From practical experience, we know that adding a lubricant to a solid-solid contact will significantly reduce friction. The reduced friction leads to less wear, heat generation and energy loss – all of which reduce operation costs and downtime. How lubricants provide these benefits will be explored in this course.
The primary function of a lubricant is to provide protection for moving parts – thereby reducing friction and wear of the machine. Cooling and debris removal are the other important benefits provided by a fluid lubricant.
Automobile Engine Piston
Lubrication is used in almost every mechanical device, such as the automobile engine, including the pistons (above) and gears (below).
Simply put, a fluid is a material that is either a liquid or gas, and fluids include air, water and oil. Most lubrication is the result of a fluid film that is in between two solid surfaces that move relative to each other. The fluid film in the lubricated area can have a thickness ranging from a few nanometers (billionths of a meter) to hundreds of microns (millionths of a meter) thick. As a point of reference, a human hair will have a diameter between 50 and 150 microns.
The most important property of a lubricant is the viscosity. Loosely defined, the viscosity is the fluid’s ability to resist motion. A high viscosity means that a fluid is thicker and does not flow as easily. For example, molasses has a much higher viscosity than water, which has a much higher viscosity than air. The viscosity of oil is usually between that of water and molasses. A higher viscosity fluid will typically make a thicker film between the moving surfaces and support greater loads.
Of course, viscosity is not a constant property. Like most fluid properties, it depends on the temperature and pressure, especially temperature. The oil in your car’s engine has a high viscosity on a cold morning before the engine is started and a low viscosity after the engine heats up.
High viscosity does not guarantee a good lubricant, though. How often have you seen molasses used as a lubricant? Chemistry of the fluid and conditions at the interface also determine the proper lubricant. These effects will be covered in a later course. For this course, we will consider only oils.
The thickness of the fluid film determines the lubrication regime, or the type of lubrication. The basic regimes of fluid film lubrication are:
In addition to fluid film lubrication, there is solid film lubrication, in which a thin solid film separates two surfaces.
The fluid viscosity, the load that is carried by the two surfaces and the speed that the two surfaces move relative to each other 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 to evaluate lubricants, to design bearings and to understand lubrication regimes, use the Stribeck Curve.
The Stribeck Curve is a plot of the friction as it relates to viscosity, speed and load. On the vertical axis is the friction coefficient. The horizontal axis shows a parameter that combines the other variables: mN/P. In this formula, m is the fluid viscosity, N is the relative speed of the surfaces, and P is the load on the interface per unit bearing width. Basically, as you move to the right on the horizontal axis, the effects of increased speed, increased viscosity or reduced load are seen. The zero point on the horizontal axis corresponds to static friction.
The combination of low speed, low viscosity 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 speed and viscosity increase, or 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.
You might notice that 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. For example, think about how much harder it is to run in a pool of water than it is to walk. Likewise, a higher viscosity will increase the fluid film thickness, but it will also increase the drag. Again, think about the difference between walking in air and walking in a pool of water.
Machinery will 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). Inspection of the Stribeck curve will show us that a machine will see the most friction and wear during start-up and shutdown.
Note: The Stribeck curve above is plotted in log-log format, so each tick represents a 10X increase over the previous interval.
We saw in the discussion of the Stribeck curve that the presence of a full fluid film and no surface contact indicates hydrodynamic lubrication. Hydrodynamic lubrication gets its name because the fluid film is produced by relative motion of the solid surfaces and the fluid pressure increase that results.
Hydrodynmic Lubrication Fluid Film
To understand hydrodynamic lubrication, we first should look at the figure above. We know that a surface will have tiny asperities or peaks that will contact if two plates are placed together. If one of the plates were to slide over the other, then friction would increase, the asperities would break and the surfaces would wear. In hydrodynamic lubrication, a fluid film separates the surfaces, prevents wear and reduces 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.
Consider two parallel plates with relative motion: if one surface is angled where the entrance area is slightly larger than the exit area, then a wedge shaped gap is created. This is a converging gap, and is the geometry necessary to produce hydrodynamic lift. Be careful though - the difference between the inlet and outlet is extremely small (a few microns at most), so the surfaces will look parallel to the naked eye. Any figures in this course or any other source will be greatly exaggerated to illustrate the concept. Surfaces that are this closely matched create a conformal contact.
Whenever a surface moves over a fluid, or a fluid flows over a surface, then the fluid immediately next to the surface will move at the same speed as the surface. So, if two surfaces move relative to each other and a fluid is present, then it will be dragged into the interface. A fluid that enters a converging gap in this manner will see a pressure increase as the gap converges, which creates hydrodynamic lift, and forces the surfaces apart like a wedge.
Hydrostatic lift is present when a higher-pressure fluid is forced between two surfaces. In this case, the surface separation is caused by the static fluid pressure, and can occur without surface motion.
