Page 33 - Improving Machinery Reliability

6 Improving Machinery Reliability

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On data sheet page 3 (Figure 1-3), the specifying engineer must determine
whether the critical speed projections made by the vendor are based on proven ana-
lytical techniques. The purchaser would be well advised to acquire, also, an under-
standing of rotor sensitivity. How serious will be the response to rotor unbalance
when there is midspan unbalance? What is the vibration response to coupling unbal-
ance? Should the user engage a consultant to perform an independent study and sub-
mit a formal report on the findings?
The reader may be interested to know that from 1960 to 1980, the hot topic was
when to use “at-speed” balancing. Fortunately, the increased availability and cost-
effectiveness of modern balancing machinery and vacuum bunkers facilitates today’s
reliability professional’s decision to use such methods. In recent times, a debate
began on how to define whether a given rotor was “rigid” or “flexible.” By IS0
Standard 1925, a rotor is rigid if:

1. It could be corrected in any two arbitrarily selected planes and
2. After that correction, its unbalance did not significantly exceed the balancing
tolerances-relative to the shaft axis-at any speed up to the maximum service
speed. These conditions approximated very closely the final supporting system.
In laymen’s terms, the rotor is rigid if its first lateral critical was above the maxi-
mum operating speed.
Thus, a rotor could be called rigid for one application (if it had a low service
speed and/or liberal balancing tolerance); whereas, for another operation-demand-
ing a higher speed and/or finer tolerance-it became flexible. By definition, a flexi-
ble rotor can operate above its first lateral critical speed.
Flexible rotors were primary candidates for high-speed balancing, assuming that
the purchaser would pay the extra cost. However, high-speed balancing can be cost-
effective. Assisted by Schenk-Trebel, a world-class manufacturer of balancing
machinery, major machinery-repair and manufacturing facilities are pursuing self-
sufficiency by acquiring at-speed balancing facilities. For example, Hickham Indus-
tries, Inc., LaPorte/Houston, Texas, began operating an “at-speed” facility in August
1996. Figure 1-4 illustrates the facility’s huge size.
Turbomachinery rotors are installed and removed by an overhead crane. This
method is safer, faster and enables more efficient floor space usage. The balancing
bunker is a vacuum chamber at one millibar of vacuum, with a pump-down time of
15 minutes. It accepts rotors up to 280 in. long by 96 in. diameter, and capacities up
to 50,000 Ibs. These rotors can be spun at 16,000 RPM; whereas, rotors in the 2,750
Ib.-league can be spun at 40,000 RPM.