Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/825468
46 | May - June 2017 | www.machinerylubrication.com stresses from rolling conditions at these surfaces will lead to fatigue wear. Mitigating Surface Interactions Friction and wear-control additives are formulated in small quantities within the base oil and have polar properties that foster metal surface attraction. These attractions are then further encouraged to chemically react with the surface as a result of the interacting conditions, which are inversely associated to the conditions leading to sufficient film thickness: higher pressure and higher temperature. When machine surfaces interact with higher pressures and temperatures, the addi- tives mitigate the typical effects of metal-to-metal contact (wear) by creating initial molecular layers on the machine surface that are more ductile. These friction- control layers directly reduce the shear strength during contact and become sacrifi- cial. The initial layers can mitigate friction by allowing the lubricant's weaker molec- ular bonds to release with less force compared to that of the strong bonds that result from the metal-to-metal asperity boundary conditions. The formation of low-shear-strength films is also influenced by the base stock type and the metallurgy of the mechanical surfaces. There are three types of lubricant addi- tives that help reduce this friction and control wear formation: friction modifiers, anti-wear additives and extreme-pressure additives. Friction Modifiers (Lubricity or Oiliness Agents) Polar compounds, such as a fatty acid added to the base oil, decrease friction at low sliding speeds by forming a soap film. They typically are used in components that are sensitive to fuel economy to reduce fric- tion and stick-slip at low speeds, such as in an engine or transmission. They act like anti-wear additives but are more effective with lighter loads and do not require high temperatures. The resulting low-shear- strength soap breaks down at higher temperatures. However, when the surface metal is more reactive to the fatty acid, creating a metal soap, the breakdown temperature is higher. Anti-wear Additives These polar compounds are typically sulfur- or phosphorus-based, such as a zinc dialkyldithiophosphate (ZDDP) type of additive. They are designed to chemically react with the metal surface only at boundary conditions. However, anti-wear additives are more effective at higher temperatures, where they become more activated and create a barrier film. ZDDP- type additives have been widely used for wear protection and are also beneficial as antioxidants in the oil. Extreme-pressure Additives (Anti-scuff Additives) Friction modifiers and even anti-wear additives become less useful and break down when surface temperatures get too high. Extreme-pressure additives, which are also sulfur- and phosphorus-based, are the best choice when high surface temperatures are expected. These additives form a low-shear-strength, soap-like film with metal surface reactions and can withstand fairly high temperatures. While the reaction is beneficial for the film to be developed, it LESSONS IN LUBRICATION Film strength can be described as the lubricant's ability to lessen the effects of friction and control wear by means other than the film thickness. Solid or semisolid soap-like chemical film Bulk lubricant Liquid or semisolid interface film Frictional Heat Load Sliding Sliding