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Figure 1: Illustration of good grease lubrication, mobility and corrosion protection. |
In the case of railroad bearing applications, the lubricant must meet all AAR requirements and perform satisfactorily under a wide range of environmental conditions without replenishment or relubrication in the field. As a practical matter, such time periods extend for the life of the wheel set, or until the bearings must be removed because of condemning limits on the wheel. Depending on mileage accumulated per year, the required grease life might range from as little as 2 years to more than 15 years.
In order to perform satisfactorily, the elastohydrodynamic ("EHD") lubrication films provided by the grease must be able to separate rolling and sliding surfaces and support hundreds of thousands of pounds per square inch of pressure without being squeezed out of the contact region. In recognition of the frequent periods of low speed operation of the railroad bearing, the grease must provide satisfactory boundary lubrication as well.
Another requirement that must be met by a grease's chemical characteristics is corrosion resistance, or protection of bearing components, in particular the rolling surfaces. In railroads 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 a moist, oxygen-rich environment. (See Figure 1)
In addition to helping resist bearing contamination itself, the grease, which lubricates the seals as well as the bearing, must be chemically compatible with the elastomeric seals so as not to degrade their performance.
Before reviewing some current field service experience, grease limitations, and efforts to develop an improved grease, we shall take a brief historical overview of the development of the current AAR M-942 grease specifications.
In the early 1950's the AAR established the first specification to control the quality of greases used in the journal bearing application. By the early 1960's the bearing manufacturers proposed that a heavier consistency grease would be beneficial and the AAR adopted a Grade B grease requirement with an as-supplied 290-32- cone penetration range. During this time, bearings on cars in interchange service required relubrication every 36 months. This requirement often caused over lubrication and, on occasion, car set out due to elevated bearing temperatures caused by grease churning. In response, the concept of No Field Lubrication (NFL) was introduced in 1976.
The longer life requirement of NFL service was supported by the goal of extending bearing fatigue life via thicker EHD films. This meant a higher viscosity base oil for the grease. However, since a higher viscosity oil might be expected to generate more heat at high speed, greater oxidation stability was required. In addition, adequate low-temperature grease performance required control of the extent to which viscosity would increase with drop in service temperature. Research indicated that the control of an oil property called viscosity index (VI) of the base oil could offset the potentially undesirable excessive increase in viscosity at low temperature. Therefore, a higher VI requirement of 80 was established.
The stability, or cone penetration measures, of a grease's hardness may be expected to change in service. As a result of reports in 1977 and 1978 of grease leaking from bearing seals that was attributed to excessive grease softening, an increase in the required dynamic mechanical stability of the grease was introduced in 1978. The cone penetration values of the grease after a 48-hour bearing vibration test was reduced from an acceptable range of 320-350 to a stiffer or harder one of 290-320, identical to the as-supplied grease. Further requirements of grease stability at high temperatures were introduced in 1989 with the addition of the elevated temperature roll stability test.
Greases approved for AAR interchange service must be smooth, well manufactured products that are free from corrosive and abrasive matter, with restrictions on certain thickeners and additives. They must exhibit properties defined by laboratory tests described in AAR Specification M-942. These test requirements are placed on the grease components, thickeners, petroleum oils and chemical additives, as well as on the bulk grease in small-scale laboratory and full-scale bearing tests. While not required, extreme pressure (EP) additives may be included in greases if they meet seal compatibility and the other test requirements.
Effectiveness in Service
What are the factors that reduce the lubrication effectiveness of AAR approved greases in service? Basically there are two primary factors: temperature extremes and contamination. In addition, currently more demanding service conditions on load, speed and life (in response to extended wheel life, Electro-Pneumatic brake developments, freight car truck design improvements, and improved maintenance practices) have also promoted the targeting of long grease life, mobility, and all-climate capability.
Elevated temperature limitations
There is an accelerated deterioration of grease properties at elevated temperatures. As has been seen, there is a reduction in oil viscosity with temperature increase and consequently, a decrease in EHD film thickness. This can lead to reduction in rolling contact element fatigue life, or an increase of raceway peeling and smearing.
