Proper Selection and Lubrication of Bearings

Written by David R. Mikalonis, SKF USA Inc.

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The primary benefit of these sets is that the 15-deg bearing is designed with considerably less internal clearance than the 40-deg 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

This bearing set encompasses a split inner ring ball bearing, or four-point contact ball bearing, which is designed to accommodate thrust loads in either direction matched with a single row angular contact ball bearing (40-deg). This arrangement is commonly used in vertical pumps to handle the primary thrust load but can also be utilized in horizontal arrangements, provided the loading is such that the split inner ring bearing does not support radial load on its own.

Because two bearings acting in tandem share the thrust load, this solution offers an extremely high thrust-carrying capacity. Reversing thrust load can be accommodated on the back side of the split inner ring bearing. These two-bearing sets behave like "triplex" sets with the added advantage of saving space and costs.

Delivering the Proper Lubrication

Proper lubrication for pump bearings is essential for reliable service, especially given some industry findings that show improper lubrication can account for more than 30 percent of bearing failures.

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 further acts to inhibit wear and corrosion and help guard against contamination damage.

Among the common methods for effective lubrication of pump bearings:

Grease. Grease is easy to apply, can be retained within a bearing's housing, and offers extra sealing protection. Depending on the rotational speeds and operating temperatures, relubrication may be required to combat short grease lives. As an attractive alternative when the operating conditions allow, sealed "greased-for-life" bearings have been developed to eliminate the need for relubrication and related maintenance tasks.
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 into the bearings. The reduced oil volume in the bearing reduces the viscous friction in the bearing system to allow higher shaft speeds and better cooling.
Oil mist and air-oil. In this case, oil is atomized and carried by an air stream to the bearing. Among all pump bearing lubrication approaches, this process generates the least amount of friction (allowing rotational speed to be based on the bearing design instead of lubrication limitations) and creates a positive pressure within the bearing housing (preventing invasive contaminants).

Regardless of the lubrication method, users should always specify lubricant according to the demands on vertical shafts and resistance to solids, pressure, temperatures, loads and chemical aggression. Where pump locations may be difficult to access, fully automatic delivery systems can be integrated to enable timely, proper and effective delivery.

Thinking "Outside the Pump"

Beyond suitable pump bearing selection and lubrication, these additional actions are recommended to further optimize pump performance:

Align systems accurately with precision tools to minimize vibration and potential bearing damage.
Conduct "energy audits" to identify opportunities for improved efficiencies in operation and costs.
Implement proactive, strategic reliability practices (including condition monitoring) to detect and remedy problems before they can escalate.
Introduce online systems with sensors for round-the-clock condition monitoring.
Involve operators in pump condition monitoring.
Promote cross-functional teamwork, communications, and feedback on pump conditions and potential corrective actions.

David R. Mikalonis is sales manager-pump industry for SKF Industrial Division, SKF USA Inc., 1510 Gehman Road, Kulpsville, PA, 215-513-4812, Fax: 215-513-4475, www.skfusa.com.

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