure of merit for larger merit. • Use a maximization optimization algorithm. When one gets into computer implementa- tion and answers are not known, minimizing instead of maximizing is the largest error one can make. FV = F1 sin θ − W = 0 FH = −F1 cos θ − F2 = 0 From which F1 = W/sin θ F2 = −W cos θ/sin θ fom = −S = −¢γ (volume) . = −¢γ(l1 A1 + l2 A2) A1 = F1 S = W S sin θ , l2 = l1 cos θ A2 = F2 S = W cos θ S sin θ fom = −¢γ l2 cos θ W S sin θ + l2W cos θ S sin θ = −¢γ Wl2 S 1 + cos2 θ cos θ sin θ Set leading constant to unity θ◦ fom 0 −∞ 20 −5.86 30 −4.04 40 −3.22 45 −3.00 50 −2.87 54.736 −2.828 60 −2.886 Check second derivative to see if a maximum, minimum, or point of inflection has been found. Or, evaluate fom on either side of θ*. θ* = 54.736◦ Ans. fom* = −2.828 Alternative: d dθ 1 + cos2 θ cos θ sin θ = 0 And solve resulting tran- scendental for θ*. shi20396_ch01.qxd 6/5/03 12:11 PM Page 4
- 5. Chapter 1 5 1-11 (a) x1 + x2 = X1 + e1 + X2 + e2 error = e = (x1 + x2) − (X1 + X2) = e1 + e2 Ans. (b) x1 − x2 = X1 + e1 − (X2 + e2) e = (x1 − x2) − (X1 − X2) = e1 − e2 Ans. (c) x1x2 = (X1 + e1)(X2 + e2) e = x1x2 − X1 X2 = X1e2 + X2e1 + e1e2 . = X1e2 + X2e1 = X1 X2 e1 X1 + e2 X2 Ans. (d) x1 x2 = X1 + e1 X2 + e2 = X1 X2 1 + e1/X1 1 + e2/X2 1 + e2 X2 −1 . = 1 − e2 X2 and 1 + e1 X1 1 − e2 X2 . = 1 + e1 X1 − e2 X2 e = x1 x2 − X1 X2 . = X1 X2 e1 X1 − e2 X2 Ans. 1-12 (a) x1 = √ 5 = 2.236 067 977 5 X1 = 2.23 3-correct digits x2 = √ 6 = 2.449 487 742 78 X2 = 2.44 3-correct digits x1 + x2 = √ 5 + √ 6 = 4.685 557 720 28 e1 = x1 − X1 = √ 5 − 2.23 = 0.006 067 977 5 e2 = x2 − X2 = √ 6 − 2.44 = 0.009 489 742 78 e = e1 + e2 = √ 5 − 2.23 + √ 6 − 2.44 = 0.015 557 720 28 Sum = x1 + x2 = X1 + X2 + e = 2.23 + 2.44 + 0.015 557 720 28 = 4.685 557 720 28 (Checks) Ans. (b) X1 = 2.24, X2 = 2.45 e1 = √ 5 − 2.24 = −0.003 932 022 50 e2 = √ 6 − 2.45 = −0.000 510 257 22 e = e1 + e2 = −0.004 442 279 72 Sum = X1 + X2 + e = 2.24 + 2.45 + (−0.004 442 279 72) = 4.685 557 720 28 Ans. shi20396_ch01.qxd 6/5/03 12:11 PM Page 5
- 6. 6 Solutions Manual • Instructor’s Solution Manual to Accompany Mechanical Engineering Design 1-13 (a) σ = 20(6.89) = 137.8 MPa (b) F = 350(4.45) = 1558 N = 1.558 kN (c) M = 1200 lbf · in (0.113) = 135.6 N · m (d) A = 2.4(645) = 1548 mm2 (e) I = 17.4 in4 (2.54)4 = 724.2 cm4 (f) A = 3.6(1.610)2 = 9.332 km2 (g) E = 21(1000)(6.89) = 144.69(103 ) MPa = 144.7 GPa (h) v = 45 mi/h (1.61) = 72.45 km/h (i) V = 60 in3 (2.54)3 = 983.2 cm3 = 0.983 liter 1-14 (a) l = 1.5/0.305 = 4.918 ft = 59.02 in (b) σ = 600/6.89 = 86.96 kpsi (c) p = 160/6.89 = 23.22 psi (d) Z = 1.84(105 )/(25.4)3 = 11.23 in3 (e) w = 38.1/175 = 0.218 lbf/in (f) δ = 0.05/25.4 = 0.00197 in (g) v = 6.12/0.0051 = 1200 ft/min (h) = 0.0021 in/in (i) V = 30/(0.254)3 = 1831 in3 1-15 (a) σ = 200 15.3 = 13.1 MPa (b) σ = 42(103 ) 6(10−2)2 = 70(106 ) N/m2 = 70 MPa (c) y = 1200(800)3 (10−3 )3 3(207)(6.4)(109)(10−2)4 = 1.546(10−2 ) m = 15.5 mm (d) θ = 1100(250)(10−3 ) 79.3(π/32)(25)4(109)(10−3)4 = 9.043(10−2 ) rad = 5.18◦ 1-16 (a) σ = 600 20(6) = 5 MPa (b) I = 1 12 8(24)3 = 9216 mm4 (c) I = π 64 324 (10−1 )4 = 5.147 cm4 (d) τ = 16(16) π(253)(10−3)3 = 5.215(106 ) N/m2 = 5.215 MPa shi20396_ch01.qxd 6/5/03 12:11 PM Page 6
- 7. Chapter 1 7 1-17 (a) τ = 120(103 ) (π/4)(202) = 382 MPa (b) σ = 32(800)(800)(10−3 ) π(32)3(10−3)3 = 198.9(106 ) N/m2 = 198.9 MPa (c) Z = π 32(36) (364 − 264 ) = 3334 mm3 (d) k = (1.6)4 (79.3)(10−3 )4 (109 ) 8(19.2)3(32)(10−3)3 = 286.8 N/m shi20396_ch01.qxd 6/5/03 12:11 PM Page 7