Lubrication Basics; How Greases Work

How Lubricants Work

Lubricants are designed to control friction and wear between two solid surfaces that are moving. This is done by varying degrees of physical separation of the surfaces, combined with chemical action at the surfaces, often referred to as boundary lubrication. Lubrication engineering is the process of selection and design of lubricants to function in a specific application or group of applications.

The contact zone is the area or space where two surfaces come together in motion. In elastohydrodynamic (EHD) conditions, the two surfaces are separated by the lubricant. EHD is achieved if the combination of speed and fluid viscosity are enough to overcome the normal load acting on the two surfaces. If the conditions are not sufficient to be in EHD, boundary contact occurs in a greater degree until the load is no longer supported by EHD forces, but by a boundary contact alone.

Lubricants come in many forms. Solid lubricants are solids that tend to reduce friction and wear in a contact. Polytetrafluoroethylene (PTFE) and polyethylene are solid lubricants that work due to a low coefficient of friction between the solid lubricant and the surfaces they separate. Molybdenum disulfide, graphite, and hexagonal boron nitride (hBN) are solids that are constructed of stronger layers that are connected weakly to each other. This structure reduces friction as the layers slide.

Liquid lubricants are composed of base oil, a blend of oils, or an emulsion. They may have additives that are soluble or dispersible in the liquid. A liquid lubricant can have a colloidal dispersion of a solid lubricant that is held in suspension. Liquid lubricants become less viscous with increasing temperature. Viscosity Index is one number that is calculated to quantify to what degree a liquid changes with respect to temperature. Liquids become more viscous under increasing pressure. These properties are important when selecting a lubricant to function in an application where the contact temperature and pressure can be very high.

Semisolid lubricants are most commonly referred to as greases. They are formed when liquid lubricant is gelled making the mixture non-Newtonian. This means that the observed or apparent viscosity of the lubricant changes depending on the amount of shear. In general, as shear rate increases, apparent viscosity decreases. Shear stability describes the ability of grease to regain its consistency after being subjected to shear. Solid lubricants are often added to grease as semisolids allow for stable suspension of incorporated solids. Additives can be dissolved or dispersed into lubricants. Additives can improve thermo-oxidative stability, friction and wear characteristics, corrosion prevention, and load carrying.

Lubricant Selection and Design

Many factors can be considered when selecting or designing a lubricant to perform in an application. The materials that come into contact are an important input. In boundary, friction is the result of load acting on two surfaces in contact due to plastic and elastic deformation of asperities on the surfaces. The area of real asperity contact increases with load. Contact between two hard surfaces results in low area and high contact pressure. If one or both of the surfaces is soft, the pressure will be lower. For instance, silicone based lubricants tend to not carry loads well and are not recommended for many metal on metal contacts. The operating environment is another important input. The temperature and exposure to oxygen, debris, dirt, moisture, and other factors can greatly affect how a lubricant performs. The duration that the lubricant must work is also part of the operating environment.

The cost of lubrication is not just the unit cost of the lubricant. Cost is also a function of the amount of lubricant needed for the application over a set period of time. If a lubricant lasts longer or less is needed, input, repair and warranties costs are reduced. Packaging and delivery method are often overlooked inputs. A central lubrication system or automated lubrication system can restrict the kind of lubricant that can be used. Package size, type, and material can affect the selection or design of a lubricant. PolySi® G-MAN™ Lubricants is uniquely qualified to assist client’s in selecting the right lubricant for their application. Our chemists and experienced sales staff can work with you from design to implementation. Call today to discuss your lubrication needs.  And remember, G-MAN™ hits the spot every time.

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