LINEAR VIBRATION

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 SUBJECT: Linear Vibration BUSINESS: All PRODUCT/APPLICATION: Generator Sets The purpose of this Engine Data Sheet is to: 1. Review the basic theory and nomenclature of vibration. A knowledge of the language of this discipline will encourage a fundamental understanding. Such terms as linear, period, displacement, velocity, and acceleration will be defined and compared. 2. Identify causes of engine-related vibration. Major contributors to engine vibration are described, as well as procedures to isolate and measure vibrations. 3. Provide possible corrective action. Excessive vibratory motion may be encountered in the engine, generator, related components, or any part of the mounting system. If left unresolved this motion may cause personnel discomfort and/or equipment damage. Balance techniques are discussed, and limits or vibratory forces are defined. FUNDAMENTALS Any mechanical system which possesses mass and elasticity is capable of relative motion. If this motion repeats itself after a given time period, it is known as vibration. A simple illustration of vibration is to suspend a weight (mass) on the end of a spring (elasticity). As long as no external force is imposed on the system, the weight remains at rest -- there is no vibration. But when the weight is moved or displaced and then released, vibration occurs. The weight will continue to travel up and down through its original Figure 1 position until frictional forces again cause it to rest. This specific illustration is an example of free vibration. If an external force continues to affect the system while it is vibrating, it is termed "forced vibration." The illustration is also an example of linear vibration. Linear vibration is an "in-line" motion and occurs as an object moves and returns along a repeatable path. Linear vibration must not be confused with the torsional (twisting) vibration motion of the crankshaft and driven rotating shafts (refer to EDS 202.1 for the subject of torsional vibration). The time required for the weight to complete one complete cycle is called a period. If the weight needed one second to complete a full cycle, the vibration frequency of this system would be one cycle per second. A system that completed its full motion 20 times in one minute would have a frequency of 20 cycles per minute, or 20 cpm. 102

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