Lecture 2 Free Vibration of Single Degree of Freedom Systems

1. Lecture 2Free Vibration of Single Degree of Freedom Systems ERT 452 VIBRATION MUNIRA MOHAMED NAZARI SCHOOL OF BIOPROCESS UNIVERSITI MALAYSIA PERLIS

2. COURSE OUTLINE • Free Vibration with Viscous Damping • Free Vibration with Coulomb Damping

3. Free Vibration with Viscous Damping

4. Free Vibration with Viscous Damping • Equation of Motion: where c = damping constant From the figure, Newton’s law yields the equation of motion: or

5. Free Vibration with Viscous Damping Assume a solution in the form: Hence, the characteristic equation is the roots of which are These roots give two solutions to Eq.(2.59)

6. Free Vibration with Viscous Damping Thus the general solution is: where C1 and C2 are arbitrary constants to be determined from the initial conditions of the system.

7. Free Vibration with Viscous Damping • Critical Damping Constant and Damping Ratio: The critical damping cc is defined as the value of the damping constant c for which the radical in Eq.(2.62) becomes zero: or The damping ratio ζ is defined as:

8. Free Vibration with Viscous Damping Thus the general solution for Eq.(2.64) is: Assuming that ζ≠ 0, consider the following 3 cases: Case1. Underdamped system For this condition, (ζ2-1) is negative and the roots are:

9. Free Vibration with Viscous Damping and the solution can be written in different forms: where (C’1,C’2), (X,Φ), and (X0, Φ0) are arbitrary constants to be determined from initial conditions.

10. Free Vibration with Viscous Damping For the initial conditions at t = 0, and hence the solution becomes Eq.(2.72) describes a damped harmonic motion. Its amplitude decreases exponentially with time, as shown in the figure below.

11. Free Vibration with Viscous Damping The frequency of damped vibration is: Underdamped Solution

12. Free Vibration with Viscous Damping Case2. Critically damped system In this case, the two roots are: Due to repeated roots, the solution of Eq.(2.59) is given by Application of initial conditions gives: Thus the solution becomes:

13. Free Vibration with Viscous Damping It can be seen that the motion represented by Eq.(2.80) is aperiodic (i.e., nonperiodic). Since , the motion will eventually diminish to zero, as indicated in the figure below. Comparison of motions with different types of damping

14. Free Vibration with Viscous Damping Case3. Overdamped system The roots are real and distinct and are given by: In this case, the solution Eq.(2.69) is given by: For the initial conditions at t = 0,

15. Free Vibration with Viscous Damping • Logarithmic Decrement: Using Eq.(2.70), The logarithmic decrement can be obtained from Eq.(2.84):

16. Free Vibration with Viscous Damping For small damping, Hence, or Thus, where m is an integer.

17. Free Vibration with Viscous Damping • Energy dissipated in Viscous Damping: In a viscously damped system, the rate of change of energy with time is given by: The energy dissipated in a complete cycle is:

18. Free Vibration with Viscous Damping Consider the system shown in the figure below. The total force resisting the motion is: If we assume simple harmonic motion: Thus, Eq.(2.95) becomes The energy dissipated in a complete cycle will be

19. Free Vibration with Viscous Damping Computing the fraction of the total energy of the vibrating system that is dissipated in each cycle of motion, where W is either the max potential energy or the max kinetic energy. The loss coefficient, defined as the ratio of the energy dissipated per radian and the total strain energy:

20. Free Vibration with Viscous Damping • Torsional systems with Viscous Damping: Consider a single degree of freedom torsional system with a viscous damper, as shown in figure (a). The viscous damping torque is given by: The equation of motion can be derived as: where J0 = mass moment of inertia of disc kt = spring constant of systemθ = angular displacement of disc

21. Free Vibration with Viscous Damping In the underdamped case, the frequency of damped vibration is given by: where and where ctc is the critical torsional damping constant

22. Example 2.11Shock Absorber for a Motorcycle An underdamped shock absorber is to be designed for a motorcycle of mass 200kg (shown in Fig.(a)). When the shock absorber is subjected to an initial vertical velocity due to a road bump, the resulting displacement-time curve is to be as indicated in Fig.(b). Find the necessary stiffness and damping constants of the shock absorber if the damped period of vibration is to be 2 s and the amplitude x1is to be reduced to one-fourth in one half cycle (i.e., x1.5 = x1/4). Also find the minimum initial velocity that leads to a maximum displacement of 250 mm.

23. Example 2.11Solution Approach: We use the equation for the logarithmic decrement in terms of the damping ratio, equation for the damped period of vibration, time corresponding to maximum displacement for an underdamped system, and envelope passing through the maximum points of an underdamped system.

