Fundamentals of Vibration

Mechanical vibration is a form of motion oscillation. it occurs in all forms of machinery and equipment. It is what you feel when you put your hand on the hood of a car, the engine of which is running, or on the base of an electric motor when the motor is running. Perhaps the simplest illustration of a mechanical vibration is a vertical spring with weight, as shown in Figure 1. In this position, the deflection of the spring from its free state is just sufficient to counterbalance the weight W. This deflection is called the static deflection of the spring. The position in which the spring is at rest is #1. The spring is then slowly extended to position #2, and released. The subsequent motion of the weight as a function of time, when there is negligible resistance to the motion, is wavy and repetitive as shown in the graph. It exhibits many of the basic characteristics of mechanical vibrations. The maximum displacement from the rest or mean position is called the AMPLITUDE of the vibration. The vibratory motion repeats itself at regular intervals (A1, A2, A3). The interval of time within which the motion sequence repeats itself is called a CYCLE or PERIOD. The number of cycles executed in a unit time (for example, during one second or during one minute), is known as the FREQUENCY. In a high-speed oscillation the frequency is high and conversely. When, as in Figure 1, the spring-weight system is not driven by an outside source, the vibration is a FREE VIBRATION and the frequency is called the NATURAL FREQUENCY of the system.