- 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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