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One of the most important things an operator can do for his machinery is to make sure it is properly lubricated. So what is a lubricant and how does it affect operations when used properly? This paper will answer these questions and more by covering the fundamentals of lubrication. We will discuss how a lubricant works to remove friction, the physical and chemical properties of the lubricant, and the many functions of a lubricant.
Many people believe that a lubricant is simply used to make things “slippery.” While it is the primary function, there are more advantages to using the right lubricant. In addition to friction reduction, it also reduces the amount of wear that occurs during operation, reduces operating temperatures, minimizes corrosion of metal surfaces, and assists in keeping contaminants out of the system. Lubricants have many properties that can be mixed and matched to meet your operating needs. For example, there are different chemicals that can be added to allow a machine to efficiently run at extreme temperatures. We can also make a lubricant more effective in protecting machine surfaces under extreme pressures. By looking at the demands of the machine, you can properly identify the type of lubricant best suited for its proper function.
To understand what lubrication is, you first need to understand why we use it. Friction is the force that resists relative motion between two bodies in contact. If friction didn’t exist, nothing would ever stop moving. We need friction to function, but there are instances where you want to be able to reduce the amount of friction present. When you rub your hands together, you create heat because of the friction between the sliding surfaces of your hands. Now imagine rubbing your hands together 3600 times a minute – your hands would be on fire! Similar heat is generated by friction in your machinery. If the lubricant in your equipment has not been appropriately selected with standard operating temperatures, load, speed, etc., in mind, catastrophic failure may result.
You could wipe your bearings or if you stop your motor, for example, and the machine is too hot, you could seize the bearings. Either way, both are costly when you consider time lost, manpower used, and new equipment purchased. In order to avoid failures of this nature, we lubricate our machinery to minimize the resistance to movement, and as a result, minimize the amount of heat produced. The heat that is produced by the equipment is transferred to the oil so that it may be removed by a lube oil cooler. There are a lot of considerations that must be applied when selecting the type of lubricant we need to use: viscosity, additives needed, properties, etc.
Reducing friction and reducing heat are only a couple of the reasons we use lubricants. If you look under a microscope at two surfaces moving across each other, you would see something that looks like two mountain ranges rubbing against one another. As this happens, pieces of the weaker material break off and create smaller abrasive particles, resulting in more broken off pieces, which go on to create more abrasion. It’s a vicious cycle, and the way we prevent this from occurring is by creating a lubrication film. Two of the preferred and most common types of fluid related lubricant films are hydrodynamic and elastohydrodynamic. Hydrodynamic films are present between sliding contacts. The most common example would be a journal bearing.
When a shaft is still, it sits on the bottom of the bearing, but when it starts to move, it tries to “climb” up the side of the bearing. Microscopic layer upon layer of the lubricant create friction with each other and form an oil wedge between the shaft and the bearing, protecting both surfaces. Elastohydrodynamic films are present in rolling contacts, such as ball bearings or roller bearings. In this situation, the softer material makes up the rolling element which actually deforms for a split second to enlarge the contact area between mating surfaces. Here, the oil film thickness is one micron or less, which brings me to another reason for lubrication. We need to minimize foreign particles that may cause damage to this area.
Now in situations where the film layer is only one micron thick, you could imagine that any contaminants that are present can create major damage, so we try to eliminate as many as possible. While we can control the amount of contamination that enters a system by using seals, filters, and other quality controls, it’s impossible to completely eliminate machinery wear, even with the best lubricant films. So what do we do with the wear particles we can’t avoid? Certain additives in lubrication will be attracted to these contaminants, suspend them in the lubricant, and transfer them to filters or other separators installed in the system where they will be removed.
Finally, most places aren’t completely unaffected by humidity. So what does it mean when water and air come into contact with metal? Corrosion, and as we all know, that’s not good for machine operation. So how does a lubricant help with this problem? There are different additives, similar in operation to the additives used for contamination control, which prevent metal surfaces from coming in contact with water. This prevents the production of rust, therefore preventing damage to the metal machine surfaces.
So a lubricant is a substance that reduces friction, heat, and wear when introduced as a film between solid surfaces. Using the correct lubricant helps maximize the life of your bearings and machinery, therefore saving money, time, and manpower, thus making operations more efficient and more reliable.
