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This results in a significant response of the entire support system during balancing. Vibration amplitude or velocity is measured at the bearing supports to determine the amount of rotor unbalance. Different rotor types of the same weight will produce different vibrations. Thus, with a soft-bearing machine, the unbalance can only be determined after calibrating the balance machine. This is done by adding known unbalance weights and measuring the response, and it closely simulates what is done when field balancing. As a result, most soft-bearing machines are used in production applications where identical parts are balanced. Most portable balance machines also are a soft-bearing type, because these machines are generally lighter and more transportable than the stiffer hard-bearing balance machines.
The support system in a hard-bearing balance machine is very stiff, and balancing is done below the natural frequency of the combined rotor support system. In these machines, strain gauge transducers mounted on the supports are typically used to determine the unbalance force.
Since unbalance is a function of speed regardless of rotor size or configuration, these machines will generally accept a larger range of rotor sizes and weights than a soft-bearing machine without any need of re-calibration.
In the article, high-speed balancing is defined as "at-speed" balancing, or balancing at the design-rotor operating speed.
For turbomachinery rotors, high-speed balancing is normally done in a vacuum chamber to reduce the power when balancing, and to reduce heating due to windage. It is not necessary to high-speed balance rigid rotors since these rotors operate below the first bending critical. A flexible rotor operates above the first bending critical.
When a rotor speed approaches a bending critical, the rotor deflects or bends. This causes a change in the rotating centerline, which can change the rotor balance. Depending on such factors as damping, unbalance distribution, and other factors, the rotor can deflect in different ways. The typical undamped mode shapes of the first several bending critical speeds are shown in Figure 6.
Figure 6: Mode shapes for flexible shaft rotors on flexible supports
When high-speed balancing, it is important to perform a rotor dynamics analysis first to understand the mode shapes, in order to determine the appropriate balance planes. To reduce vibrations as the rotor goes through a critical speed and to reduce vibrations at operating speed, correction weights might need to be added near the location of maximum deflection.
For the two rotor examples discussed in the presentation, the first bending critical is at the same plane as the disc. However, if one of these rotors operated near the 2nd critical speed, the location of peak deflection is different. In this case, balance weights might also need to be added at these additional locations.
Thus, with high-speed balancing, it might be necessary to balance in additional balance planes to ensure low vibrations at all speeds.
High-speed balancing results in a better balance quality than low-speed balancing only, but it is time consuming and expensive. Is high-speed balancing really necessary?