Know Your Kappa Values
EP Editorial Staff | June 15, 2020
If your bearings require grease lubrication, reliable long-term performance is determined by the Kappa value.
By Mark Barnes, PhD CMRP, Des-Case Corp.
Selecting the correct lubricant for rotating or reciprocating assets is the most fundamental aspect of any reliability-focused lubrication program. Unfortunately, for grease-lubricated assets, it’s quite common to select the incorrect lubricant.
Unlike bearings lubricated with oils, where most OEMs provide a good starting point, many grease-lubricated bearings do not come with a lubricant recommendation, and for good reason. In an oil-filled gearbox, input/output speeds and load are all known. That’s not necessarily the case when rolling-element bearings are used in a slow-turning conveyor or a high-speed fan. Since lubricant selection is always based on choosing the correct viscosity and additive package for the required load and speed, selecting the right grease for a 60-rpm conveyor pulley bearing is very different from that for a 3,600-rpm direct-drive fan.
Kappa is Key
To select the proper grease for element bearings, we need to understand how a lubricant separates moving surfaces under operating loads and speeds. The term for this is film thickness, which is a measure of how far apart the metal surfaces are under operating loads and speeds. Too close together and wear will occur. Too far apart and fluid friction will create churning, causing operating temperatures to rise.
To determine optimum oil-film thickness, lubrication engineers often use the Kappa value of the grease/bearing at operating speed and temperature. The concept of Kappa applies equally to oil- and grease-lubricated bearings but is most useful when calculating the appropriate grease. Simply put, Kappa is the ratio of the actual viscosity of the base oil, within the grease at operating temperature, to the minimum viscosity required to support a dynamic load within a bearing to provide full film lubrication (Figure 1).
At Kappa values less than 1.0, asperities on rolling-element surfaces and raceways will contact each other since the lubricant viscosity is too low to create complete separation, an effect known as boundary lubrication. As base-oil viscosity increases, so too does the Kappa value for the same bearing under identical operating conditions. Once Kappa exceeds 1.0, complete bearing separation is achieved. Kappa values in the 1.5 to 2.0 range indicate “full-film” elasto-hydrodynamic (EHD) separation.
For higher-speed oil-lubricated bearings, operating under constant speed, Kappa values in the 1.5 to 2.0 range are recommended. For grease-lubricated bearings, operating at slower (or variable) speeds, Kappa values in the 2.0 to 4.0 range may be more appropriate. Where shock loading may occur, even higher Kappa values are sometimes used.
The Kappa value of a grease/bearing combination can be determined by calculating the minimum viscosity at operating conditions, using the nomograph shown in Figure 2, p. 20, and comparing the minimum viscosity to the actual viscosity of the base oil at operating temperatures. When using graphs such as Figure 2, it’s important to understand that minimum does not equate to optimum.
Lubricating a bearing with an oil of the same viscosity as the minimum viscosity calculated from Figure 2 will achieve a Kappa value of 1.0, which may seem good in theory but leaves no margin for error for slight changes in operating load or speed. In practice, aim for a Kappa value in the 2.0 to 4.0 range. This requires a higher viscosity base oil. To illustrate the importance of the Kappa value to bearing life and equipment reliability, consider the following two examples.
Overhead crane wheel-drive bearing
An injection-molding facility used a series of small overhead cranes to deliver plastic pellets to specific machines and workstations. Because of the relatively small size of the cranes and their inaccessibility for routine maintenance, “sealed-for-life” bearings were installed on all components, including the wheel-drive bearings. Despite being pre-lubricated with grease by the bearing manufacturer, bearing failures were commonplace, often within 3 to 6 months. Based on bearing failure analysis, incorrect lubrication was suspected to be the root cause.
By comparing the size of the bearings, which had an OD of 250 mm, with the linear speed of the crane, which varied from zero at idle to a top speed of 5 m/sec., the rotational speed of the bearings could be calculated to range between 0 and 382 rpm.
After talking with the bearing manufacturer, it became apparent that the standard sealed-for-life bearings used were typically designed for electric motors running in the 1,800 to 3,600 rpm range and were prefilled with a low-viscosity-base-oil grease consistent with higher rpm applications. By calculating the minimum viscosity requirements at linear speeds between 1 and 5 m/sec. (equivalent to rotational speeds between 76 and 382 rpm) from Fig. 2, it was determined that the bearings were functioning at a Kappa range between 0.5 and 1.8. The bearings only operated with a Kappa greater than 1 at linear speeds greater than 3 m/sec., which represented just a small fraction of the actual operating time. By switching to same-sized sealed bearings, pre-lubricated with a grease formulated with a higher viscosity base oil, and including the use of EP additives to assist during stop-start motions, a five-fold increase in bearing life was achieved.
High-Speed Fan Bearings
A papermill was experiencing high operating temperatures and excessively high failure rates on direct-drive fans used to vent the Machine Room. To try to correct the problem, greasing intervals were increased, and a high-quality synthetic paper-machine grease was used, with little gain.
Suspecting a lubrication issue, the grease specification was reviewed. Comparing the viscosity of the paper-machine grease, at an operating temperature of 210 F, with the minimum viscosity for a 6313 bearing rotating at 3,600 rpm, calculated from Figure 2, it was determined that the fan was operating at a Kappa value in excess of 7.0. An ideal Kappa for a high-speed fan is 1.5 to 2.5. By switching to an electric-motor grease with a base-oil viscosity closer to 110 cSt (after first checking grease compatibility data), operating temperatures decreased by 35 F and bearing life increased fivefold.
Multipurpose Grease Scourge
One of the primary reasons grease-selection mistakes are so prevalent is the over-reliance on multipurpose grease. Multi-purpose (EP 2) grease is, by far, the most commonly used grease, and for good reason. Formulated with shear stable thickeners, base oils in the range of 220 to 320 cSt, and anti-wear or mild EP additives, multipurpose grease is well suited for “average” applications such as pillow-block bearings on conveying systems, pins and bushings, or slides and ways.
Make no mistake, multipurpose does not mean “all-purpose.” When used to lubricate electric motors, multipurpose greases typically have two to three times higher viscosity than recommended for motors operating in the 1,800- to 3,600-rpm range. This equates to a Kappa value in the 5.0 to 7.0 range. When the base-oil viscosity is this high, it can cause a motor to run hot and increase energy consumption. These conditions deteriorate the motor-winding insulation.
Aside from electric motors, the second most common misapplication of multipurpose grease is in couplings. Greased couplings require specialized greases matched to the type of coupling and formulated with base oils and thickeners that have a similar specific gravity to prevent separation under high centrifugal force. Using a multipurpose grease in a high-speed coupling can result in separation of the base oil from the thickener at higher rpms, causing oil to leak out of the coupling, resulting in lack of lubrication.
For rotating and reciprocating assets, selecting a lubricant with the correct viscosity is the most important factor for machine reliability and longevity. When it comes to greased bearings, there is no such thing as “one size fits all.” Use multipurpose grease judiciously and always know your Kappa. EP
Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp., Goodlettsville, TN (descase.com). He has 21 years of experience in lubrication management, oil analysis, and contamination control and has published more than 150 technical articles and white papers.