24. Example 2.11Solution Since , Hence the logarithmic decrement becomes From which ζ can be found as 0.4037. The damped period of vibration given by 2 s. Hence,

25. Example 2.11Solution The critical damping constant can be obtained: Thus the damping constant is given by: and the stiffness by: The displacement of the mass will attain its max value at time t1, given by This gives: or

26. Example 2.11Solution The envelope passing through the max points is: Since x = 250mm, The velocity of mass can be obtained by differentiating the displacement: as When t = 0,

27. Free Vibration with Coulomb Damping

28. Free Vibration with Coulomb Damping • Coulomb’s law of dry friction states that, when two bodies are in contact, the force required to produce sliding is proportional to the normal force acting in the plane of contact. Thus, the friction force F is given by: where N is normal force,μ is the coefficient of sliding or kinetic frictionμ is usu 0.1 for lubricated metal, 0.3 for nonlubricated metal on metal, 1.0 for rubber on metal • Coulomb damping is sometimes called constant damping

29. Free Vibration with Coulomb Damping Consider a single degree of freedom system with dry friction as shown in Fig.(a) below. • Equation of Motion: Since friction force varies with the direction of velocity, we need to consider two cases as indicated in Fig.(b) and (c).

30. Free Vibration with Coulomb Damping Case 1. When x is positive and dx/dtis positive or when x is negative and dx/dt is positive, the equation of motion can be obtained using Newton’s second law (Fig.b): Hence, where ωn = √k/m is the frequency of vibration A1 & A2 are constants

31. Free Vibration with Coulomb Damping Case 2. When x is positive and dx/dt is negative or when x is negative and dx/dt is negative, the equation of motion can be derived from Fig. (c): The solution of the equation is given by: where A3 & A4 are constants

32. Free Vibration with Coulomb Damping Fig.2.34 Motion of the mass with Coulomb damping

33. Free Vibration with Coulomb Damping • Solution: Eqs.(2.107) & (2.109) can be expressed as a single equation using N = mg: where sgn(y) is called the sigum function, whose value is defined as 1 for y > 0, -1 for y< 0, and 0 for y = 0. Assuming initial conditions as

34. Free Vibration with Coulomb Damping The solution is valid for half the cycle only, i.e., for 0 ≤ t ≤ π/ωn. Hence, the solution becomes the initial conditions for the next half cycle. The procedure continued until the motion stops, i.e., when xn ≤ μN/k. Thus the number of half cycles (r) that elapse before the motion ceases is: That is,

35. Free Vibration with Coulomb Damping Note the following characteristics of a system with Coulomb damping: The equation of motion is nonlinear with Coulomb damping, while it is linear with viscous damping The natural frequency of the system is unaltered with the addition of Coulomb damping, while it is reduced with the addition of viscous damping. The motion is periodic with Coulomb damping, while it can be nonperiodic in a viscously damped (overdamped) system. The system comes to rest after some time with Coulomb damping, whereas the motion theoretically continues forever (perhaps with an infinitesimally small amplitude) with viscous damping.

36. Free Vibration with Coulomb Damping Note the following characteristics of a system with Coulomb damping: The amplitude reduces linearly with Coulomb damping, whereas it reduces exponentially with viscous damping. In each successive cycle, the amplitude of motion is reduced by the amount 4μN/k, so the amplitudes at the end of any two consecutive cycles are related: As amplitude is reduced by an amount 4μN/k in one cycle, the slope of the enveloping straight lines (shown dotted) in Fig 2.34.

37. Free Vibration with Coulomb Damping • Torsional Systems with Coulomb Damping: The equation governing the angular oscillations of the system is and The frequency of vibration is given by

38. Free Vibration with Coulomb Damping and the amplitude of motion at the end of the rth half cycle (θr) is given by: The motion ceases when

39. Example 2.14Pulley Subjected to Coulomb Damping A steel shaft of length 1 m and diameter 50 mm is fixed at one end and carries a pulley of mass moment of inertia 25 kg-m2 at the other end. A band brake exerts a constant frictional torque of 400 N-m around the circumference of the pulley. If the pulley is displaced by 6° and released, determine (1) the number of cycles before the pulley comes to rest and (2) the final settling position of the pulley.

40. Example 2.14Solution (1) The number of half cycles that elapse before the angular motion of the pullet ceases is: where θ0 = initial angular displacement = 6° = 0.10472 rad, kt = torsional spring constant of the shaft given by

41. Example 2.14Solution and T = constant friction torque applied to the pulley = 400 N-m. Eq.(E.1) gives Thus the motion ceases after six half cycles. (2) The angular displacement after six half cycles: