By PPI Special Report
BRUSSELS,
June 13, 2011
(RISI) -
A common question maintenance personnel ask bearing companies when their equipment temperature is high or rises is: What is the maximum temperature that your rolling bearings can handle? The better question to ask is: What is the maximum temperature that the bearing system can handle?
When thinking about temperatures it is important to think in terms of a complete bearing system.
A complete rolling bearing system includes a rolling bearing, a lubricant, and in most cases a lubricating system. The lubrication of ball and roller bearings is accomplished with oils, greases, or dry solid films [graphite, molybdenum disulfide (MoS2), polytetrafluoroethylene (PTFE), oil impregnated polymer oil, etc]. Rolling bearings carry the load of rotating equipment, but can not function for very long without a lubricant. If the lubricant fails the bearing will fail shortly thereafter. Lubricants are often the root cause of failure when a bearing system's temperature rises.
This article is designed to help maintenance personnel think of maximum temperatures that a bearing system can endure under use, rather than think of temperatures in relation to only a bearing and only the lubricant and lubricant system.
The reaction of AISI 52100 bearing steel to heat
The steel used in rolling bearings, both through hardened and case carburized steels, is processed to a minimum hardness of Rockwell C 60. According to the American Bearing Manufactures Association (ABMA) the maximum operating temperature of through hardened steels, AISI 52100 is 160°C (320°F), 440-C is 180°C (356°F), and M50 is 320°C (600°F). In general for all steels, as the temperature exceeds 200°C (392°F) the hardness begins to decrease. Thus, the rolling bearing life decreases as temperatures rise beyond 200°C (392°F). Figures 1 and 2 shows AISI 52100 properties at various temperatures.
Dimensional stability VS. temperature
As any metal heats up a material phase change occurs and the dimensions become unstable. In other words the bearing parts expand. Figures 3 and 4 show the maximum dimensionally stable temperature for various steels used in rolling bearings.
Since the most common method to measure the temperature of a bearing is by reading the temperature on the outside of housing, it is important to remember that the temperature at the bearing surface is 10 to 15° F lower than bearing temperature.
Lubrication in a bearing system
Rolling bearings can't function reliably without a lubricant. A rolling bearing system includes a rolling bearing, a good lubricant, and a lubricating system. The lubrication of ball and roller bearings is accomplished with oils, greases, or dry solid films. When a machine's temperature rises, the main concern for maintenance personnel should not be the maximum allowable temperature the rolling bearing can accommodate. The main concern should be the maximum temperature the entire bearing system can handle. The main focus should be on the maximum temperature that the lubricant and/or the lubricating system can endure because their failure will cause the rolling bearing to fail. A lubricant's failure due to a high temperature is difficult to detect. Therefore, it is important to monitor the temperature of a lubricant.
Without a good supply of oil a rolling bearing will fail before its potential life (L10 life) due to friction and wear between the rolling element and the raceways. Oil between the rolling element (ball, spherical roller, cylindrical roller, etc.) and the raceway reduces friction and wear, and thus reduced heat generation.
Oil can be supplied to a rolling bearing either directly or through the use of grease. Oils used to lubricate a rolling bearing include base oil and sometimes additives. Greases are made up of base oil (65-95%), thickener (3-30%), and additives (0-15%). Sometimes up to 5% of a solid lubricant is added to grease. It is the base oil in grease that does the work in a bearing system, not the thickener, nor the additives. A thickener holds the oil and the additives improve the properties of the thickener. The oil does the work for the rolling bearing, the thickener keeps the oil in place, and the additive can enhance the performance of the oil and/or thickener.
Whether oil or grease is used to lubricate a rolling bearing, both can only endure so much heat before they lose their effectiveness in the bearing system. Figure 5 shows maximum temperatures of various oils.
The type and amount of thickening agent used (organic, inorganic, metal soap) and the base oil type (mineral or synthetic oil) and its viscosity determine the upper and lower operating temperature limits of lubricating greases. Figures 6 and 7 show the maximum temperatures of greases by thickener, base oil, and soap, and a comparison of thickeners.
