LINEAR VIBRATION (cont)

LINEAR VIBRATION (cont) - TM-55-1930-209-14P-9-4_144

TM-55-1930-209-14P-9-4 Water Purification Barges (NSN 1930-01-234-2165) Electric Power System Manual
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LINEAR VIBRATION TESTING AND ADJUSTING Figure 2 The total distance traveled by the weight, that is, from one peak to the opposite peak, is referred to as the peak-to-peak displacement. Figure 3 This measurement is usually expressed in mils, where one mil equals one thousandth of an inch (0.001 in). It can be used as a guide in judging vibration severity. Both average or root-mean-square (rms) are sometimes used to measure vibration. rms = 0.707 times peak of vibration. These readings are meaningful in theoretical discussions, but are of limited practical value. Another popular method used to determine the magnitude of vibration is to measure the vibration velocity. Note that the example is not only moving, but also changing direction. This means that the speed of the weight is also constantly changing. At its limit of motion, the speed of the weight is "0." As it passes through the neutral position, its speed or velocity is greatest. Figure 4 The velocity is an extremely important characteristic of vibration but, because of its changing nature, a single point has been chosen for measurement. This is the peak velocity and is normally expressed in inches per second peak. Vibration acceleration is another important characteristic of vibration. It is the rate of change of velocity. In the example, not that peak acceleration is at the extreme limit of travel where velocity is "0." As the velocity increases, the acceleration decreases until it reaches "0" at the neutral point. Acceleration is normally referred to in units of "g" (peak) where one "g" equals the force of gravity at the earth's surface. (980.665 cm/se, 386.09 in/s2, 32.174 ft/s2 Displacement, velocity, and acceleration a all used by vibration experts to diagnose particular problems. Displacement measurements tend to be a better indication of vibration under condition of dynamic stress and are, therefore, most commonly used. Caterpillar has standardized on displacement measurement. Velocity is a direct measure of vibration and, as such, provides the best overall indicator of machinery condition. It does not, however, reflect the affect of vibration on brittle material. The relationship between peak velocity and peak-to- peak displacement can be found by the following formula: 103

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