Certification Matters, Part II: Review Of Bearing Principles

Key to the reliability of equipment and processes everywhere, these components present special lubrication-related challenges.

This article is the second in an ongoing series focusing on the major components of the lubrication certification exams administered by the Society of Tribologists and Lubrication Engineers (STLE) and the International Council of Machinery Lubrication (ICML). (Please refer to pgs. 10-14, LMT January/February 2011 for more information on STLE and ICML certifications.) In the March/April 2011 issue, we discussed “The Fundamentals of Lubrication.” Here, we explore the most common equipment component. Bearings are found on all types of machines. How they are treated is critical to the reliability and uptime of systems and processes across the board.   

Remember: While this series of articles is based on the content of STLE- and ICML-suggested training modules, they’re published here only as an informational framework for individuals seeking lubrication certification. Candidates will need to to engage in substantial additional study to develop the degree of in-depth knowledge that’s required to pass a certification exam.

Bearings have three major functions: 1) to reduce friction; 2) to support a load; 3) to maintain alignment. The two major types are journal (sleeve) and rolling element (also called anti-friction bearings).

Journal bearings
Journal bearings (as shown in Fig. 1) have a larger surface area and carry heavier loads than rolling-element bearings. They are employed in turbines, compressors, transportation equipment and many other applications where support of a heavy load is required. Characteristics and features of journal bearings include:

• The most common journal bearing is the “split” (or “segment”) type, wherein the top half can be readily separated from the bottom half.
• To protect the journal (or shaft) from wear, the inner surface of a journal bearing is soft and sacrificial. This soft inner surface allows particles to become embedded in it, thereby minimizing shaft damage from particulate contaminants. The most common material used on the inner surface is called Babbitt. Tin-based Babbitt, consisting of tin, copper and antimony, has replaced lead-based Babbitt and is now the most popular type.
• Journal bearings are very effective at handling radial loads (i.e., perpendicular to the shaft). Large journal bearings are lubricated with a circulation system where oil is introduced through a hole on top of the bearing and distributed along the bearing via axial grooves for constant loads. (Some use circumferential grooves for variable loads.) It’s important that the grooves be placed away from the load zone. Some smaller journal bearings—such as those found in large pumps, electric motors and process-steam turbines—are lubricated by the use of a slinger ring on a shaft in an oil bath. The ring is 1.5 to 2.0 times larger than the shaft and rotates along with it. The oil is thrown from the ring, which is immersed in the oil to a depth of 1/8” to 3/8” from the inside bottom of the ring on the shaft and distributed to the bearing. The limiting factors on the use of slinger rings are the speed and viscosity of the oil.
  Since journal bearings don’t handle thrust or axial loads that occur parallel to the shaft, thrust bearings must be used with them for these applications. For large equipment like compressors and turbines, Kingsbury bearings are employed. These bearings feature a tilted-shoe design that’s positioned on a shaft close to a collar and lubricated hydrodynamically through a circulation system.
• A journal bearing is lubricated by the rotating shaft forming an oil wedge between itself and the bearing. Boundary, mixed and hydrodynamic lubrication regimes occur as shaft speed increases. The major variables in achieving a hydrodynamic regime through the increase in the fluid film are viscosity and speed—which are related directly to the film thickness and indirectly to the load. Load is another variable that is indirectly related to film thickness.

Journal-bearing failure modes…
Figures 2 and 3 show two journal-bearing failure modes. The fretting damage in Fig. 2 results from vibration in stationary bearings causing metal-to-metal contact between the shaft and bearing inner surface. The fatigue damage reflected in Fig. 3 can be caused by the generation of surface and subsurface cracks through overload or bridging of a particle between the shaft and bearing surfaces. Such conditions lead to spalling (the release of material causing pits on the bearing surface). Other failure modes include:

• Babbitt fatigue
• Lack of lubrication and improper grooving to distribute lubricant
• Babbitt wiping with rotor contact
• Abrasive particle damage
• Varnish (especially on thrust bearings)
• Electrostatic discharge damage
• Cavitation
• Improper installation

Fig. 4.  The type of rolling element incorporated in a bearing is what gives the bearing its speed and and load-carrying ability.

Rolling-element bearings
Rolling-element bearings are classified into two major families: ball and rolling element. While these families have a lower load-carrying ability than journal bearings, they often are operated at higher speeds because of lower surface contact. As shown in Fig. 4, the type of rolling element that a bearing employs is what gives the bearing its speed and load-carrying ability.

The bearing that can operate at the highest speed is the ball type—because of the minimal surface contact between ball and the raceway. The spherical and tapered rolling-element types, however, have greater load carrying ability.

Unlike journal bearings, the rolling-element bearings can handle some thrust load. Both angular contact ball and tapered roller bearings can handle moderate levels of thrust but in only one direction. Therefore, they need to be paired to handle thrust in both directions. Some rolling-element bearings are designed to handle only thrust and no radial loads. The most common rolling-element bearing is the deep groove single-row ball bearing illustrated in Fig. 5.

As shown in Fig. 5, the major components of a rolling-element bearing are the inner ring, outer ring, rolling element and cage. The only bearing that has no inner ring is the needle type, where the elements are directly attached to the shaft.

