• Used successfully for short rotors such as:
  -overhung fans up to 2m diameter,
  -pump impellers on mandrels (using a pot magnet as the swing weight).

• Also on smaller fans with two bearings that are nearer to "long" rotors.

• How used for a 10-stage pump of the ring-section design (where each impeller is assembled in turn with its matching stationary section):
  -Parallel ways were clamped to the bed of a milling machine.
  -Balance correction was made on the adjacent machine using an end milling cutter.
  -A table was calculated and drawn up giving the depth of cut required in each case.
  -Position was marked clearly on the impeller: machinist cut the required mass from the back shroud.

• Rotor must be in rolling element bearings, or set up on a mandrel and parallel ways, and not affected by crosswinds.

• For in situ balancing, take usual safety precautions so that the machine cannot be switched on while the work is in progress.

• Mark numbers around the rotor, evenly spaced. For bladed rotors, mark at the blades.

• Choose swing weight to fasten (by clamp or magnet) on rotor, to give to-and-fro oscillation time of about 20 seconds. (This gives some allowance for stopwatch reaction time).

• With the swing weight fixed to rotor at Position 1, rotate the rotor so that the weight is at one horizontal position and stationary.

• Release the weight and let the rotor swing freely under its own inertia -do not push it in any way.

• Time the to-and-fro oscillation from the horizontal position.

• Rotate the rotor so that the weight is at the horizontal position on the other side, and repeat.

• Take the average time of swing for this position

• Plot on a graph of time vs position (with position numbers evenly spaced: position 1 appears twice-at the start and the end of the position axis).

• Move swing weight to Position 2, etc. and repeat process.

• Connect the graphed points with a smooth sine wave shape graph, and read off the maximum and minimum timed of swing: T and t.

• Calculate the size of balance weight required from: - Balance weight = Swing weight ×

• Check the result if required by repeating the swing test for 3 spaced points.

• Make balance weight of the required mass, and fix it on the rotor at position of longest swing time. Allow for mass of weld.

• If the correction is to be made by removing mass from the rotor, remove it from the position of quickest swing.

Reader tip provided by, Ray Beebe, Senior Lecturer, Monash University

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