Other than releasing the suspected resonant foot bolt and removing the shims, are there any other methods to correct this problem? The motor in question has two sets of bolt holes per foot, thus having a fairly long foot pad. The equipment packager made use of only the most outer bolt holes for the supporting base. This allowed me to insert a 1.5" x 17" x 22" plate under the motor, bolted to the inner pad bolts. It hasn't helped one bit. As soon as that bolt comes into contact, 1x climbs to 0.3 ips...only 0.08 ips when loose.

Yvan,

Looks like modern motor designs more and more rely on the foundation to support the frame, we have a design around here where the motor feet are covering more than 80% of the length of the motor. In most cases where the foundation is reasonable it is enough to make sure that all feet have full contact for the complete length to the base. If the foundation is less than perfect like having 3 steel piles in a row 12 meter long/deep the cost effective way was to reduce speed and mount a gearbox since the design from start was like 120% "overspeed" with a VFD. You could always try with another motor make that have a different design but that is a gamble, sometimes work. Right now we are looking at a case where the original motors on 4 fans worked fine but was not the "hi efficency" version required so they were swapped to that kind from another brand and now they are all bad.... I am sometimes surprised that the motors these days can be transported to site, maybe we should leave the crate on if that´s the only thing that support them? Olov

Yvan,
You didn't mention if this was an increase in 2x line frequency vibration, but I will make that assumption
The reason you see this problem more and more on new motors has to do with meeting new efficiency ratings. In order to achieve this, the air gap has been made much tighter. That means a greater effect from any air gap deflection issues.
I have seen bases that may have been acceptable in the past cause a softfoot problem that was only uncovered when BOTH foot bolts on the same side rail were loosened (dropped from 0.3 in/sec to 0.02 in/sec)
With such long feet, I suspect you may have a 'sprung foot'. Have you tried step shimming the foot (feet) to handle any planarity situation? You also don't mention if this major reduction is from loosening only one foot.

Good morning fellows. Actually, no there was no significant change in 2x amplitude which in fact is of very low amplitude comparatively speaking. In fact, as the 1x amplitude increases, the 2x decreases. I should mention that this 2 pole motor is on a VFD. The motor operates a Kobelco rotary screw. As it is brought on line, it begins at 2400 rpm, slowly ramps up. At 3450, it comes into resonance, 1x amplitude climbs and an otherwise steady phase angle begins to shift. At max speed of 3580, phase has now shifted nearly 90 degrees. In order to prove resonance, I over drove the VFD beyond 60 Hz and got the motor up to 4000 or so rpm. Then, I nearly had the expected 180 degree shift as I expected when passing fully through resonance. And, as expected 1x amplitude begins to drop. The rear right foot when released, cause an amplitude drop from 0.3 ips to 0.08 ips. There was a 0.004" "mushyness" which I measured with a dial indicator. I even pulled the 7-8 shims which made up the stack and replaced it with a nice 3 shim stack, cleaned the foot and base surfaces, added the required 0.004". When checked again, the dial still shows a 0.003" climb. Although when the foot is released, the shims don't simply slide out from under the foot, the foot must be pryed up and the aid of a vice grip is needed to "slide" them out. I do realize that the shim gap between the motor foot and base isn't perfectly parallel, but what skid mounted compressor package is? I've even conducted some bump tests on the motor and the base. The motor end bell (both ends) show clear resonance in the 3600 cpm area, the base doesn't. The motor manufacturer seems hesitant to admit that they had a problem. They've sent a shop test report which indicates that the motor runs at 0.04 ips, on rubber pads. That is in fact about the same amplitude that I record at that speed, with the right rear leg hanging, but the instant that the bolts is even hand tightened, all heck breaks loose. Anyways, in desperation to resolve this, the equipment packager may try to convince me to have gussets, supports and all kinds of stuff welded on the base. I am resisting this because I simply feel that it will not help. As of right now, I have the foot released, the motor is running smoother that it ever has, the operators are happy, but when the politicians get involved....?

