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Hi guys,

We have a horizontal overhung pump motor arrangement. Recently we have observed 2xLF on the motor in horizontal direction and the same 2xLF is also seen on the pump. (Its not 2X. We took high res data and confirmed its 2xLF)

Motor horizontal - 4.5 mm/s - Only 2xLF peak, vertical and axial values are around 1 mm/s

Pump horizontal - 7 mm/s - only 2xLF peak, vertical and axial values are around 2 mm/s

Motor solo was taken and the maximum vibration was 3.5 mm/s with 2xLF as the only peak. So the first thing we do is check for soft foot. By dial method on each foot and loosening the bolt and checking the dial reading, we haven't found any soft foot. The dial was always around 0.01 mm or 0.02 mm.

So we started to loosen each bolt in running condition to see if the vibration is changing. But no change. However when all the bolts are loosened then the vibration all of a sudden goes down to 0.3 mm/s

(Also we involved civil guys to check for the natural frequency of the foundation and after their evaluation, it is found that the natural frequency of the foundation is way below the 2xLF.)

And the solo run of the motor at our workshop is also very less at 0.5 mm/s. So we are thinking the motor foot is ok but the equipment base is not. We are planning to check the blue matching of the motor foot on the foundation.

Any thoughts?

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Just a thought.....I can picture a situation where the the location of the plunger of the dial indicator may not respond at all if the back/hard-to-reach section of the feet is not mating properly (I believe some call it angular soft foot). With the fasteners all loose, you could try to see if a (say a 0.002") shim can be inserted in any of the four corners of each foot.  The fact that the 2E goes down suddenly when bolts are loose indicates to me you've narrowed your search to this as the cause.

Curious, your second sentence says you recently observed this occurring. Does this mean it was not showing up previously? If so, what recent changes might have happened? or, was it always there and not detected?

Negash,

For a soft foot to result in a 2 LF, it needs to influence the air gap. Otherwise, regardless of the way it is measured or its value, the soft foot by itself will not produce a high 2 LF. The effect depands on the way the structure will deform.

Maintenance staff usually pay attention to what they "see". The base/foundation of a motor, when the motor is removed, is exposed and can be easily inspected for dirt, cracks erosion etc. The lower face of the motor's foot, on the other hand, is not visible except during the removal period (by a crane for instance) unless the maintenance staff pay special attention. At least, this is what I noticed.

After all hold-down bolts were undone and the vibration is low, did you observe the vibration as you tighten the bolts again? 

Do you have another identical motor (healthy) next to this, perhaps as a stand-by? Maybe you can swap the motors to eliminate/confirm the possibility of base related issue. It might me less expensive than sending the motor back to the shop for testing /inspection.

 

Regards- Ali M. Al-Shurafa 

@Registered Member The 2XLF was there earlier too but the amplitude was less for us to take any action. And the stand by pump also has the 2XLF as the dominant peak but at a very low amplitude.

@Registered Member Yes all 4 bolts tightened vibration is high. If we loose them one by one no major change observed. But all 4 loose saw a drop in vibration. And swapping the motor with the standby is not possible in running plant as it requires at least one pump running. Motor inspection at workshop will be cheaper for us as we have a contract with them. And we are not suspecting any air gap variation as the motor was tested at our in house workshop and at a third party workshop. Both the locations reported very low solo run vibrations.

@Registered Member its a 62 kW motor. Sorry I don't have a picture with me right now.

Yasaswy posted:

@Registered Member its a 62 kW motor. Sorry I don't have a picture with me right now.

I have a personal experience before regarding small  size motors that the distortion is caused by the electrical supply cable. I cannot exactly remember what design (whether supply cable on the side or at the top) caused it. When you said all feet loosen and vibration down, it rang a bell to me. But off course it might not be your case.

Last edited by Registered Member

We have one electromotor 110kw (simense) and pump (KSB) quite similar to this problem in company, The electromotor vibration is 18mm/s in the horizontal direction (but in the vertical and axial direction about 2 mm/s.  all 4 bolts tightened vibration is high. If we loose them one by one no major change observed. But all 4 loose saw a drop in vibration. 
we checked structure, it was ok

we checked soof foot , they were ok

we checked crack on soof foot and case with PT test but they were ok

we dont know , what do we do?!!!