All lubricants start with a base oil. There are three types: mineral, synthetic, and vegetable. In industrial applications, we mostly deal with mineral and synthetic, so I would like to focus on these. Mineral oil comes from crude oil and the quality depends on the refining process. There is a grading scale for oil and different applications require different oil quality. Mineral oil is mainly made up of four different types of molecules – paraffin, branched paraffin, naphthene, and aromatic. Paraffinic oils have a long, straight chained structure, while branched paraffinic oils are the same with a branch off the side. These are used mainly in engine oils, industrial lubricants, and processing oils. Naphthenic oils have a saturated ring structure and are most common in moderate temperature applications. Aromatic oils have a non-saturated ring structure and are used for manufacturing seal compounds and adhesives. Synthetic oils are man-made fluids that have identical straight chained structures, much like the branched paraffinic oils. One of the benefits of a synthetic is that the molecular size and weight are constant while mineral oils vary greatly; therefore the properties are very predictable.
So why don’t we use synthetic oils all the time if we know exactly what it’s going to do? While there are many advantages to using a synthetic, there are almost as many reasons to not use it. The best quality mineral oil is mostly made up of paraffinic oils, like those in synthetic oil. So, in many applications, mineral oil is just as good as synthetic, and in these applications is most likely the preferred base due to synthetic’s high cost, toxicity, solubility, incompatibility, and hazardous disposal. However, in extreme applications where a high flash point, low pour point, fire resistance, thermal stability, high shear strength, or high viscosity index is needed, a synthetic may be just what’s required.
We briefly discussed a couple of the additives that are used with a base oil in order to improve performance, but I’d like to expand on the most common additives now. The most important property to look at when choosing a lubricant is its viscosity. This is the oil’s resistance to shear and flow. The simplest way to describe viscosity is to relate it to substances that we are familiar with. The higher an oil’s viscosity, the slower it flows. Molasses, for example, has a very high viscosity while baby oil has a very low viscosity. The viscosity required for an application depends on the speed, operating temperature, and type of bearing as well as the type of component, like a gearbox versus a motor. Working hand in hand with viscosity is the viscosity index, which relates change in viscosity due to temperature. The higher the viscosity index, the less viscosity is affected by temperature. This property can be improved with a viscosity index additive. Rust inhibitors protect surfaces against rust by forming a thin water repelling film on the metals surface. Dispersants help protect components against abrasion from wear products by enveloping particles and suspending them in the oil so that they may be easily flushed and removed from the system. Antiwear and extreme pressure (EP) additives react with a component’s surfaces to form a thin protective layer to prevent metal-to-metal contact. This is especially helpful in situations where there is high pressure or a lot of stop and start evolutions. Detergents work to neutralize acids and clean surfaces where deposits may be detrimental. Finally, defoamants weaken the surface tension of bubbles so that they may break easily and minimize foaming.
For any given oil, the ingredients are the base oil and the additives. The only difference for grease is that it also has a thickener. This is most commonly described as “the sponge that holds the lubricant.” Up to thirty percent of grease is made up of the thickener which is either a simple or complex soap. Simple soap is made up of long fibers and has a smooth, buttery texture. Examples of simple soaps are lithium, polyurea, calcium, and silica. Complex soap is made up of short and long fibers and has a more fibrous texture. Some examples are aluminum, sodium, and barium.
There are benefits of using a grease as opposed to oil in certain applications. Grease seals out contaminants, is better suited for insoluble solid additives like molybdenum disulfide and graphite, and has better stop-start performance because it doesn’t drain away like oil, for a lower chance of a dry start. However, the thickness of grease limits bearing speed, reduces cooling of components, makes for difficult sampling and analysis, and makes it difficult to determine the proper amount of grease that needs adding. This is something that must be taken into consideration when deciding if oil or grease would be better suited for the application.
With a basic understanding of lubrication, you can see there are quite a few advantages of using the proper lubrication in your machines. Higher efficiency, longer life, better reliability, and less money spent on maintenance are goals that every company strives to achieve. Learning more about proper lubrication programs and applying everything you learn will make these goals easily within reach.