Illustrated Vibration Diagnostic Chart Sample
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PROBLEM SOURCE TYPICAL SPECTRUM PHASE RELATIONSHIP REMARKS

MASS UNBALANCE
A. FORCE UNBALANCE


Force Unbalance will be in-phase and steady. Amplitude due to unbalance will increase by the square of speed below first rotor critical (a 3X speed increase = 9X higher vibration). 1X RPM always present and normally dominates spectrum. Can be corrected by placement of only one balance correction weight in one plane at Rotor center of gravity (CG). Approx. 0 degree phase difference should exist between OB & IB horizontals, as well as between OB & IB verticals. Also, approx. 90 degrees phase difference between horizontal and vertical readings usually occurs on each bearing of unbalanced rotor (+30degrees).

B. COUPLE UNBALANCE

Couple Unbalance results in 180 degrees out-of-phase motion on same shaft. 1X RPM always present and normally dominates spectrum. Amplitude varies with square of increasing speed below first rotor critical speed. May cause high axial vibration as well as radial. Correction requires placement of balance weights in at least 2 planes. Note that approx. 180 degrees phase difference should exist between OB & IB horizontals, as well as between OB & IB verticals. Also, approx. a 90 degrees difference between the horizontal and vertical phase readings on each bearing usually occurs (+30 degrees).

MISALIGNMENT
A. Angular Misalignment


Angular Misalignment is characterized by high axial vibration, 180 degrees out-of-phase across the coupling. Typically will have high axial vibration with both 1X and 2X RPM. However, not unusual for either 1X, 2X or 3X to dominate. These symptoms may also indicate coupling problems as well. Severe angular misalignment may excite many 1X RPM harmonics. Unlike Mechanical Looseness Type 3, these multiple harmonics do not typically have a raised noise floor on the spectra.

B. Parallel Misalignment

Offset Misalignment has similar vibration symptoms to Angular, but shows high radial vibration, which approaches 180 degrees out-of-phase across coupling 2X often larger than 1X, but its height relative to 1X is often dictated by coupling type and construction. When either Angular or Radial Misalignment becomes severe, they can generate either high amplitude peaks at much higher harmonics (4x - 8X), or even a whole series of high frequency harmonics similar in appearance to mechanical looseness. Coupling type and material will often greatly influence the entire spectrum when misalignment is severe. Does not typically have raised noise floor.

MECHANICAL LOOSENESS





Note Raised Noise Floor Indicating Looseness




Mechanical Looseness is indicated by either Type A, B or C vibration spectra.
Type A is caused by Structural looseness/weakness of machine feet, base plate or foundation; also by deteriorated grouting, loose hold-down bolts at the base; and distortion o the frame or base (i.e., soft foot). Phase analysis may reveal approx. 90 degrees to 180 degrees phase difference between vertical measurements on bolt, machine foot, base plate or base itself.
Type B is generally caused by loose pillow block bolts, cracks in frame structure or in bearing pedestal.
Type C is normally generated by improper fit between component parts, which will cause many harmonics due to nonlinear response of loose parts to dynamic forces from rotor. Causes a truncation of time waveform and a raised noise floor in the spectrum. Type C is often caused by a bearing liner loose in its cap, a bearing loose and turning on its shaft, excessive clearance in either a sleeve or rolling element bearing, a loose impeller on a shaft, etc. Type C Phase is often unstable and may vary widely from one measurement to next, particularly if rotor shifts position on shaft from one startup to next. Mechanical Looseness is often highly direction and may cause very different readings when comparing levels at 30 degree increments in radial direction all the way around one bearing housing. Also, note that looseness will often cause sub harmonic multiples at exactly or 1/3X RPM (.5X, 1.5X, 2.5X, etc.).


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