Oh, OLI and Ron, I should thank-you for coming to the rescue so quickly.

quote:

At max speed of 3580, phase has now shifted nearly 90 degrees

Yvan, the "180 degree phase shift with resonance" that we here so much is a bit misleading. If you have 180 phase shift, it means you are well above the 1st critical, NOT running AT resonance. If you are running at resonance, there has been a 90 degree phase shift, which seems to be what you describe. So at the maximum speed, you are probably running right at resonance.

Can you start this machine at less than the initial 2800 rpm? If so, you can record and plot phase as you bring the unit up slowly to maximum speed (or beyond). Then repeat it with the offending foot "loose". This should give you a handle on what is going on.

"Foot related resonance" (didn't Ralph Buscarello coin that phrase?) can be a bear to resolve.

Rusty, I agree with you. The motor runs at resonance as it approaches maximum amplitude at 3580 rpm and 90 degree phase shift. I did exactly what you suggested, plot phase with foot tight and released and concluded the it is "foot related resonance". And yes it was Ralph who coined the term. As a young analyst starting out, he warned that it would be a rare occurence but ones that we would remember. In this case, my client (the packager) had sent my findings and data to the motor manufacturer. Apparently, they were very receptive to the results and agreed that leaving the foot hang would likely be best. That is how this motor is currently running, and everyone involved is smiling.....life is good

Yvan,

Leaving the motor foot hanging is fine for testing, but is not a long-term solution. The resonant structure should be modified to correct the problem.

Walt

quote:

Leaving the motor foot hanging is fine for testing, but is not a long-term solution.

In a 'perfect world' (unlimited resources), I'd agree with this statement. But if this is a fairly small machine with little starting torque, I'm not sure that running with one foot loose is a problem. The problem is probably in the motor, not the base or structure. Eventually the motor will be replaced, and the problem will likely disappear.

But hey, that's just one man's opinion.

You could try that plastic shim stuff that may have some more springy effect than the steel. Olov

Yvan,
We had the same issue last year with an expander generator. It runs at 3,000 rpm. It seemed to be running just over the resonant frequency. On coast down, the vibrations increased from 5mm/sec to 11mm/sec at about 2,600 rpm.
We asked the OEM to test the genny at their shop, which they did, but the increase in vibs remained during the coast down and the phase shifted 180 deg. They finally admitted that there was a fault with the design of the genny. We run it with one foot free. It runs better with the diagonally opposite foot free also but the engineers were uncumfortable with that arrangement, so we have a double nutted stud, slightly nipped. The OEM were subject to a management buy-out after the genny was ordered, so after agreeing there was a design problem they could only offer a redesign if it was paid for.
I havn't seen the ordering specifications although I was told that the limit to the vibrations was 3mm/sec, which seems a bit rough for new equipment, plus the coast down readings were available but our projects people hadn't asked for them. Hmmm.
Regards,
Joe Mc Cormack

I'm unsure if there is such a thing as foot related resonance. If your 180 shift is at or within 10% of your 3580 RPM then it's resonance too close to design speed.

If loosening a foot bolt (CW rotation - bolt on the right) reduces vibration then it's an induced stress problem. What happens to the location of the phase shift? No change! No relation to the foot. Then address the foot problem as it need correcting.

Ever notice turbines having only 3 legs or feet? Why; a three legged stool doesnt' wobble as all feet sit equally on the floor. Four legs and a shim may be required to remove the wobble; same with machinery. And when addressing soft foot also look for wedge foot and you may want to consider "soft-shoe" shims - they do work. I've been using plastic shims since 1967 and soft-shoe shims for a dozen years. Plastic shims do not have a springy characteristic as I can tell.

Sam,
I hear what your saying and I respect your opinion, however, I'm sure that the frame of the genny was in resonance and not the rotor.
Two diagonally feet were jumping and the other two were stable. I sat the genny on top of lead car seals, then measured them and had shims manufactured with the appropriate angles to allow the feet to sit without any softness or uneveness. I never did get to the bottom of it, but it's running now and is on the back burner until it misbehaves.
Regards,
Joe Mc Cormack

