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 (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:
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:
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 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:
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 ndm 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…
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:
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:
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.
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.
This “Certification Matters” series continues in the July/August issue with a discussion of the “Basic Principles of Gears.”