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.
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