An index of the ability of a lubricant to minimize wear at applied loads in the 4-Ball EP Test (ASTM D2596). The applied load is increased step-wise, until welding (seizure) occurs. The load wear index is calculated based on the wear scars of the 10 highest non-seizure loads. See ASTM D2596 for further details.
Any material that is used or applied to surfaces in order to reduce friction and/or wear that can occur when two surfaces are in contact and undergo relative motion. Lubricants are available in various forms: liquids, greases, dry films, and coatings.
A lubricant that is a solid to semi-fluid dispersion of a thickening agent (thickener) in a liquid. A lubricating grease may be formulated with additives that impart special properties such as resistance to oxidation or wear. An alternative definition (Vold and Vold, 1952) states, “A grease is a lubricant that has been thickened in order that it remain in contact with the moving surfaces and not leak out under gravity or centrifugal action, or be squeezed out under pressure.”
The physical arrangement of thickener particles or fibers in a lubricating grease. The nature, form, and stability of this arrangement determine the appearance, texture, and physical properties of the grease.
The use or application of a material (lubricant) to reduce friction and/or wear that can occur when two surfaces undergo relative motion while in contact under an applied load. Friction and lubrication performance depend upon the relative speed of the surfaces, the lubricant viscosity, and the applied load. There are four major lubrication regimes or types of behavior.
- Boundary Lubrication – The lubricant film is too thin to form a fluid layer that completely separates two surfaces. Asperities on the surfaces collide. Friction and wear depend upon the presence of chemical additives that adsorb and form molecular layers on the surfaces.
- Elastohydrodynamic Lubrication – Elastohydrodynamic or EHD occurs in rolling element bearings and certain gears when very high loads are concentrated on small surface areas. Under these conditions, the surfaces deform elastically or flatten to increase the surface area that bears the load. Lubricant is trapped between the surfaces, and its viscosity can increase under the pressure. As a result, the lubricant is able to form a hydrodynamic film and separate the surfaces.
- Hydrodynamic Lubrication – Under appropriate conditions, the relative motion of two sliding surfaces causes a continuous fluid film to form and completely separate the surfaces. This requires a balance between the sliding speed, the applied load, and the lubricant viscosity. Fluid Film Lubrication is another name for this type of lubrication.
- Mixed Lubrication – A lubrication regime where the load is supported partially by a fluid film and partially by the surfaces in contact
- Hydrostatic Lubrication – Lubricant is supplied under pressure to a plain bearing. This applied pressure forces the lubricant to form a continuous fluid film that completely separates the surfaces. Hydrostatic lubrication occurs typically during start-up of plain bearings. Hydrodynamic lubrication becomes effective when plain bearings are in motion.
The intensity of light reflected by lubricating grease, i.e., its sheen or brilliance. Luster should be described as follows:
- Bright – Reflects light with a relatively strong intensity.
- Dull – Reflects light with a relatively weak intensity. Some greases with a high water content may have a dull luster. Certain thickeners and fillers give a grease a characteristic dull luster.