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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 14, Mental Approach

Section 1, Most Usual Reasons for Vibration Increase (for Rotors that Once Ran Smoothly)

1 x rpm Vibration Increase

Change in key length not properly compensated.

Balancing arbors, mandrels and other tooling may not be the same size as the final shaft size in use. (A mil or two can cause a large error.)

Wrong balancing method (e.g. single plane balance instead of two plane).

Rotor blade or tooth broken off.

The rotor may have shifted on the shaft slightly -- radially and/or cocked.

A rotor's individual part may have shifted slightly within the rotor.

One or more bearing housing or other hold-down bolts are loose, creating a resonant situation.

Fly ash or other pollutants accumulate on fan blades (or section of fly ash breaks loose).

Shifting loose particles, such as rust, scale or dirt collected in various rotor cavities.

Water condensation accumulated in rotor voids or hollow fan blade.

Radially, repositioned loose motor or generator pole piece.

Loose or defective motor rotor bars (revealed only under load).

Electrical short in motor or generator's armature or field.


2 x rpm, Sometimes 3 or 4 x rpm or Other Harmonic Vibration Increase

Coupling alignment changed. Or, the alignment itself did not actually change. Instead, the
coupling may be binding or seizing due to coupling gear teeth or grid wear.

Coupling lubricant may have degenerated. If vanepass frequency, check for a too low flow rate.

Coupling match marks (index marks) were assembled out-of-phase.

Higher Frequencies Vibration Increase

Defective or improperly lubricated rolling element bearings.

Gear teeth worn, damaged or improperly lubricated.

Increased displacement at 1 x rpm, may have caused gearmesh amplitude increase.


Slightly Under ½ x rpm or Exactly ¼ x, ? x, ½ x, ¾ x rpm Vibration Increase

Loose bearing or other looseness (exactly ½ x rpm).


Erratic, Random, Very Wide Frequency Range Vibration Increase and Noise

(Usually low amplitudes) -- cavitation developed.

Textbook Index


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