Page 312 - Machinery Component Maintenance

294 Machinery Component Maintenance and Repair
IS0 2953 suggests the solid roll-type rotors, with the largest one
weighing 1100 lb. For larger rotors (or even at the 1100 lb level) a dumb-
bell-type rotor may be more economical. This also depends on available
material and manufacturing facilities.
Critical are the roundness of the journals, their surface quality, radial
runout of the test mass mounting surfaces, and the axial and angular lo-
cation of the threaded holes which hold the test masses. For guidance in
determining machining tolerances, refer to the section on Test Masses.
Before using a proving rotor, it will have to be balanced as closely to
zero unbalance as possible. This can generally be done on the machine to
be tested, even if its calibration is in question. The first test (Umar Test)
will reveal if the machine has the capability to reach the specified mini-
mum achievable residual unbalance, the second test (UR Test) will prove
(or disprove) its calibration.
Whenever the rotor is reused at some future time, it should be checked
again for balance. Minor correction can be made by attaching balancing
clay or wax, since the rotor will probably change again due to aging,
temperature distortion or other factors. The magnitude of such changes
generally falls in the range of a few microinches displacement of CG,
and is not unusual.

Test Masses

Test masses are attached to a balanced proving rotor to provide a
known quantity of unbalance at a precisely defined location. The rotor is
then run in the balancing machine at a given speed and the unbalance
indication is observed. It should equal the unbalance value of the test
mass within a permissible plus/minus deviation.
Since the rotor with test masses functions as a gage in assessing the
accuracy of the machine indication, residual unbalance and location er-
rors in the test masses should be as small as possible. The test procedure
makes allowance for the residual unbalance in the proving rotor but not
for test mass errors. Therefore, the following parameters must be care-
fully controlled to minimize errors.

1. Weight of test mass
2. Distance of test mass mounting surface to proving rotor shaft axis
3. Distance of test mass center of gravity (CG) to mounting surface
4. Angular position of test mass
5. Axial position of test mass

Since all errors are vector quantities, they should be treated as was
done in the error analysis in the section on balancing arbors, Le., ad-