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CHAPTER OBJECTIVES

CHAPTER OBJECTIVES. Discuss effects of applying torsional loading to a long straight member Determine stress distribution within the member under torsional load. Determine angle of twist when material behaves in a linear-elastic.

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CHAPTER OBJECTIVES

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  1. CHAPTER OBJECTIVES • Discuss effects of applying torsional loading to a long straight member • Determine stress distribution within the member under torsional load • Determine angle of twist when material behaves in a linear-elastic. • Discuss stress distributions and stress concentration caused by torsional loadings

  2. CHAPTER OUTLINE • Torsional Deformation of a Circular Shaft • The Torsion Formula • Power Transmission • Angle of Twist • Stress Concentration

  3. Circles remain circular T T Longitudional Lines become twisted Radial lines remain straight Before deformation After deformation 5.1 TORSIONAL DEFORMATION OF A CIRCULAR SHAFT • Torsion is a moment that twists/deforms a member about its longitudinal axis • By observation, if angle of rotation is small, length of shaft and its radius remain unchanged

  4. g 5.1 TORSIONAL DEFORMATION OF A CIRCULAR SHAFT r = 0; g=0 r= r ; g= gmax The shear strain g increases as radius increses

  5. f(L) 5.1 TORSIONAL DEFORMATION OF A CIRCULAR SHAFT f(L) > f(x) The angle of twist f(x) increases as x increses

  6. A g Deformed plane dx D B r df  = (/2)  lim ’ Undeformed plane d dx  = CA along CA BA along BA  c ( )  = max 5.1 TORSIONAL DEFORMATION OF A CIRCULAR SHAFT • By definition, shear strain at A is Let x  dx and   = d BD =  d = dx  • Sinced / dx =  / = max /c Equation 5-2

  7. c ( )  = max max c ∫A2dA  = Shear stress varies linearly along each radial line of the cross section 5.2 THE TORSION FORMULA • For solid shaft, shear stress varies from zero at shaft’s longitudinal axis to maximum value at its outer surface. • Due to proportionality of triangles, or using Hooke’s law and Eqn 5-2,

  8. Tc J max = 5.2 THE TORSION FORMULA • The integral in the equation can be represented as the polar moment of inertia J, of shaft’s cross-sectional area computed about its longitudinal axis max = max. shear stress in shaft, at the outer surface T = resultant internal torque acting at cross-section, from method of sections & equation of moment equilibrium applied about longitudinal axis J = polar moment of inertia at cross-sectional area c = outer radius pf the shaft

  9. T J = 5.2 THE TORSION FORMULA • Shear stress at intermediate distance, 0  c • The above two equations are referred to as the torsion formula • Used only if shaft is circular, its material homogenous, and it behaves in an linear-elastic manner

  10. 2 J= c4 5.2 THE TORSION FORMULA Solid shaft • J can be determined using area element in the form of a differential ring or annulus having thickness d and circumference 2 . • For this ring, dA = 2 d • J is a geometric property of the circular area and is always positive. Common units used for its measurement are mm4 and m4.

  11. 2 J= (co4  ci4) Tci J tmax = tmax tmax T tmax Tr J Tco J T Shear stress varies linearly along each radial line of the cross section = max= 5.2 THE TORSION FORMULA Tubular shaft

  12. 5.2 THE TORSION FORMULA Absolute maximum torsional stress • Need to find location where ratio Tc/J is maximum. • Draw a torque diagram (internal torque  vs. x along shaft). • Sign Convention: T is positive, by right-hand rule, is directed outward from the shaft. • Once internal torque throughout shaft is determined, maximum ratio of Tc/J can be identified.

  13. tA tB 75 mm 15 mm EXAMPLE 5-1 Shaft shown supported by two bearings and subjected to three torques. Determine shear stress developed at points A and B, located at section a-a of the shaft.

  14. Section a-a EXAMPLE 5-1 Internal torque Bearing reactions on shaft = 0, if shaft weight assumed to be negligible. Applied torques satisfy moment equilibrium about shaft’s axis. Internal torque at section a-a determined from free-body diagram of left segment.  Mx = 0; 4250 kN·mm  3000 kN·mm  T = 0 T = 1250 kN·mm

  15. tA tB 75 mm 15 mm EXAMPLE 5-1 Section property: J = /2(75 mm)4 = 4.97 107 mm4 Shear stress: Since point A is at  = c = 75 mm A = Tc/J = ... = 1.89 MPa Likewise for point B,at  = 15 mm B = T /J = ... = 0.377 MPa Directions of the stresses on elements A and B established from direction of resultant internal torque T.

  16. P = T (d/dt) P = T P = 2fT 5.3 POWER TRANSMISSION • Power is defined as work performed per unit of time • Instantaneous power is • Since shaft’s angular velocity  = d/dt, we can also express power as • Frequency,f,of a shaft’s rotation is often reported. It measures the number of cycles per second and since 1 cycle = 2 radians, and  = 2f, then power becomes Equation 5-11

  17. P = 2fT or nin rpm J c T allow = 5.3 POWER TRANSMISSION Shaft Design • If power transmitted by shaft and its frequency of rotation is known, torque is determined from Eqn 5-11 • Knowing T and allowable shear stress for material, allow and applying torsion formula,

  18. J c T allow = 5.3 POWER TRANSMISSION • For solid shaft, substitute J = (/2)c4 to determine c • For tubular shaft, substitute J = (/2)(co2  ci2) to determine co and ci

  19. EXAMPLE 5-2 Solid steel shaft shown used to transmit 3750 W from attached motor M. Shaft rotates at n= 175 rpm and the steel allow = 100 MPa. Determine required diameter of shaft to nearest mm.