Figure 8 gives temperature ranges of solid lubricants.
Oil viscosity and temperature
Another thing to consider with high temperatures in a bearing system is the oil's viscosity. Viscosity is the measure of the relative resistance of a fluid to flow at a given temperature: the higher the viscosity, the greater its resistance to flow. Viscosity is measured either in centistokes (cSt) or Saybolt Universal Seconas (SUS). With greases the viscosity number is the flow rate of the base oil, not the thickener.
A viscosity of a lubricant should be sufficient to separate parts under operating conditions, but not so high that extra drag is created. As the temperature rises the viscosity of oils lowers. The higher the temperature, the higher the viscosity needed in a bearing system.
Viscosity Index is the measurement of the rate of change of viscosity with temperature: the higher the viscosity index, the more gradual the rate of change, Fig. 9.
Generally in high temperature applications, if the oil or grease used has a lower viscosity, the bearing system will fail prematurely. As can be expected, generally in a high temperature application a higher viscosity oil or grease should be used. Figure 10 shows oil viscosity vs. temperature for various oils.
Temperature effect on the life of oil
The most important property of oil, from a quality standpoint, is its chemical or oxidation stability. Heat is primarily an accelerator of oil oxidation. The rate of any chemical reaction, including the oxidation of hydrocarbons will double for every 18° F (10° C) increase in temperature. It is estimated that the life of an oil is decreased 50% for every 18° F (10° C) temperature rise above 140° F (60° C). At temperatures greater than 248° F (120° C) oxidation greatly affects grease life. The formula for this would be:
Lt = α * L
where:
Lt = Oil life due to temperature greater than 60° C (140° F)
α = Temperature life factor
L = Expected life of oil at or below 60° C (140° F)
Lubricaion system considerations
No matter what is used to lubricate a rolling bearing in a high temperature application and/or environment, the lubrication system can either extend the life of a rolling bearing or decrease it. A bearing system must be designed to assist the bearing in meeting its potential life. Attention to the lubrication system should never be overlooked or ignored.
Several oil systems can be used to remove heat from a bearing system. Grease and dry solid film do not assist in removing heat from a rolling bearing. The primary consideration with greases is the re-lubrication method and interval.
In the higher speed ranges of operation, too much grease will cause overheating. The amount of grease that is appropriate for a particular high speed application can only be determined by experience. In general, about half to a third of the open area in a bearing should be filled with grease on startup. If excess grease in the bearing causes overheating, it will be necessary to remove some grease from the bearing. A slight show of purged grease at the bearing seals is normal and also helps keep contaminants out of the bearing. More grease can be tolerated and is generally desirable in low speed applications.
When establishing grease re-lubrication schedules, a small amount of grease at frequent intervals is preferable to large quantities at infrequent intervals. Figures 13-16 can be used to determine lubrication intervals.
There are several lubrication systems when it comes to oil; sump (bath), circulation through sump, circulation through sump with cooling, jet (injection) into bearing, combination of bypass circulation with cooling and jet, and mist (spot). An oil bath system can be used to lubricate a bearing, but in general, it does not assist in removing the heat from a bearing system. The oil level for a sump bearing should come up to the middle of the rolling element when it is at its lowest position.
The oil flow rate for a circulating oil system can only be determined by experience.
For a paper machine's dryer section, Fig. 17-18 can be used. (Note: Oil flow rates are per bearing on drive side of machine. Except where condensate and blow-through steam is removed through operating (tending) side journal, flow rate for front bearing is 60-75% of these values.)
Conclusion
A complete rolling bearing system includes a rolling bearing, a lubricant, and in most cases a lubricating system. The lubrication of ball and roller bearings is accomplished with oils, greases, or dry solid films [graphite, molybdenum disulfide (MoS2), polytetrafluoroethylene (PTFE), oil impregnated polymer oil, etc]. Rolling bearings carry the load of rotating equipment, but can not function for very long without a lubricant. If the lubricant fails the bearing will fail shortly thereafter. Lubricants are often the root cause of failure when a bearing system's temperature rises.
Article contributed by NSK Engineering
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