A. While investigating increased 2*LF vibration on a 7000hp 3600 rpm horizontal sleeve bearing motor, a mechanical soft foot check was performed by loosening hold-down bolts and checking deflection at top of the feet. The results are as follows:
NE - 0 mil
NW - 1 mil
SW - 4 mil
SE - 6 mil

It is an unusual pattern to have two soft feet on the same side, would you agree? Do you have any idea what might cause this pattern?

Attached is a photo which I believe may have some clues. (by the way, I am holding back a little bit of information intially... this is a little bit of a quiz to start with before I get to my real questions).

SUFPPHOTO.ppt (838 Kb, 105 downloads)

El-Pete,

It is not a high amount of soft-foot, and I would not expect it to be an issue regarding motor case distortion. It is not simple soft-foot, since the diagonals (NE-SW and NW-SE) are not similar values. Could be caused by angled foot (front-rear or side-side) or by spring-like shims. I was working on a diesel engine recently that had torch-cut thick steel shims that had a crown and were acting like a spring. Bolt-binding is another cause, since the bolt can be angled in the hole. THe photo looks like the two-bolt long-shim feet that were discussed in another thread.

Walt

Thanks Walt. You're right, this is the same one as in the other thread with 2 bolts per corner, 24" shims installed.

I would be interested to hear any other interpretations about that soft foot pattern.

Here is what the seasoned millwright who took these readings told me:

• The big terminal box hanging on the right side (north side) of the stator frame tends to pull the feet on the left side (south side) up. It is not exactly hanging, because it has some small angle iron supporting from below, but if that angle iron is not sized/positioned exactly right (notice there is a little bit of a tilt), the box may put some load on the stator frame.
• Although the millwright did not put any shim corrections in when he took these measurements for us yesterday, he told me he has attempted to correct this type of soft foot on this machine and others with large term box in the past and the results are always the same.... he ends up chasing it around and the soft foot never goes away.

It makes sense to me. What do you guys think?

Another piece of data - we have done a running soft foot check by loosening one foot (both bolts) at a time and the 2*LF vibration did not decrease.

Also we checked the airgap with long feeler gages from each end and results were good (difference between max and min is less than 5% of the average)

Is there any easy way to check this theory? I was wondering about a soft foot check with machine secured and dial indicator and loosening both of the left (south) feet at the same time. If the box really is pulling, there should be an even bigger change then.

I was also thinking about jacking up on the box a little bit while running to see if vibration goes down..., but a little nervous about that... there is 13.8kv passing through that box to the motor...woudlnt want anything to break loose while we're standing right there. Maybe instead we should jack up with machine secured while the south feet are loose with dial indicator on the foot and see if the soft foot indication decreases.

Any other suggestions for checks we might do?

Do you think the unusual soft foot reading and/or terminal box might have anything to do with elevated 2*LF (jumped recently) or not?

This message has been edited. Last edited by: electricpete,

El'Pete,

That's a lot of bull from the millwright!! How much does the "hollow" terminal box weigh compared to the total motor weight. If it was 10% I'd be very surprised.

I am not sure what the real problem is with the motor, but why invest the time to "jack the terminal box"; just plain crazy!

I don't think the soft foot values are correlated to the 2xLF increase. Not all types soft-foot significantly change the air gap. Was the motor removed, or moved for alignment and then the 2xLF increase, or the 2xLF simply went up for no obvious reason or no repairs were made? I'd be looking all over the motor case and terminal box for a natural frequency near 120 Hz.

Walt

Thanks for your coments Walt.

It certainly could be bull, but I am keeping an open mind.

The terminal box I agree is probably far less than 10% of motor weight. But the motor is "centered" on its supports while the terminal box is overhung (if in fact some of the terminal box weight is being carried by the motor frame).

I do wonder about what is the mechanism by which frame distortion can cause 2*LF when it has been proven the airgap is fairly symmetric But I have a hunch that the frame distortion causing 2*LF in other 2-pole motors does not cause that vibration as a result of changes in airgap, I think it acts thru some other mechanism. I have a little bit of an idea how that might happen for NEMA frame motors (we have talked about the interface between the o.d. of the core and the cylindrical bore of the frame before), but it's a little harder for me to picture for large horizontal motors such as this where I believe the stator core is bolted to the bottom of the frame. Has anyone ever measured airgap variation associated with a soft foot condition? Has anyone seen foot conditions related to 2*LF on large horizontal motors such as this?

Jacking the terminal box just means putting hydraulic hand-operated jack and some timber spacers underneath to apply pressure and look for a change. Maybe we can get a jack with a pressure gage so we can determine the force and only apply a force equal to the estimated weight of the terminal box. It does not seem like a tremendous amount of work to me, but I want to think about my approach (and talk it over here) before I request it. Probably better to do the off-line version as mentioned above.

This message has been edited. Last edited by: electricpete,

As to the terminal box, is it rigid conduit, or flex?

It is flex conduit below the term box, but there is also some metal channel supporting the term box from below.

Are the terminal box supports bolted down. If the box is pulling then by loosening the support bolts (or the conduit) it may (a long shot) lift up the supports if the bolts are loosened up.

Like I said a long shot but who knows??

What is the rotation of the motor in comparison to the side that is lifting? Could this be a torque issue? (Don't know just questions).

