Page 293 - Machinery Component Maintenance

Balancing of Machinery Components 275

1. The balancing speed should be as low as possible to decrease cycle
time, horsepower requirement, wind, noise, and danger to the oper-
ator.
2. It should be high enough so that the balancing machine has suffi-
cient sensitivity to achieve the required balance tolerance with ease.

However, there is one other important consideration to be made before
deciding upon a balancing speed substantially lower than the rotor’s ser-
vice speed; namely, is the part (or assembly) rigid?
Is the Rotor “Rigid”?

Theoretically it is not, since no workpiece is infinitely rigid. However,
for balancing purposes there is another way of looking at it. (See defini-
tion of “Rigid Rotor” in Appendix 6A.)
Any rotor satisfying this definition can be balanced on standard balanc-
ing machines at a speed which is normally well below the service speed.
When selecting the balancing speed, consider the following guidelines:
1. Determine the proper balance tolerance by consulting Table 6-5 and
subsequent nomograms.
2. Select the lowest available speed at which the balancing machine
provides at least 1/4 in. amount-of-unbalance indicator deflection or
5 digital units of indication for the required balance tolerance. It is
usually of no advantage to select a higher speed for achieving
greater sensitivity, since the repeatability of a good quality balanc-
ing machine is well in line with today’s exacting balance tolerances.
Whether a given rotor can be termed “rigid” as defined in Appendix
6A depends on numerous factors that should be carefully evaluated. For
instance:
1. Rotor configuration and service speed. Technical literature pro-
vides reference tables which permit approximating the critical
speed of the first flexural mode from the significant geometric rotor
parameters (see Appendix 6D). In most cases it can be assumed that
a rotor can be balanced successfully at low speed if its service speed
is less than 50 percent of the computed first flexural critical speed.
2. Rotor design and manufacturing procedures. Rotors which are
known to be flexible or unstable may, nevertheless be balanced sat-
isfactorily at low speed if certain precautions are taken. Rotors of
this type are classified as “quasi-rigid rotors.”