Elevated temperatures increase the propensity of grease to slump and purge out of the seals. Also, the undesirable oxidation of conventional grease and its components increases exponentially with temperature increase. For example, above approximately 160 F, every 18 F temperature increment is estimated to shorten the potentially effective grease life by 50%. Further, there is a maximum short-time tolerance of grease meeting this specification to temperatures of 280 F. At such high temperatures chemical additive effectiveness also deteriorates rapidly with exposure time.
Extreme low temperature limitations
There are operating limits for greases currently meeting the AAR specification at the other end of the thermal operating range, in low temperature environments. At very low environmental temperatures (-50 F and lower) the races may become dry of lubricant and unprotected. At these low temperatures, oil may not bleed from the stiff, channeled (immobile) grease preventing the replenishment of lubricant films and as a result the rolling surfaces can be "starved" of the critical oil film. The absence of such films with their protective additives can lead to rapid corrosion and rust depending on subsequent service or storage conditions.
A cup roll track showing corrosion at roller spacing due to lubricant "starved" condition, is shown in Figure 2
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Figure 2: Illustration of corrosion on cup raceway due to lubricant "starved" condition. |
Cold start ups at extreme low temperatures also means there is usually no effective seal lip lubrication. In fact, elastomeric seal lips can suffer significant wear and tear damage in such start up conditions.
Finally, at very low environmental temperatures, the starting or rolling resistance may be very high. In fact, under certain extreme conditions of cold temperature, load, and wheel/rail friction, the high viscous resistance of the roller bearing grease may be sufficient to actually cause wheel sliding. Other low temperature grease characteristics were cited in an ASME Rail Transportation Division technical paper by Leedham and Glass in 1992.
Grease Contamination
The other major factor limiting the lubrication performance of AAR greases, or any grease, and a significant contributing cause of roller bearing failure, is contamination by foreign matter (dirt, abrasives, etc.), aggressive chemicals, or water. Such contaminants in the grease can cause raceway corrosion, indentation and wear, as well as seal lip wear. This situation can be aggravated through the normal, repeated heat-up/cool-down cycle the bearing experiences in service. Shrouded end caps and backing rings help to prevent contaminants from reaching the sealing elements.
Field Service Experience and Special Tests
Cooperative Problem-Solving Approach - In this area of bearing performance, Brenco has worked cooperatively with customers and grease suppliers to identify and solve field problems. The results of these collaborative efforts have been better performance-based requirements and improved products and maintenance practices. The following are some examples of our cooperative problem-solving efforts using currently approved greases.
"Dry Race" Problem - The dry race conditions and corrosion damage in the bearing that were previously described in the extreme low temperature and contamination section, were observed in bearings not only from low temperature operating environments, but in general service as well. As a result, Brenco has experimented with several of the currently AAR approved greases in various service environments over the past five years, moving towards more fluid (mobile) greases with certain proven EP additives. In the following three sections, we describe some of that experience with railroads and fleet operators.
CN Experience - The Canadian railroads identified corrosion and water etching in their bearings as their biggest bearing problem. CN North America noticed a large difference in the performance of bearings reconditioned by one of their bearing shops versus another. The only probable cause they could find was that the two shops used different AAR approved greases. Bearings reconditioned in one shop using a stiffer AAR grease had the greater percentage of water etched and corroded bearings, while the other shop was using a more fluid AAR grease and had far fewer corrosion problems.
In 1989, a five-year grease test was started by CN, with Brenco participation, to prove whether or not there was better performance with the more fluid grease and to establish the further potential for improvement with EP grease. In-service comparison of 400 6 X 11 non-EP lubricated bearing and 400 bearing with EP grease was conducted. Interim as well as final tear down inspections revealed that the EP greased bearing had remained in excellent condition, with very thick lubricating films on the bearing components and no signs of water etch or corrosion, despite the harsh operating environment and corrosive atmosphere.
The bearings lubricated with the non-EP grease continue to show circumferential wear. This indicates accelerated wear on the rolling surfaces during boundary lubrication conditions. In addition, these bearings did contain water etching and corrosion damage.
