- Buyer's Guide
Bearing topics and lubricant application topics overlap in process pumps. The main issue here is that not all pumps are designed and sold with provisions ensuring that lubricant is consistently reaching the bearings. Many pumps will benefit from thoughtful upgrading; simply repairing these pumps and restoring the mechanical assembly to as-bought condition won’t reduce the risk of failure. The relevance of this statement is best understood by observing the disappointingly high rate of repeat pump failures in industry. Repeat failures can occur only if the root cause of a problem hasn’t been found or, if it is known, someone deliberately chose not to remedy the problem.
Among the often-overlooked fundamental causes of repeat process pump failures is cage-induced windage, the unidirectional blower effect of slanted ball separators (cages) in angular contact bearings. The effect of this windage on oil flow illustrates the interdependence of bearing design and lubrication matters. Needless to say, concerns for bearing initial cost and the often misguided desire to standardize or consolidate product selection are responsible for bad bearing choices. Not all pump designers are aware of this particular fact. This article brings it to your attention because best-of-class pump users make it their business to upgrade the design and move beyond mere restorative maintenance efforts.
Oil bath or oil sump lubrication is one of the oldest and simplest methods of oil lubrication; only grease lubrication is older than oil bath lube. The bearing rolling elements pass, or “plough”, through a portion of this oil sump as the shaft revolves (Figure 1). Oil bath lubrication is feasible unless and until too much frictional heat is generated by the plowing action of rolling elements at excessively high speeds. Because heat reduces oil film strength and accelerates the rate at which oil oxidizes, the oil bath lube method is avoided on process pumps whenever DN (the inches of shaft diameter [D] multiplied by shaft revolutions per minute [N]) exceeds 6,000.
To illustrate the DN approach, it can be reasoned that a two-inch shaft at 1,800 rpm, with its DN value of (2)(1,800) = 3,600, would operate in the suitable-for-oil-bath zone, where lubricant would reach the rolling elements and where (see Figure 1) no oil rings would be needed. However, pumps incorporating a two-inch shaft operating at 3,600 rpm (DN = 7,200) would use oil rings (sometimes called “slinger rings”) to lift or spray the oil from a sump with its level maintained below the bearing. Less frictional heat results from lower oil levels (Figure 2) than from an oil level reaching the center of the bearing elements (Figure 1).
The DN limit of 6,000 is an experience-based value that takes into account real-world conditions of misalignment and a host of other factors that make the actual pump environment different from test-stand practices. Be prepared to have it disputed by pump manufacturers for the usual reasons.
Figure 1. A typical pump bearing housing with oil level reaching to the center of the lowermost rolling elements. Here, keeping the DN values below 6,000 reduces the risk of oil overheating.
A bearing housing with lower oil level and intended for DN values in excess of 6,000 is shown in Figure 2. Bearings with DN values in excess of 6,000 will require the addition of either a flinger disc (as shown in this figure) or an oil ring (Figure 3) or similar lube application component to dependably lift or spray-feed oil into the bearings. However, oil rings are potentially vulnerable components. They will not interact the same with lubricants of different viscosities or at different immersion depths. Unless used on perfectly horizontal shaft systems, oil rings will run downhill and then often make contact with the bearing housing.
Note that already in the 1970s, a then-prominent U.S. pump manufacturer recognized the pitfalls of oil rings. Its advertising literature pointed out that this company’s reliable pumps incorporated an “anti-friction oil thrower ensuring positive lubrication to eliminate the problems associated with oil rings”. Some European-made pumps have avoided the pitfalls of oil rings by incorporating flinger discs; they have had good success for decades.
To resist deformation while operating, oil ring manufacturers must include an annealing step to relieve stresses. Oil rings tend to become progressively more unstable as DN values approach or exceed 8,000. Instability means that the oil rings skip, skew, misalign and abrade. While the oil ring shown on the left side of Figure 3 shows the chamfered edges and moderate contact pattern of an oil ring in its near-new condition, the one on the right has abraded to the point where the chamfers are no longer visible. Good maintenance practice would be to measure the new ring at assembly and to again measure it at the time of repair. The width difference represents lost metal; the lost metal became an oil contaminant and will have caused the bearings to fail prematurely.
To get oil rings to function as designed, the shaft system must be almost perfectly horizontal. Ring immersion in the lubricant must be in the right range – usually close to 5/32 of an inch or 8 to 10 millimeters below the oil level. Moreover, to avoid ring abrasion and dangerous oil contamination, ring eccentricity must be within 0.002 inches (0.05 mm), and surface finish should be reasonably close to 32 and, at most, 64 RMS. Oil viscosity should be in close range of typical ISO VG 32 properties and temperatures must be in the moderate range. These different and equally important parameters are rarely all within their respective desirable range in actual operating plants. If several of the individual parameters are just “borderline acceptable”, oil rings will intermittently malfunction. That, then, is an elusive failure cause.
It has been pointed out that grooved oil rings perform slightly better than the plain or flat oil ring variety. Also, certain plastics perform a little better than the brass or bronze rings typically found in pumps. It is more commonly known that oil rings work better with ISO Grade 46 oil than with ISO Grade 68 oil. (A well-formulated synthetic Grade 32 may be required to suit both the constraints of oil rings and the needs of a particular pump bearing).
