CEMB USA N100 - Manual (Page 56)

C - 2
A rapid guide to interpreting a spectrum
7) f is much greater
than n, not an
exact multiple
Damaged roller bearings. Unstable frequency, intensity and phase. Axial
vibration.
Excessive wear on sleeve
bearings. Completely or locally defective lubrication.
Audible screech.
Belts too tight. Characteristic audible screech.
Multiple belts not
homogeneous. Run between the belts.
Low load gears. Teeth knock together because of insufficient load;
unstable vibration.
Rotors with blades for fluid
management (cavitation,
reflux, etc.). Unstable frequency and intensity.
f = n x number of blades x number of channels.
Frequent axial vibration.
8) f = natural frequency
of other parts
Excessive play on sleeve
bearings. Oil whip caused by vibrations in other parts.
Check with stroboscope.
Belts disturbed by vibrations
from other parts. Examples: eccentric or unbalanced pulleys,
misalignments, rotor unbalances.
9) f unstable with
knocking
Multiple belts not
homogeneous.
Belts with multiple joints.
Unstable intensity.
10) f = n
c
n ≠ n
c
( n
c
= critical speed of shaft)
Roller bearings.
For rotors above the 1 st critical speed.
(n
r
= mains frequency)
Electric motors, generators.
Harmonics also present.
12) f = f
c
< n or f = 2 f
c
Belt with defective elasticity
in one area. f
c
is the belt frequency.
f
c
= π D n / l (D = pulley diameter; l = belt length).
Considerable axial vibrations, more than 10% of the transverse vibration, may be caused
typically by:
- misalignment (more than 40%);
- shaft inflection, especially in electrical
motors;
- defective thrust bearings;
- elliptic eccentricity in the electric motor
rotor;
- forces deriving from tubing;
- distorted foundations;
- wear in stuffing box seals, etc.;
- rotor side rubbing;
- defective radial bearings;
- defective coupling;
- defective belts.