Utilities Manager: Fighting Friction In Rotating Equipment
Kathy | November 1, 2008
Any time you knock out friction around your operations, you’re on your way to reducing energy consumption. Bearings are a good place to start.
Whenever rotating machinery is turning, friction can become a “spoiler”—potentially threatening the operation, reliability, productivity and service life of assets. For bearings, friction is problematic, as it can contribute to increased wear, generate unwanted heat and higher operating temperatures, limit speeds and power and reduce overall energy efficiency. The mission is to mitigate the negative effects.
Anti-friction rolling bearings (ball and roller types) provide a first step on the road to solutions. In contrast to plain (or sliding) bearings, with their sliding and frictionprone surfaces, ball and roller bearings inherently will minimize friction by removing almost all sliding between bearing surfaces and replacing the major internal contact areas with rolling interfaces. However, even with the benefits from one element rolling (not sliding) over another, some friction will occur with ball and roller bearings.
Specifically, multiple sources of friction can be pinpointed. Friction can be generated at the rolling contacts, in the contact areas between rolling elements and cage (as well as in the guiding surfaces for the rolling elements or the cage), in the lubricant and in contact seals where applicable.
Even though industry has devised calculations to determine “frictional moments” in advance, friction always can increase. The problem, though, can be managed by taking advantage of friction-reducing materials and designs for bearings and the proper selection and quantity of lubrication.
Getting your bearings
In keeping friction at bay, rolling bearings must always be subjected at least to a given minimum load to allow for proper rolling element rotation and lubricant film formation in rolling contact areas. A general rule of thumb: Loads corresponding to roughly 0.02 times the dynamic radial load rating should be imposed on roller bearings and loads corresponding to 0.01 times the dynamic radial load rating should be placed on ball bearings.
Generally, roller bearings can support heavier loads than similarly sized ball bearings, and bearings incorporating a full complement of rolling elements can accommodate heavier loads than corresponding caged bearings. Ball bearings are used mostly where loads will be relatively light or moderate. For heavy loads and where shaft diameters are large, roller bearings typically will be specified.
Once the ideal load has been established for proper rotation and lubricant film formation, opportunities to minimize friction can be developed with the bearings themselves. For example, materials used to manufacture rolling bearing rings, rolling elements and cages can play a vital role in reducing the amount of friction. Bearing grade ceramics (silicon nitride) have helped create the category of hybrid bearings, which combine the silicon nitride rolling elements with steel rings to exhibit demonstrable advantages compared with conventional all-steel bearing counterparts. Among benefits, the ceramic balls are roughly 40% less dense than steel balls. This reduces centrifugal force and enables the bearings to run faster and with likely less friction at higher speeds.
In addition, due to higher values for the modulus of elasticity of ceramics and the increased stiffness this provides, hybrid bearings feature smaller contact areas. This, too, favors a reduction in the rolling and sliding friction components.
As another example of a material solution, polymer cages introduce superior friction properties compared with conventional steel or brass counterparts. Such PEEK (polyetheretherketone) cages additionally can operate at higher speeds, perform at higher temperatures and offer enhanced resistance to aggressive agents.
Specialized coatings similarly can be enlisted in the fight against friction. Examples include a low-friction coating that can be applied on a bearing’s inner surfaces. Compared with standard uncoated types, bearings with the coating will generate less friction (and resulting heat) and can better tolerate potential damage from contamination and marginal lubrication. (They also are better equipped to resist wear, operate at higher speeds, accommodate higher loads and perform even during periods of insufficient lubrication.)
Bearing engineering, too, has kept pace in efforts to reduce friction. An entirely new generation of “energy-effi- cient” bearings has been developed as part of a system solution. They incorporate an optimized bearing raceway shape to minimize friction torque (or friction loss); a uniquely compatible grease minimizes friction torque associated with grease thickener and oil viscosity; and a polymer cage serves to reduce ball cage friction loss and channels more effective grease migration inside the bearing.
Once any bearing is installed and operating, users should be aware of the negative effects relating to the issues of clearance and/or misalignment. When bearing internal clearance is reduced due to high operating temperatures or high speed limits, friction will increase. Proper internal clearance should always be maintained. Misalignment, too, typically will increase friction, and self-aligning bearings offer one remedy to help solve the problem.
Looking at lubricants
Lubricants for bearings primarily deliver a separating film between a bearing’s rolling elements, raceways and cages. The film serves to prevent metal-to-metal contact and the resulting friction that otherwise would generate excessive heat that could cause wear, metal fatigue and potential fusing of the bearing contact surfaces. (Adequate lubrication for bearings further acts to inhibit wear and corrosion and help guard against contamination damage.)
The friction torque in a bearing will be lowest with a quantity of the lubricant with the correct viscosity (relative resistance to flow) sufficient only to form a film over the contacting surfaces. The friction will increase with greater quantity and/or high viscosity of lubricant. With more than just enough to form a film, the friction torque also will increase with the speed. The lesson? Lubricant with the correct viscosity for an application in the proper quantity will help succeed in keeping friction in check.
Grease has emerged as the preferred lubricant for rolling bearings, in part because grease is easy to apply, can be retained within a bearing’s housing and offers protective sealing capabilities. Oil represents another often-used alternative.
When grease lubrication is used and a bearing has just been filled— or refilled—with the recommended amount of grease, the bearing can show considerably higher frictional values during the first several hours or days of operation (depending on the speed) than may have been calculated originally. This is because the grease takes time to redistribute itself within the free space of the bearing and/or housing; meanwhile, it is churned and moved around. After this “running-in” period, however, the frictional moment will align with similar values as oil-lubricated bearings—and, in many cases, even lower values are possible.
Especially in high-speed and/or high-temperature applications, a major function of the lubricant is to remove heat, and circulating oil lubrication will be necessary. But, beware that excessive quantity of lubricant within a bearing’s boundary dimensions can increase friction and heat generation. This indicates that as much as possible, a proper flow rate of lubricant through the bearing should be obtained consistent with good lubricant film formation and heat removal.
In fighting friction, users can turn to these and other strategies for all the associated benefits. Regardless of the equipment applications, be they electric motors, fans, compressors, pumps, gearboxes or others, users should likewise consider turning to an experienced bearings manufacturer to help factor friction out of service. UM