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 .

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

Could be Oil Whip, Whirl or Bearing Flutter.

Martin.

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

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