What is it called?

That's a good question. The shaft, no matter the source, appears to go through a rapid chatter sequence. The drive shaft on my Kubota tractor does it. It runs in a plastic bushing, which has worn slightly. In my mind's eye I can see it bouncing around the bushing, touching here and there, but I'm at a loss to explain why it introduces so much (apparent) friction.

Harold

I believe (but maybe wrong) that the combination of rpms, friction, stiction and how loose the bearing is creates a resonant frequency causing the chatter and slowing of RPM's. It probably makes noise at any speed, but our default human hearing capacity limits what we can hear. I'm sure that others can describe the phenomenon better.

lane

I had an MGB that "buzzed at certain RPMs then smoothed out above that speed. I always thought it was call "critical speed"

John

Might be something like screetching chalk on a blackboard. Maybe the impact force of the chatter is enough to squeeze out the lubricant so the parts "dig in" to each other and then break free.

Jeff

You might call it resonance. The problem, of course, is that the bearing is a little (maybe quite a little) worn. In "normal" operation, the shaft is pushed against the bearing in one direction by the ordinary operating forces of the system. But, at one particular speed, you hit the natural frequency of the shaft and it bounces around like a vibrating violin string. And it takes power to accelerate the center of mass of the shaft back and forth. Even though the distance the center of mass of the shaft travels is small, given the speed at which it vibrates, the power required is considerable and that power is, of course, taken from the rotory motion of the shaft. Hence the appearance of "friction." But, if it were actual friction, the power consumed would be turned into heat.

In the case of the vibrating violin string, the string moves rapidly through the center of its motion and slows as it reaches its limit, stretching like a spring as it deflects. The power it takes to stretch the string is returned as it swings back through the center. But the vibrating shaft is constrained by the bearing, so the power used to accelerate its center of mass is lost when it hits the other side. And when the vibrating shaft bounces against the bearing, there is considerably more force between the shaft and the bearing, hence there is more real friction as well. The combination of the two can result in quite a bit of power being taken out of the system.

Jerry

The effect is known as precession. The lubricant layer breaks down, usually because the bush is excessively worn and the lubricant can escape from the high pressure area that normally exists. The shaft contacts the bearing surface and rolls around the inside of the bearing surface. The precession is in the opposite direction to the shaft rotation and causes large vibrations which absorb a lot of energy. I've seen this effect happen in air bearings which have worn excessively large allowing the air film to escape. Martin

Precession is the term I'm familiar with as well and agree with Martin's explanation.

Precession is also the reason that tight fits are required between the shaft and inner race of ball bearings where the inner race is rotating with respect to load direction. If there is any clearance between the shaft and race they'll roll relative to one another (precess), resulting in fretting and ultimate failure.

A spring that's a close fit on a shaft will often show an exaggerated example of this behavior if given a spin with the palm of your hand. When I was a kid there was a crib in our house with bumper springs on the sliding side gate. If you spun a spring just right while releasing it at the top of the shaft it'd make a satisfying whir and would almost seem to levitate as it precessed around and moved very slowly down the shaft.

Ned Simmons

Huh! Man, how many of us have seen that in one way or another. Didn't realize it had a name, though. Thanks, Ned and Martin, for a simple and easy to understand explanation. Now if I can just remember what it's called.

Harold

Thanks, everyone for answering the post - - even if it was just to second the need for a name. I'm familiar with precession, but only as applied to astronomy and the slow change in direction of the earth's center of rotation. The effect certainly fits the physical action you have described, and so we have a name. I was kind of hoping for some less scientific name like "squeeging" or some such expression that almost mimics the sound. But I'll go for "precession" since that's what is actually happening.

Thanks again,

Chuck Olson

THATs where I heard of precession before. Thanks Chuck, I've been reading all the posts on precession and knew I had heard that term before but in a different context. I used to read astronomy stuff all the time and was even going to build my own telescope at one time.

Lane

It also is used to describe motion in gyroscopes.

My father just used to refer to it as "Oh sh_t". He was a millwright, and it meant he'd missed something in a maintenance inspection.

| >>In article , | >> snipped-for-privacy@ntlworld.com says... | >>

| >>>"Chuck Olson" wrote in message | >>>news: snipped-for-privacy@comcast.com... | >>>

| >>>>What is it called when a shaft running in sleeve bearings suddenly | >

| > makes a | >

| >>>>loud grating sound and slows way down? I have an electric fan that | >

| > does | >

| >>>this | >>>

| >>>>when I start it up on medium power, but it manages to get through the | >>>>critical speed quickly when started up on high power and runs quietly. | >

| > I | >

| >>>>know all it needs as a couple of shots of oil in the right places, so | >

| > it's | >

| >>>>not that big a problem, but I just want to know what the phenomenon is | >>>

| >>>that | >>>

| >>>>makes the sound and greatly increased friction, and if there is a name | >

| > for | >

| >>>>it, what it's called. | >>>>

| >>>>Thanks, | >>>>

| >>>>Chuck | >>>>

| >>>>

| >>>

| >>>The effect is known as precession. | >>>The lubricant layer breaks down, usually because the bush is excessively | >>>worn and the lubricant can escape from the high pressure area that | >

| > normally | >

| >>>exists. | >>>The shaft contacts the bearing surface and rolls around the inside of | >

| > the | >

| >>>bearing surface. The precession is in the opposite direction to the | >

| > shaft | >

| >>>rotation and causes large vibrations which absorb a lot of energy. I've | >

| > seen | >

| >>>this effect happen in air bearings which have worn excessively large | >>>allowing the air film to escape. | >>>Martin | >>>

| >>>

| >>

| >>Precession is the term I'm familiar with as well and agree | >>with Martin's explanation. | >>

| >>Precession is also the reason that tight fits are required | >>between the shaft and inner race of ball bearings where the | >>inner race is rotating with respect to load direction. If | >>there is any clearance between the shaft and race they'll | >>roll relative to one another (precess), resulting in | >>fretting and ultimate failure. | >>

| >>A spring that's a close fit on a shaft will often show an | >>exaggerated example of this behavior if given a spin with | >>the palm of your hand. When I was a kid there was a crib in | >>our house with bumper springs on the sliding side gate. If | >>you spun a spring just right while releasing it at the top | >>of the shaft it'd make a satisfying whir and would almost | >>seem to levitate as it precessed around and moved very | >>slowly down the shaft. | >>

| >>Ned Simmons | >>

| >>

| >

| >

| > Thanks, everyone for answering the post - - even if it was just to second | > the need for a name. I'm familiar with precession, but only as applied to | > astronomy and the slow change in direction of the earth's center of | > rotation. The effect certainly fits the physical action you have described, | > and so we have a name. I was kind of hoping for some less scientific name | > like "squeeging" or some such expression that almost mimics the sound. But | > I'll go for "precession" since that's what is actually happening. | >

| > Thanks again, | >

| > Chuck Olson | >

| >