3 lobe journal bearing question

[Hesam267]

we had 3 lobe journal bearing in a machine with 45000 rpm. after many years bearing change with new one but second bearing was not 3 lobe like another one and it change wrongly. actually it was simple journal bearing just with 3 oil grooves and it was just look like 3lobe bearing!!!
but machine started and it seem no problem on vibration, temperature and etc. i must add 45000 rpm is vendor critical speed and over speed is 35000 rpm actually. now this is my question: we did mistake but why everything is good? it may be make problem for us in future? it may make problem in start-stop moments?

 

[ronfrend]

The main reason for having a non-circular bearing is to avoid oil whirl. With a well designed "simple" journal bearing (your words) the maximum pressure in the lubricant happens somewhat after the bottom centre line of the bearing (say 7 0'clock if the shaft is running clockwise). As the speed increases more oil is drawn into the oil wedge so the resultant higher oil pressure tends to push the shaft in the direction of rotation until a new equilibrium is reached with the pressure x area on the left side = the pressure x area on the right side. As the speed increases further the position of highest pressure keeps moving round until eventually gravity causes the shaft to drop and the high pressure starts to whirl around the shaft giving massive vibration amplitudes.
The asymmetric bearing avoids this by increasing the gap between the shaft journal and the bearing so the high pressure either collapses at higher speed or (more often, thankfully) it moves back to its prior position.
What you need to look out for is an unstable vibration that might set in either due to increase in speed, change in oil viscosity, increase in bearing wear or even a change in load.
I have placed a simple powerpoint slide to show the relationship between shaft rotation and position of peak pressure.
I hope that helps. It's a rather long winded explanation of a fairly straightforward phenomenon but be careful.

Ron Frend

http://www.predicon.net

 

[Strong]

Ron,
Your file has only one slide!
Walt

 

[ronfrend]

Hi Walt,

Sorry but this was just a quick dirty animation of hydrodynamic lube so, yes, it's just one slide.

Ron Frend

http://www.predicon.net

 

[Strong]

Thanks Ron, I did find the animation preview feature in PowerPoint.
Walt