Momentary locking of 'loose' stator. 1

[Biggadike]

A few years ago I encountered a strange problem which I have yet to explain:
A friction clutch was being run at 2800 rpm (Lenze series 4 as it happens). It was only rated for 1400 rpm but Lenze said that only the life would be affected. Fine.
The stator was a very loose (good clearance) fit and rested lightly on the rotor with a PTFE coating between them. It was stopped from rotating by a peg-in-slot arrangement, again a loose fit.
What we found was that on some clutches, a momentary lock-solid would occur when the clutch was run up. This would literally bend the retaining peg and give out a loud squeel consistent to a total lock. On inspection everything was free moving again and with the clutch in your hand it was impossible to get the two surfaces to lock.
I could only put the whole thing down to some harmonic vibration problem - we cured it by giving a close fit on the location pin to stop vibration.
A collegue said he'd experinced the same thing with a component in a VCR.
Has anyone got a propper name for this phenomenon and even better, a way of predicting when it will strike?

 

[Metalguy]

I don't know what they call it, but the same thing happens if you put a spinning grinding-point or carbide burr into a hole which is just a little larger. The first contact makes the point/burr ricochet around the ID, and it will shatter a grinding point. The burrs just tear up the hole, and make the tool vibrate so strong it hurts your hands.

We had the same thing happen on a much larger scale some years ago. An essential water pump (nuclear power-LPSI) which was undergoing a 100-start sequence suddenly wouldn't start-made an awful noise, and the 500 HP motor tripped on high current. There was a small space between the pump and motor (vertical shaft), and the shaft could easily be turned by hand afterwards. This was repeated quite a few times-same result. The only thing noticeable during a pump teardown was a regular-spaced series of marks all around the ID of the lower wear ring (K-monel). I had the ring changed to a "slippery" SS, Nitronics 60 (love the stuff). During the next 100-start sequence, it started something like 98 times OK. Then, same problem.

Turned out to be a case where, on motor-start (right across a 4160 volt line, IIRC), the rotor would actually bend within the two motor bearings and slam the bottom of the impeller into the lower wear ring, and it would bang around for the few seconds it took to trip the breaker, making only a few slow revolutions while making "the noise".

The fix? We simply used an 800 HP motor! But not for brute force. The 800 HP motor had a thicker rotor shaft, which didn't bend enough for the impeller to touch the wear ring during start!

 

[Shortstub]

Metalguy,
I'm curious about your experience. Did the motor in question have a 3-legged rotor spider? Or was it a 4-legged spider? Finally, were you certain there were no broken rotor bars?

 

[Metalguy]

I don't know what a rotor spider is, but I suppose I could find out how amny legs it had.

The motors (we installed at least 3 new motors) didn't have anything wrong with them-that was the first suspicion.

This was a baffling problem for a while.

 

[borjame]

If I'm reading these right, it sounds like the shaft is bending due to vibrations (Biggadike is right). The 1400 RPM rates shaft was never intended to run at 2800 RPM and was probably never checked at 2800 RPM. The shaft could be "wobbling" b/c of an imbablance that you don't notice until higher RPM where the forcing function from the higher RPM rotation matches a shaft natural frequency. As far as predicting, if it's due to an imbalance, you're out of luck unless you do a vibrational analysis on each part (attach accelerometers and hit the thing with a big mallet, record the natural displacements, fourier transforms, etc, etc, etc).

 

[Biggadike]

Borjame,

The funny thing about this situation is that the locking element was the stator. This is a very light item which sits loosly on the rotor.
If the motor shaft wasn't balanced, it would provide the forcing function for the stator, gravity would probably provide the return. Watching the thing move, it didn't look like it was vibrating, just that tiny level of vibration that you get when something is sitting on a fast moving shaft. Looking at it, it was more the variation in friction on the PTFE interface which was causing tiny levels of pulsing. The you'd get a bigger pulse as if it had just caught on something but recovered before locking and after a while it would lock catastrophically.
The thing is there was so much clearance that the bending shaft analogy doesn't work. Also, the length of contact is small - about 40mm - so bending shouldn't be an issue.

 

[Shortstub]

Biggadike,

Some call them 'arms', others 'legs'.

Did the other two machines exhibit like symptoms?

BTW, is 'Biggadike' italian for as large dam?

 

[Biggadike]

Shortstub -

With my problem I had a clutch so no arms or legs. My problem was seen on perhaps 1 in 10 units.

BTW - 'Biggadike' is my last name. Its English but comes from the time when the Dutch engineers were brought in to drain the fens of Lincolnshire. The dikes are the drainage ditches. Its interesting that it translates to something similar in Italian.

 

[Metalguy]

Biggadike, It doesn't translate to Italian. I think it was just a joke. <g>

 

[Biggadike]

Metalguy,..

Second rule of comedy: If you have to explain it, it isn't funny.

Actually, it probably was funny but I wasn't expecting to see humour on an engineering forum. Now I know.

 

[Metalguy]

Humorless engineers? I'll tell that to my boss next time he says (after reading one of my reports), &quot;What are you, a clown?&quot;.

No, he really doesn't, but I did have a high school math teacher (didn't like either him or the subject), who once asked me, after looking at my horrible homework assignment, &quot;What are you, a comedian?&quot; <g>