Among the many inspection, rework, and assembly processes involved in overhauling a turbine engine, one of the most critical and least understood is balancing.
Engines running well-balanced components operate more smoothly, quietly, more efficiently, and produce more power than those with excessive vibration. Considering the large number of components that stack up to assemble a turbine engine there are numerous opportunities for vibration to be induced. Many of these cannot be controlled. Fortunately, the unbalance in the main engine rotors can be reduced.
Engine component balancing is fairly straightforward, once the basics are understood and the correct equipment is used. Basically, the balancing process is a method of moving a rotor's mass center so that it coincides with the rotational center.
Engine rotating components are all bearing mounted on a shaft, thus they have a fixed rotational center. The mass center, however, is the centerline around which the rotor would spin if it were in free space, unrestricted by the journals. If these centers (mass and rotational) are not in the same place, the rotor still wants to rotate around the mass center, and will try to move there when spinning. This attempt to move toward the mass center shows up as vibration at the bearing journals, and is called unbalance.
Unbalance is typically measured in weight-radius units. For instance, if a given rotor is perfectly balanced, and a 1 gram weight is placed 1 inch (radially) from the centerline, the rotor will be unbalanced by 1 gram-inch. That same weight placed at a 2-inch radius will double the unbalance to 2 gram-inches. It is important to remember that the amount of material to be added or removed will decrease as the correction radius increases ? the same correction farther out on the part makes a bigger change in the unbalance.
There is a common misconception that the amount of unbalance in a rotor gets larger as the rotor spins faster. Generally this is not true. A rigid rotor that is out of balance by 1 gram-inch at 1,000 rpm will be out of balance by 1 gram-inch at 50,000 rpm. The force created by that 1 gram-inch unbalance will increase exponentially ? as the speed doubles, the force increases by a factor of four. But the absolute amount of unbalance does not change, as long as the rotor shape remains constant as speed increases. This means that balancing speed is generally not critical, as long as the correct balancing tolerance can be achieved. The rotor must spin fast enough to be stable, to load any loose blades, and to get adequate vibration for the balancing machine to read. Beyond that, it is mostly operator preference, safety, and production concerns that determine balancing speed.
In the design
When engine rotors are designed, the engineers realize that balancing must be done, so included in the designs are places on the rotors where balance corrections can be made. These balancing "planes" are specific areas on the rotor where material may either be removed or added to change the mass center of the part to bring it into balance. Material removal is generally made by grinding, sometimes drilling into the balancing plane to make a balance correction. Rotors designed for balancing by material addition typically have special weights and rivets or bolts that are attached in the appropriate areas on the part.
Designers will call out the allowable residual unbalance - the balance tolerance - in the overhaul manual. A specification such as ".001 ounce-inch," or ".1 gram-millimeter" will usually be noted in the balancing specifications. Overhaul manuals also typically show how balancing corrections should be made, and where they are allowable.
When taken down to its most basic elements, all balancing is simply a matter of ?where, and how much.? Where (at what angular location on the rotor) must a correction be made, and how large (or small) must that correction be. Any balancing machine and related tooling is simply a method of supporting and spinning the part so that it can tell the operator these two things - where and how much.
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.
In general, the balancing process is simply a series of steps to take "where and how much" from a machine readout and apply it to a physical rotor. Verifying the balancer's readings can be done at any time to assure proper performance - the balancing technician's version of "trust but verify."
The challenges encountered during balancing are usually due to rotor configuration or assembly, journal finish or condition, or a mechanical or electronic adjustment that is off on the machine.
Balancing is not a process that requires no thinking or judgment, but to a mechanic with some experience in how the rotors fit together and work, and some basic training in balance theory, the basics can be quickly learned. From that point, experience becomes the best teacher, and the balancing technician continues to strengthen the depth of his or her understanding and ability.