The beneficial effect of the AAR approved EP grease on the basic corrosion resistance that the grease imparted to the race and roller surfaces was striking. This current railroad grease now in use with EP additives was formally granted AAR unconditional approval in March 1992. This grease passed all AAR tests, including a two-year Canadian National Railroad controlled service test. The EP additive in this grease has shown no evidence of reducing bearing life. This was a concern many years ago with some of the first formulations employing EP additives. The current formulation has been tested since 1986 and is now in use in a number of countries including Canada and the U.S. without any reported problems. It is the standard specification for all bearings supplied to the Canadian National, who to date have only excellent experience with the grease. Such an EP grease has also been used in locomotive wheel bearing for many years.
Dow Experience - A similar test of the potential of EP grease to improve corrosion resistance began at Dow Chemical in 1992. Bearings were supplied with non-EP grease and with EP grease. Sample inspections were made in October 1993, September 1994, and September 1995. Again, the superior lubrication performance of the EP formulation was noted, with no sign of water etch or corrosion damage. These bearings were in "like new" condition after many months of service, the grease was in excellent condition and showed very good mobility throughout the bearing. The EP grease successfully addressed a very serious concern that the Dow and Brenco team had identified with the high percentage of corroded bearings appearing in their demanding service.
Alaska RR Experience - Perhaps the harshest operating environment for the freight car journal roller bearing operating in North America is on the Alaska Railroad. In addition to very cold start-up conditions, the bearings are periodically subjected to moist sea air during long periods of car storage in the Anchorage port area. These conditions have produced a high incidence of raceway corrosion, as well as raceway smearing, in bearings lubricated with one of the stiffer AAR approved greases. Brenco and the Alaska Railroad have conducted field studies with different greases and seals to identify and solve this problem. The most recent tests with the softer, more fluid AAR approved grease and Brenco seals indicates that the water etching and corrosion damage have been totally eliminated. This observation is base d on a tear down inspection of 14 bearings after 20 months of service (as much as 100,000 miles of service).
Ten of these bearings did, however, show signs of raceway and roller smearing, similar to that shown previously with the other grease. Since further improvements to the grease to resist smearing were required, the EP version of this grease has also been put into test in Alaska.
Limits and Conclusions on Existing AAR Specification Greases - The above experience, as well as other field and laboratory test results, has led Brenco to certain conclusions on both the adequacy of the current AAR grease specification and the performance limits of the current generation of metallic soap type greases.
Limitations of Current AAR Spec. - It has become clear that the current laboratory oriented testing specification is not discriminating enough to assure adequate lubricant performance in the field for all the approved greases. Even within the current specification framework, there is need for additional tests, such as the Federal oil bleed test and a cold temperature penetration test, to assure an optimum amount of oil bleed and cold grease mobility for adequate surface protection and lubrication under adverse conditions. Also it appears that the stringent requirements on grease stability at elevated temperatures, added due to industry reaction to the leaky seal problems of the 1970's, may no longer by necessary given the improvement in sealing technology.
Limitations of Current Grease Types - As recognized above, even the most successful of the current generation of greases under the AAR specification are limited in their ability to satisfy performance demands throughout the full range of operating temperatures, speeds, loads and harsh environments for long bearing life (up to 600,000 service miles).
Promising Developments with a New Type of Grease
To meet the increased lubrication demands (that have exceeded even the improved versions of the current generation of greases), Brenco is conducting research on advanced grease types with fundamentally superior lubrication performance capabilities.
Polyurea Grease Introduction - Whereas the current AAR approved greases are a combination of metallic soaps and mineral oils, we believe a preferred grease of the future may be one of the new family of polyurea greases.