You should make your own posting and reference the original post. 

What is the shaft speed?

What is the vibration frequency?

Can you feel motion across motor base and grout top?

What is the condition of the grout under motor and anchor bolts?

Can you post photos of the motor, base and foundation?

Has good shaft alignment been done?

Has a structural impact test been conducted to detect resonance?

Walt

 

3DD2B42B-D46B-4A0C-B744-E0DF417F76DA@Registered Member posted:

You should make your own posting and reference the original post. 

What is the shaft speed?

What is the vibration frequency?

Can you feel motion across motor base and grout top?

What is the condition of the grout under motor and anchor bolts?

Can you post photos of the motor, base and foundation?

Has good shaft alignment been done?

Has a structural impact test been conducted to detect resonance?

Walt

 

shaft speed is 2990 rpm

vibration frequency is 2XLF (100 Hz) 

Yes i have photo and i will send

when electromotor dis couple or couple not different on vibration

We Strengthened the structure  but not changing on vibration

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  • 3DD2B42B-D46B-4A0C-B744-E0DF417F76DA
@Registered Member posted:

Hi guys,

We have a horizontal overhung pump motor arrangement. Recently we have observed 2xLF on the motor in horizontal direction and the same 2xLF is also seen on the pump. (Its not 2X. We took high res data and confirmed its 2xLF)

Motor horizontal - 4.5 mm/s - Only 2xLF peak, vertical and axial values are around 1 mm/s

Pump horizontal - 7 mm/s - only 2xLF peak, vertical and axial values are around 2 mm/s

Motor solo was taken and the maximum vibration was 3.5 mm/s with 2xLF as the only peak. So the first thing we do is check for soft foot. By dial method on each foot and loosening the bolt and checking the dial reading, we haven't found any soft foot. The dial was always around 0.01 mm or 0.02 mm.

So we started to loosen each bolt in running condition to see if the vibration is changing. But no change. However when all the bolts are loosened then the vibration all of a sudden goes down to 0.3 mm/s

(Also we involved civil guys to check for the natural frequency of the foundation and after their evaluation, it is found that the natural frequency of the foundation is way below the 2xLF.)

And the solo run of the motor at our workshop is also very less at 0.5 mm/s. So we are thinking the motor foot is ok but the equipment base is not. We are planning to check the blue matching of the motor foot on the foundation.

Any thoughts?

We have same this problem 

"We Strengthened the structure  but not changing on vibration" What exactly was done?

It is difficult (especially without experience) to modify a structure without understanding how it is moving/deflecting. An operating deflection shape (ODS) test with two accelerometers, or an impulse-response test with instrumented hammer and accelerometer are two methods to quantify structural deflection.

I could tell stories of how this exact problem was solved several times. If you want to try an experiment to add mass to detune the structure, then here is a practical method that has worked:

Remove the two motor lift-lug eye-bolts and get two bolts with same threads of suitable length

Make two steel weights (start with about 25-lbs each and about 50-lbs maximum) from steel blocks or plates (less than 12-inches square)

Make hole in center of each weight (drill or torch-cut) for bolt

Bolt each weight tight to motor case at the two lift-lug locations

Run motor with base bolts tight with no soft foot present. Note that shims should be snug at all four base locations when base bolts are loose to minimize soft foot.

If the vibration level at 2x line frequency (100 Hz) is substantially reduced, then a natural frequency was present and controlled. Either leave weights in place, with appropriate engineering review, or remove weights and evaluate other vibration control methods.

If you try this, then please provide before and after vibration measurements.

Walt

 

@Registered Member posted:

"We Strengthened the structure  but not changing on vibration" What exactly was done?

It is difficult (especially without experience) to modify a structure without understanding how it is moving/deflecting. An operating deflection shape (ODS) test with two accelerometers, or an impulse-response test with instrumented hammer and accelerometer are two methods to quantify structural deflection.