Yvan,
The motor manufacturer is correct and you are correct. All two bearing motor designs have two natural frequencies that are considered rigid body modes. The rotor/shaft is NOT bending, but it is 'bouncing' in it's bearings. The first mode has the ends of the shaft in phase and the second mode they are out of phase, but not bending. These modes are considered support dependent. The natural frequencies above these are considered rotor stiffness dependent are therefore called bending modes, where the shaft bends.
The rigid body modes are highly influenced by support stiffness. The stiffness of the bearing in series with the end bracket, in series with the motor base, in series with the floor, etc. etc. The weakness point in the stiffness series is what governs where these two modes will end up.
Testing the motor on rubber is all about moving both of these rigid body modes well below running speed so they don't influence the final vibration test. Once this motor is placed on the OEM's equipment, it is their job to insure that the stiffness is such that the modes do not coincide with running speed. Unfortunately, with a VFD, it is next to impossible to accomplish this.
As I see your situation, you have two options;
Soften the base support until the rigid body modes are below your minimum operating speed (put the entire skid on vibration isolators), or trim balance whatever you can at speed to reduce the 1x component. Remember to do this balancing WITHOUT phase, since the natural frequency will drive your balance software crazy. Use the four run method.
Good Luck!

quote:

Remember to do this balancing WITHOUT phase, since the natural frequency will drive your balance software crazy.

This would be a top candidate for my Old Wives Tale list.

quote:

quote:
Remember to do this balancing WITHOUT phase, since the natural frequency will drive your balance software crazy.


This would be a top candidate for my Old Wives Tale list.

or is that TAIL? I hope no old wives (or the younger ones, either) believe this. Successful balancing of machines near, below, or above natural frequencies can be achieved better using phase and amplitude.

Yvan,

Isn't foot related resonance just an indication of improperly designed supporting structure for this particular motor? With stiffness added when bolt gets tightened up it unfortunately hits the resonance. If a bump test is done at both condition this will obviously become apparent. And if there is a possibility of conducting modal testing, location for changing stiffness can be determined.

Rusty,
I am surprised at you. Bill, well I understand. Never? and I mean NEVER, had a balance job have you chasing in circles due to running on a critical? Well, then I must just have been the only one unfortunate enough to have seen more than a few of these.

At the critical or near, I certainly would try to use shutdown information. Also, I would work hard to duplicate operating contions. I would run the machine until steady state was reached.

As to the 4-run method, don't forget that the amplitudes are changing near the critical also. Phase is just as reliable. The rate of change of phase with respect to speed variation depends upon damping as does the rate of change in amplitude.

If the readings are varying I average them, look for circles on the polar plot (vs time), etc. In other words, I pay attention to the details of the job, as I gather Rusty does.

In fact phase may be more critical in these instances.

Bill,
I can only respond by saying, what works, works. As far as 'paying attention to detail', I don't know what you mean. I know that amplitude is also affected by running at or near critical, and this may mean that my typical trial weight would be much too heavy and a lighter one is called for, but I have found that dealing with only one of the two parameters (amplitude and phase) when dealing with these type of issues has worked extremely well.
Obviously, not intended for multi-plane applications.

Ron, phase information does not change very much when running at a critical. The perception is there is a rapid, 180-degree (or whatever) phase shift as you go through resonance and that is just not the case, not even in theory. Amplitude information cannot be trusted because the amplification factor often does change drastically near a critical. I use the phase information the balancing program returns, but usually not the amplitude info.... you have to "sneak up on" the correct amount of correction weight. At least that's my experience.

Once had a set of 6500 h.p. I.D. fans at the power plant that ran near a critical (as designed, out of the box). Adding as little as 500 grams weight (on a 16,000 lb. rotor) could move the critical above or below the running speed. You could trust the phase info, but not the correction weight info. We finally learned to "detune" the resonance before balancing by "adjusting" the outlet diffuser (using turnbuckles at first, then later on just the diffuser roller jackbolts). It didn't take much buildup on these fans to get them out of balance. We learned to relax the diffuser after we balanced it, and then we could adjust it again after about 3 months to get the vibration back down. This 30-minute process could be done several times before it stopped working, enough to last the 18 months between outages. It was cheaper than the $500,000 per fan that the manufacturer wanted to fix their defective design. Not sure what they did after I left, but this worked for the 10 years I was there.