Description of Special Polyurea Greases - In general this particular new grease consists of a proprietary polyurea thickener made of primary amines (a class of organic compounds derived from ammonia by replacement of one or more hydrogens or organic radicals) and diisocyanates (a compound that contains two nitrogen-carbon-oxygen or isocyanate groups). In simple terms, the thickener is a type of plastic - a close cousin of polyurethane. However, this particular thickener, unlike many other polyurea thickeners, is very shear stable and resists age hardening. It protects the base oil from oxidation unlike the lithium thickener now being used which actually promotes oxidative degradation of the oil. These two grease structures are shown in Figures 3a and b
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Figure 3A: Lithium Complex Grease |
Figure 3A: Polyurea Grease |
Figure 3 A & B: Grease Fiber Structures |
Additives have been selected to be compatible with the seal polymer, and provide optimum protection for the grease and bearing components. The additive package provides an extremely low coefficient of friction for steel sliding contacts, resulting in significantly lower bearing operating temperatures. Excellent rust preventing properties are also key features of this new grease. Good low temperature grease mobility and high temperature performance have been optimized for long service life.
Performance Potential - Some polyurea greases with appropriate additives have shown improved life, friction and temperature reduction and a greater range of useful low and high temperature operation than traditional metallic soap greases in automotive and other applications. Brenco has also accumulated promising railroad bearing performance data from preliminary laboratory, test machine and field service testing of a polyurea grease developed jointly with Witco Corporation to address these specific railroad bearing lubrication and corrosion protection needs.
This experience includes bearing test machine data at 125% AAR rated load and 100 mph for 125,000 miles showing the polyurea grease lubricated bearing components to be in "new" condition after testing and the bearing exhibited lower operating temperatures. Figures 4a and b provide a comparison of inner races from cone assemblies lubricated with polyurea grease and with a benchmark AAR approved grease. The inner race lubricated with the AAR standard grease shows light staining and scoring.
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Figure 4 A & B: Side by side comparisions of cone raceways.
AAR approved, non E-P grease on left vs. Polyurea grease on right. |
Size 6 1/2 X 12 bearings lubricated with this grease have also been operated at the AAR Transportation Technology Center in Pueblo, Colorado in limited service at 286,000 pound load with similar good results. The seals are also compatible with this grease.
Brenco has now entered into captive service testing of this grease, including extreme cold climate operation, with the anticipation that it will not only successfully meet the new rigorous service demands, but will lower operating temperature and rolling resistance, while being compatible with existing seal compounds and useable with light-contact and co-contact seals. We believe this grease has the potential to be a million-mile railroad bearing grease. We will be reporting on the details of this joint Brenco / Witco development in a future Technical Forum.
Issues to Resolve
The advanced polyurea grease under joint development by Brenco and Witco meets or exceeds all the current AAR specification requirements, with the exception of the hot roll stability limits. Because this fundamentally different type of grease actually extends performance limits in bearing lubrication, we believe this particular specification requirement is inappropriate for this grease.
After the 96-hour, Elevated Temperature Roll Stability Test, required in the AAR specification, the polyurea grease did, in fact, exhibit greater cone penetration than allowed in the current specification. However, it is significant that, in contrast to all soap based greases, this polyurea grease maintains a gel structure at higher penetration values and is not pourable even up to the 400 to 435 worked penetration range.
Conclusions
With the current generation of greases, improvements in lubrication effectiveness and corrosion resistance can be made with the appropriate selection of a grease with optimum stability and oil bleed. These greases may contain a proper EP additive package to further enhance performance in heavy haul service.
Increasing service demands and the need for timely introduction of promising new types of greases with greater capability may require changes in the AAR grease specification to allow realization of the bearing's full potential. We fully agree with the spirit of AAR's stated policy, promoted several years ago, to develop appropriate performance based specifications, and we will continue to work actively towards that end.
With the superior bearing products available today, utilizing the cleaner steels and tighter tolerances created by improved technology; there is a real opportunity to deliver a bearing that will go a million miles before it requires reconditioning. However, a major limitation in our current bearing technology is the lubricant. To achieve our goal, we must develop a grease that will last a million miles, in all climatic conditions. While certain versions of the current generation of AAR specification greases are showing good performance in most present service conditions, we believe that advanced, new greases - such as the special polyurea grease described in this report - are needed if we are to be assured of attaining our ultimate goal, a million maintenance-free miles.
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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