As pointed out earlier it should be precisely confirm unbalance is the only problem before implementing the balancing, as there are many different problems which are having similar vibration characteristics to unbalance. Yet, after the confirmation, all attempts to balance have failed, the next recommended step is to check the rotor for repeatability of unbalance readings. We have to simply stop the rotor and remove all trial weights and balance correction weights and rerun the rotor to the condition it was in when original readings were taken. Compare readings taken in this run with the original unbalance readings. They should be the same. These exercise may be done several times to verify the repeatability of the unbalance readings. If the two sets of readings differ significantly in amplitude and/or phase, this may be due to one of the following factors.
1. The actual configuration or shape of the rotor may not have been stabilized when the first set of original unbalance readings was taken. The rotor may have had a temporary 'sag', which has disappeared now that it has had an opportunity to to run.
2. The unbalance condition of the rotor may be changing from one run to the next. For example, a rotor that is loose on the shaft may assume a slightly different position on the shaft each time it is started and stopped. In addition, fans with hollow blades or hollow shafts may have accumulated dirt or water that changes location each time the rotor is started or stopped.
3. If nothing of the above observed, the rotor should be thoroughly examined for possible cracks on the shaft or rotor. If a repeatability check reveals that the rotor is repeating the unbalance readings from run to run some other problem is usually the reason and a more thorough analysis should be carried out to determine the cause.
1. The actual configuration or shape of the rotor may not have been stabilized when the first set of original unbalance readings was taken. The rotor may have had a temporary 'sag', which has disappeared now that it has had an opportunity to to run.
2. The unbalance condition of the rotor may be changing from one run to the next. For example, a rotor that is loose on the shaft may assume a slightly different position on the shaft each time it is started and stopped. In addition, fans with hollow blades or hollow shafts may have accumulated dirt or water that changes location each time the rotor is started or stopped.
3. If nothing of the above observed, the rotor should be thoroughly examined for possible cracks on the shaft or rotor. If a repeatability check reveals that the rotor is repeating the unbalance readings from run to run some other problem is usually the reason and a more thorough analysis should be carried out to determine the cause.
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