Balance is essential in achieving optimum performance and efficiency for nearly any part that rotates or spins. Unbalance produces a variety of negative effects, including vibration. Vibration-induced shaking of a part wastes energy, increases noise, boosts stresses on associated components, and accelerates bearing wear. Eventually, vibration can degrade an entire machine and significantly raise maintenance and replacement costs.
Unbalance can result from factors such as manufacturing variations, material inconsistencies, and asymmetrical design of the part. It can be corrected on a balancing machine. The machine spins the part while sensors measure the part’s speed, its phase, and the forces caused by disproportionally distributed mass. Force data is converted to a measurement of the location of the unbalance in one or more defined radial planes. After machine balancing, the part can then be manipulated (mass added or removed) and retested on the machine until satisfactory balance tolerance is achieved.
Balance tolerances for aerospace gas turbine engines are shrinking in response to the development of quieter and more fuel-efficient powerplants such as the new Pure Power from Pratt & Whitney, the Leap series from GE, and Trent XWB from Rolls-Royce.
New, smaller engines designed for regional jets feature tighter manufacturing tolerances that shrink clearances and magnify the effect of unbalance-generated vibration. They have smaller, lighter, faster-spinning components. And because the centrifugal force resulting from unbalance increases with the square of speed, unbalances that formerly were considered minor become significant.
The tolerance eccentricities (displacement of the mass axis from the spin axis) of legacy engines were often measured in thousandths of an inch; in comparison, many of the newer-generation engine components have eccentricities measured in millionths of an inch. For example, tolerances for a new series of integrally bladed compressor rotors from one of the industry's signature engine programs equate to an eccentricity of 0.000112 in; the maximum allowable distance between the rotor's ideal spin axis and its mass axis is 0.000112 in or less.
State-of-the-art balancing machines are capable of measuring eccentricities in millionths of an inch. A typical aerospace-industry-standard requirement is that the machine be capable of an unbalance reduction ratio (URR) of at least 95%. This means that, provided the operator makes the balance correction specified by the machine, the unbalance detected upon a subsequent run will represent a reduction of at least 95% from the prior measurement.
Considering the URR specifications, it theoretically should take only a few runs before the tolerance is achieved. However, an operator often will find that the process reaches a point where the machine is no longer responding as expected, almost as if the axis of rotation were changing randomly. The closer the operator gets to the tolerance, the more erratic the problem becomes, forcing the operator to chase the unbalance. Like play on a slot machine, the part is run over and over with the hope that one of the corrections will provide the desired result. This hit-or-miss approach, in addition to being questionably accurate, greatly increases engine repair and maintenance turnaround time.
A variety of factors can cause unexpected shifts in a part’s spin axis. There can be inaccuracies in the machine’s sensing systems, the tolerances being pursued might be so small that they exceed the machine’s sensitivity, the operator’s corrections may be in error, part fixturing can be incapable of repeating the required axis, or the components of the rotor might not repeat well. Machine or tooling wear or misadjustment also can cause results to vary.
An often-overlooked source of poor repeatability is the fixture or tooling used to mount the part on the machine. Tooling first of all must hold the part securely and prevent it from shifting from run to run. Then, if adding or subtracting mass to the part requires removing it from the machine, it is essential that the part be replaced in exactly the same position as when it was removed.
Repeatability is everything; the balancing process can be no more accurate than the tooling’s ability to define the spin axis. Otherwise, the process is an effort to hit a moving target.
While the manufacturing and assembly tolerances for tooling used with legacy engines were measured in thousandths of inches, tooling capable of meeting the demands of today’s cutting-edge engines must be built to repeat to less than 20% of the balancing tolerance. In accordance with standard metrology practices, engine manufacturers require that tooling repeat to a minimum of 4X tighter than the balance tolerance. So, in the case of the previously mentioned integrally bladed compressor rotors with a maximum eccentricity allowance of 0.000112 in, tooling must repeat to 0.000028 in, or about three-quarters of a micron.
As allowable eccentricities and repeatability standards contract, a need arises for balancing tooling that is custom-engineered and -manufactured to hold specific parts. Vibration Solutions North, for example, specializes in designing and building balancing tooling and total tooling packages focused on specific parts and assemblies.
In addition to providing tooling intended to maximize part-holding security, the company also addresses the issue of repeatability with a tooling-mounting system called Kin-Dex. The system uses patented kinematic location principles to ensure repeatability of 0.00020 to 0.00040 in and also streamlines the mounting/dismounting process to save time in the balancing process.
George Allen, Vice President and Director, Balance Tooling Products and Services, Vibration Solutions North (and Chairman of SAE Balance Committee EG1-A), wrote this article for Aerospace Engineering.