Does anyone know if soft foot can cause a peak at 2x line frequency in the axial direction? I've just had a motor fitted which has a peak of 4.5mm/s RMS at 6000cpm (UK 50Hz supply) axially. It also appears in the Mtr NDE at 2.2mm/s. This is very close to 6x rpm, but a hi-res spectrum shows it not to be a shaft harmonic.

I believe that there may be an issue with one foot being boltbound, and I know from experience that the feet on this motor are particularly flimsy, so soft foot is quite likely.

Any thoughts would be appreciated, thanks.

Al,

Is it a sleeve bearing motor? Is the magnetic center set properly?

Good Luck,

Danny

This is a standard AC motor with deep groove ball bearings: DE 6313, NDE 6312.

Unfortunately, as a mechanical guy, I wouldn't know where to begin to check magnetic centre. I'd like to think that the overhaul shop got it right, but will believe anything in this game.

I'll try some running soft foot checks next week and see what the results are.

AL,

Tha magnetic center is only a concern on sleeve bearing motors.

Could you slacken one foot at a time whilst taking a reading? If the level drops or vanishes then you can confirm the soft foot condition - with the machine running. Obviously only if it is safe to do so.

I agree with Danny's comments on magnetic center.

Good suggestion by MPH.

Bolt bound is a condition that prevents proper alignment but not necessarily induces stress. Induced stress from soft foot can cause 2X line or line sync freq if pronounced enough.

The suggestion of loosening one bolt at at time is good; most always only 2 adjcent feet need loosening as there is a hard diagonal. However, you may want to loosen the two that are fartherest between centers - on a CW mtr the two on the right may be the safest (torque consideration).

I've just carried out a running soft foot check. When I started I was getting approx 6mm/s at 6000cpm axially on the motor DE. After loosening each bolt individually and then trying all possible combinations of 2 bolts without a massive improvement, I removed the known bound bolt entirely. Then I released each of the remaining bolts individually until I eventually managed to reduce the peak to 1.4mm/s. The two offending bolts were both on the same side of the motor and not diagonally opposite. Obviously the problem hasn't been cured, but at least it's been identified.

I've advised getting the bolt necked and addressing the soft foot.

Thanks to everyone for their input.

This message has been edited. Last edited by: Big Al,

Al,

To me the question as to 'Why 2xLF present in axial?' still remains in effect (AL, does some 2xLF present also in radial? My guess is "yes"). The physics behind axial component it is unclear since 2xLF forces in theory are radial by nature. But this motor proved the opposite Apparently under certain condition an axial component will appear as well! May be it is due to off-magnetic-center condition, as Danny mentioned, even in a motor with ball bearings (assembly error)? Think of magnetic field having a 2xLF component and being NOT squared with the rotation axis.

Big Al,

A resonant motor frame can sure cause 2xLF axial vibrations. I have solved a few of these problems. Loosening of the base bolts may simply indicte that the natural frequency changed and has nothing to to do with soft-foot. If vibraiton is still high after correcting shims and base bolt binding, then I suggest a natural frequency test.

Walt

David,

Yes the same frequency is present radially. I didn't explain correctly in my initial post. The 2.2mm/s on the motor NDE was indeed radial.

I've just been notified that corrective action has been carried out (necked bolt fitted, re-aligned, soft foot check) and I'll be taking some new redaings later today.

I experienced probably softfoot problem coming at 2X shaft in horizontal direction. It was confirmed by loosening the motor base bolts and taking vibrations. in this condition, 2X component disappeared but appears when bolts tightened. soft foot confirmed and corrected by machining and levelling both motor foot & base. but when vibrations masured again, 2x horizontal component again appeared. high resolution spec. confirms it is 2x shaft only. what to do now. it is 150 kw AC induction motor driving a centrifugal pump.

Anurag

I can't say I ever remember seeing 2x running speed that varied with loosening feet.

Just thinking through it, here are my ideas fwiw:

Since it's not twice line frequency, but it varies strongly with loosening/tightening feet, then one would suspect some kind of resonance.

To confirm, you might try gently prying and wedging on the base and motor while running to see if any significant change in vibration. (Observing safety precautions in case your prybar slips)

Also what would excite a resonance at 2x? Either looseness or misalignment I would guess.
TIOMOAICBTWA

By the way, feel free to start a new thread when you have a question like this... it can get confusing talking about two different machines in one thread.

How does a belt drive motor respond to the running soft foot check? If you loosen the bolts on the side of the motor opposite the belt drive won't belt tension pull the foot up?
I have had some belt drive motors I tried this with and when I loosened the bolt opposite the belt vibration decreased but even after shimming it always came back.

The readings following corrective action are still showing a very prominent (although slightly reduced)peak at 6000cpm axially, and also on the NDE radially. The corresponding velocity waveform shows an almost perfect sine wave at this frequency.

I know that it's foot related, either soft foot or foot related resonance. That has been proven by releasing the feet, but unfortunately I can't work out what to do next. I've got a lot of faith in the guy who did the soft foot/alignment checks, and from personal experience of trying to align these motors myself, I know that the feet are quite flexible which makes things very difficult.

For now I'm going to continue to monitor and watch for deterioration, but any suggestions are welcome.

Ok, here are my 2 rupias.

It could be a resonance, or

We always get high "axial" 2xLF on a motor when the frame is distorted. So how can you have no soft foot and still have high axial 2xLF? The frame can still be distorted if the base is not flat.

If you are a brave person (or dumb like me) and when the operators aren't looking, you could use a pry bar and slide in shims to get the best result. I have never done this on a belt drive, though!

by the way doesn't it take about 7000 rupias to make a dollar?

Here's an update. Things are getting worse.

I've just carried out a bump test and identified a natural frequency in the motor at 6000cpm in the axial direction. The radial natural frequencies are nowhere near this. This explains the high axial readings at 2x line frequency. Congratulations to the people who suggested resonance.

Ok, so what's exciting the natural frequency in the first place. I feel that I've got an electrical issue (2x line frequency with sidebands of 1x rpm), which isn't necessarily caused by soft foot. I've released all bolts individually and in all combinations of two, and although I can alter the amount of vibration by doing this, I think it's because I'm altering the natural frequency. All shims remain firm and I can't get any feeler gauges in.

This motor has been sitting around in the stores for years and I'm wondering if something may have affected it (moisture perhaps?)and some of the connections are not what they should be. Any thoughts? I'm on the verge of calling a motor change.

My gut feel given that we know there is a resonance is that the resonance is the main cause of the problem and the 2*LF forces are just a normal part of what motors do.

But, a quick check for balanced stator currents might be fairly easy, once you find a safe place to get access for your clamp-on. Or with the machine off-line check resistive and inductive balances.

My gut feel has been wrong many times before and I have learned to pay attention to other folks' gut feel. Unbalanced voltages and currents (for example due to high resistance connection) can in theory lead to 2*LF axial vibration. The attached paper provides a math discussion of this exact phenomenon.

Axial_Magnetic_Forces_In_Induction_Machines_at_Twice_Line_Frequency__Good.pdf (195 Kb, 28 downloads)

Thanks for the paper Pete. I've got to admit that I skipped briefly through the maths (I have enough headaches with the vib analysis) , but the conclusions are interesting.

The electrical dep't have already checked out the phase balance and everything is ok.