Extending the Life of Pump Bearings Through Maintenance and Technology

Pumps & Systems, August 2008

Progressive maintenance practices are vital to maximize pump life, minimize downtime and avoid costly repairs and lost production. It is good practice to plan the detailed groundwork of maintenance programs, along with contingencies for various maintenance events to minimize downtime.

Good maintenance programs should encompass maintenance activities for all possible failure modes, such as wear, seal damage and bearing failure. Bearing maintenance is one opportunity in pump maintenance optimization since bearings are vital components in a pump rotating system. Additionally, bearing technologies can significantly reduce pump maintenance. State-of-the-art, severe duty material technologies can yield superior wear resistance and longer fatigue life.

Centrifugal pumps, the most common industrial pumps in the world, come in a variety of configurations and sizes. Figure 1 shows a common overhung impeller centrifugal pump. This system permits the shaft to rotate freely while minimizing mechanical loss. It typically includes two or three bearings. The inboard side (non-drive end) bearing supports part of the radial load caused by the cantilevered impeller and is able to float axially to allow for thermal expansion of the shaft. The outboard side (drive end) bearing also supports the radial load, but it is fixed to keep axial shaft movement to a minimum. This maintains an acceptable clearance between the impeller and housing.

For these bearings to continuously perform their function, it is important to know how they commonly fail and what to do to prevent or predict that failure. This opens the door toward consistent reliability, potentially extending the life of the pump bearing system beyond conventional standards.

Figure 1. Diagram of typical overhung impeller centrifugal pump

Instead of being reactionary, maintenance should be predictive, preventive and proactive. We will discuss pump bearing failure modes to help improve and develop the preventive and proactive approach of pump bearing maintenance.

Causes for Bearing Failure

The most common causes for bearing failure are contamination and poor lubrication; however, excessive loading and bearing damage prior to operation-during assembly or handling-can also be culprits. In many cases, clear indications point to the root cause, so try common solutions first.

When more than one failure mode exists, more thought must be used to determine the sequence of failure. This is not always easy because there are multiple possible causes for every failure mode. Use what is known about the application and reference the pump manufacturer's specifications and recommendations to eliminate as many situations as possible. Good failure mode analysis pays dividends and should support the development of maintenance strategies.

 Figure 2 explores some common bearing failure modes.

Contamination

Contamination usually occurs in three forms: dust/dirt, metallic particles and fluids, such as water or chemical solutions. It is highly recommended to keep the bearing in its packaging until it is ready for installation. This eliminates the chance for dust particles in the air to stick to open type bearings, which typically have a rust preventive oil coating. When handling an unpacked bearing, beware of dirt, grease or other contaminants on hands, rags and surfaces that can make their way into the pump or the raceways of the bearing.

Once contaminants enter the internal components of a bearing, a chain reaction starts. When a rolling element runs over small particles, it creates dents that result in high, localized stresses on the raceway and the rolling element. This will lead to further denting, fatigue cracks, spalling and eventually early bearing failure. Once spalling occurs, small metallic particles from the bearing can enter the oil and lead to further damage. It is just as important to make sure that the lubricating oil is kept clean as it is to ensure contaminated oil is not supplied to the pump. Transfer oil in clean containers and avoid using dirty rags when handling funnels or replacing caps.

In some applications, fluid contamination can be a big issue. Seals are important to keep the lubrication in and the pumped medium out. Regularly check any joints or connections that could allow water or contaminants into key mechanical locations of the pump. If contamination might be an issue, regular lubrication analysis can be performed.

To check for dirt or metallic particles, run the pump to mix the lubricant and take a sample from the top of the reservoir. To check for water contamination, let the lubricant settle and take a sample from the bottom of the oil sump to see if water is present. If fluid contamination exists, follow the pump manufacturer's oil servicing instructions for ensuring contaminated oil is replaced with clean oil and filled to the correct level. Bearing life can be extended on problematic applications where contamination is difficult to control with specialized bearing material heat treatments that offer superior wear resistance. 

