Balancing Act

Among the many inspection, rework, and assembly processes involved in overhauling a turbine engine, one of the most critical and least understood is balancing.

The balancing process

Rotors to be balanced should be as fully assembled as possible, as mounting additional rotating parts after balancing will affect the balance of the rotor. They must be supported by their bearing journals, and any nonrotating components (nozzles, shrouds) must be properly supported to avoid interference. Once correctly mounted in the balancing machine, the machine is adjusted so that its electronics paints a ?picture? of the rotor - where the balancing planes are, what the tolerance is, how unbalance is to be corrected, etc. This prepares the machine to give the operator meaningful information - where and how much.

Once initial unbalance readings are obtained, it is advisable for the machine operator to evaluate the size of the corrections needed. An unusually large initial unbalance may be an indication of another problem - excessive runout, improper alignment, faulty components, etc. A balancing machine operator who understands how the entire system works can be a very effective inspector for the entire assembly process.

When it has been determined "where and how much" must be added or removed, the operator begins making corrections to the rotor. Most engine rotors are two plane balanced, meaning that there are two distinct areas on the rotor where corrections must be made to properly balance the part. A rotor with one plane balanced and one still out is not a good rotor.

The actual balancing process is iterative - rarely is one set of corrections made on a rotor to completely balance it. Generally, the initial corrections are larger, to get the rotor unbalance closer to tolerance, then smaller and smaller amounts are removed as the operator "sneaks up" on the tolerance. Final corrections may be extremely small.

Adding and removing weight

When balancing by adding material, the process is fairly simple. The operator may, if so desired, weigh rivets or other weights and select one that corresponds to the required weight at the balancing radius. Weights may be temporarily placed on the rotor, and the part re-spun for an inspection run. Adjustments to weight size and/or position may be made to further reduce the unbalance.

When material is removed to balance rotors, it is slightly more complex. It is difficult to guess the amount of material removed by a grinder, and once removed the material cannot be replaced. Additional caution must be exercised to avoid over-correcting or removing material from the wrong location. Small corrections are usually made until the operator gets a "feel" for what the rotor needs.

Anyone who has balanced a rotor has encountered a condition called "chasing." This is where the location (angle) of the unbalance shifts after a correction has been made. After a second correction the location shifts further in the same direction, and so forth. Eventually, the operator can wind up correcting halfway around the part and questioning his or her sanity. The cause of this is fairly straightforward.

All balancing machines indicate the point of unbalance related to some reference mark. If the balancer is slightly misadjusted, it will give an angle readout that is slightly skewed from the actual point of unbalance. The operator, with absolute faith in his equipment, corrects in the indicated place, and the balancer (and rotor) responds by shifting the location. In this case, the operator must check and verify that the angle reader on the balancer is properly set. This is a simple adjustment, and is part of the machine setup, but it often gets overlooked.

Once a rotor has been corrected so that the balancing machine is indicating residual unbalance levels within the manufacturer's tolerance, the rotor is balanced. Additional balancing to even lower levels is not required, though many engine builders report better performance and lower test cell vibrations on engines where they have balanced the rotors closer than specified in the overhaul manual.

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