Rolling-element bearing manufacturers classify bearing life as the amount of time any bearing will perform in a specified operation before failure. They typically use the L-10 rating for this determination.

The L-10 rating is defined as the number of revolutions that 90% of a group of identical bearings under identical conditions will endure before the first sign of fatigue failure occurs. Fatigue is defined as when a spall with an area of 0.01 in² or more develops regardless of the bearing size. Two of the major factors that influence bearing life are speed and load. Life is inversely proportional to speed. Doubling speed lowers bearing life by 50%. Load is even more detrimental to bearing life. By doubling the load, bearing life is reduced by nearly 85%.

Lubrication of rolling-element bearings…
The lubrication regime for rolling-element bearings differs from that of journal bearings. Take a ball bearing as an example: The contact between the ball and raceway—called the “point contact”—is quite small. This generates high pressures because the load is carried through the ball and, thus, supported by just a small surface area. The oil is trapped between the ball and raceway into a film thickness less than one micron and behaves like a solid to provide protection. The large pressures trapping the oil film result in deformation of the ball and raceway to support the load. This lubrication regime is called elastohydrodynamic, and it occurs primarily where there is rolling contact in non-conforming surfaces. Desired properties of rolling-element-bearing lubricants are summarized in Table I.

Selection of the correct viscosity is the most important consideration in lubricating a rolling-element bearing. Major bearing manufactures have minimum requirements for viscosity at the operating temperature. For example, the minimum requirements for the following bearings are:

• Ball Bearing -13.2 cSt
• Cylindrical Roller -13.2 cSt
• Spherical Roller -20 cSt

Normally, the “K factor”—the use of a higher viscosity than the minimum calculated—is applied when it comes to lubrication of rolling-element bearings. Some bearing manufacturers recommend 2.0 to 4.0 times the calculated viscosity to extend bearing life. This results in higher heat generation from the thicker oil and greater energy consumption. More typical values used are 1.2 to 2.0 times calculated viscosity. A more accurate way for determining the proper viscosity than using minimum recommended values involves the bearing speed factor:

By using the bearing speed factor formula and referring to the manufacturers’ tables, a more accurate viscosity can be determined for a bearing at the operating temperature. To determine the correct viscosity, you must convert the viscosity at the operating temperature to the viscosity at 40 C by using the viscosity temperature table for the particular lubricant base stock type.

The bearing speed factor number can also be useful in determining the limiting speeds for the use of grease. For example, 350,000 nd_m is the maximum speed for grease- lubricated ball bearings and 150,000 is the limiting speed for spherical roller bearings. Of course, there are exceptions to this rule. Special greases with low-viscosity oils have been used in ball bearings with speeds up to 1,000,000.

Rolling-element bearing failure modes…

• Poor maintenance
• Poor design
• Ineffective sealing
• Electrical arching across bearing
• Wrong bearing for application
• Overload or excessive speed
• Insufficient lubrication
• Incorrect lubrication
• Oil deterioration
• Temperature variation
• Contamination
• Incorrect assembly/installation
• Misalignment
• Incorrect clearances
• Improper seating
• Vibration
• Fatigue

There clearly are many ways bearings can fail—the most common being contamination- and lubrication-related. Unfortunately, even a perfectly lubricated and maintained bearing will eventually fail through fatigue. Failure modes are classified in the following categories:

• Fatigue
  • Subsurface
  • Surface-Initiated
• Wear
  • Abrasive
  • Adhesive
• Corrosion
  • Moisture
  • Fretting
• Electrical Erosion
  • Excessive Voltage
  • Current Leakage
• Plastic Deformation
  • Overload
  • Debris Indentation
• Handling Indentation
  • Fracture
  • Forced
• Fracture
  • Forced
  • Fatigue
  • Thermal Cracking

Rolling-element bearings are usually temperature-mounted with an interference fit. Temperature-mounting methods include oven, induction-heater and oil-bath.
NOTE: DO NOT USE A TORCH!

Handle with care
When working with bearings, the following best practices should be employed:

1. Store bearings in a clean environment.
2. Dunk bearings to clean them.
3. If you drop a bearing, discard it.
4. Stack bearings no more than five high.
5. Store bearings a minimum of one foot off concrete.
6. If you touch a bearing, oil it.
7. For safety reasons, don’t air-spin a bearing.
8. Assemble a bearing with assembly lube.
9. Micrometer the fits on 1/8ths.
10. Read fits to 0.0001 inches.

Conclusion
Bearings are critical components in all types of machinery and processes. Basic understanding of them is essential in applying lube best practices and enhancing reliability.

For more details on bearing lubrication and the selection of correct viscosity, refer to previous articles in this publication. (You can search archives on www.lmtinfo.com and www.mt-online.com.) LMT

Acknowledgements
The author wishes to thank Bob Matthews of Royal Purple for sharing his bearing knowledge and allowing the use of his best practices for bearing care in this article.

Coming Up
This “Certification Matters” series continues in the July/August issue with a discussion of the “Basic Principles of Gears.”

Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training for operations around the world. Telephone: (281) 257-1526; email: rlthibault@msn.com.