Page 326 - Machinery Component Maintenance

308 Machinery Component Maintenance and Repair

Figure 6-36. Inboard and outboard rotors with narrowly spaced correction planes.

Experimental Determination of Tolerances
For reasons of rotor type, economy, service life, environment or oth-
ers, the recommended tolerances may not apply. A suitable tolerance
may then be determined by experimental methods. For instance, a sample
rotor is balanced to the smallest achievable residual unbalance. Test
masses of increasing magnitude are then successively applied, with the
rotor undergoing a test run under service conditions before each test
mass is applied. The procedure is repeated until the test mass has a no-
ticeable influence on the vibration, noise level, or performance of the
machine. In the case of a two-plane rotor, the effects of applying test
masses as static or couple unbalance must also be investigated. From the
observations made, a permissible residual unbalance can then be speci-
fied, making sure it allows for differences between rotors of the same
type, and for changes that may come about during sustained service.

Applying Tolerances to Rotor Assembly Components

If individual components of a rotor assembly are to be pre-balanced (on
arbors for instance), the tolerance for the entire assembly is usually dis-
tributed among the components on the basis of the weight that each com-
ponent contributes to the total assembly weight. However, allowance
must be made for additional unbalance being caused by fit tolerances and
mounting surface runouts. To take all these into account, an error analy-
sis should be made.

Testing a Rotor for Tolerance Compliance

If the characteristics of the available balancing equipment do not per-
mit an unbalance equivalent to the specified balance tolerance to be mea-
sured with sufficient accuracy (ideally within * 10 percent of value), the