I could tell stories of how this exact problem was solved several times. If you want to try an experiment to add mass to detune the structure, then here is a practical method that has worked:

Remove the two motor lift-lug eye-bolts and get two bolts with same threads of suitable length

Make two steel weights (start with about 25-lbs each and about 50-lbs maximum) from steel blocks or plates (less than 12-inches square)

Make hole in center of each weight (drill or torch-cut) for bolt

Bolt each weight tight to motor case at the two lift-lug locations

Run motor with base bolts tight with no soft foot present. Note that shims should be snug at all four base locations when base bolts are loose to minimize soft foot.

If the vibration level at 2x line frequency (100 Hz) is substantially reduced, then a natural frequency was present and controlled. Either leave weights in place, with appropriate engineering review, or remove weights and evaluate other vibration control methods.

If you try this, then please provide before and after vibration measurements.

Walt

 

 

Foundation test: We evaluated the moisture and quality of the concrete under the equipment

Strengthen structure: We added a 10 kg weight to the base with 4 bolts, to strengthen the structure and change the resonance, but the vibrations did not change at all.

My device hasn’t ODS test, If necessary, we must provide another new device

In adjusting soft food when the electromotor is discouple, we checked each bolt with the least shim, but vibrations increase when the third or fourth bolt is tightened.

Notice: At the start of the electromotor, when the motor power is star mode, the vibrations are 4 mm/s in the horizontal direction, but after 10 seconds, when power is in the triangular mode, the vibrations suddenly increase to 16-18 mm/s

I'm sure the vibration frequency with high resolution is 2XLF (100-101 Hz), The amplitude of vibrations (2X) is shorter.

I didn't understand how your experimental test was done. Please let me know a little more clearly (or in a simple photo).

"We evaluated the moisture and quality of the concrete under the equipment" Not much use for evaluating vibrations.

"We added a 10 kg weight to the base with 4 bolts, to strengthen the structure and change the resonance, but the vibrations did not change at all." The weight you installed was probably not effective in changing natural frequency. A simple "bump" test could indicated this.

Bolt the weights (steel block or stack of plates) in place of the lift-lugs as shown.

Walt

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@Registered Member posted:

"We evaluated the moisture and quality of the concrete under the equipment" Not much use for evaluating vibrations.

"We added a 10 kg weight to the base with 4 bolts, to strengthen the structure and change the resonance, but the vibrations did not change at all." The weight you installed was probably not effective in changing natural frequency. A simple "bump" test could indicated this.

Bolt the weights (steel block or stack of plates) in place of the lift-lugs as shown.

Walt

We did your experiment and added mass

we added (4 mass 10 kg )

vibration change from 16.2 to 9.8 (vibration is been better but isn’t in standard rang)  

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Mohammad, 

It looks like a resonance case. A practical solution can be stiffening the base but I prefer to having an impact test and more checks prior to the modifications.

If this was not a problem during the commissioning, a structural problem might have occurred and inspection is necessary. Cracks, sheared anchor bolts and loose nuts are typical.

 

Regards- Ali M. Al-Shurafa

@Registered Member posted:

Mohammad, 

It looks like a resonance case. A practical solution can be stiffening the base but I prefer to having an impact test and more checks prior to the modifications.

If this was not a problem during the commissioning, a structural problem might have occurred and inspection is necessary. Cracks, sheared anchor bolts and loose nuts are typical.

 

Regards- Ali M. Al-Shurafa

We checked:

Foundation test: We evaluated the moisture and quality of the concrete under the equipment

Strengthen structure: We added a 10 kg weight to the base with 4 bolts, to strengthen the structure and change the resonance, but the vibrations did not change at all.

In adjusting soft food when the electromotor is discouple, we checked each bolt with the least shim Thickness, but vibrations increase when the third or fourth bolt is tightened.

we checked crack on case electromotor and structure by PT test ( blew picture  

I was really confused0F60E768-2370-4032-A0D0-4060D19CDFEF

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