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

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

LINEAR VIBRATION (cont) - TM-55-1930-209-14P-9-4_146

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 Misalignment or Unbalance Most linear vibrations of generator sets are caused by misalignment or unbalance of the rotating members. This results in first order vibration which can be corrected in the field. The vibration motion is relatively constant over the speed range but exceeds accepted limits. For generator set applications this may be determined by operating between 45 Hz to 65 Hz. Load does not change vibration motion caused by unbalance. Figure 5 Resonance Large vibration motion (amplitude) occurs within a narrow speed range. This can occur on the generator set and/or the attached equipment, such as piping, air cleaners, etc. When vibrations "peak out" in a narrow speed range, the vibrating component is in resonance. Resonance occurs when the disturbing force in the engine has the same frequency as the natural frequency of the part which is vibrating. The force of the vibration is, therefore, added to each cycle of the vibration part causing magnification of the original vibration. Figure 6 Torque Reaction The vibration motion increases as load is applied. With a two-bearing generator, it can be caused by insecure mounting of the engine or generator to its base and/or by a base not sufficiently rigid to withstand the associated forces. Figure 7 Torque reaction problems are not found with close- coupled two-bearing or single-bearing generators. The rigid joint between the flywheel housing and the generator body is generally adequate to withstand torque. Assuming that a two-bearing generator is mounted on a weak base, first order motion and orders related to the number of cylinders firing with one crankshaft revolution are due to torque reaction which, in turn, causes misalignment. In this case, the first order motion would be most prevalent. A few engines do not follow the cylinder firing rule. These are the D346 and 0379 with second order and the D349 and 0399 with fourth order, in addition to the first order motion. Certain specific causes of linear vibration can be further identified: Vibration Frequency Correctable Causes 1/2 x Engine rpm Misfiring of One or (1/2 Order) More Cylinders 1 x Engine rpm Unbalance, Misalignment (1st Order) Out of Time Balance Weights (3408), Crank- case Overfill 2 x Engine rpm Unbalance, Out of Time (2nd Order) Balance Weights, Rotating Twice Engine Speed (3304, D346, D379 1-1/2, 2-1/2, 3rd Normal Cylinder Combus- and Higher Orders tion (Not Correctable) 105

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