Torsional Vibrations

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Torsional Vibrations: ---When a shaft is transmitting torque it is subjected to twisting of torsional deflection; and if there are cyclic variations in the transmitted torque the shaft will oscillate, that is twist and untwist. Assumption: Mass moment of inertia of the disk is large compared with the mass moment of inertia of the shaft. For the torque exerted by the rod: T = I * α Therefore d 2θ −K θ = I t dt 2 Where Kt = torsional spring constant of the shaft The negative sign is used because T is opposite in sense to θ. ∴ d 2θ K + t θ = 0 dt 2 I It is a homogeneous differential equation. Solution is æ K ö æ K ö θ = ç t ÷ + ç t ÷ C1 cosç t÷ C 2 sinç t÷ è I ø è I ø Boundary conditions: (1) θ = 0 when t = 0 (2) θ = A when sqrt(Kt/I) * t = π/2 ∴ C1 = 0 and C2 = A and the solution becomes æ ö ç Kt ÷ θ = A sinç t÷ è I ø This is a simple harmonic motion. K ω = t n I Since Kt = T/θ and θ = (TL)/(GJ), so Kt=(JG)/L Therefore π 4 ω = d G n 32 I L Where ωn = natural frequency of torsional vibration, rad/s fn = natural frequency in cycles/sec = ωn /(2 π) d = shaft diameter, m G = modulus of elasticity in shear for shaft material (79.3 x 109 Pa for steel) I = mass moment of inertia of disk about the x-axis = Mr2 L = shaft length, m Torsional Vibration of a Shaft with Two Disks When disk 1 is rotating in a counterclockwise direction, disk 2 must be rotating in a clockwise direction. The natural frequency can be calculated be considering the system as composed of two single mass systems where the shaft consist of two lengths l1 and l2 and their ends meet at the plane of zero motion, or node. The frequency of the two masses is the same. Since Kt = (GJ)/L, ω = J G = J G n l l 1 I1 2 I 2 Thus l = l = l − l 1 I1 2 I 2 I 2 ( 1 ) ∴ I l l = 2 1 + I1 I 2 (This gives the position of the node.) therefore J G I + I I + I ω = 1 2 = K 1 2 n l t I1 I 2 I1 I2 Disk 1 and 2 will oscillate with amplitudes A1 and A2, respectively. The node will be nearer the disk with the larger moment of inertia. Examples: (1) motor driving a generator (2) turbine driving a generator (3) motor driving a centrifugal pump (4) radial aircraft engine driving a propeller Stepped Shafts Substitute Kt into equation for ωn of two disks system. .

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