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Balancing Overhung Rotors

The information provided is for help with Commtest Instruments products:

vb instrument

QUESTIONHow can I balance an overhung rotor?

ANSWEROverhung rotors have characteristics such as Disk Skew and Gyroscopic Effects thatcan make them hard or impossible to balance with standard single or dual planebalancing techniques. This FAQ describes two effective techniques for balancing anoverhung rotor; one for single plane balancing and another for dual plane balancing.

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

A = bearing furthest from the rotor

B = bearing closest to the rotor

C = inboard on the rotor

D = inlet/outboard on the rotor

Rotor showing measurement points and correction planes

Using Single Plane Balancing - Static then Couple

The following method resolves static and couple imbalances separately. The staticcomponent 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 1accelerometer 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 atrial weight on plane D, also place an equal weight 180 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 180

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opposite. Use only the weight and location of the plane D weight whenentering 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. Thismethod can balance an overhung rotor in four runs (not counting trim balances).Essentially, perform a dual plane balance; however, when placing trial or trim weightson plane D also place an equal weight on plane C in a location 180 opposite theweight 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 theleft 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 equalweight in plane C 180 opposite. Use only the weight and location of theplane D weight when entering data into the instrument. Take the 'right' trialreading 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 theinstrument in the 'left' correction plane. For the 'right' correction plane applythe weight in plane D as indicated by the instrument, and also add an equalweight 180 opposite in plane C, i.e. for every balance and trim balance youwill be applying two weights to plane C and one to plane D.

You can use the instrument's 'combine weights' function to combine thetwo weights in plane C into one i.e. use one heavier weight instead of twoindividual 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 beforecarrying out lengthy balancing procedures.