Oil whirl within Journal Bearings

Oil whirl is probably the most common cause of subsynchronous instability in hydrodynamic journal bearings. Typically, the oil film itself flows around the journal to lubricate and cool the bearing. This develops an average speed slightly less than 50 percent of the journal surface speed (Figure 1).

Normally, the shaft rides on the crest of an oil pressure gradient, rising slightly up the side of the bearing somewhat off vertical at a given, stable attitude angle and eccentricity. The amount of rise depends on the rotor speed, rotor weight and oil pressure. With the shaft operating eccentrically relative to the bearing center, it draws the oil into a wedge to produce this pressurized load-carrying film.

Figure 1. Oil Film Within a Journal ¹

If the shaft receives a disturbing force such as a sudden surge or external shock, it can momentarily increase the eccentricity from its equilibrium position. When this occurs, additional oil is immediately pumped into the space vacated by the shaft. This results in an increased pressure of the load-carrying film, creating additional force between the oil film and shaft. In this case, the oil film can actually drive the shaft ahead of it in a forward circular motion and into a whirling path around the bearing within the bearing clearance. If there is sufficient damping within the system, the shaft can be returned to its normal position and stability. Otherwise, the shaft will continue in its whirling motion, which may become violent depending on several parameters.

- James E. Berry, Technical Associates of Charlotte, PC

Oil Whip Instability

Oil whip occurs on those machines subject to oil whirl when the oil whirl frequency coincides with and becomes locked into a system’s natural frequency (often a rotor balance or critical speed frequency). If we take an example, when the rotor speed increased to just above 9,200 RPM, its speed increased to 2X its first balance natural frequency. At this time the oil whirl which was approximately 43 percent of RPM, was brought into coincidence with this critical speed. The oil whirl was suddenly replaced by oil whip - a lateral forward precessional subharmonic vibration of the rotor. At this point, the oil whip frequency remains the same, independent of the rotor RPM. Note that the oil whip frequency never changed even though the machine continued up in speed to 12,000 RPM. When a shaft goes into oil whip, it's dominant dynamic factors become mass and stiffness in particular; and its amplitude is limited only by the bearing clearance. If it's not being corrected, oil whip may cause destructive vibration resulting catastrophic failure – often in a relatively short period of time.

So make sure:

1. check your bearings (i.e clearance etc)
2. oil specs, viscosity-wise and type
   
3. oil pressure and temperature
4. leakage
5. internal damping

A slight mistake will cause you thousands of dollars. Sometimes 30 seconds mistake will cost you 100k (if it involves compressors) and it will be your most expensive lesson ever. Common mistake is temperature, so watch your temp!!

Knowledge is power.