Need some of ur Thoughts on this problem.

2000Kw, 1500 RPM Alternator driven by steam turbine, gearbox and gear coupling.Axial vib (DE & NDE) max 13 mm/sec at full load purely 1X drops down to 3.5 mm/sec at no load radial readings dont change much max about 4 mm/sec.

No Thermal issue cause vib increases instantly on loading also axial vib highest at 12 o clock position (13 mm/sec) and 5 mm/sec at 6 o clock position). Axial readings at DE & NDE are in phase.

Mechanical unbalance ruled out cause speed is constant at all loads. Could it be an electrically related problem or else what could it be.

Can you provide any recent data both radial and axial data. Hisotrical data will also help. From what you describe, it could mean several things from loose bearing fits, cocked bearing, cracked shaft, locked coupling....etc

Hi Dave,

This is a brand new alternator, problem occured during commissioning of the unit, shop test of the gen perfectly ok all vibes normal(with diffrent driver and setup), can post the plots when i get back to my office.

levels at Drive end & Non drive end nearly similar H- 2.1 V- 4.0 & A- 12.5 mm/sec at full load, axial vib reduces immediately to about 3.0 mm/sec when load is removed and about 8 mm/sec at half load, no thermal effects as the change is instantaneous.Axial vibes are in phase at both ends.

My doubt is also on the locked coupling but i find it strange the radials are not high and the relation between vib levels and load is nearly linear.

From the arrangement you describe I would suspect that the gearbox is a double helical design. In this arrangement usually only one rotor of the gearbox has a thrust bearing and the other rotor is free to float relative to the "held" rotor. Unfortunately axial spacing becomes very critical. If the "free" rotor has any external applied thrust then the torque split is not shared 50/50 across the two helices. This will often cause a jump in axial amplitude as load is increased.

If you can confirm that the gearbox is a double helical then I'd recommend you review the axial spacing setup.

The link http://www.dresser-rand.com/e-tech/PDF%20Files/tp083.pdf will take you to an excellect article titled "Axial Alignment and Thermal Growth Effects on Turbomachinery Trains with Double-helical gearing."

Thanks john it appears to be a double helical gear design im waiting for the drawings from the gearbox manufacturer. i think what u say is quite logical. Have to wait for another seven days as the gen cannot be stopped now due to operational requirements