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shi20396 chương 17

There are practical limitations on doubling the iconic scale. We can double pulley diameters and the center-to-center distance. With the belt we could: • Use the same A-3 belt and double its width; | Chapter 17 17-1 Preliminaries to give some checkpoints 1 VR 2 Angular velocity ratio 0.5 Hnom 2 hp 1750rev min C 9 12 108 in Ks 1.25 nd 1 dmin 1in Fa 35 lbf in y 0.035 lbf in3 f 0.50 b 6 in d 2 in from Table 17-2 for F-1 Polyamide t 0.05 in from Table 17-4 Cp 0.70. w 12ybt 12 0.035 6 0.05 0.126 lbf ft 6d 3.123 rad exp f6 4.766 perhaps _ ndn n 2 1750 V Y2 916.3 ft min 12 w V 2 a Eq. e Fc 4 v c 32.174 60 0.126 32.174 916.3 2 0.913 lbf 60 Ans. T 63025 2 1.25 1 90.0 lbf. 2T A F d 1750 2 90 90 lbf 2 Eq. 17-12 F a bFaCpCv 6 35 0.70 1 147 lbf Ans. F2 F1a - AF 147 - 90 57 lbf Ans. Do not use Eq. 17-9 because we do not yet know f . Eq. i Fi F1a F2 - Fc 147 57 - 0.913 101.1 lbf Ans. . 1 lnP F1 a - Fc 6d L F2 - Fc ln 3.123 147 - 0.913 57 - 0.913 J 0.307 The friction is thus undeveloped. b The transmitted horsepower is AF V 33 000 H nfs H K Hnom Ks 90 916.3 33 2.5hp A 5 1 2 1.25 From Eq. 17-2 L 225.3 in Ans. z _ 3C2w c From Eq. 17-13 dip 2 Fi where C is the center-to-center distance in feet. 3 108 12 2 0.126 dip ------2 101 n------- 0.151 in Ans. . 432 Solutions Manual Instructor s Solution Manual to Accompany Mechanical Engineering Design Comment The friction is under-developed. Narrowing the belt width to 5 in if size is available will increase f . The limit of narrowing is bmin 4.680 in whence w 0.0983 lbf ft Fc 0.712 lbf T 90 lbf in same AF Fi a - F2 90 lbf Fi 68.9 lbf Longer life can be obtained with a 6-inch wide belt by reducing Fi to attain f 0.50. Prob. 17-8 develops an equation we can use here F AF Fc exp f 6 - Fc i exp f 6 - 1 F2 F1 - A F F1 F2 Fc Fi a 114.7 Ibf F2 24.6 Ibf f f 0.50 dip 0.173 in Fi F1 - Fc F2 Fc f ln J dd 3 CD 12 2w dip 2 Fi which in this case gives F1 114.9 Ibf F2 24.8 Ibf Fi 68.9 Ibf Fc 0.913 Ibf f 0.50 dip 0.222 in 2 So reducing Fi from 101.1 lbf to 68.9 lbf will bring the undeveloped friction up to 0.50 with a corresponding dip of 0.222 in. Having reduced F1 and F2 the endurance of the belt is improved. Power service factor and design factor .