  20. T allow  c4 2 c J c = = EXAMPLE 5-2 Power, P = 3750 W = 3750 N·m/s Thus, the torque is = 204.6 N·m c = 10.92 mm Since 2c = 21.84 mm, select shaft with diameter of d = 22 mm

  21. T(x) dx J(x) G L ∫0  = 5.4 ANGLE OF TWIST • Angle of twist is important when analyzing reactions on statically indeterminate shafts  = angle of twist, in radians T(x) = internal torque at arbitrary position x, found from method of sections and equation of moment equilibrium applied about shaft’s axis J(x) = polar moment of inertia as a function of x G = shear modulus of elasticity for material

  22. TL JG T  = f L T TL JG   = 5.4 ANGLE OF TWIST Constant torque and cross-sectional area If shaft is subjected to several different torques, or cross-sectional area or shear modulus changes suddenly from one region of the shaft to the next, then apply Eqn 5-15 to each segment before vectorially adding each segment’s angle of twist:

  23. f +f(x) +T(x) +f(x) +T(x) Positive sign convention for T and f 5.4 ANGLE OF TWIST Sign convention • Use right-hand rule: torque and angle of twist are positive when thumb is directed outward from the shaft

  24. The gear attached to the fixed-end steel shaft are subjected to the torques shown in the figure. If the shear modulus of elasticity is G= 80 GPa, and the shaft has a diameter of 14 mm, determine the displacement of the tooth P on gear A. The shaft turns freely within the bearing at B. EXAMPLE 5-3

  25. By inspection, the displacement of tooth P on gear A, sP, is shown in the figure. EXAMPLE 5-3 sP = fAr

  26. Internal torque in each segment: TDE EXAMPLE 5-3 TAC = +150 N.m TCD = –130 N.m TDE = –170 N.m

  27. Angle of twist at the end of shaft or gear A: TDE TACLAC JG TL JG  = A = TCDLCD JG TDELDE JG + + fA = –0.212 rad EXAMPLE 5-7

  28. sP = (0.212 rad)(100 mm) EXAMPLE 5-3 fA = –0.212 rad (Negative sign indicates the sense of fA is CW) The displacement of tooth P on gear A, sP, is then sP = fAr = 21.2 mm

  29. coupling keyways Shoulder fillet 5.5 STRESS CONCENTRATION • Red dots on cross-section indicate where maximum shear stress will occur • This maximum shear stress can be determined from torsional stress-concentration factor, K

  30. K max = K Tc J r d 5.5 STRESS CONCENTRATION • K, can be obtained from a graph as shown • Find geometric ratio D/d for appropriate curve • Once abscissa r/d calculated, value of K found along ordinate • Maximum shear stress is then determined from

  31. 5.5 STRESS CONCENTRATION IMPORTANT • Stress concentrations in shafts occur at points of sudden cross-sectional change. The more severe the change, the larger the stress concentration • For design/analysis, not necessary to know exact shear-stress distribution on cross-section. Instead, obtain maximum shear stress using stress concentration factor K • If material is brittle, or subjected to fatigue loadings, then stress concentrations need to be considered in design/analysis.

  32. EXAMPLE 5-4 Stepped shaft shown is supported at bearings at A and B. Determine maximum stress in the shaft due to applied torques. Fillet at junction of each shaft has radius r = 6 mm.

  33. EXAMPLE 5-4 Internal torque By inspection, moment equilibrium about axis of shaft is satisfied. Since maximum shear stress occurs at rooted ends of smaller diameter shafts, internal torque (30 N·m) can be found by applying method of sections

  34. EXAMPLE 5-4 Diameter: D = 2C = 2 x 40 = 80 mm d = 2c = 2 x 20 = 40 mm Polar moment of inertia:  2  2 J= c4 =(20x10-3)4 = 25.13 x 10-7 m4

  35. EXAMPLE 5.18 (SOLN) From shaft geometry, we have 2(40 mm) 2(20 mm) D d = = 2 r d 6 mm 2(20 mm) = = 0.15 Thus, from the graph, K = 1.3 Maximum shear stress max = K(Tc/J) = ... = 3.10 MPa

  36. CHAPTER REVIEW • Torque causes a shaft with circular cross-section to twist, such that shear strain in shaft is proportional to its radial distance from its centre • Provided that material is homogeneous and Hooke’s law applies, shear stress determined from torsion formula,  = (Tc)/J • Design of shaft requires finding the geometric parameter, (J/C) = (T/allow) • Power generated by rotating shaft is reported, from which torque is derived; P = T = 2fT

  37. T(x) dx JG L ∫0  = TL JG   = CHAPTER REVIEW • Angle of twist of circular shaft determined from • If torque and JG are constant, then • For application, use a sign convention for internal torque and be sure material does not yield, but remains linear elastic

  38. CHAPTER REVIEW • If shaft is statically indeterminate, reactive torques determined from equilibrium, compatibility of twist, and torque-twist relationships, such as  = TL/JG • Solid noncircular shafts tend to warp out of plane when subjected to torque. Formulas are available to determine elastic shear stress and twist for these cases • Shear stress in tubes determined by considering shear flow. Assumes that shear stress across each thickness of tube is constant

  39. CHAPTER REVIEW • Shear stress in tubes determined from = T/2tAm • Stress concentrations occur in shafts when x-section suddenly changes. Maximum shear stress determined using stress concentration factor, K (found by experiment and represented in graphical form). max = K(Tc/J) • If applied torque causes material to exceed elastic limit, then stress distribution is not proportional to radial distance from centerline of shaft

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