FUNCTIONS OF A ROLLER BEARING LUBRICANT

Even though the railroad journal roller bearing is inherently an "anti-friction" bearing with precision ground, hard, smooth rolling elements (races and rollers), there exists high pressure contact of microscopically rough, steel surfaces in relative motion in the extremely demanding railroad environment. Therefore, a satisfactory lubricant is required to reduce wear, friction, and corrosion and consequently to achieve expected bearing fatigue life.

The function of the lubricant is to separate the rolling contact surfaces at points of high pressure contact and to minimize friction from surface roughness which can lead to higher contact stresses and surface damage. The lubricant films that separate these surfaces may range from molecularly thin films ("boundary" lubrication) to full fluid films (so called elastohydrodynamic - EHD) which are typically of the order of 20 millionths (0.5 microns) of an inch in thickness. An intermediate lubrication regime ("mixed" lubrication) can also exist, depending on lubricant properties, operating conditions like speed and temperature, and surface finish.

A description of the relative motions between the components of a tapered roller bearing and an explanation of how lubricant films (including EHD films) can be developed and maintained, even at high pressure rolling contacts, are beyond the scope of this particular Forum. It is sufficient here to be aware that the development of such films is dependent primarily on the rolling and/or sliding speeds of the contacting components and a lubricant property called "viscosity." Viscosity of the lubricant is a measure of its resistance to flow that depends on the temperature and pressure in the contact area.

FULL LUBRICANT FILMS SUPPORT LONG LIFE

If the lubricant can form EHD films under normal operating conditions, then the full rolling contact fatigue potential of the bearing can be obtained. In the full-film condition, fatigue failure will be due entirely to subsurface stresses (see BRENCO Technical Forum 90-1) or to stress concentrations around surface defects. Therefore, we strive to promote the use of lubricants that will support a full-film state and thus achieve expected bearing life.

OTHER LUBRICANT FUNCTIONS

There are other lubricant functions that are necessary for optimum bearing performance beyond the full film condition discussed above. One of these is the reduction of sliding friction (such as at the roller end - cone rib interface) and wear when full film lubrication cannot be achieved. In these cases, the molecular structure and chemical characteristics of the lubricant are more pertinent than the strictly fluid mechanical properties. Special lubricant surface chemical reactivity is desirable to enhance boundary lubrication which is essential to prevent bare steel asperity contact and welding. The lubricant must also be chemically stable so that undesirable by-products are not formed.

Another function, served by a lubricant's chemical characteristics, is corrosion resistance or protection. In railroad service, even with effective seals, the steel surfaces must be protected from rust and oxidation accelerated by moisture condensation that can occur on bearing cool down and breathing action in moist, oxygen-rich environments.

Finally, in addition to the resistance to bearing contamination itself, the lubricant must be chemically compatible with the elastomeric seals so as to not degrade their performance.

In railroad bearing applications, the lubricant must perform all these functions and meet all requirements without replenishment or relubrication in the field. This may mean a service life of up to 25 years, in an environment where ambient temperatures range from -50 degrees F to 120 degrees F and at speeds as high as 80 MPH - a big requirement indeed! What lubricant can do all this?

WHAT IS GREASE?

The type of lubricant that can meet this railroad lubrication challenge is grease. But not any grease. Basically, grease is a semi-solid lubricant that is mostly oil (approximately 90%) with a dispersed metallic soap thickener and appropriate chemical additives. The liquid, usually a mineral or petroleum oil, provides the lubrication, and the thickener primarily holds the oil in place and provides varying resistance to flow.

Grease is one of the oldest lubricants used by man as well as a modern one continually undergoing development and finding use even in aerospace applications. It has a history of use as a journal lubricant as far back as 1400 B.C. An analysis of deposits from the axle of an Egyptian chariot indicated the presence of fats and lime and therefore an early use of calcium soap grease.

GREASE COMPONENTS

It is appropriate to discuss grease in detail in terms of its three major components: base oil, thickener, and additives.

BASE OIL
Refined petroleum oils are used as the base oil in most of the greases produced today because they offer a good combination of performance characteristics and cost effectiveness. The synthetic oils used in some lubricating greases are usually chosen because of the specific properties which they contribute to this grease. These properties include lower and/or higher operating temperature ranges than provided by petroleum oils. For example, greases made from certain synthetic oils can routinely meet performance requirements from -100 degrees F to 450 degrees F under lightly loaded conditions.

THICKENERS
Although roller bearing greases may look and feel smooth and "buttery," they have a distinct fibrous structure caused by their "thickeners" as shown in the electron photomicrographs of Figure 1. The thickener, besides controlling many grease properties, also forms a crystalline lattice which is responsible for the semi-solid form of the grease.

ADDITIVES
Chemical additives of many types are often needed to augment or improve performance, or meet special needs. Some additives modify soap; others enhance natural characteristics of the oil, giving it longer life or improving the oil's ability to protect against damage to the rolling contact surfaces.

Antioxidants - Oxidation inhibitors are the most common additives and must be selected to match the individual grease. The objective of these inhibitors is to protect the grease during storage and in service.

Rust and Corrosion Inhibitors - Corrosion can be caused by breakdown of the grease, or by contamination. Under extremely wet or humid operating conditions, the performance of most greases can be improved by a rust inhibitor. Therefore, most high-quality greases contain rust and corrosion inhibitors.

