We have a HT motor of 2900 kw,with a journal bearing having 22mm axial float.Arrangemet is such that motor drive gear box and gear box drives lobe type compressor.

Gear box having a double raw tapper roller bearing.Gear box input shaft having .15 mm axial float.

our problem is that Motor vibration is high.
Motor solorun test is OK less than 1.5 mm/sec.
we are using pinbush type flexible coupling.

Motor manufacturer observation is that gear box because of axial clearance motor vibration is high.

Please suggest,
(1)Cause of motor vibration high
(2)Which bearing to be use for this type of drivers.

22 mm or 0.87 inches is a rather large axial float, but it is still not unreasonable for a large motor. I am not sure what you mean by "pin-type" coupling. The motor rotor should be set in the center of its float (11 mm) and then the motor is moved axially as necessary to get the correct shaft end-gap before connecting the coupling. The shafts should be aligned to precision tolerances for both offset and angle. Cold alignment offsets may be needed to accomodate differential thermal growth and pump piping movement. I always orient the shafts so that single keys are 180-degress apart to minimize unbalance. The gearbox provides the thrust restraint for the motor if diaphram, disk-pack or limited end-float gear coupling types are used to limit motor axial movement.

You simply need to focus on all aspects of motor axial position and shaft alignment to minimize vibrations.

Walt

I'm not familiar with that coupling.

Google shows what the coupling looks like
http://www.renold.com/Products/Couplings/Industrial/PinandBushCoupling.asp

Here is what "Couplings and Joints" by Mancuso says about it:

quote:

"II. PIN-AND-BUSHING COUPLING
In a pin-and-bushing coupling (Figure 9.1 A), resilient drive is provided by rubber-cushioned sleeve bearings. Misalignment is accommodated by compression of the rubber bushing. End float is accommodated by the sliding of hardened-steel drive pins in the bronze inside diameter on the rubber bushing. No lubrication is required. This type of coupling not only transmits torque and accommodates for misalignment and axial travel but also absorbs small amounts of torsional vibration. Pin-and bushing couplings are generally used on small-horsepower applications such as pumps, hoists, conveyors, and paper converting equipment. Misalignment capacity, angular up to 1 degree and offset up to 0.003 in. times the coupling outside diameter. Couplings range from 4 to 15 in. with torque capacities to 125,000 in.-lb."

First they said it was generally limited to small horsepower applications, which this obviously is not. But then they said torque capacities up to 125,000 in-lb ~ 10,000 ft-lbf which can certainly be a much bigger machine.

One essential feature of the coupling is that it must have limited endfloat to prevent the motor from drifting too far axially. I assume most couplings have that, but it's not clear from this picture whether this type does or not. Do you think this is what the motor manufacturer meant by "Motor manufacturer observation is that gear box because of axial clearance motor vibration is high"? Or something else?

Certainly Walt's comments about misalignment are good ones.

A a few more questions:
What does the vibration look like? 1x? Directional? Axial? Occurs immediately upon startup or takes awhile? Higher on motor than gearbox? Was the solo test run done in-place or at a shop?

This message has been edited. Last edited by: electricpete,

Torsional vibration is another possible cause of the high motor vibrations. A torsionally resilient coupling combined with strong torque pulsations from a lobe-type compressor/blower is a good candidate for torsional vibrations. I have measured torsional vibrations on these type of machines up to 2500 hp. If all other issues can be resolved and problem is still present, then measure torsional vibrations or simply change to a different coupling type.

Walt