Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1378657
ML www.machinerylubrication.com | May - June 2021 | 23 ML Asperities A sperities are the rough, microscopic peaks on what is otherwise seemingly smooth surface. Even if the surface has been machined to a mirror finish, it will still have asperities. is is the exact point that friction and wear begin. When the asperities on each surface contact each other, friction is created. is is also the point where fluid friction comes into play in the form of good old lubrication. In industrial lubrication, we pay a lot of attention to fluid friction, but, believe it or not, we deal with all four types of friction on an almost daily basis. We simply may not see it because it is happening within a compo- nent — like ball bearings rolling inside of the race in their housings or sliding friction at the pitch line in the gear teeth of your gearboxes. As friction continues, some asperities become dislodged and tumble in the gaps between surfaces, creating a third body. is gives way to a term known as "three-body abrasion." is third body represents the aggregate of all particles between the two moving surfaces and with a relative motion of its own. Friction Factors Friction modifiers are mild wear and friction control additives that work at the beginning of mixed-film lubrication. ere are many factors that go into deciding to use friction modifiers in addition to extreme pressure (EP) or anti- wear (AW) additives. Temperature, for example, is a very critical data field to consider when optimizing lubri- cants. Operational temperature and ambient temperature both affect friction and are very important for specific applications. Two of the biggest factors referenced when selecting the right lubricants are the speed and load of the application. e higher the speed, the more fluid friction. erefore, the lower your viscosity should be in the lubricant to avoid unnecessary fluid friction. at is one reason why a lot of higher speed pumps and blowers use lubricants with an ISO 46 or 68 Viscosity Grade. e next big factor that affects friction is load characteristics, including operational load and shock loads. If the load exceeds the recommended capacity of the component, then boundary conditions will result, and friction will occur. Also, the type of relative motion that interacts between the surface asperities plays a role, such as rolling surface on a ball bearing or sliding surface on a journal bearing. Lastly, the characteristics of the lubricant (base oil viscosity, base oil type and what additives are in the lubricant) can all affect friction or the coefficient of friction. e challenge when it comes to lubrication is to reduce the friction as much as possible by either eliminating the factors that negatively affect the surface in rela - tive motion or at least attempting to control as many of those factors as possible. Viscosity of the oil is the primary influencer of this, while friction modifiers and other wear and friction control additives help when ideal load, speed and temperature conditions are not met, such as during machine start-ups. Friction Modifying Additives 1. Organic Friction Modifiers (OFMs) a) OFMs are amphiphilic surfactants, like fatty acids, often produced as a byproduct of fats and vegetable oils. b) OFMs are common and important addi- tives in engine oils today. c) ey adsorb on metal surfaces to form incompressible layers (monolayers) which prevent asperity contact and reduce friction and wear. 2. Oil-Soluble Organomolybdenum Additives a) Originally developed for use as anti-wear (AW) additives. b) Commonly utilized as gear oil. 3. Functionalized Polymers a) Fou nd to a d sorb spe ci f ic a l ly on polar surfaces. (Stribeck) Friction Z = Viscosity N = Speed P = Load Without friction modifier Friction modifiers work here Contact Temperature Friction Cool Hot Mineral Oil With friction modifier Relative temperature where modifier becomes ineffective A microscopic view of these surfaces in relative motion reminds us, no matter how well-machined, that each surface contains tiny, jagged asperities. Friction Modifiers lower friction, reduce energy consumption and prevent stick-slip oscillation noise. They reduce friction regardless of whether wear is involved.