Extreme Pressure (EP) Additives - These additives provide improved load carrying ability and give added protection under shock loads. For roller bearings experiencing high thrust loads, EP additives are often needed to prevent scoring and wear of roller ends. In this and other extreme pressure conditions, the EP agents react with steel surfaces to form a surface or interface (boundary) lubricant which will act like a solid lubricant and prevent metal-to-metal contact or welding.

Not only do EP additives possess different load carrying capabilities and thermal stability, but they also can beneficially affect the grease in other areas such as rust and corrosion protection. However, care must be taken to assure that the additives or their decomposition products do not react adversely with lubrication seal polymers or adversely affect the ability of the grease to hold its gel structure. If this gel structure is destroyed, the base oil and thickener will separate rapidly.

MAJOR GREASE PROPERTIES

Unique to greases are the following characteristics or properties:

Penetration is the measure of hardness or "consistency" and is defined as "the depth in tenths of a millimeter that a standard cone penetrates the sample under prescribed conditions of weight, time and temperature" (ASTM Method D217). The photographs in Figure 2 show a cone penetration test in progress. Inasmuch as agitation or working may alter the hardness of a grease, penetration values are reported on either unworked or worked samples. The latter values are most common and are the basis of the National Lubricating Grease Institute (NLGI) number classifications which are shown in Figure 3.

As an example, the current AAR grease specification requires a worked penetration of 290 - 320 which is an intermediate range between NLGI numbers 1 and 2.

Dropping Point is the temperature at which a grease turns from a semi-solid to a liquid state. It is significant only in that an operating temperature above the dropping point may cause permanent thickener separation or alternation of grease properties. It is not necessarily a measure of the useful upper temperature limit of a product, as very rapid thermal or oxidative degradation of the product may occur at temperatures considerably below the dropping point.

NLGI CLASSIFICATION OF GREASES

Mechanical Stability of a grease, which is its resistance to hardening or softening as it is churned (worked) in a bearing, is important. Unstable greases whose characteristics change drastically in service can result in poor lubrication or other operational problems. The AAR modified elevated temperature roll stability test (ASTM D1831) is used to characterize railroad roller bearing greases.

Lower Temperature Properties are significant in applications where torque requirements are critical, such as cold train starts, and in applications where the ability of the oil to bleed out and coat the races and rollers thus allowing boundary lubrication to develop, is important. Low temperature properties are a function of grease grade, soap type and the viscosity of the mineral oil component.

Water Tolerance is a critical property in wet applications. Certain thickener types, such as sodium soap, tend to absorb water and wash out of a bearing while others, such as calcium soaps are highly water resistant. Additives in specific greases may also have a substantial effect on water tolerance.

Bomb Oxidation Stability is a measure of the oxidation stability of greases in a specific test: ASTM D942. In this test, the sample is placed in a container, or bomb, which is then charged with oxygen and pressurized; a constant elevated temperature is maintained. Oxidation stability is expressed in terms of pressure drop over a given time period.

EFFECT OF GREASE THICKENER ON FLUID FILM BEHAVIOR

The flow or rheological behavior of a grease differs from that of its base oil mentioned previously and as seen graphically in Figure 4. In effect, grease has a "yield point", as opposed to oil, or an apparent viscosity which depends on shear rate.

The fundamental relationship of viscosity and shear rate for a lubricating grease is shown in this Figure. It illustrates that a grease behaves as a semi-solid, and exhibits a very high viscosity at a low shear rate, and yet its viscosity approaches that of the oil in the grease at high shear rates. This property of behaving as a semi-solid at rest but as an oil under shear is the fundamental reason that greases are used. Thus, the grease can remain in contact with the rolling contact surfaces of the bearing, even at rest, and provide the essential lubricating oil when required.

Figure 4

But how does this unique flow behavior of grease affect the important lubricating characteristic of film thickness in the critical rolling contact? Recent research on EHD film thickness has included grease as well as the base oils. The film thickness with grease is usually greater than with the base oil alone, although there may be some tendency of "starvation" due to the relative lack of mobility of the grease or lack of oil bleed. In some cases, the increase in effective viscosity of the grease is several-fold while in others soap make very little difference to film thickness.

SUMMARY

In this first Technical Forum on grease lubrication we have outlined the fundamentals of roller bearing grease lubrication, described grease composition and properties and tests used to characterize the capabilities of a wide range of available greases. As semi-solid lubricants, greases have a complex structure consisting of metallic soap thickeners, base oil and various chemical additives. We must select and use the correct grease with the required fluid mechanical and chemical properties for a particular operating range. Such a grease can create the required thin films to separate the microscopically rough surfaces of rolling elements at critical contact points and allow realization of full bearing life.

We will use this understanding of fundamentals in a subsequent Technical Forum to describe the specific requirements, specifications and limitations of greases used in railroad journal roller bearings.

The Technical Forum is an information resource for the rail industry and is provided as a courtesy of Amsted Rail Group. Suggestions, inquiries or comments are welcomed and should be directed to:

Editor, Technical Forum
BRENCO, Incorporated
P.O. Box 389
Petersburg, Virginia 23804
804-863-1713

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