Page 292 - Machinery Component Maintenance

274 Machinery Component Maintenance and Repair

Example:

Machine specification limits Wn2 to 2400 - lo6 lb n2.
A given symmetric rotor weighs 1200 lb, and is to be balanced at 800
rpm. Its Wn2 value is:

Wn2 = 1200 80O2 = 768 10”

Therefore, the balancing speed of 800 rpm falls well within the capa-
bilities of the machine.
For nonsymmetrical load distribution between the supports, and for
outboard rotors, the following formula provides a fast approximation of
(a) the maximum permissible balancing speed in a soft-bearing machine,
and (b) the maximum balancing speed in a hard-bearing machine at
which permanent calibration in the A-B-C mode is maintained.

we = w [ L
D
+ ll2+
Where: We = Weight equivalent to be used in Wn2 formula, (lb).
W = Weight of rotor, (lb).
s = Distance from the rotor CG to the nearest support.
(If the CG is outboard of the supports, s is positive; if
the CG is inboard, s is negative.)
D = Distance between the supports.

Determining the Right Balancing Speed

The question is often asked whether a given rotor such as a crankshaft,
fan, roll or other rotating component should be balanced at its respective
service speed. The answer, in most cases, is no. The ncxt question, usu-
ally, is why not? Doesn’t unbalance increase with the square of the rota-
tional speed? The answer, again, is no. Only the centrifugal force that a
given unbalance creates increases proportionately to the square of the
speed, but the actual unbalance remains the same. In other words, an
ounce-inch of unbalance represents a one ounce unbalance mass with its
center-of-gravity located at a one inch radius from the shaft axis, no mat-
ter whether the part is at rest or rotating (see also earlier in this chapter
on “Units of Unbalance”).
What balancing speed should be used then? To answer that question,
consider the following requirements: