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.