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our 15MW motor has higher vibration(pk-pk) on DE - x - 20µm and y - 35µm at 1400rpm. we have seen it running beffore over years @ x 10 and y 10µm.

Absolutely no vibration change on the gearbox side - coupling is a lamiflex coupling.

so we checked the alignment - per laser and with 4x dial gauges to double check - and we saw that the motor is vertical 0,3mm deeper than the gearbox and horizontal 0,1 mm out of range. Initially the motor was calculated and hopefully alingned to be 0,3mm higher than the gearbox and 0,0 at horizontal

we will realign it anyways but i wanted to know based on your experience - how much influence will/can have this missalignment on the vibrations?

thanks in advance



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Just to clarify, by "15 MW" you mean 15 megawatts (about 21000 HP)?  This would probably mean fluid film bearings throughout.

Can you provide a sketch of the machine?  And very specifically show the hand of assembly of the gearbox and the directions of rotation.  I'm wondering if you've taken into account the movements of the gearbox rotor in their bearing due to the forces generated by gear load.

thanks john,

yep 15 MW means 15 Megawatts with sleeve bearing(checked and OK - new OIL!) - enclosed see the scetch with initial measurements, not de actual ones - direction of the motor is anticlockwise when you have the motor in the back

thanks in advance!horizontalvertikal


Images (2)
  • horizontal
  • vertikal

You sketch shows the gear to the left of the pinion as I look into the low speed shaft  With anticlockwise rotation, due to forces from the mesh, the pinion will move up and right.  The gear will want to go down at about an angle corresponding to 7:30 on a clock face.  It actually will roll up the left side of the bearing.  Based on the rating and motor speed I estimate the journal size on the low speed shaft at about 250mm to 300mm (10 to 12 inches) so bearing clearance is likely about 0.5 mm (16 - 20 mils).  So the upward motion of the low speed due to it rolling up the side of the bearing should be relatively small.  The growth of the casing will be the much larger factor.

Depending on who made the gearbox you should find anticipated growths on what is termed the dimension drawing.  Have you verified what you are calculating against that diagram.  I've attached a sample of the diagram I'm referring to.  The units on the measurement are inches by the way.



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  • Capture

I have no idea how the word "dilatation" applies.

In aligning a gearbox with fluid film bearings, if the desire is to optimize alignment, then there are four movements that need to be accounted for:

Vertical thermal growth of the casing

Horizontal thermal growth of the casing

Vertical motion of the rotor within the bearing due to gear generated forces

Horizontal motion of the rotor within the bearing due to gear generated forces

Those four movements determine that "operating point" on the graphic you've provided.  I assume that the four quantities tabulated are the variables I've mentioned but can't be certain due to the partial graphic.

There is one quantity that can sometimes be neglected, that being the horizontal thermal growth of the casing.  Usually a gearbox casing is doweled to its soleplate directly under one of the two rotors and often the pinion.  If the gearbox casing is doweled to the sole plate under the gear (as opposed to pinion) centerline then it can be ignored; only the pinion would be affected by the growth.  If the dowel pin is positioned under the pinion, the the growth should be accounted for in doing alignment to the gear.

If you have records of the alignment, you should determine if the alignment has changed.  If the alignment has changed, investigated causes to correct as needed.

There should be a tolerance to the desired cold alignment.  0.1 mm may easily be acceptable.  A difference of 0.6 mm may be unacceptable, but check.


well we have the record from the initial alignment - and it seems that it has changed as initially the motor was higher than the gearbox - and we will realign it but from your experience -> will it solve our problems with the vibration on DE bearing side of the motor?

the flex coupling we use should also compensate some misalignment ...

Simon, as I am reading this topic, it is impossible to say it will solve the problem. Go step by step, take systematic approach. If it doesn't solve the problem, you will make further steps. The re-alignment won't make any harm that's for sure.

Another point, actually two, you should have a similar graphic relative to the motor that shows the various thermal growths and rotor movement in the motor bearings. That should be incorporated into your efforts that are striving for shafting that is reasonably co-linear while operating.

The other point is when interpreting these rotor movement diagrams, check closely for any conditions on temperature.  As an example, at my long ago former employer, the diagram was based on a nominal 100 deg F temperature rise.  In actual service, if only a 70 deg rise was experienced, then the thermal growth portion of the total movement had to be reduced proportionally.

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