Here is the bottom line for the truly reliability-focused: Because oil ring behavior is very difficult to control, some reliability-focused purchasers try to avoid them. These users often specify and select pumps with large-diameter flinger discs in DN applications where lower oil levels are needed. Small-diameter flinger discs are sometimes used in oil bath applications. Labeled “oil throwers” in Figure 4, small-diameter discs are intended to simply prevent temperature stratification of the oil. Without them, hot oil would tend to float at the top of the oil sump. Due to their relatively small diameter, they aren’t the equivalent of large-diameter discs. Where the latter throw oil into the bearings for the purpose of lubrication, the former are used in bearing housings with oil levels reaching the center of the lowermost bearing balls. They serve to keep oil temperatures uniform in pumps with DN values below 6,000.
Figure 2. Bearing housing with oil level lowered to accommodate high DN values. A flinger disc lifts or sprays oil into the bearings. Note that oil pressure/temperature-equalizing passages must be provided at each bearing.
Note that the double-row radial bearing in Figure 4 was, in the 1960s, called a “line bearing”, and take a careful look at the balance holes in the bearing housing near the top of each of the two bearings. Then-prominent manufacturer Worthington Pump Company knew the importance of achieving pressure balance throughout the entire pump bearing housing. Oil leakage risk past the oil seals (lip seals in this very old design) was greatly reduced by keeping all pressures equal. Pressure balance is also important because it largely neutralizes the potential windage effects of certain slanted bearing cage configurations or angularly inclined cage orientations.
Flexible flinger discs are sometimes used to enable insertion in the simplest bearing housings. In these, the housing bore diameter is smaller than the flinger disc diameter. To accommodate the preferred solid steel flinger discs, bearings must be cartridge-mounted. With the cartridge design depicted in Figure 2, the effective bearing housing bore (i.e., the cartridge diameter) will be large enough to allow insertion of a steel flinger disc with the needed diameter.
Of course, providing such a cartridge will add to the cost of a pump. Yet, in the overwhelming majority of cases, the incremental cost of the cartridge design will be low compared to what it costs to repair a pump. The same can be said for balance holes or passageways; they are needed to ensure housing internal pressure and temperature equalization, and add pennies to the cost of a pump. Internal pressure and temperature balance between the central volume of the bearing housing and the spaces between bearings and housing end caps is essential. This requirement appears to be disregarded in some pump models. In some cases, it would be best to provide equalization passages at both top and bottom. Lack of these passages is one of the explanations for oil leakage and overheated oil.
Overheated oil and/or oil contaminated with slivers of black O-ring material will result in “black oil.” This is an important comment since old-style bearing protector seals are often designed with a dynamic O-ring in close proximity to sharp-edged O-ring grooves.
Also, note that the 1960s-vintage bearing housing of Figure 4 shows lip seals and water-cooling provisions. The lip seals shown would no longer be acceptable; modern bearing protector seals (shown in figures 1 and 2) would be used instead. Cooling water was deleted from pumps with rolling element bearings in the early 1970s.
Figure 3. An oil ring in as-new (wide and chamfered) condition on the bottom, and one in abraded (narrow and chamfer-less) condition on the top.
Cooling is still found in pumps with high speed and heavily loaded rolling element bearings. But cooling of the oil is very rarely needed and often of no benefit in installations with rolling element bearings. Suppose a cooling jacket restricts the bearing outer ring from free thermal growth in all radial directions. However, the bearing inner ring heats up and grows, causing bearing internal clearances to vanish. An excessive preload could result.
Similarly, immersing cooling coils in the oil will cool not only the oil but the air in the bearing housing. Such cooling then tends to promote moisture condensation and harmful oil contamination. Therefore, cooling water has been deleted from every pump with rolling element bearings at many best-of-class (BOC) locations. Since the late 1970s, there no longer is cooling water on bearing housings of pumps with operating (fluid) temperatures up to and including 740 degrees Fahrenheit (394 degrees Celsius) in modern BOC oil refineries.
Because cooling water ports are shown on a pump drawing, the user is led to believe that such cooling is either needed or helpful. Commenting again on Figure 4, when it was discovered that cooling was no longer needed, BOC companies began to leave these cooling water drains open. With modern synthetic lubricants and properly selected rolling element bearings, cooling is no longer used in process pumps. So, irrespective of lube application method, cooling will not be needed on rolling element bearings as long as high-performance synthetic lubricants are utilized.
With sleeve bearings, cooling is still used in order to maintain optimum oil viscosity through close temperature control. This provides a reasonably stable environment for oil rings in pumps equipped with sleeve bearings. Circulating systems are the primary choice for large pumps that incorporate these bearings.
Very large process pumps use sleeve bearings and circulating oil systems. In circulating systems, the oil can be passed through a heat exchanger before it is returned to the bearing. Pressurization is needed to move oil through filters and exchangers, but the bearing itself is rarely fully pressurized. In some sleeve bearing systems, oil rings lift the oil and deposit it onto the shaft surface. In other sleeve bearings, an oil spray from suitably placed nozzles is directed to oil grooves with good effect and very high reliability.
Bibliography and Suggested Reading About the Author Heinz Bloch works as a consultant for Process Machinery Consulting. He is the author of more than 400 technical papers and similar publications. He has written 17 books on practical machinery management and oil mist lubrication published by major engineering publishers. To learn more, e-mail Heinz at email@example.com or visit www.heinzbloch.com.
Figure 4. This 1960s-vintage bearing housing uses “oil throwers” to keep the oil at uniform temperature. Note the pressure balance holes provided at the top of the radial and thrust bearings. (Source: Worthington Pump Installation and Operating Manual, 1966)