Plain bearings in orbit

[Sam StonesParticipant @samstones42903]

Gentlemen,

What is the term usually used to describe the condition when a shaft begins to whirl or orbit (instead of sliding smoothly), inside a plain bearing?

It’s a noisy condition which is often cured with just a drop of oil, or a new bearing with a better fit.

Regards,

Sam

[Sam StonesParticipant @samstones42903]

Looking for the term to describe a noisy bearing

[SpringbokParticipant @springbok]

Shafted…!!

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Sam ,

Cyclic instability is correct though most people call it bearing knock .

Several variants :

(1) Stable continuous action – like epicycic gears .

(2) As (1) but unstable – happens sometimes not others .

(3) Either of (1) or (2) but incomplete – shaft moves back and forth around an arc only .

(4) Stable discontinuous action – where shaft jumps from one side of bearig to other . Usually associated with crankshafts .

(5) Chaotic – shaft just jumps around randomly .

Some of the above are simple reponse effects like crankshaft pulling repeatedly in alternate directions and some are resonance effects .

For each situation , and normal stable running , there is a different way in which any lubricant present flows in the bearing and forms the bearing cushion .

Regards ,

Michael Williams .

[DoddyParticipant @doddy]

Eccentricity may manifest itself as shaft to bore mis alignment (STBM) and/or dynamic run out (DRO).

Even with a good oil supply, high speed turbine shafts can suffer "oil whirl" leading to major vibration issues

Eccentricity and dynamic run out a usually due to mis alignment, more a "sloppy fit" in this case

[Ian S CParticipant @iansc]

The term here would be "Munted", which is another way of saying "stuffed". Ian S C

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Sam, maybe it could be termed as Axial Oscillation.

Regards Nick.

[blowlampParticipant @blowlamp]

Could be Oil Whip, Whirl or Bearing Flutter.

Martin.

[BazyleParticipant @bazyle]

The correct term is "Precession". A secondary definition to the more common association with gyroscopes and planets.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Sam's original question impied a dry running or trace oil condition .

To be clear – in any unstable bearing there can be two different sets of action going on :

(1) Those actions due to hydrodynamic effects in the oil film .

(2) Those actions due to the dynamics of the whole shaft and to the geometry of the bearings and journals .

In any practical situation (1) and (2) of course interact .

Purely for interest there is a variant of bearing instability where the shaft shuttles back and forth axially .

MW

[Sub MandrelParticipant @submandrel]

In my workshop I call that a 'precision running fit'

Neil

[Ian S CParticipant @iansc]

Sounds like the big end on some of my hot air engines, I just call it free running, sometimes it does get a bit extreme, one did get to .020" on a 1/4" crank, Brass bush very little wear, unknown grade steel pin took all the wear. Ian S C

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