Page 19 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

directly measure force. Excessive force can lead to improper lubri-
cation in journal bearings, and result in failure. The dynamic force
created by the vibration of a rotating member can directly cause
bearing failure. Generally a machine can withstand up to eight
times its designed static load before bearing failure occurs. How-
ever, overloads as little as 10% can cause damage over an ex-
tended period of time. Although this seems insignificant, it can be
shown that small unbalances can easily create sufficient dynamic
forces to overload the bearings.

HOOKE’S LAW

Referring to Figure 1-7, a metal bar is
stretched a distance dL, by applying a weight
W.
Graphing the amount of stretch versus
the amount of applied weight produces a
stress-strain curve as shown in Figure 1-8. In
the elastic zone, if the weight is doubled, the
stretch is also doubled. When the weight is
removed, the bar returns to its original
length.
Hooke’s Law states that the amount of
stretch, or elongation, is proportional to the
applied force.
Stated as an equation: Figure 1-7. Bar
being Stretched
with a Weight
F = (constant)(dL) (1.2)

However, if the force applied is too large, the metal bar will
reach its yield point, and when the force is removed, the bar will
have a permanent elongation. The bar was stretched beyond its
elastic limit.
Figure 1-9 illustrates the permanent offset caused by apply-
ing a force beyond a material’s elastic limit. Note the yield point