Is it possible the weight of the box combined with the torque needed to turn this rotor is causing the side of the motor to flex to the point of causing an airgap problem? Measuring in a static state might not be giving a true gap reading when power is on. Of course "static" is the only physical way to measure.

Is there some way you can measure the side of the motor on startup, where the box is, to see if it is moving inward or outward (not as in vibration, but staying inward or outward under load)?

Good comments. The soft foot check with dial indicators on the feet was conducted with the machine off, so there was no influence from torque at that moment. But it may be interesting as suggested to monitor for movement between off-line and running. Good point that a static airgap check doesn't guarantee the condition of the airgap when running.

I've always used 3 mils a max tolerance. Did you check wedge foot? A 7000 hp mtr is kinda of heavy.

Regardless; I would correct. On motors of this size I've always shot for zero and if the base is of good integrity it is usually fairly easy to get.

I had in mind a limit of 2 mils or 3 mils at top of the foot requires correction.

But we didn't correct. The millwright indicated that he believed the condition couldn't be corrected by shimming based on his past experience, so he didn't try this time around. No-one has run across a soft foot due to frame loading that couldn't be removed by shimming?


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The vibration on this motor at 2*LF recently increased from approx 0.05 -> 0.10 ips on the outboard horizontal (1H) and 0.11 -> 0.15 ips on the inboard horizontal (2H).

Now here are some perplexing observations. While running the motor uncoupled during plant outage (nothing else running), we noted:

• The concrete foundation does not show any noticeable vibration
  
• There is a very noticeable vibration walking on the gravel around the coupling/pump (went away when motor was subsequently stopped).
  
• Several vertical structural members embedded into the gravel in the same area were vibrating heavily. One was measured and was 0.3 ips at 2*LF. No obvious connection to pump or piping. [CORRECTION - THESE VERTICAL SUPPORTS SUPPORT PUMP PIPING. WE DIDN'T NOTICE VIB ON PIPING OR PUMP BUT DIDN'T REALY LOOK - PUMP WAS UNCOUPLED FROM MOTOR AT THE TIME.] Went away when motor was subsequently stopped.
  
• Motor bearing housing vibration as mentioned above was lower ~ 0.15 ips. Went away immediately upon power-down

I do know the core is fastened to the bottom of the frame and if vibration originates in the core, it is much more likely to transmit force to the foundation without going through the bearing housing. It seems as if the foundation is causing 2*lf vibration of gravel and structural members around it but the foundation itself is not vibrating. How could that be?

Pretty bizarre, huh? Anyone seen that before or have any ideas?

This message has been edited. Last edited by: electricpete,

Just my thoughts and probabilities,

1- Is there any loose pack of gravel slide causing Electric cable pull at NE or NW ?

2- Is there is any other environmental changes causes cable pull Which causes soft foot at SE and SW?
Proposed Diagnosis
Inspect the power cables at terminal box for any induced Physical tension and/or disconnect the Power cable and check the soft foot using dial indicator

This message has been edited. Last edited by: KJAP,

I would collect data from the piping and supports. I also assume they're imbedded in the gravel. Is the pipe running into ground also. Sonic velocity may be a factor. I've seen this. IN addition: I would ensure that no pipe strain exists.

Mr.Pete,2*LF means not always stator rotor air gap.it could be harmonic distortion.do you have any dc supply / battery back up control panel near this 7000 hp motor control panel then it could be due to that!secondly,we can also support the terminal box externally to avoid the tilting / pulling effect-cordially,pal

I'm sorry, I have to again correct the info I posted 23 April 2008 07:49 AM

After looking a little further today with the motor running coupled to the pump, only one of the supports is vibrating at a frequency 2*LF. I had previously felt the others and assumed the others were vibration at the same frequency, but they are in fact vibrating at other frequencies. The one that is vibrating 0.3 ips at 2*LF is embedded into the gravel but has only about 2" gravel between it and the foundation. The foundation itself has 0.005 - 0.02 ips at 2*LF on a few locations. I now conclude that that the 0.3ips at the one support is just resonant amplification of that frequency at that location.

Laying the accelerometer on top of the gravel near the coupling (where you can feel vibration with your feet) shows a predominant frequency of 2*LF with an indicated magnitude of 0.02 ips (although not sure if this is an accurate magnitude reading since the magnet is not attached to the ground/gravel).

I did map the long side of the frame of the motor which is vibrating at 2*LF. It does not vary much vertically. Horizontally on the frame it does vary from very low at the ends to 0.2 ips in the center. This is probably just what you expect when a plate is supported on the edges... vibrates most on the middle... probably doesn't tell us much.

In summary, I don't see much in my measurements of 2*LF at locations other than the bearing housing that tells me anything interesting.

This message has been edited. Last edited by: electricpete,

El'Pete,

It's a pretty large motor, so did you reach all the way up the sides and also measure across the top of motor case? Did you measure vibrations on the terminal box? I am skeptical that you can feel vibration at 0.02 ips with your feet (barefoot?) on ground; perhaps it is lower frequency vibrations that you actually feel with feet. What type of concrete foundation is under motor and pump:
a) direct to bedrock
b) cap on timber or concrete piles
c) mat or floating on compacted soil

I have seen foundation-related machine vibrations change with level of ground water (water table).

If your millwrights did not place a shim pack for each foundation bolt (8-total), then it is no wonder why they can't remove the soft-foot with the over-size shims.

Walt