Shop Balance Machine

This is where most balancing of rotors takes place. The machines range from heavy, direct-driven to the light, belt-driven type. There are numerous shop-made versions to balance a particular type of rotor, especially for the high speed, very light rotors. The machines spin the rotors at a speed considerably less than operating speed. The force generated by the unbalanced rotor, recalling the simple force equation is measured. Because of the range of rotor weights, it is not difficult to see why several sizes of balance machines are needed. Also, the engineer's worry of balancing a light rotor in a too-large machine is well-founded because the sensitivity changes with the construction. The forces generated by the small rotor on the big machine do not cause much reaction and, therefore, lose sensitivity.

The reaction of the bearings is measured on the machine dials, or on the CRT, if a computer-assisted machine is used. Balancing is not considered much of a problem if the planes in which the unbalance is located are known, as shown in the simple two-disk rotor used at the beginning of this section. However, consider a centrifugal compressor having eight impellers, all with cover discs, a balance piston, thrust collar, and coupling hub. It is extremely difficult to look at bearing readouts and decide which plane or even which impeller, notwithstanding that a given impeller cover or back shroud may be the item needing correction. This is the dilemma of the service shop.

For new rotors, where the elements have not yet been put on the rotor, other techniques can be used. First, the components can be individually balanced on a precision mandrel. Precision means that the runout is a few tenths of a mil (.001 inch). The runout high spot should be scribed on the mandrel. The new component now can be reasonably well-balanced. As the component is removed from the mandrel, the mandrel mark should be transferred to the component. When all the components are completed, the shaft is checked for runout. The high spot should be marked. As the components are stacked onto the shaft, the marks on the shaft are aligned with those transferred to the component. This works well with keyless rotors (no key between shaft and component). Experience has shown that in most cases with keyless rotors when the stacked rotor is put in the balance machine and checked, the residual unbalance is within the acceptable tolerance. If not, the rotor must be unstacked and the problem located. It must be remembered, however, if the components were properly balanced and the rotor comes out with unbalance, there must be a prob lem. To begin to balance the assembled rotor means unbalancing some components and covering up a problem. It should also be remembered that balancing in the wrong plane will permit operation with two unbalanced couples canceling each other out at the shaft ends, giving no apparent unbalance. With the couples still in the rotor, there are residual moments that will add unwanted stress to the shaft. If taken to the extreme, shaft breakage could result.

For rotors with keys, an alternative method also endorsed by API, is progressive balancing. In this method, the components are stacked onto the rotor one or two (but never more than two) at a time. The balance correction may only be made to the element added. When the rotor is completely stacked, only very minor trim balance should be required. If more balancing is indicated, unstacking is required to prevent compromising already balanced elements. Since most balancing is done using clay to determine the amount of unbalance, and then corrections per formed by grinding material from the element, a poor job will leave a very sad looking element. More material in another location will need removal to correct the errors of the previous job.

With both balance methods cited, unstack if things appear to go wrong; don't grind a balanced impeller. One common problem is the manner in which a component is installed. The components are fitted to the shaft with an interference fit, even when keys are employed. To get the component on the shaft, the bore must be dilated. This is normally done by heating the component. It is tricky, at best, to get the bore to open evenly. It is even more difficult to get the cooling to take place uniformly. If it does not, the component becomes cocked and either locally distorts the shaft (bows) or has excessive run out. This can be detected before the balance step, but because humans are involved, not everything that is supposed to happen will happen. If the balancer is well-disciplined, he will detect the problem, but if not, the component gets a "butcher" job.

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