Sunday, October 12, 2008

balancing steps

Component balancing:

For shaft rotors, which comprise of more than one component, it is vital to balance all of the major components individually before assembly. This is done because if the rotor is fully assembled, there is no way to know exactly what contribution each component part is making to the total measured unbalance vector. In addition, if a large unbalance exists in one of the major components, within the rotor, the rotor shaft may flex at this point during high speed operation and cause significant damage to the rotating and stationary parts.

Each major rotor component must be individually balanced on a precision ground mandrel (note that expanding mandrels are not acceptable for this purpose). The balance mandrel should be ground between centers to assure concentricity of all diameters throughout its length as well as to assure a good smooth surface. After grinding, the mandrel must be precision balanced. A trial bias weight may be used to raise the observed residual unbalance readout of the balancing machine. The desired balance result is such that no matter at what angular location the bias weight is added, the balance readout is always the same. In this case the residual unbalance of the precision mandrel is as close to zero as possible. The rotor component should always be mounted to the mandrel with an interference fit, never a sliding or loose fit. If the rotor component has a key fit to its shaft, than the balancing mandrel should also have a matching keyway. After each component is shrunk on its mandrel, the axial and radial runouts should be checked to ensure that the mounted impeller or hub is not cocked on its mandrel prior to component balancing. As a general rule, runout should not exceed 0.16 mm/meter of diameter.

Progressive component stack balancing (sequential balancing):

After individual balancing of all major rotor components, the rotor must be progressively stack balanced as each major component is assembled onto the rotor shaft. Progressive or stack balancing is necessary due to the deformation of components during assembly. Components with unequal stiffness in all planes, such as those with single keyways, may deform when shrunk onto the rotor shaft. For such components, considerable deformation and resultant unbalance can occur between mandrel balancing using a light shrink fit and stack balancing on the job shaft with a heavy shrink fit.

Progressive balancing is accomplished by stacking no more than two rotor components at a time onto the rotor shaft. Component axial and radial runouts should be checked against mandrel runouts, as each component is start. In general, the start component runouts should match those runouts recorded with the components on the mandrel.

As each rotor component is start into position and the runouts checked as acceptable, the rotating assembly is to be placed in the balancing machine and trim balanced (if required) as necessary to achieve the balance tolerance. Balance weight correction is to be performed only on the most recently stacked component.

After the rotor is completely stacked, trim balancing, if required at all, should be very small to meet the tolerance of permissible residual unbalance. As a general rule of thumb, the remaining residual unbalance in the rotor should not exceed two times the residual balance tolerance prior to trim balancing.

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