ENT345 Mechanical Components Design

Transcription

1 1) LOAD AND STRESS ANALYSIS i. Principle stress ii. The maximum shear stress iii. The endurance strength of shaft. 1) Problem 3-71 A countershaft carrying two-v belt pulleys is shown in the figure. Pulley A receives power from a motor through a belt with the belt tensions shown. The power is transmitted through the shaft and delivered to the belt on pulley B. Assume the belt tension on the loose side at B is 15 percent of the tension on the tight side. a) Determine the tensions in the belt on pulley B, assuming the shaft is running at a constant speed. b) Find the magnitudes of the bearing reaction forces, assuming the bearings act as simple supports. c) Draw shear-force and bending moment diagrams for the shaft. If needed, make one set for the horizontal plane and anther set for the vertical plane. d) At the point of maximum bending moment, determine the bending stress and the torsional shear stress. e) At the point of maximum bending moment, determine the principal stresses and the maximum shear stress. Solution Assume the belt tension on the loose side at B is 15 percent of the tension on the tight side. T 2 = 0.15 T 1 UniMAP Page 1

2 a) Determine the tensions in the belt on pulley B, assuming the shaft is running at a constant speed. T = 0 (300 45)N(125)mm + (T 2 T 1 )N(150)mm = Nmm + (0. 15T 1 T 1 )N(150)mm = Nmm T 1 = 0 UniMAP Page 2

3 T 1 = 250Nmm On pulley B T 2 = (0. 15)250Nmm = Nmm T = T 1 + T 2 = = Nmm b) Find the magnitudes of the bearing reaction forces, assuming the bearings act as simple supports. Y N 45 N 31.82N T 2 =0.15 T 1 X N 300 N A 45 B Z N T 1 M oy = 0 345cos45 (300) (700) + R Cz (850) = 0 R Cz = N F z = 0 R Oz 345cos = 0 R Oz = N M oz = 0 345sin45 (300) + R Cy (850) = 0 R Cy = N UniMAP Page 3

4 F y = 0 R Oy + 345cos = 0 R Oz = N y O A B C X 300 mm 400 mm 150 mm UniMAP Page 4

5 Z N O A B C X 300 mm 400 mm 150 mm c) Draw shear-force and bending moment diagrams for the shaft. If needed, make one set for the horizontal plane and anther set for the vertical plane. y O A B C X 300 mm 400 mm 150 mm V (N) 86.1 O UniMAP Page 5

6 M (Nm) O x Z N O A B C X 300 mm 400 mm 150 mm V (N) O UniMAP Page 6

7 M (Nm) O x d) At the point of maximum bending moment, determine the bending stress and the torsional shear stress. The critical location is at A where both planes have the maximum bending moment. Combining the bending moments from the two planes; M = ( 47.37) 2 + ( 32.16) 2 = Nm The torque transmitted through the shaft from A to B is T = (300 45)(0.125) = Nm The bending stress and the torsional stress are both maximum are on the outer surface of a stress element. σ = Mc I = 32M πd 3 = 32(57.26) π(0.020) 3 = 72.9x106 Pa = 72.9MPa τ = Tr J = 16T πd 3 = 16(31.88) π(0.020) 3 = 20.3x106 Pa = 20.3MPa UniMAP Page 7

8 e) At the point of maximum bending moment, determine the principal stresses and the maximum shear stress. σ 1, σ 2 = σ x 2 ± ( σ x 2 ) 2 + (τ xy ) 2 = ± ( ) 2 + (20.3) 2 = σ 1 = 78.2MPa σ 2 = 5.27MPa τ max = ( σ x 2 ) 2 + (τ xy ) 2 = ( ) 2 + (20.3) 2 = 41.7 MPa UniMAP Page 8

9 2) Problem 3-73 A gear reduction unit uses the countershaft shown in the figure. Gear A receives power from another gear with the transmitted force F A applied at the 20 pressure angle as shown. The power is transmitted through the shaft and delivered through gear B through a transmitted force F B at the pressure angle shown. a) Determine the force F B, assuming the shaft is running at a constant speed. b) Find the magnitudes of the bearing reaction forces, assuming the bearings act as simple supports. c) Draw shear-force and bending-moment diagrams for the shaft. If needed, make one set for the horizontal plane and another set for the vertical plane. d) At the point of maximum bending moment, determine the bending stress and the torsional shear stress. e) At the point of maximum bending moment, determine the principal stresses and the maximum shear stress. UniMAP Page 9

