Journal bearings are a very simple form of bearings because they contain no rolling elements. The journal is the portion of the shaft that slides over the bearing surface. The advantage with journal bearings is that they are inexpensive, simple and have a high load bearing capacity. If maintained properly, these types of bearings can have a very long service life.
Some forms of journal bearings contain a lubricant between the journal and the bearing wall; this essentially creates a frictionless bearing that is extremely useful for high power machinery that experience large loads.
Journal bearings can be lubricated hydrostatically or hydrodynamically. Hydrostatically lubricated bearings have an external pump that maintains a static amount of pressure in the lubricant, whereas in hydronynamic bearings the pressure in the lubricant film is maintained by the rotation of the journal.
Oil whip can occur due to the design of the journal bearing, misalignment of the shaft, or due to a sudden shock or impact to the machine. If the shaft moves off its center due to load, eccentricity, or imbalances, it will create a large gap on one side of the bearing and a small gap on the opposite side, as shown in the figure below.
Figure 1: Uneven gaps between the shaft and bearing that may lead to oil whirl and oil whipThe lubricant rotates at an average speed which is less than half of the shaft speed; it speeds up as it travels through the narrow gap between the shaft and bearing, and slows down as it exits the gap. The speeding up and slowing down causes turbulence in the fluid known as oil whirl. The shaft rotates with this wave of turbulence and induces a sub-synchronous vibration which is approximately 40-50% of the running speed.
When a machine that shows signs of oil whirl is run above its second critical speed, the oil whirl frequency approaches the first critical speed; a resonance condition arises under these conditions and leads to oil whip. Oil whip is a more severe condition that can cause significant damage to the machine if left untreated.
The proper design and use of lubricated journal bearings can prevent oil whirl and oil whip. Some of the parameters that affect the fluid stability and lead to these phenomena can include:
- Excessive wear of the bearing
- Large clearances between the bearing and shaft
- Changes in the fluid pressure of the lubricant
- Uneven loading and misalignment of the shaft
- Lubricant leakages