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Balance Overhung Rotors

QUESTION

How can I balance an overhung rotor?

ANSWER

Overhung rotors have characteristics such as Disk Skew and Gyroscopic Effects that can make them hard or impossible to balance with standard single or dual plane balancing techniques. This FAQ describes two effective techniques for balancing an overhung rotor; one for single plane balancing and another for dual plane balancing.

Use the diagram below to follow the steps outlined in each method.


Rotor showing measurement points and correction planes

A = bearing furthest from the rotor
B = bearing closest to the rotor
C = inboard on the rotor
D = inlet/outboard on the rotor

Using Single Plane Balancing - Static then Couple

The following method resolves static and couple imbalances separately. The static component is balanced first before dealing with any couple unbalance.

Step 1 - Static Balance
• Connect the channel 1 accelerometer to bearing B (closest to the fan) and perform a single plane balance on plane C only (closest to the bearings).

Step 2 - Couple Balance
• When the vibration level at bearing B is satisfactory move the channel 1 accelerometer to bearing A (furthest from the fan).
• If the vibration level of bearing A is not acceptable perform a single plane balance on plane D using 'couple weights'. This means that when placing a trial weight on plane D, also place an equal weight 180º opposite on plane C.
• When the instrument calculates the required balance weight, place it as directed on plane D and place an equal weight on plane C in a location 180º opposite. Use only the weight and location of the plane D weight when entering data into the instrument.

Step 3 - Final Static Balance
• When the vibration level at bearing A is satisfactory move the channel 1 accelerometer to bearing B (closest to the fan).
• If the vibration level of bearing B is not acceptable perform another normal single plane balance in plane C (i.e. repeat step 1).

Dual Plane Balancing for Faster Results

The following method combines the static and couple balance operations into one. This method can balance an overhung rotor in four runs (not counting trim balances). Essentially, perform a dual plane balance; however, when placing trial or trim weights on plane D also place an equal weight on plane C in a location 180º opposite the weight positioned on plane D.

1. Connect the two accelerometers on bearings A and B. Initiate a dual plane balance and collect the initial readings.
2. For the 'left' trial weight apply a known weight in plane C and take the left trial reading.
3. If you have 'remove trial weights' selected remove all trial weights.
4. For the 'right' trial weight apply a known weight in plane D and an equal weight in plane C 180º opposite. Use only the weight and location of the plane D weight when entering data into the instrument. Take the 'right' trial reading and let the instrument perform balance calculations.
5. If you have 'remove trial weights' selected remove all trial weights.
6. Balance the rotor by applying the weight in plane C as indicated by the instrument in the 'left' correction plane. For the 'right' correction plane apply the weight in plane D as indicated by the instrument, and also add an equal weight 180 º opposite in plane C, i.e. for every balance and trim balance you will be applying two weights to plane C and one to plane D.

You can use the instrument's 'combine weights' function to combine the two weights in plane C into one i.e. use one heavier weight instead of two individual weights.

The techniques outlined above are effective in reducing vibration on imbalanced rotors. If vibration persists this may be because the problem is not caused by imbalance. We advise you to check the machine to eliminate other probable causes of vibration before carrying out lengthy balancing procedures.