There are two primary types of balancing machines; a Soft Bearing Machine and a Hard Bearing Machine.  Both types have advantages and disadvantages. Here are the differences:

Soft Bearing Balancer

A soft bearing balancer allows the rotor ends to move freely in the horizontal direction of the balancing stand.  This type of balancer allows the rotor to turn at much slower speeds than the rotor’s operational speed.  The balancing procedure is almost identical to field balancing and a calibration or trial weight is used to test the response of the rotor.  In this way, each rotor balance is in essence self-calibrated.  Like field balancing, multiple runs are required and the correction and trim weights are applied until the rotor meets the acceptance criteria.  As long as the speed is above the resonance of the soft work supports, and not at the rotor’s critical speed, the response will be linear and very accurate. Some of the largest steam turbines in the country have been balanced using soft bearing work supports resting on railroad tracks.  These rotors are balanced at speeds around 30 RPM.  If one is concerned about the number of runs in a stand, then a hard-bearing machine might be preferred.

Soft Bearing Balancing Stand
Image Source: IRD LLC – “Soft-Bearing vs. Hard-Bearing Balancing Machines”

Hard Bearing Balancer

A hard bearing machine fixes the rotor ends to the balancing stand pedestals.  This system only requires one run to determine unbalance and correction weights.  A hard bearing stand measures force, rather than motion like the soft bearing machine.  If one knows the force and angle of the unbalance plus the weight of the rotor, a correction can be calculated.  The advantage is that only one run is required to determine correction weights.  However, because the hard-bearing machine measures force directly, the accuracy is sensitive to speed. If the speed of a rotor doubles, the force increases by a factor of 4.  Thus the higher the speed, the higher the measurable force and the better the accuracy of the balancing stand.  One may be nervous about running rotors such as fans at higher speeds due to wind forces.  In this case, a soft bearing machine would be better.

Hard Bearing Balancing Stand

Download our 5-Step Balancing Procedure for a simple and effective procedure for doing field balancing.

Preparing for the Balancing Job: A Few Things to Keep in Mind

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by Gary James CRL