Oil whirl is a problem related with sleeve type of bearings. This vibration occurs only in the machines equipped with pressure lubricated sleeve bearings & operating at relatively high speed – usually above the second critical speed of the rotor. Oil whirl vibration is often quite severe, but easily recognized because the frequency is slightly less (5 – 8%) than one half the rpm of the shaft.
The mechanism of the oil whirl can be explained by referring to the diagram below.
Under the normal operation, the shaft of the machine will rise up the side of the bearing slightly. How far the shaft will rise depends on the shaft rpm, rotor weight & oil pressure. The shaft, operating at an eccentric position from the bearing centre, draws oil into a wedge to produce a pressurized load bearing film. If the eccentricity of shaft within the bearing is increased from this normal operating position, say be external shock or the load transient, additional oil will be immediately be pumped in to fill the space vacated by shaft thus increasing the oil film supporting the shaft. This oil film may drive the shaft in a whirling motion around the bearing. If damping in the design is sufficient enough then the system will return to its original position otherwise the whirling motion will continue.
Alternately, a lightly loaded bearing may rise under the normal conditions decreasing the clearance above the bearing to a point where an oil wedge forms forcing the shaft back down. In doing so the clearance is restored at the top of the bearing & the oil wedge fails removing the downward pressure.
Steam Turbine bearings are susceptible to oil whirl as they tend to have larger than normal clearances to allow for high oil flows for the cooling.
Causes
Normally associated with the poor bearing design. Other problems are
- excessive bearing wear
- increase in lube oil pressure
- Change of oil viscosity
- External excitation caused by a transmitted in vibration at the natural oil whirl frequency.
Temporary Remedies
Temporary remedies include
- Changing the oil temperature or viscosity
- Increase bearing loading by introducing slight misalignment.
- Scrapping the sides of bearing or grooving the bearing surface to disrupt the lubricant wedge
Improved Bearing Design
Shorter bearings increase bearing load which can help prevent oil whirl
Lemon Pip Design
This is achieved by machining the two shells whilst shims are fitted between the faces
Tilting Pad – Michell type
A thermocouple is fitted to the lowest pad
Grooved
Loss of the bearing material means reduction in load carrying capacity
Nut Cracker
Camella
Formed by boring non-concentric circular bearing surfaces in a bearing allowing the formation of 3 wedges whilst maintaining the correct bearing clearance
