Section 11 — Balancing Rotors215The centrifugal force of a heavy spot on the rotor that causes the rotorto vibrate increases with the square of the rotational speed. Thevibration level of the rotor may be acceptable at one speed but not atanother. Therefore, it is important to always allow the rotor to settle toits normal operating speed before taking balancing analysismeasurements.Whether a rotor should be balanced in one plane or two depends on thedimensions and operating speed of the rotor. The table outlines acommonly used guideline.Rotor Length toDiameter RatioOperating Speed Balancing Technique0.5 or less1000 RPM or less Single-plane balancingMore than 1000 RPM Dual-plane balancingMore than 0.5150 RPM or less Single-plane balancingMore than 150 RPM Dual-plane balancingNOTE: Before attempting to balance a rotor, you must confirm thatthe cause of vibration is uneven mass distribution in the rotor.Good balancing results can only be obtained if vibration is causedby uneven rotor mass distribution. Attempting to balance a rotorwith other problems will generally NOT reduce the vibration level.Balancing ProcessYou can balance a rigid rotor in one or two planes. This involves:• Setting Balancing Parameters — Select the units and balancingmethod.• Initial Reading — Measure the initial imbalance.• Trial Reading — Attach a trial weight to the balancing planeand take another measurement. For dual plane balancing, dothe same for the second plane.• Balancing — Attach correction weights to the balancing plane(s)as recommended by the instrument.• Trim Balance — Take a measurement in each plane to confirmthe rotor is balanced. Any residual imbalance can be removedvia additional trim balance cycles.
