Before actually attempting to balance a rotor, there are few preliminary factors that need consideration. The most important include:
1. Determining the type of unbalance: This is important in deciding whether a single or two plane approach will be needed. If in doubt, use a two plane approach. A single plane problem can be solved by the with a two plane approach. However, a two plane problem can not be solved with a single plane approach.
As a general guideline the length to diameter (L/D) ratio of a rotor can be useful to determine whether single plane or two plane approach should be adopted for balancing, which is described as follows:
L/D ratio Single Plane Two plane
<0.5> 1000
>0.5 rpm <> 150
2. Calculating the amount of trail weight: Great care must be taken in selecting the size of trial weights. If the weight is too small, no significant change in the unbalance readings will result and a run will have been wasted. On the other hand, if the trial weight is too large for the machine being balanced, the unbalance forces generated by the trial weight may cause extensive damage to the machine.
As a general rule, a trial weight that produces a minimum of a 30% change in amplitude and / or a 30 degree change in phase from the original unbalance readings will help insure satisfactory results. A common approach for selecting a trial weight is to use one that will produce an unbalance force equal to 5-10 % of the rotor weight supported by the bearing.
3. Correction method: Correction can be done either by weight addition or by weight removal. Weight addition is preferred than weight removal for greater accuracy. In large rotors slots for weight addition are given in pre-determined balancing planes. If weight addition is not feasible, great care should be taken when weight removal.
Balancing may be done in-situ (i.e. in installed condition) or in a balancing shop depending on certain factors like unbalance severity, type of rotor, planes of correction, rotor weight, downtime of the machine etc. The above mentioned criteria and balancing types applies to both of these. In shop balancing, measurements and calculations are done by the balancing machine software while in in-situ balancing portable vibration analyzers or polar charts may be used.
1. Determining the type of unbalance: This is important in deciding whether a single or two plane approach will be needed. If in doubt, use a two plane approach. A single plane problem can be solved by the with a two plane approach. However, a two plane problem can not be solved with a single plane approach.
As a general guideline the length to diameter (L/D) ratio of a rotor can be useful to determine whether single plane or two plane approach should be adopted for balancing, which is described as follows:
L/D ratio Single Plane Two plane
<0.5> 1000
>0.5 rpm <> 150
2. Calculating the amount of trail weight: Great care must be taken in selecting the size of trial weights. If the weight is too small, no significant change in the unbalance readings will result and a run will have been wasted. On the other hand, if the trial weight is too large for the machine being balanced, the unbalance forces generated by the trial weight may cause extensive damage to the machine.
As a general rule, a trial weight that produces a minimum of a 30% change in amplitude and / or a 30 degree change in phase from the original unbalance readings will help insure satisfactory results. A common approach for selecting a trial weight is to use one that will produce an unbalance force equal to 5-10 % of the rotor weight supported by the bearing.
3. Correction method: Correction can be done either by weight addition or by weight removal. Weight addition is preferred than weight removal for greater accuracy. In large rotors slots for weight addition are given in pre-determined balancing planes. If weight addition is not feasible, great care should be taken when weight removal.
Balancing may be done in-situ (i.e. in installed condition) or in a balancing shop depending on certain factors like unbalance severity, type of rotor, planes of correction, rotor weight, downtime of the machine etc. The above mentioned criteria and balancing types applies to both of these. In shop balancing, measurements and calculations are done by the balancing machine software while in in-situ balancing portable vibration analyzers or polar charts may be used.
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