Figure 1. ANSI standard pumps typically rely on such bearings to handle thrust loads.
Design variations are becoming more popular. Examples include steeper (40°) contact angles to deliver increased thrust capacity, machined brass cages to offer robust performance under heavy-duty and poor-lubrication conditions, reduced axial internal clearances to promote load sharing between the two rows of balls and a reduced possibility of skidding in the inactive ball set, and ABEC-3 (P6) tolerances to contribute better control of the bearing’s mounted condition and smoother bearing operation.
Specialized angular contact ball bearing sets. For pumps with minimal thrust, paired 15° angular contact bearings generally suffice. However, for high-thrust-load conditions, use high-performance matched sets of 40° and 15° angular contact ball bearings to provide improved robustness by reducing the susceptibility of ball skidding in the inactive bearing. These designs are intended for centrifugal pumps that don’t reverse or only periodically reverse.
The primary benefit of these sets is that the 15° bearing is designed with considerably less internal clearance than the 40° bearing, making it less susceptible to centrifugal and gyroscopic forces producing ball sliding and shuttling, while providing additional radial stiffness to maintain integrity of the shaft and seals.
Split-inner-ring angular contact ball bearing sets. These, by matching a single-row 40° angular contact ball bearing with a split-inner-ring ball bearing or four-point contact ball bearing, are designed to accommodate thrust loads in either direction. This arrangement commonly is used in vertical pumps to handle the primary thrust load but also can be utilized in horizontal arrangements, provided the loading is such that the split-inner-ring bearing doesn’t support radial load on its own.
Because two bearings acting in tandem share the thrust load, this arrangement offers an extremely high thrust-carrying capacity. Reversing thrust load can be accommodated on the backside of the split-inner-ring bearing. These two-bearing sets behave like “triplex” sets with the added advantage of saving space and costs.
Supplying the lubrication
Proper lubrication for pump bearings is essential for reliable service — improper lubrication accounts for more than 30% of bearing failures, according to some studies.
Good lubricants primarily provide a separating film between a bearing’s rolling elements, raceways and cages to prevent metal-to-metal contact and undesired friction that otherwise would generate excessive heat that could cause wear, metal fatigue and potential fusing of the bearing contact surfaces. Adequate lubrication for bearings also acts to inhibit wear and corrosion and help guard against contamination damage.
The common methods for the effective lubrication of pump bearings include:
Grease. Easy to apply, grease can be retained within a bearing’s housing and provides extra sealing protection. Depending on the rotational speeds and operating temperatures, relubrication may be required to combat short grease life.
When the operating conditions allow, “greased-for-life” bearings, which eliminate requirements for relubrication and related maintenance tasks, can offer an attractive alternative.
Oil bath. This option establishes an oil level at the center of the bearing’s bottom rolling element and represents the comparative baseline of bearing friction among the lubrication methods. Best results over time can be achieved using a constant-level oiler.
Oil ring. In this method, an oil ring is suspended from the horizontal shaft into an oil bath positioned below the bearings. The rotation of the shaft and ring flings oil from the bath onto the bearings. The lower oil volume in the bearing reduces the viscous friction in the bearing system to allow higher shaft speeds and better cooling.