The most important aspect of lubrication is viscosity oil’s resistance to flow or “thickness.” Finding the right viscosity for an application is more science than art.
In order to determine what lubricant to use, it is best to first understand what we mean by elastohyrodynamic lubrication, and why it’s important to your equipment.
EHL is a type of lubrication that combines the elastic deformation of contacting surfaces with the hydrodynamic action of a lubricant under pressure to provide a thin film of oil that separates and protects the surfaces. For industrial systems the EHL regime is present at a certain set of speeds. In fact, most wear occurs at start-up and shut-down, when the equipment is operating. Think of the tires on your vehicle on a wet and rainy day. The tire has treads to give it grip and channel the water away from the contact surface. At room temperature (25°C /77°F/298K) water (the lubricant in this situation) has a viscosity of near 1 centistoke (cSt). The common accepted speed where a tire can begin to hydroplane is 35 mph. This is where the tire has enough rotational speed to force water between its surface and the road surface. The tire is essentially floating on the water surface, completely separated from the road by a layer of water. This situation is analogous to hydrodynamic lubrication in boundary lubrication, increasing the potential for wear to occur.
The conditions for elastohydrodynamic lubrication to occur are slightly different. At certain speeds and loads, the pressure exerted on the fluid in the contact zone is great enough that the fluid becomes stiffer than the surfaces it is separating. The effect? The fluid actually causes the surfaces to deform. Let’s think back to the tire example. If EHL were to occur, the water would actually get so stiff that it would deform the tire. In the case of a bearing under EHL conditions, the lubricant is actually deforming the metal of the bearing in the contact zone.
Now let us think of the parameters that impact lubrication viscosity choice: speed, load, and temperature. With EHL, we are doing the equivalent of trying to calculate which viscosity of oil is needed to have your bearings, gears, or rotating equipment hydroplane, so the least amount of friction and wear is occurring when the system is operating under EHL conditions.
Think back to the tire example. In the same way that we know when the tire will hydroplane based upon the speed in RPM and temperature (remember as an oil gets warmer it “thins” out) we can determine which viscosity is needed to fully separate the rotating component from its housing so minimal wear is occurring during constant operation based on the speed of rotation and the temperature of the lubricant.
For questions in determining if your system needs to operate in EHL conditions contact WP Group's expert lubrication engineer team on 0800 980 6172.
"Elastohydrodynamic Lubrication" by, Bill Becker (adapted) used with permission of the Mobil SHCTM Club (all rights reserved.)