Page 294 - Machinery Component Maintenance

276 Machinery Component Maintenance and Repair

Examples:

A gas turbine compressor assembly, consisting of a series of
bladed disks which can all be balanced individually prior to rotor
assembly. Considerable effort has been made by the turbine de-
signers to provide for accurate component balancing so that stan-
dard (low speed) balancing machines can be employed in produc-
tion and overhaul of these sophisticated rotor assemblies.
A turbine rotor with flexible or unstable mass components, such
as governors or loose blades. To obtain, at low balancing speed, a
position of governor or blades which most nearly approximates
their position at the much higher service speed, it may be neces-
sary to block the governor or “stake” the blades.
A large diesel crankshaft normally rotating in five or even seven
journals. When running such a shaft on only two journals in a
balancing machine, the shaft may bend from centrifugal forces
caused by large counterweights and thus register a large (errone-
ous) unbalance. To avoid these difficulties, the balancing speed
must be extremely low and/or the shaft must be supported in the
balancing machine on a rigid cradle with three, five, or even
seven precisely aligned bearings.
Rotors which can not be satisfactorily balanced at low speed, re-
quire special high-speed or “modal” balancing techniques, since
they must be corrected in several planes at or near their critical
speed(s) .2

Flexibility Test

This test serves to determine if a rotor may be considered rigid for bal-
ancing purposes, or if it must be treated as a flexible rotor. The test is
carried out at service speed either in the rotor service bearings or in a
high-speed, hard-bearing balancing machine.
The rotor should first be balanced fairly well at low speed. Then one
test mass is added at the same angular position in each end plane of the
rotor near its journals. During a subsequent test run, vibration is mea-
sured on both bearings. Next, the rotor is stopped and the test masses are
moved to the center of the rotor, or where they are expected to cause the
largest rotor distortion. In a second run the vibration is again measured at
the bearings. If the total of the first readings is designated A, and the total
of the second readings B, then the ratio of (B-A)/A should not exceed
0.2. Experience has shown that, if the ratio stays below 0.2, the rotor can