tailieunhanh - VIBRATION AND SHOCK

In any structure or assembly, certain whole-body motions and certain deformations are more common than others; the most likely (easiest to excite) motions will occur at certain natural frequencies. Certain exciting or forcing frequencies may coincide with the natural frequencies (resonance) and give relatively severe vibration responses. We will now discuss the much-simplified system shown in Fig. | CHAPTER 23 VIBRATION AND SHOCK Wayne Tustin Equipment Reliability Institute Santa Barbara California VIBRATION ROTATIONAL IMBALANCE VIBRATION MEASUREMENT ACCELERATION MEASUREMENT 661 SHOCK MEASUREMENT AND ANALYSIS 692 668 SHOCK TESTING 695 673 SHAKE TESTS FOR ELECTRONIC ASSEMBLIES 705 681 VIBRATION In any structure or assembly certain whole-body motions and certain deformations are more common than others the most likely easiest to excite motions will occur at certain natural frequencies. Certain exciting or forcing frequencies may coincide with the natural frequencies resonance and give relatively severe vibration responses. We will now discuss the much-simplified system shown in Fig. . It includes a weight W it is technically preferred to use mass M here but weight IF is what people tend to think about a spring of stiffness K and a viscous damper of damping constant C. K is usually called the spring rate a static force of K newtons will statically deflect the spring by 3 mm so that spring length I becomes 8 I. In English units a force of K lb will statically deflect the spring by 1 in. This simplified system is constrained to just one motion vertical translation of the mass. Such single-degree-of-freedom SDF systems are not found in the real world but the dynamic behavior of many real systems approximate the behavior of SDF systems over small ranges of frequency. Suppose that we pull weight IV down a short distance further and then let it go. The system will oscillate with W moving up-and-down at natural frequency fN expressed in cycles per second cps or in hertz Hz this condition is called free vibration. Let us here ignore the effect of the damper which acts like the shock absorbers or dampers on your automobile s suspension using up vibratory energy so that oscillations die out. f may be calculated by V- 77 y w It is often convenient to relate fN to the static deflection 8 due to the force .