I wonder if somebody have ever taken an orbit with prox probes of a rigid shaft resting on antifriction bearings.

In case of a sleeve bearing when, say imbalance is present, a circular orbit of the shaft CL will be produced as oil support layer changes its thickness within one revolution.

In case of an antifriction bearing I can speculate only that nothing of this kind happens, since it is unlikely that imbalance force will lift the shaft completely off the rollers. Imbalance force though may move the shaft sidewise thus moving the load zone. Of course, I assume that races and rollers are absolutely non-compressable.

Why I am asking all that? Just curious... Trying to understand how shaft relative motion is converted into bearing support absolute movement. Why "excessive" bearing clearance causes non-harmonic motion of the bearing support. There may not be shaft relative movement at all. It may or may not have any implications as well. One thing I can see is that if no shaft orbit is present, then bearing support absolute movement and shaft absolute movement is the same unlike that in a sleeve bearing.

Anyway, how shaft orbit looks like in an antifriction bearing?

Will appreciate your opinion.

I have never taken orbits on rolling bearing machines. I have some rambling comments...

There can be some internal clearance present in rolling bearings although not as much as sleeve.

The effect of gravity may be similar to the effect of a preload often described for sleeve bearings. It may tend to change the shape of the orbit from an an oval towards a half-oval or a sideways figure 8 or a smile.

Also I think it may be possible for that force to cause the rotor to orbit through the full bearing clearnace even with unbalance force a small fraction of rotor weight. Consider the dynamics. If you were to set the rotor in motion in frictionless bearing (no damping in supports etc), then there would be no force required to keep it going. There is energy changing back and forth between kinetic and potential. The bearing walls provide the force to accelerate in circular motion. Real world adds friction and springiness of rotor and bearing support and motion of the bearing. How much force required in that real world to make it orbit within the full clearance ? I suspect if you are near a machine critical then very little unbalance force (much less than rotor weight) is required to give that response.

How much unbalance force required to cause a full clearance orbit when far away from critical? I don't know but it seems like it can be much less that rotor weight. Consider that as the clearance gets smaller, less and less force is required.

This message has been edited. Last edited by: electricpete,

I dealt with machines mounted on rolling element bearings and monitored using relative probes. They were huge motors driving huge cooling sea water vertical pumps.

Unbalance was seen as circular orbit however its magnitude was not as if the bearings were fluid film bearings. Though I've my reservation on this configuration, I noticed an advantage. Because these pump's bearings were just bushings and they were not accessible at all to normal vibration monitoring, the relative vibration on the shaft gives an idea of the bearings condition.

To me, the relative probes could be useful even if the shaft is born on rolling element bearings if and only if the distance between the bearing and probe is studied carefully to reflect the elasticity of the shaft under normal magnitude of the expected forces acting on the shaft. Yet, I do not recommend this set up without very solid justification.

Thanks- Ali M. Al-Shurafa