Page 333 - Intro Predictive Maintenance

324 An Introduction to Predictive Maintenance

most likely lost material and must be replaced. If imbalance occurs with a new belt,
it is defective and must be replaced. Figure 14–40 shows a spectral plot of shaft rota-
tional and belt defect (i.e., imbalance) frequencies.

Resonance. Belt resonance occurs primarily when the natural frequency of some
length of the belt is excited by a frequency generated by the drive. Occasionally, a
sheave may also be excited by some drive frequency. Figure 14–41 shows a spectral
plot of resonance excited by belt-defect frequency.

Adjusting the span length, belt thickness, and belt tension can control belt resonance.
Altering any of these parameters changes the resonance characteristics. In most appli-
cations, it is not practical to alter the shaft rotational speeds, which are also possible
sources of the excitation frequency.

Resonant belts are readily observable visually as excessive deflection, or belt whip. It
can occur in any resonant mode, so there may or may not be inflection points observed
along the span. Figure 14–42 illustrates first-, second-, and third-mode resonance in
a belt span.

Tension. Loose belts can increase the vibration of the drive, often in the axial plane.
In the case of multiple V-belt drives, mismatched belts also aggravate this condition.
Improper sheave alignment can also compromise tension in multiple-belt drives.

Wear. Worn belts slip, and the primary indication is speed change. If the speed of the
driver increases and the speed of the driven unit decreases, then slippage is probably
occurring. This condition may be accompanied by noise and smoke, causing belts to
overheat and be glazed in appearance. It is important to replace worn belts.