High Speed Balancing

The at speed balance (high speed balance) facility was originally used in Europe and is a rather sophisticated and costly installation. Time on the machine is expensive, partly due to the owner having a large investment to amortize and partly because the machine requires more than just an operator.

The installation consists of a chamber large enough to house the rotor and bearing pedestals, with ample room for the technicians to add the trial weights and do the grinding necessary. The rotor assembly is moved into the chamber on tracks. There are other versions where the chamber opens from the top to permit an overhead crane to lower the rotor assem bly. There is a large vacuum pump to remove the air for the rotor runup The vacuum is necessary to reduce the windage and friction of the elements moving in an open area at operating speed. The horsepower to spin the rotor would be significant, and the heating in a closed chamber would prohibit such an operation unless completely evacuated. The drive must be variable-speed, and there must be a lubrication system to service the bearings of the rotor undergoing balance, drive equipment, and vacuum pumps. Normally the drive is a motor with a gear. To get a wider speed range, some facilities keep several gear sets for the different speed ranges. The rotor monitoring equipment is elaborate. It includes a computer to do the data logging and the calculations necessary to determine weight locations. To make a weight change, the vacuum on the chamber must be relieved. After the change, the chamber must be re-evacuated and the run repeated.

Why does anyone need this? Not all rotating equipment practitioners agree on the use of the at speed balance. There is one fear that high speed balancing will cover up a problem. Those in favor of the high speed balance argue that since most rotors operate above a bending critical or pos sibly above more than one bending critical, that balancing should take into account the rotor's mode shape at its operating speed. On the positive side, repaired rotors not given a mechanical ran test may be operated at rated speed. While an oblique pro, cocked components will sometimes straighten when run at speed.

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