Torsion of Shafts Shafts are structural members with length significantly greater than the largest cross-sectional dimension used in transmitting torque from one plane to another. Learning objectives Understand the theory, its limitations and its applications for design and analysis of Torsion of circular shafts. Develop the discipline to visualize direction of torsional shear stress and the surface on which it acts. 5-1
C5.1 Three pairs of bars are symmetrically attached to rigid discs at the radii shown. The discs were observed to rotate by angles φ 1 = 1.5, φ 2 = 3.0, and φ 3 = 2.5 in the direction of the applied torques T 1, T 2, and T 3 respectively. The shear modulus of the bars is 40 ksi and the area of cross-section is 0.04 in 2. Determine the shear strains in bars, CD, and EF. 5-2
Internal Torque T = ρdv = Equation is independent of material model as it represents static equivalency between shear stress and internal torque on a cross-section ρτ θ d 5.1 5-3
C5.2 hollow titanium (G Ti = 36 GPa) shaft and a hollow luminum (G l = 26 GPa) shaft are securely fastened to form a composite shaft as shown in Fig. C5.2. The shear strain γ θ in polar coordinates at the section is γ θ = 0.05ρ where ρ is in meters and the dimensions of the cross-section are d i = 40 mm, d l = 80 mm and d Ti = 120 mm. Determine the equivalent internal torque acting at the cross-section. Titanium ρ luminum θ d i d Ti d l Fig. C5.2 5-4
Theory Objective Theory for Circular Shafts (i) to obtain a formula for the relative rotation (φ 2 - φ 1 ) in terms of the internal torque T. (ii) to obtain a formula for the shear stress τ θ in terms of the internal torque T. y z r T 2 2 5-5
Kinematics Original Grid Deformed Grid o, o Initial position 1, 1 Deformed position o 1 1 o ssumption 1 ssumption 2 ssumption 3 Plane sections perpendicular to the ais remain plane during deformation. (No Warping) On a cross-section, all radials lines rotate by equal angle during deformation. Radials lines remain straight during deformation. φ = φ( ) φ is positive counter-clockwise with respect to the -ais. y γ θ ρ θ γ θ 1 C z Δ ssumption 4 Strains are small. γ θ = Δφ dφ ρ d ρ R γθ γ ma ρ 5-6
Material Model ssumption 5 Material is linearly elastic. ssumption 6 Material is isotropic. From Hooke s law τ = Gγ, we obtain: τ θ = dφ Gρ d θ Failure surface in aluminum shaft due to τ θ τ θ τ θ Failure surface in wooden shaft due to τ θ Sign Convention Internal torque is considered positive if it is counter-clockwise with respect to the outward normal to the imaginary cut surface. Positive τ θ Positive T Positive τ θ Positive T Outward normal Outward normal 5-7
Torsion Formulas ssumption 7 T Gρ 2 dφ = d = dφ d d Gρ 2 d Material is homogenous across the cross-section. dφ T = ------ d GJ J is the polar moment of inertia for the cross-section. The quantity GJ is called the torsional rigidity. π Circular cross-section of radius R or diameter D, J = --R 4 = -----D π 4. 2 32 τ θ τ θ = Tρ ------ J τ ma ρ ssumption 8 Material is homogenous between 1 and 2. ssumption 9 ssumption 10 φ 2 φ 2 φ 1 = dφ = φ 1 The shaft is not tapered. 2 1 T ------ d GJ The eternal (hence internal) torque does not change with between 1 and 2. φ 2 φ 1 T( 2 1 ) = ------------------------- GJ 5-8
Two options for determining internal torque T T is always drawn in counter-clockwise direction with respect to the outward normal of the imaginary cut on the free body diagram. Direction of τ θ can be determined using subscripts. Positive φ is counter-clockwise with respect to -ais. φ 2 φ 1 is positive counter-clockwise with respect to -ais T is drawn at the imaginary cut on the free body diagram in a direction to equilibrate the eternal torques. Direction of τ θ must be determined by inspection. Direction of φ must be determined by inspection. Direction of φ 2 φ 1 must be determined by inspection. Torsional Stresses and Strains In polar coordinates, all stress components ecept τ θ are assumed zero. Shear strain can be found from Hooke s law. Direction of τ θ by inspection Torsional Shear Stress 5-9
C5.3 Determine the direction of shear stress at points and (a) by inspection, and (b) by using the sign convention for internal torque and the subscripts. Report your answer as a positive or negative τ y. y T Class Problem 1 C5.4 Determine the direction of shear stress at points and (a) by inspection, and (b) by using the sign convention for internal torque and the subscripts. Report your answer as a positive or negative τ y. y T 5-10
C5.5 Determine the internal torque in the shaft below by making imaginary cuts and drawing free body diagrams. 20 kn-m 12 kn-m 10 kn-m C D 0.5 m 18 kn-m 1.0 m 0.4 m 5-11
Torque Diagram torque force diagram is a plot of internal torque T vs. Internal torque jumps by the value of the eternal torque as one crosses the eternal torque from left to right. n torsion template is used to determine the direction of the jump in T. template is a free body diagram of a small segment of a shaft created by making an imaginary cut just before and just after the section where the eternal torque is applied. Template 1 Equation Template 1 Template 2 Template 2 Equation T 2 = T 1 T T et 2 = T 1 + T et C5.6 Determine the internal torque in the shaft below by drawing the torque diagram. 20 kn-m 10 kn-m 12 kn-m C 18 kn-m 0.4 m D 0.5 m 1.0 m 5-12
C5.7 solid circular steel (G s = 12,000 ksi) shaft C is securely attached to two hollow steel shafts and CD as shown. Determine: (a) the angle of rotation of section at D with respect to section at. (b) the maimum torsional shear stress in the shaft (c) the torsional shear stress at point E and show it on a stress cube. Point E is on the inside bottom surface of CD. 5-13
C5.8 The eternal torque on a drill bit varies as a quadratic function to a maimum intensity of q in.lb/in as shown. If the drill bit diameter is d, its length L, and modulus of rigidity G, determine (a) the maimum shear stress on the drill bit. (b) the relative rotation of the end of the drill bit with respect to the chuck. L q 2 ------ L 2 in.lb/in Fig. C5.8 5-14
Statically Indeterminate Shafts oth ends of the shaft are built in, leading to two reaction torques but we have only on moment equilibrium equation. The compatibility equation is that the relative rotation of the right wall with respect to the left wall is zero. Calculate relative rotation of each shaft segment in terms of the reaction torque of the left (or right) wall. dd all the relative rotations and equate to zero to obtain reaction torque. C5.9 Two hollow aluminum (G = 10,000 ksi) shafts are securely fastened to a solid aluminum shaft and loaded as shown Fig. C5.9. Point E is on the inner surface of the shaft. If T= 300 in-kips in Fig. C5.9, Determine (a) the rotation of section at C with respect to rotation the wall at. (b) the shear strain at point E. T 4 in 2in C E D 24 in 36 in Fig. C5.9 24 in 5-15
C5.10 Under the action of the applied couple the section of the two tubes shown Fig. C5.10 rotate by an angle of 0.03 rads. Determine (a) the magnitude maimum torsional shear stress in aluminum and copper. (b) the magnitude of the couple that produced the given rotation. aluminum F copper F Fig. C5.10 5-16