Excessive Loading

Excessive loading can drastically lower bearing life. This can occur due to the following: misalignment, cavitation, flow is well above or below the pump best efficiency point (BEP), the bearing is exposed to temperatures above tolerable levels. When the flow rate is less than 50 percent of BEP, not all of the medium will exit. This creates an irregular distribution of pressure on the impeller, which can result in high radial loads that the bearings must carry. Check with the pump manufacturer to determine the BEP and recommended flow rates.

Excessive heat can be transferred to the bearing rings from the shaft and the housing. Although methods for bearing cooling exist, the inner ring may still expand, creating less clearance between the raceways and rolling elements. This will result in higher contact pressure and reduced bearing life. When anticipated, these conditions can be accommodated for by selecting a bearing with a higher than normal clearance. However, if too much clearance is provided, the bearings will not be able to maintain pump shaft rigidity. Misalignment, seal leakage and increased impeller/housing wear can occur as a result.

Misalignment

Misalignment commonly occurs when the pump shaft is coupled to the motor shaft. The coupling will pass reactionary loads to the pump and motor bearing systems. The reactionary loads will differ based on the type and material of the coupling.

Misalignment can also occur when the pump temperature rises from ambient temperature to operating temperature. To check for this, run the pump. Once it reaches operating temperature, turn off the pump and check the alignment. If necessary, realign the shafts. Follow the pump manufacturer's recommendations for proper alignment techniques.

  

  

Checkpoints Prior to Assembly

Prior to pump assembly, consider these bearing related matters. Shaft and housing fits vary depending on size, temperature and loading conditions. An oval housing will create an uneven distribution of contact pressure on the bearing rolling elements. Excessively loose shaft and housing fits can lead to creep and fretting on the bore or outer diameter of the bearing rings, eventually resulting in vibration and bearing failure. Bearing shoulders must be perpendicular to the axis of rotation to prevent misalignment. Bearing experts often work with pump manufacturers in the design process to ensure robust specifications are established for bearings and associated interfaces. Verify that the shaft and housing fits are correct and within the pump manufacturer's specifications.

Internal damage to bearings can occur during pump assembly. Exert force only on the bearing ring being mounted during installation. Installing the bearing incorrectly by exerting force on the free ring can produce axial brinells on the raceway shoulders. One method to avoid creating axial brinells during mounting is to expand the bearing inner ring by heating it to 230-deg F to 250-deg F. Axial brinells can also be created by shock loading the bearing, such as dropping it or hitting it with a hammer. Axial brinells can result in denting on the rolling elements and transfer marks on the raceways, which can significantly reduce bearing life.

Steps to Avoid Vibration

Vibration can result from many causes. It has already been mentioned that improper alignment, contamination and damage from mounting can lead to vibration. It is also critical to check the shaft straightness. This can be checked by laser aligning or using a pair of dial indicators and magnetic bases.

Vibration can also be transferred by a faulty motor or imbalance of the pump rotating assembly. Vibration and temperature monitors can assist in data acquisition at the bearing locations. Monitors can indicate atypical pump operation, thereby avoiding more serious pump issues prior to a potential catastrophic failure.

Conclusion

The initial cost of a pump is small in proportion to the overall life cycle cost of a pump. Energy and maintenance are the primary cost drivers over a pump's life. Maintenance should be tracked and recorded on a regular basis. Historical maintenance data is the basis for reliability analysis and maintenance optimization. These general pump bearing practices can increase pump application life.

Advanced technologies such as ultraclean steels, optimized contact angles and specialized heat treatments can extend bearing fatigue life two to five times over conventional bearings without changing the bearing size. As maintenance and reliability techniques improve, keep in mind what technology can do to provide improved reliability for difficult applications, extend pump life dramatically and ensure confidence for mission critical applications.