Figure 1. In a typical large-pump bearing housing, two oil rings dip in the oil and fling the lubricant into a slanted trough. The oil runs downhill and enters the bearings. Two limiter screws and two oil ring observation ports are indicated.
- Problems in the pump’s drive end might be the cause of repeated failures.
- Bearing protector seals of an appropriate design can promote better bearing lubrication and improve pump reliability.
- The manner in which oil gets to the bearings plays an important role in reliability.
Repeated failures are evidence of undiscovered or unresolved flaws. That said, you should remember that repeat failures can happen only for one of two possible reasons:
- the true root cause of failure remains hidden
- the true root cause is known but isn’t pursued.
Flawed drive-end components in process pumps are little understood, although they contribute to some elusive repeat failures that involve bearings in these fluid movers. Here are some pointers and explanations.
Avoid excessive heat
Some oil-lubricated pumps operating at 3,600 rpm can produce excessive bearing heat if the oil level is set to reach the center of the bearing balls at the 6 o’clock position. Choose a considerably lower oil level to avoid the “plowing effect” of rolling elements that produce frictional power loss and heat. However, with oil lubrication and lowered oil levels, the oil must be lifted and somehow guided into the bearings.
Two choices present themselves: oil rings or slinger rings (Figures 1 and 2) and flinger discs (Figure 3). In the first instance, an oil ring of a given size and weight might have been designed for ISO Viscosity Grade (VG) 32 oil but can’t perform optimally with ISO VG 68. Always be mindful that synthetic ISO VG 32 might be well suited for pumps, whereas a mineral ISO VG 32 lubricant wouldn’t have adequate film strength and film thickness in warmer climates.
Figure 3: A flinger disc is used for lifting oil from the sump. The angular contact thrust bearing set is placed in a cartridge. Upon assembly, this provides an opening of sufficient diameter for the disc to pass through.
Lubricating oil might have to be conveyed or “lifted” from the oil sump into the bearings. Free-running oil rings often are used to do this lifting. However, there are caveats and elusive sources of bearing problems with free-running oil rings.
- An oil ring sometimes leaves its shaft groove path and gets wedged in one of the long limiter screws.
- Oil rings tend to contact a housing’s internal parts if the shaft isn’t truly horizontal. Figure 4 shows an oil ring moving all over the place, probably intermittently touching the inside walls of the bearing housing.
- An oil ring can malfunction unless oil viscosity and oil levels are maintained within a narrow acceptable range.
- Oil ring eccentricity must be maintained within 0.002 in. Bad oil rings can exceed this standard by a factor of 30.\An oil ring shouldn’t distort, which means that it should be manufactured in three stages: (1) pre-machining, (2) stress-relief annealing, (3) finish machining. Cheap oil rings spell trouble.
- There also is sensitivity to bore surface roughness and oil ring bore finish. Both must be approximately 16 to 32 RMS.
Figure 4. Oil rings are sensitive to shaft horizontality and other geometric variables. Markings and measurements can highlight the extent of abrasive wear.
Regardless of the lubrication application method used, additional risk issues should be considered. Many pump thrust bearings use cages (ball separators) that are angularly inclined, and the thrust bearings in figures 1 and 2 are among these. Cages in angular contact bearings can act as small impellers that promote “windage flow” from the smaller toward the larger of the two cage diameters.
There should be an oil return slot (approximately 3/16 in. wide and 1/8 in. deep) at the 6-o’clock position of each bearing housing bore. Verify that these passages exist on any pump that is oil (sump) lubricated. Note that the passage is missing from the thrust bearing side of Figure 2 and from both bearings in Figure 3. Oil getting trapped between the bearing and its housing end cap will overheat.