The mathematical equation that describes the fluid pressure as it relates to surface motion, film thickness and viscosity, the Reynolds equation, was developed by Osborne Reynolds over 115 years ago. In its full form, the Reynolds equation is very complicated and difficult to solve; however, the equation can be simplified to solve many problems in lubrication. The Reynolds equation itself is beyond the scope of this course.
A bearing is a machine component that supports or bears a load on a moving interface. You may be familiar with ball bearings, thrust bearings or journal bearings, all of which are common examples of fluid film bearings. Fluid film bearings are divided between hydrodynamic, hydrostatic and elastohydrodynamic bearings. Hydrodynamic bearings get load support by hydrodynamic lift. The most recognizable hydrodynamic bearings are slider bearings and journal bearings.
A simple description of a slider bearing is that of a block moving over a stationary surface on a thin fluid film. In a slider bearing, the moving surface will “slide” over the stationary surface – hence the name. This configuration is used to provide load support for a number of machines.
A “thick” fluid film, low friction and no wear are the defining characteristics of hydrodynamic lubrication, which generally occurs at conformal contacts. A lubricated nonconformal contact will experience elastohydrodynamic lubrication (EHD).
Rolling element bearings include many types of ball bearings and roller bearings, and provide load support through elastohydrodynamic lubrication.
Roller Element Bearing
As the name suggests, roller element bearings have rolling elements that carry the load. The elements can take many shapes like balls or cylinders, or shapes in between, but will always have nonconformal contacts and elastohydrodynamic lubrication. The nonconformal contact is usually not a ball-on-flat since the rolling element may ride on a curved surface (the race). But, the curvature of the race is considerably less than the ball so that the contact is still nonconformal.
The rolling elements and race of a rolling element bearing are made from hardened steel that is able to withstand the extreme pressures of the nonconformal contact. The bearing materials and increase in fluid viscosity allow rolling element bearings to smoothly and reliably support loads in a wide range of applications where a rotating shaft is present, including automobiles, pumps, compressors and turbines.
The next page features a visual illustration of the concepts in hydrodynamic lubrication presented above. The video shows a narrated demonstration of the actual contact area of a ball on disk under conditions of hydrodynamic lubrication at various speeds and loads. You will need QuickTime (or similar viewing software) to view this clip. Click on the QuickTime link below to download this free software if you need it. This video contains about 6 megabytes. It may take as much as 30 minutes to download on phone connections as slow as 24,000 bps, or about 2 minutes on a DSL line. For best results, download completely before starting. It will take about 7 minutes to play.
Boundary lubrication occurs when the lubricating film is about same thickness as the surface roughness such that the high points (asperities) on the solid surfaces contact. This is generally an undesirable operating regime for a hydrostatic or hydrodynamic bearing, since it leads to increased friction, energy loss, wear and material damage. But, most machines will see boundary lubrication during their operating lives, especially during start-up, shutdown and low speed operation. Special lubricants and additives have been developed to decrease the negative effects of boundary lubrication.
Boundary lubricants generally have long, straight, polar molecules, which will readily attach themselves to the metal surfaces. The lubricant molecules will form a thick protective layer that resembles a molecular shag carpet (below).
The thin layers keep the metal surfaces from contacting, but the boundary lubricant layers will contact each other causing wear. The sacrificial wear of the lubricant layer will reduce metal wear and prolong the life of the machine.
Mixed lubrication occurs between boundary and hydrodynamic lubrication, as the name would suggest. The fluid film thickness is slightly greater than the surface roughness, so that there is very little asperity (high point) contact, but the surfaces are still close enough together to affect each other. In a mixed lubrication system, the surface asperities themselves can form miniature nonconformal contacts. As we saw previously, nonconformal contacts lead to EHD. But since we are dealing with asperities, not ball bearings, the effect is localized. This phenomenon is termed micro-elastohydrodynamic lubrication.
Greases consists of a solid soap such as calcium or lithium soap or in some cases a fine clay that forms a matrix in which a liquid lubricant is dispersed. The matrix does not aid lubrication but is a reservoir that releases lubricant to the contact area. The liquid lubricant can contain boundary and EP additives, as well as solid lubricants such as graphite and molybdenum disulfide.
Grease Gelling Agents - Soap and soap complex
Thickeners, while not contributing much toward lubrication, impart unique properties to the grease affecting its applicability in certain applications or environments. Of these the lithium and so-called lithium complex thickened greases are the most common.
Another class of thickeners are the nonsoap thickeners. These are usually used in applications where the temperatures are high causing the other types of thickeners to soften excessively. This can allow the grease not to stay in place or can even cause it to lose its thickness permanently.
You have completed the materials for Fundamentals of Lubrication and Tribology III - Lubrication.
We hope that you have achieved the learning objectives listed on the Overview page of this module such that you are able to:
If you would like additonal information, there is some very good literature (books, journals and papers) that illustrates the application of lubrication principles. Any good introductory tribology or lubrication book will have a section on lubrication fundamentals.
You can also check out several References in the Resurces section of this course.
We would like to thank the following people for their valuable contributions to this course: