We have a soft-bearing balance machine. When balancing small motor rotors, we commonly remove the rolling element bearings and set the rotor shaft directly on the balance machine rollers. A discussion, this morning, left us wondering which is the best practice. The two opposing views are:
1. Mount the rotor in the stand directly on the portion of the journals that contact the bearings. This best reflects the condition of the motor once in service.
2. Mount the rotor in the stand such that the balance machine rollers ride on a machined portion of the shaft away from the area that contacts the bearings. This may even be over a portion of the shaft that has a different radius than the bearing journal. This protects the bearing journal from damage due to contact with the balance machine rollers. Differences in shaft concentricity or dimension are so insignificant between the bearing journals and other parts of the machined shaft that they will not corrupt the balance.

Opinions...thoughts?

Thanks in advance,
George

George - it is always best to run on the journals, due to the fact that you want to balance around the same centerline that the part will see when it is in place. I know you feel that the machined portion you mention has no difference, but even if the centerlines are as little as 1/10 mil different, it can affect the balancing reading, especially when trying to achieve tolerances as tight as 4W/N. You might experience some discoloration of the journal, but if your soft bearing suspension roller bearings are in good flat condition you should not see any journal damage. Foreign material is the only thing that would typically make marks on the journal.

George,
Earl makes a very good point. I worked in an electric motor repair shop with a soft bearing machine and we either balanced on the journals or over the years we collected a supply of used bearings still in good shape and we would install those bearings on the journal and balance in them.
I left that shop for about 5 years and when I came back they were balancing on the fits you refer to and were having a lot of problems getting the motors to run at acceptable levels, especially 3600 RPM motors which we balanced to 4W/N. I believe in many cases they were balancing a rotor on these non-concentric fits and were actually do more harm than good.
I posted a question on this forum a few years back about the effects of balancing on a non-concentric fits and was even given a name for what this is. It is called mass center displacement. I was also given a formula for calculating how much effect this will cause and it may still be on my computer at home. Unfortunately, or actually fortunately for me, I no longer work for that shop so I do not have the formula here at work. You might try a search in the posting for "balancing on a non-concentric fit".

Ronnie

http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...341035831#8341035831

This is the topic I was refering to.

Ronnie