Dry Bearing Shaft Whirl? (Basics)

[arple]

Hi,

We have a rulon bearing (dry, teflon impregnated bushings) supported shaft which vibrates, and appears to make significant additional drag, a very significant issue with a wind measuring instrument (anemometer). I have been searching for info on the physics of this and the closest I have come so far are articles on oil whirl/whip, etc. However, since this bearing does not involve fluids (other than air) I doubt they apply.

During testing, the shaft is rotating at a wide variety of speeds from 2 rps up to 10 rps. However, the vibration (rattle) has its highest amplitude in the 8.5rps region. I can clearly see the shaft "whipping" by watching a spot on the center blur as it goes into vibration but I have not been able to identify the "orbit" pattern/direction. The rattle frequency does not follow the shaft rotation speed - it remains at 8.5Hz.

The rotor structures are very flexible; there are bound to be resonances in these frequency bands but, so far, altering conditions by adding masses, balancing/imbalancing, changing stiffnesses, changing bearing clearances, bearing mount stiffness, etc. have failed to produce any significant changes in the "resonant" frequency of 8.5Hz. However, the vibration amplitude does respond ("measured" by listening to the sensor mount with a strapped-on microphone) and, in fact, so does the amount of time the vibration is heard (vibration comes and goes, can be continuous, pulse in and out, or be quiet until perturbed then go crazy).

Any ideas on what phenonema are we seeing here? What key words should I be using to search for this info? Thoughts on other tests to identify isssue? (limited resources - free-running,variable frequency strobe, variable source of "wind", o-scope, machine shop)

Thanks!

 

[sms]

Sounds like a rub issue. Try searching on rubs in machinery or full annular rub. Be carful about your search terms, you may end up with unintended results.

Here is one link:

http://www.hindawi.com/GetArticle.aspx?doi=10.1155/S1023621X03000058

-The future's so bright I gotta wear shades!

 

[rob768]

if it has no distinct vibration pattern, it may be a slip-stick phenomena. Directly related to rub. You should be able to recognize this by inspecting the bearing surface. Look for local wear patterns or surface distortions.
It may also just be a "plain" lateral vibration problem. you can quite easily check this with a small finite element model.

 

[TPL]

Using a strobe to 'look' at what is happening would be useful - put some reference marks around the shaft (indelible pen), then slightly detune the strobe from running speed to see if you can better understand what is hapenning.

You mentioned orbits: have you considered the use of a pair of proxmity probes hooked up to a scope to look at an actual shaft orbit: along with a tacho pulse, this would provide a good insight into the phenomenon you are seeing.

Your running speed range is 120rpm to 600rpm and you suspect a resonance at 510rpm: is it possible that your rotor is sufficiently flexible to have a critical speed this low?