May 17, 2012
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William Crosher: Tooth Tips
Proper balance is one of the critical keys in designing and manufacturing high-speed gear units. This installment discusses how this can be achieved.
Unbalance is the most common cause of serious vibration in high-speed gear units. When present the vibration results in bearing, tooth, and seal damage. Unfortunately there is no practical way to relate the balance of the gears with the vibration. Even a perfectly machined solid disc can have an unbalanced rotation due to the non-homogeneity of the material or eccentricity because of clearances between the shaft and gear. The material problem causes a shift from the geometric center. The shaft clearance moves the geometric center away from the rotational axis. Many of these state of the art drives are used for power generation, and a series of tests in the 1990s proved conclusively that unbalance and the resulting vibration could result in significant energy losses. Losses from 15-25 percent were recorded. When there are heavy spots at opposite ends of the gear, and the mass centerline intersects the rotating centerline, the unbalance can only be detected by spinning the gear and measuring the resultant unbalance forces or the vibration. All gears and shafts should be balanced when used for high-speed operation. Balancing involves both high and low speed. Low-speed balancing ensures that neither the machine nor the gears are affected by vibration during the high-speed balancing process. Typically, low-speed balancing takes place between 100 and 1,000 rpm. The stiffness of the high-speed balancing machine may differ from the field installation, and in turn affect the critical speeds that were observed. High-speed balancing gives superior results and is a requirement for many drives, but it is time consuming and expensive.
Several standards have been produced on the subject of balancing. Principally, in the gear field, the use of API standards had been predominant, such as API CRE Subcommittee of Mechanical Equipment (Rev. 22 Nov. 1999, API-613 and 617). API 613-Feb. 2003 requires all gear wheels to be multi-plane dynamically balanced. When there is a single keyway it is to be filled with a fully crowned half-key. The weight of all half-keys used is recorded. The “half-key convention” is fully described in ISO 8821. API 617 results in a residual unbalance of < 10 percent of the rotor weight. The maximum residual unbalance is calculated from four times the journal static loading in pounds times the maximum continuous rpm. API-684 is a “Tutorial on the API Standard Paragraphs Containing Rotor Dynamics and Balancing.” For the most part these standards are based on practical field experience. ISO-1940-1 defines balance quality and determination of residual unbalance, and ISO 13691 is for High-Speed Special Purpose Gear Units. Both standards are less conservative than API. ISO 11342 second editions includes methods and criteria for the mechanical balancing of flexible rotors. The U.S. Navy MIL-STD-167 was developed to make it more difficult for sonar operators to detect machinery vibrations, and thereby track naval vessels. The calculations are based on rotors operating above 1,000 rpm. Lloyd’s Register for the Classification of Ships similarly requires balancing for all gears running in excess of 1,000 rpm with minor exceptions. AGMA standard 6011-1/03 for high speed helical gear units with one stage at speeds > 4,000 rpm requires all gear rotating elements to be multi-plane dynamically balanced and does not deviate from similar standard requirements. Three separate unbalanced tolerance charts have been established for commercial gear sets, military gear drives, and precision drive systems.
The balancing machine must have been recently calibrated within the accuracy tolerances set by its manufacturer. Typically the balancing machines are classified as soft or hard bearing. In the soft bearing machine the rotor is mounted on a flexible support system. The natural frequency when combined with the rotor is below the balancing speed. The machine must also be sensitive enough to provide reliable and repeatable unbalance data. Vibration amplitude or velocity is measured at the bearing supports. The hard bearing machines generally take larger and heavier rotors. Because the support system is rigid, strain gage transducers are used to measure the unbalance force. ISO 2953 is a standard for the calibration and performance of balancing machines.
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is former director of the National Conference on Power Transmission, as well as former chairman of the AGMA's Marketing Council and Enclosed Drive Committee. He was resident engineer-North America for Thyssen Gear Works, and later at Flender Graffenstaden. He is author of the book Design and Application of the Worm Gear.