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Thus for a rotor with a single disc mounted equally between bearings, the maximum residual unbalance in each plane would be half the calculated total residual unbalance.
To compare the results from these standards, the balance requirements for a 1,000 pound, 15,000 rpm between-bearing compressor rotor will be compared. In this example, the journal static weight of each bearing will be 500 pounds.
Permissible residual unbalance = 4W/N = 4*(500/15,000) = 0.13 oz-inches in each plane
Permissible residual unbalance = 4W/N = 4*(1000/15,000) = 0.27 oz-inches in each plane
Using a grade 2.5, Uper = G * W/N = 2.5*6.015*1,000/15,000 = 1.0 oz-inches
Uper1 o = 0.5 * Uper = 0.5*1 = 0.5 oz-in
Uper2 = 0.5 * Uper = 0.5*1 = 0.5 oz-in
The most conservative criteria in this example are from API-617. For new machines, specifying the rotor balance is not as important as specifying the maximum permissible vibration levels during shop tests and in the field. For rotor repairs, it is more critical to specify the balance criteria, since the rotor might not be shop tested in the machine.
The appropriate balance standard should be selected based on the user's experience. In the example above, the writer would recommend using the API-617 balance criteria. It is more conservative and it is the writer's experience that it is not incrementally more difficult or costly to balance to this standard in comparison to other standards.
The balance machine also must be sensitive enough to provide good, repeatable residual unbalance data. In many cases, a rotor is repaired and balanced at a non-OEM shop. In this case, the other test that might be considered is the API-617 residual unbalance test. This test is used to confirm the rotor residual unbalance and ensure that the balance machine is sensitive enough to accurately measure the residual unbalance.
This test consists of adding a trial weight at a radial position that equates to two times the 4W/N balance criteria (four times 4W/N for soft bearing balance machines - discussed further below). The rotor is then test run in the balance machine and the resultant unbalance is measured and recorded. This test is repeated with the trial weight moved to six more equally spaced radial positions, each at the same radius. The last location is the same as the first location (i.e. 0º and 360º).
The measured unbalance amplitudes vs. trial weight locations are then plotted on polar graph paper. If the plot does not approximate a true circle and/or encircle the center of the polar plot, or if the balance does not repeat at the starting location, then the balance machine is not sensitive enough to measure the unbalance and/or the rotor is not properly balanced.
Balance machines are typically classified as soft bearing or hard bearing. In a soft-bearing balance machine, the rotor is mounted on a flexible support system. The natural frequency of the combined support system and rotor is lower than the balancing speed.