10 F B Y F A Z a) Determine the force F B, assuming the shaft is running at a constant speed. T = (cos20 )(300) F B (cos25 )(150) = 0 F B = N b) Find the magnitudes of the bearing reaction forces, assuming the bearings act as simple supports. M oz = (sin20 )(400) 22810(sin25 )(750) + R Cy (1050) = 0 R Cy = 8319 N F y = 0 R Oy 11000(sin20 ) 22810(sin25 ) = 0 R Oy = 5083 N UniMAP Page 10

11 M oy = (cos20 )(400) 22810(cos25 )(750) R Cz (1050) = 0 R Cz = N F z = 0 R Oz 11000(cos20 ) (cos25 ) = 0 R Oz = 494 N y 8319 O A B C X 400 mm 350 mm 300 mm UniMAP Page 11

12 Z O A B C X 400 mm 350 mm 300 mm c) Draw shear-force and bending-moment diagrams for the shaft. If needed, make one set for the horizontal plane and another set for the vertical plane. d) V (N) O M (Nm) x V (N) 9843 UniMAP Page 12

13 O M (Nm) 3249 O -198 d) At the point of maximum bending moment, determine the bending stress and the torsional shear stress. The critical location is at B where both planes have the maximum bending moment. Combining the bending moments from the two planes; M = (2496) 2 + (3249) 2 = 4097 Nm The torque transmitted through the shaft from A to B is T = 11000(cos20 )(0.3) = 3101 Nm The bending stress and the torsional stress are both maximum are on the outer surface of a stress element. UniMAP Page 13

14 σ = Mc I = 32M πd 3 = 32(4097) π(0.050) 3 = 333.9x106 Pa = 333.9MPa τ = Tr J = 16T πd 3 = 16(3101) π(0.050) 3 = 126x106 Pa = 20.3MPa e) At the point of maximum bending moment, determine the principal stresses and the maximum shear stress. σ 1, σ 2 = σ x 2 ± ( σ 2 x 2 ) + (τ xy ) 2 = ± ( ) + (126.3) 2 = σ 1 = 376MPa σ 2 = 42.4MPa τ max = ( σ 2 x 2 ) + (τ xy ) 2 = ( ) + (126.3) 2 = 209 MPa f) The endurance strength of the shaft: Material: AISI 1040 CD with S ut = 590 MPa Diameter of shaft: d = 50 mm Machined or cold-drawn a = 4.51 and b = S e = 0.5(590) = 295 MPa k a = as b ut = (4.51)(590) = Surface factor k a = The loading situation is rotating bending. k b = ( d ) = ( ) = S e = k a k b S e = ( )( )(295) = MPa. The endurance strength is S e = MPa UniMAP Page 14

15 3) Problem A gear-reduction unit uses the countershaft depicted in figure. Find the two bearing reactions. The bearings are to be angular-contact ball bearings, having a desired life of 40 kh when used at 200 rev/min. Use 1.2 for the application factor and a reliability goal for the bearing pair of Select the bearings from Table 11-2 Solution UniMAP Page 15

16 F B Y F A Z b) Determine the force F B, assuming the shaft is running at a constant speed. T = (cos20 )(300) F B (cos25 )(150) = 0 F B = N c) Find the magnitudes of the bearing reaction forces, assuming the bearings act as simple supports. M oz = (sin20 )(400) (sin25 )(750) + R Cy (1050) = 0 R Cy = N F y = 0 R Oy 1068(sin20 ) (sin25 ) = 0 R Oy = N UniMAP Page 16

17 M oy = (cos20 )(400) (cos25 )(750) R Cz (1050) = 0 R Cz = N F z = 0 R Oz 1068(cos20 ) (cos25 ) = 0 R Oz = N UniMAP Page 17

18 y O A B C X 400 mm 350 mm 300 mm Z O A B C X 400 mm 350 mm 300 mm R o = (R oy ) 2 + (R oz ) 2 = ( ) 2 + ( ) 2 R o = = N R c = (R cy ) 2 + (R cz ) 2 = (807. 7) 2 + ( ) 2 R o = = N UniMAP Page 18

19 F D O = (1. 2)( ) = N F D C = (1. 2)( ) = N x D = L Dn D 60 L R n R 60 = (40000)(200)(60) (10 6 = 480 ) Realibility for R A and R B are R = 0.95 = (C 10 ) O = [ [ln ( )] ] 1 3 = N Bearing Type is angular contact bearing at O, series number Bore : 12 mm OD : 32 mm Width : 10 mm Fillet radius : 0.6 mm Shoulder diameter: d s = 14.5 mm, d H = 28 mm (C 10 ) O = = N [ [ln ( )] ] Bearing Type is angular contact bearing at C, series number Bore : 30 mm OD : 62 mm Width : 16 mm Fillet radius : 1.0 mm Shoulder diameter: d s = 35 mm, d H = 55 mm 1 3 UniMAP Page 19