High-operating temperature can spell trouble for journal bearings. Here are some ways to avoid it.
M. M. Khonsari
Director, Center for Rotating Machinery
Mechanical Engineering Dept.
Louisiana State Univ.
Baton Rouge, La.
E. R. Booser
Journal bearings that overheat signal-a problem with lubrication, misalignment, contamination, or overload. Elevated operating temperatures can damage bearings, seals, and shafting, which may lead to severe vibration, and even catastrophic failure of equipment.
The good news is proper design can prevent most problems. In particular, look at temperature limits of bearing materials. Tin and lead babbitts commonly used in oil-film bearings, for example, start to plastically flow when local fluid pressure approaches compressive yield strength. This can happen even under normal loads when operating temperatures reach about 300°F. Therefore, keep peak temperatures below 250 to 275°F in such bearings to allow for transient loads.
Bearings made from certain plastics and carbon composites can tolerate higher temperatures than babbitt types, but when run without lubrication a high thermal-expansion coefficient often drives the design.
In other cases, lubricants rather than bearing materials limit operating temperatures. Of the mineral-based oils, recent hydrocracked turbine oils are the most stable with temperature, while inexpensive "once-through" oils like those used in mist and drip feeds are the least. Synthetic oils work well at extremely high or low-operating temperatures and can be formulated for special properties such as fire resistance. But oil stability depends on operating conditions and environment. For example, synthetic (and conventional) oils specially formulated for use in automotive and diesel engines may rapidly foam and form emulsions when used in turbines, compressors, pumps, and electric motors.
Regardless of the application, calculate bearing peak temperatures during the design phase to prevent temperature-related failures in the field. Then monitor operating temperature of installed bearings if possible. The temperature of oil exiting a bearing is a good indicator of bearing mean temperature. Typically a 20°F increase in mean temperature indicates possible bearing problems. Locate temperature sensors in a bearing bore (in the vicinity of minimum film thickness) for a more sensitive indicator of bearing health. Oil-feed pressure and temperature sensors can monitor the integrity of oil-supply systems.
Minimum film thickness, hmin, is an important metric in oil-fed bearings because an insufficient amount promotes elevated temperatures and wear. Excessive levels, on the other hand, promote vibration instability. Minimum film thickness should be about 0.0007 to 0.0012 in. for babbitt bearings under steady loads at speeds of 500 to 1,800 rpm, conditions typical of electric motors and driven equipment. Journal bearings in large turbine-generators run at 1,500 to 3,600 rpm need an hmin of 0.003 to 0.005 in. For journal bearings in automotive and reciprocating aircraft engines (with highly finished bearing surfaces) that range is about 0.0001 to 0.0002 in.
A rule of thumb says diametral clearance the space between bearing and shaft surfaces when the two parts are concentric to one another should be about 0.002 in. per inch of diameter. But check that a given clearance does not cause unacceptably high operating temperatures. Insufficient clearances are exacerbated by differential thermal expansion between a warmer journal and bearing constrained in a cooler housing, which in extreme cases can temporarily close clearances and seize shafts. Large bearings are particularly vulnerable at machine start-up because it may take 30 min or more for housing temperatures and internal clearances to reach equilibrium.
PV factor provides a measure of temperature limits in dry and semilubricated bearing materials, where P = projected unit load (psi) and V = surface velocity (ft/min). For example, a 0.25-in.-diameter shaft spun at 200 rpm under a 40-psi load gives a PV = 2,080. Assume the shaft is run in a phenolic bearing. Phenolic resins have a maximum PV = 5,000 and a corresponding temperature rise = 170°F from an 80°F (limiting temperature = 250°F). So expected temperature rise in this case = 170
3 (2,080/5,000) = 71°F.
However, PV factor should serve only as a guideline. Surface finish of bearings and shafts, shaft/bearing combinations, and plastic fillers, all influence operating temperatures. New designs should be simulated or prototyped first before production begins.
Ways to limit bearing peak temperatures
- Increase internal clearances
- User lower viscosity oil
- Reduce unit lodging by increasing axial bearing length (lowers eccentricity ratio). Temperature rise is roughly proportional to unit load in many cases.
- Boost cooling-oil flow through bearings by raising feed pressure or by adding internal grooves to bearing surfaces.
- Lower frictional heating with grease or add lubricating solids to bearing materials
- Improve heat transfer from bearings to surroundings
- Lower unit lodging, P
For additional details see authors' Applied Tribology-Bearing Design and Lubrication, John Wiley & Sons Inc., New York, 2001.