Page 313 - Machinery Component Maintenance

Balancing of Machinery Components 295

justed by the RSS method. The resulting probable maximum error should
ideally not use up more than one tenth of the reciprocal of the specified
Unbalance Reduction Ratio factor. For example, if a URR of 95 percent
is to be proven, the total test mass error from parameters 1 to 5 should
not exceed 0.1 - 5 percent = 0.5 percent of the test mass weight.
Often test masses need to be so small that they become difficult to han-
dle. It is then quite common to work with differential test masses, i.e.
two masses 180” opposite each other in the same transverse plane. The
effective test mass is the difference between the two masses, called the
“differential unbalance.” For instance, if one mass weighs 10 grams and
the other 9, the difference of 1 gram represents the differential unbal-
ance.
When working with differential test masses, the errors of the two com-
paratively large masses affect the accuracy of the differential unbalance
in an exaggerated way. In the example used above, each differential test
mass would have to be accurate within approximately .025 percent of its
own value to keep the maximum possible effect on the differential unbal-
ance to within 2 - 0.25 percent = 0.5 percent. In other words, if the op-
posed masses are about ten times as large as their difference, each mass
must be ten times more accurate than the accuracy required for the differ-
ence.

Test Procedures

To test the performance of a balancing machine, IS0 2953 prescribes
two separate tests, the U,,, Test and the Unbalance Reduction Test. The
origin and philosophy behind these tests and their purpose were ex-
plained. Here are the actual test procedures:

U,,, (or Traverse) Test
I. Perform the mechanical adjustment, calibration and/or setting of
the machine for the particular proving rotor being used for the test,
ensuring that the unbalance in the rotor is smaller than five times
the claimed minimum achievable residual unbalance for the ma-
chine.
2. Put 10 to 20 times the claimed minimum achievable residual unbal-
ance on the rotor by adding two unbalance masses (such as balanc-
ing clay). These masses shall not be:
in the same transverse plane
in a test plane
at the same angle
displaced by 180”