Page 112 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

A bent shaft exhibits vibration characteristics very much like
those of misalignment. However, it is easily detected when align-
ment attempts fail to reduce the vibration amplitude.
Misalignment results in two primary forces, one in the radial
direction, and the other in the axial direction. This is true even
with flexible couplings where the misalignment is within the tol-
erances of the coupling.
As with unbalance, the amplitude of the vibration will in-
crease as the problem is worsens. Therefore, the more misalign-
ment, the more vibration. The key to detection of misalignment or
a bent shaft is the high axial vibration.
In general, the vibration is at 1 × rpm, but when the misalign-
ment is severe, second order (2 × rpm) and even third order (3 ×
rpm) vibrations can occur.

MISALIGNMENT IN BELT DRIVES

Misalignment of sheaves sprockets and V-belt drives also
produces these vibration characteristics. This type of misalign-
ment results in severe wear to chains, V-belts, sprockets, and
sheaves. Misalignment in V-belt drives that employ multiple belts
can create a host of other problems as well. Misalignment will
alter the tension of the different belts, so some may slip and others
may be overloaded. This can obviously lead to premature belt
failures.
Generally, these belts come as a set and thus their replace-
ment cost is normally high. Multi-belt systems should always be
replaced as a set, since the individual belts are matched to each
other at the manufacturer. In addition, the cost of down time and
labor to change a single belt vs. replacing the entire set is usually
nil. However, if only a single belt were replaced and another belt
in the set were to fail in a few hours of operation, these costs
would far offset the price of the set of belts.
One method of detection, is to draw a chalk mark across the
set of belts when the unit is off. Using a strobe light, the frequency