Page 296 - Intro Predictive Maintenance

Failure-Mode Analysis 287

Table 14–1 Vibration Troubleshooting Chart

Frequency of
Dominant Vibration
Nature of Fault (Hz = rpm. 60) Direction Remarks
Rotating Members 1 ¥ rpm Radial A common cause of excess vibration in
Out of Balance machinery
Misalignment & Usually 1 ¥ rpm Radial A common fault
Bent Shaft Often 2 ¥ rpm &
Sometimes 3 & 4 ¥ rpm Axial
Damaged Rolling Impact rates for Radial Uneven vibration levels, often with
Element Bearings the individual & shocks. °Impact-Rates:
(Ball, Roller, etc.) bearing components° Axial Concoct Angle Impact Rates 1 (Hz)
Also vibrations at Ball Die For Outer Race Detect 1(Hz) = n1 11 – DD Con l1
•
2
PD
(BD)
very high frequencies Pitch For Inner Race Detect 1(Hz) = n1 11 – DD Con l1
•
2
PD
Die 2
DD 2
(20 to 60kHz) (PD) For Ball Detect 1(Hz) = n1 (1 – ) Con l
•
2
PD
n = number of balls or rollors
l n = rotating rpm./s between
inner & outer races repsor
Journal Bearings Sub-harmonics of Primarily Looseness may only develop at operating
Loose in Housings shaft rpm, exactly Radial speed and temperature (e.g.,
1/2 or 1/3 ¥ rpm turbomachines)
Oil Film Whirl or Slightly less than Primarily Applicable to high-speed (e.g., turbo)
Whip in Journal half shaft speed Radial machines
Bearings (42% to 48%)
Hysteresis Whirl Shaft critical speed Primarily Vibrations excited when passing through
Radial critical shaft speed are maintained at
higher shaft speeds. Can sometimes be
cured by checking tightness of rotor
components
Damaged or Worn Tooth meshing Radial Sidebands around tooth meshing
Gears frequencies (shaft rpm & frequencies indicate modulation (e.g.,
¥ number of teeth) Axial eccentricity) at frequency corresponding to
and harmonics sideband spacings. Normally only
detectable with very narrow-band analysis
Mechanical 2 ¥ rpm
Looseness
Faulty Belt Drive 1, 2, 3 & 4 ¥ rpm Radial
of belt
Unbalanced 1 ¥ rpm and/or Primarily
Reciprocating multiples for higher Radial
Forces order unbalance
and Couples
Increased Blade & Vane Radial Increasing levels indicate increasing
Turbulence passing frequencies & turbulence
and harmonics Axial
Electrically 1 ¥ rpm or 1 or 2 Radial Should disappear when turning off the
Induced Vibrations times sychronous & power
frequency Axial