Bearing topics and lubricant application topics overlap in process pumps. The main issue is that not all pumps are designed and sold with provisions ensuring that lubricant is consistently reaching the bearings.1 Many pumps will benefit from thoughtful upgrading; simply repairing these pumps and restoring the mechanical assembly to as-bought condition will not reduce failure risk.
The relevance of this statement is best understood by considering the disappointingly high rate of repeat pump failures in industry. Repeat failures can occur only if (a) the root cause of a problem has not been found or (b) if the root cause is known and someone deliberately chose not to take remedial action.
Among the often overlooked fundamental causes of repeat failures of process pumps we find cage-induced windage, which is the uni-directional blower effect of slanted ball separators (also called “cages”) in angular contact bearings.1,2 The effect of this windage on oil flow illustrates the interdependence of bearing design and lubrication matters. Needless
Lubricant level and oil application
Oil bath or oil sump lubrication is one of the oldest and simplest methods of oil lubrication, and 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 greatly 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 2? 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, as shown in Figure 1, no oil rings would be needed. However, pumps incorporating a 2? 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 disregarded and even disputed by pump manufacturers for the usual reasons …
Issues with oil rings
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 here) or an oil ring (Figure 3) or similar lube application component to dependably lift or spray-feed oil into the bearings. But oil rings are potentially vulnerable components. They will not interact in the same way 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. We should 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”.2 Some European-made pumps have avoided the pitfalls of oil rings by incorporating flinger discs; they have had good success for many decades.