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Oil Whirl

Oil whirl is a condition peculiar to journal bearings. It manifests itself as a vibration of less than

one-half rotational speed. It is caused by a lightly loaded bearing riding up on its high-pressure

wedge and going up over the top and around. The journal is actually revolving around inside

its bearing opposite the direction of rotation at about 45 percent of the rotating speed (Fig - 1 ).

The 45 percent, or less than one-half rotation speed, comes from the average fluid velocity with

some slippage.

Figure 1 Pump cavitation symptoms are broadband vibrations typically from 3000 to 5000 Hz.

Figure 2 Oil Whirl

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Figure Shaft Motion During Startup

Figure Oil Film Within a Journal

The cure is to increase the load on the bearing. This usually requires a redesign of the bearing

and is best handled by the bearing or machine supplier. Some redesigns are narrower bearings,

axial grooves, pressure dams, lobed journals, or tilting pad bearing. The narrower bearing

increases the load on the journal. The other redesigns break up the symmetrical oil flow pattern.

There are some temporary measures that can be taken to alleviate oil whirl. This is a destructive

condition, and temporary relief is usually required until a permanent fix can be redesigned. The

temporary measures are to change the oil viscosity, by adjusting the oil temperature or a

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different oil. Another temporary measure is to run the machine in a more loaded condition.

Introducing some small misalignment to load the bearings and reduce oil whirl has been applied

successfully as a temporary measure.

Oil whirl is aggravated by excessive bearing clearance. This should be recognized as a condition

of looseness. The motion of the shaft within its journal can be reduced by increasing the load on

the machine. This pins the shaft against one side and limits its freedom of motion. Therefore, a

test for oil whirl is to increase the load on the machine and observe a decrease in the

subsynchronous vibration. Another test is to measure the bearing clearance. This can be done

by lifting the shaft with a pray bar or jack and measuring the movement with a dial indicator. A

general guideline for journal bearing clearance is:

0.002 + 0.001(d) in

Where d = shaft diameter, in

For example, a 6-in-diameter shaft should have a diametral clearance of

0.002 + 0.001(6) = 0.008 in

Figure - 3 shows a condition of oil whirl on a 400-hp motor hermetically sealed in a centrifugal

compressor. The motor turned at 3,600 rpm (60Hz). The 22.5-Hz vibration is 38 percent of the

running speed of 60 Hz. This is due to oil whirl in the journal bearings caused by excessive

clearance. At higher loads this 22.5-Hz amplitude decreases. This 22.5 Hz also decreased when

these worn bearings were replaced with new ones that had a smaller clearance.

This section on oil whirl has been brought up so that you can recognize the symptoms as a sub

synchronous vibration at less than one half rotation speed. This condition appears only with

journal bearings, typically on larger machines.

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Figure 3 Spectrum from a 400-hp motor showing oil whirl at 22.5 Hz.

When oil whirl becomes severe, there is a potential for the shaft to rub the inside of the journal.

This causes friction and subsequent localized heating. A rub is a symptom and can be caused by

other factors, most notably heavy imbalance or severe misalignment. The rub is a metal-to-

metal contact and as such, shows up best in the time domain. In the frequency domain it is not

so clear. The friction is a broadband high frequency vibration that can excite resonances. Themost notable effect of rubbing is a localized temperature rise followed by metal particles in the

oil. Both of these should be used as supporting evidence to confirm a suspected rub. The

analysis of rubbing based on vibration spectral data is not defined well enough to analyze this

condition with confidence. The best indicator, in terms of vibration, is an increase in the overall

vibration level, followed by observing the time domain view for evidence of metal-to-metal

contact.