CEMB USA N500 - Manual (Page 81)

A rapid guide to interpreting a spectrum
C
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Appendix C
A rapid guide to interpreting a spectrum
TYPICAL CASES OF MACHINE VIBRATIONS
1. PRELIMINARY RAPID GUIDE
Measured values during control:
f = vibration frequency [cycles/min] or [Hz]
s = shift amplitude [µm]
v = vibration speed [mm/s]
a = vibration acceleration [g]
n = piece rotation speed [rpm]
Frequency data Causes Notes
1) f = n Unbalances in rotating
bodies.
Rotor inflection.
Resonance in rotating bodies.
Intensity proportional to unbalance, mainly in the radial
direction, increases with speed.
Axial vibrations sometimes sensitive.
Critical speed near n with very high intensity.
Roller bearings mounted with
eccentricity.
Misalignments.
Recommend balancing the rotor on its own bearings.
Considerable axial vibration also present, greater than
50% of the transverse vibration; also frequent cases of
f = 2n, 3n.
Eccentricity in pulleys, gears,
etc. When the rotation axis does not coincide with the
geometric axis.
Irregular magnetic field in
electrical machines. Vibration disappears when power is cut off.
Belt length an exact multiple
of the pulley circumference. Stroboscope can be used to block belts and pulleys at
the same time.
Gear with defective tooth. An unbalance vibration often also intervenes.
Alternating forces Second and third harmonic present
2) f ≅ n with knocking Mechanical unbalance defect
superimposed on irregular
magnetic field. In asynchronous motors, the knocking is due to
running.
3) f ≅ (0,40 ÷ 0,45) n Defective lubrication in
sleeve bearings. For high n, above the 1° critical level.
Check with stroboscope.
Precision journal movement (oil whirl).
Faulty roller bearing cage. Check for harmonics
4) f = ½ n Mechanical weakness in
rotor.
Sleeve bearing shells loose.
Mechanical yield.
This is a sub-harmonic, often present but hardly ever
important.
f = 2n, 3n, 4n and semi-harmonics also often present.
5) f = 2n Misalignment.
Mechanical looseness.
There is strong axial vibration.
Loose bolts, excessive play in the mobile parts and
bearings, cracks and breaks in the structure: there are
upper grade sub-harmonics.
6) f is an exact multiple
of n
Roller bearings misaligned or
forced in their housings.
Defective gears. Frequency = n x number of spheres or rollers.
Check with stroboscope.
f = z n (z = number of defective teeth).
Because of general wear, teeth badly made if z = total
number of teeth.
Misalignments with excessive
axial play. Often caused by mechanical looseness.
Rotors with blades (pumps,
fans). f = n x number of blades (or channels)