In our prior posts, we looked at the factors to consider when selecting the right lubricant for your application. In this AST Bearings blog installment, we will discuss the Stribeck Curve, an important tool engineers use to evaluate lubricants, design bearings, and understand lubrication regimes.
Lubricating Film: A Must for Long Life in Ball Bearings
Long bearing life is predicated upon proper lubrication -- a lubricating film present and separating the metal surfaces. In the case of radial ball bearings operating in electric motors or other devices running at similar speeds, proper lubrication means the presence of an EHD (Elastohydrodynamic) film. Bearing life calculations assume the presence of this film.
The ABMA (American Bearing Manufacturers Association) standard 9 is used to calculate the basic rating life for ball bearings. The method includes adjustment factors for reliability, special bearing properties, and operating conditions. The adjustment factor a3 would be used for operating conditions and would be less than 1 if the kinematic viscosity of the lubricant drops below 13 centistokes (cSt) or if the rotational speed is very slow (meaning no EHD film formation). The adjusted rating life could be 20% to 50% that of the basic rating life calculated.
Film Formation and the Stribeck Curve
The fluid viscosity, the load that is carried by the two surfaces, and the speed that the two surfaces move relative to each other all combine to determine the thickness of the fluid film. This, in turn, determines the lubrication regime. How these factors all affect the friction losses and how they correspond to the different regimes is shown on the Stribeck curve. Engineers use this tool to evaluate lubricants, to design bearings and to understand lubrication regimes.
The combination of low fluid viscosity, low speed, and high load will produce boundary lubrication. Boundary lubrication is characterized by little fluid in the interface and large surface contact. We can see on the Stribeck curve that this results in very high friction.
As the fluid viscosity and speed increase, and/or as the load decreases, the surfaces will begin to separate, and a fluid film begins to form. The film is still very thin, but acts to support more and more of the load. Mixed lubrication is the result, and is easily seen on the Stribeck curve as a sharp drop in friction coefficient. The drop in friction is a result of decreasing surface contact and more fluid lubrication.
The surfaces will continue to separate as the speed or viscosity increase until there is a full fluid film and no surface contact. The friction coefficient will reach its minimum and there is a transition to hydrodynamic lubrication. At this point, the load on the interface is entirely supported by the fluid film. There is low friction and no wear in hydrodynamic lubrication since there is a full fluid film and no solid-solid contact.
The Stribeck curve shows the friction increasing in the hydrodynamic region. This is due to fluid drag (friction produced by the fluid) - higher speed may result in thicker fluid film, but it also increases the fluid drag on the moving surfaces. Also, a higher viscosity will increase the fluid film thickness, but it will also increase the drag.
Machinery will typically see boundary lubrication at start-up and shutdown (low speeds and thin film), before transition to hydrodynamic lubrication at normal operating conditions (high speeds and thick film). A review of the Stribeck curve shows us that a motor or machine sees the most friction and wear during start-up and shutdown.
Hydrodynamic lubrication gets its name because the fluid film is produced by relative motion of the solid surfaces and the fluid pressure increases that result. The surfaces will have tiny asperities (peaks) and direct contact should be avoided. If one surface slides over the other, then friction increases, the asperities would break off and the surfaces will wear. In hydrodynamic lubrication, the fluid film separates the surfaces, preventing wear and reducing friction.
The hydrodynamic film is formed when the geometry, surface motion, and fluid viscosity combine to increase the fluid pressure enough to support the load. The increased pressure forces the surfaces apart and prevents surface contact. Therefore, in hydrodynamic lubrication, one surface floats over the other surface. The increase in fluid pressure that forces the surfaces apart is hydrodynamic lift.
The final installment in the AST Lubrication for Life blog will detail a variety of application examples to show how different environments can affect lubrication performance. Don’t miss Part 5 coming November 20.