Industrial lubricants are oils, fluids, greases, and other compounds that reduce friction, binding, wear, or exclude moisture. Their primary function is to displace solid surfaces with a fluid film. They are otherwise used to modify surface properties, regulate temperature, or remove debris. The ideal lubricant minimalizes solid-to-solid contact and reduces any amount of friction that would otherwise impede movement and induce wear.

Lubrication Regimes

The thickness of a fluid film is dependent on a fluid’s viscosity, the load between the mating surfaces, and the speed at which those surfaces move with respect to each other. There are four lubrication regimes, also referred to as modes of lubrications, defined by the thickness of the fluid film as follows:

Hydrodynamic lubrication regimes exist when the fluid film supports the load eliminating all solid surface interactions. This lubrication regime exists when the geometry, surface motion, and fluid viscosity create enough fluid pressure to cause hydrodynamic lift.

Elastohydrodynamic lubrication regimes are formed when higher loads exist or when there is nonconformal contact between the lubricated surfaces. This causes a localized load-bearing area that imposes some amount of elastic strain across the solid surfaces. The extreme pressures experienced affect the viscosity of the lubricant. This change in viscosity allows the fluid to support the load, effectively lubricating point contacts in applications such as ball and roller bearings.

Mixed lubrication regimes insufficiently separate solid surfaces allowing some amount of solid surface interaction. As the name suggests there is an active fluid film that lubricates the surface while asperities, which are surface irregularities, cause what can be referred to as micro-elastohydrodynamic lubrication.

Boundary lubrication regimes allow the asperities to support the load, as the surface roughness exceeds the thickness of the fluid film. In these circumstances the lubricant no longer acts directly to limit friction. To effectively lubricate surfaces experiencing a boundary lubrication regime, extreme-pressure (EP) additives, boundary additives, or solid lubricants can be used to decrease the negative effects of the solid surface contact.

Stribeck Curve

The stribeck curve is a graphical representation of how friction relates to fluid viscosity, load, and speed. It is used to illustrate the coefficient of friction under various operating conditions. The lubrication parameter, directly related to fluid film thickness, is a function of speed, load, and viscosity, and is shown on the x-axis. An increase in viscosity, a decrease in load, or an increase in speed each reflects a larger fluid film thickness and a larger lubrication parameter. The coefficient of friction has a steep drop off between boundary and mixed lubrication regimes, a minimal value as it enters hydrodynamic lubrication regimes, and then begins to increase thereafter due to the negative effects of fluid drag.

Image Credit: InTech

Selection Criteria

Industrial lubricants are available in a variety of forms, compositions, viscosities, and may be application specific.

Product Form

Industrial lubricants include low viscosity oils, high viscosity oils, greases, and solid lubricants.

Low viscosity oils offer the least resistance to movement. The reduced shear stress minimizes friction. Load bearing capabilities and fluid film thickness are also reduced. Low viscosity oils may contain additives to prevent lubrication failure during periods of high load or at low speeds. 

High viscosity oils exhibit higher shear stresses and thicker fluid films. The thicker fluid film is able to support greater loads and function at lower speeds, although their resistance to flow increases the amount of friction between the mating surfaces.

Greases are semi-solids formed by the dispersion of a thickening agent in a base fluid. The thickening agent serves as a matrix that holds the lubricant in place, while supplying some amount of ingress protection. The oil or base fluid is the active lubricating agent.

Solid, or dry film lubricants,  disperse a coating that excludes moisture and reduces friction. Solid lubricants may also contain corrosion inhibitors and are generally used in high temperature applications where liquid lubricants break down.

Composition

Lubricating oils often consist of natural oils, synthetic silicone oils, petroleum-based compounds, or combinations with wax or solid lubricant dispersions.

Petroleum and mineral oil lubricants are functional fluids derived from petroleum, a naturally occurring hydrocarbon-based fluid with various molecular weights. Petroleum-based lubricants tend to be more viscous, temperature sensitive, and possess a lower break down temperature than synthetic or solid lubricants. Petroleum products are refined from a base stock that is either aromatic, napthenic or paraffinic in origin.

Synthetic lubricants do not contain a petroleum or mineral oil base, but provide exceptional fire resistance and cooling performance. They are based on synthetic compounds such as silicone, polyglycol, esters, digesters, chlorofluorocarbons (CFCs) and mixtures of synthetic fluids and water. The characteristics, cost, and heat transfer performance of semi-synthetic fluids fall between those of synthetic and soluble oil fluids.

Solid lubricants are used to effectively lubricate surfaces in high temperature and high pressure environments. Graphite and molybdenum disulfide (MoS₂) are the most common dry lubricants while boron nitride, polytetrafluorethylene (PTFE), talc, calcium fluoride, cerium fluoride, and tungsten disulfide are also used.

Applications

Lubricants are designed to produce a desired effect in known operating conditions. They are often application specific as each application requires unique properties under diverse environmental conditions.

Specifications

Important properties for industrial lubricants include kinematic viscosity, viscosity index, pour point, flash point, autogenous ignition temperature (AIT), and special features.

Properties of lubricants. Video Credit: Mr. Joses

• Kinematic viscosity is the time required for a fixed amount of fluid to flow though a capillary tube under the force of gravity.
• The viscosity index scale describes the change in viscosity at two temperature extremes: 210° F (98.9 ° C) and 100° F (37.8 ° C).
• Pour point, the lowest temperature at which a fluid flows, is typically 15° F to 20° F below the lowest end-use temperature.

• Flash point is the lowest temperature at which a liquid can give off sufficient vapors to form an ignitable mixture in air near the surface of the liquid.

• Autogenous ignition temperature (AIT) is the temperature at which ignition occurs spontaneously.

Features

Extreme pressure (EP) additives include chemically-active agents (sulfur, phosphorous, chlorinated compounds) that are reactive and form a film preventing seizure, sticking, or surface adhesion in high pressure applications.

Micro-dispersions are oils or lubricating fluids that contain a dispersion of solid lubricant particles, such as polytetrafluoroethylene (PTFE), graphite, molybdenum disulfide, or boron nitride (BN) in a mineral, petroleum, or synthetic oil base.

Low-foaming or non-foaming additives break out entrained air. Entrained air can cause pump damage due to cavitation. Foaming can also reduce the cooling ability and the bulk modulus (or stiffness) of the fluid.

High water content fluids (HWCF) are natural oils, water-soluble fluids, soap complexes, or waxes that provide exceptional fire resistance. 

Dielectric greases and insulating fluids are insulating oils, greases, transformer oils, and fluids that have a high dielectric strength and are used in transformers, capacitors, EDM machining, and other electrical device applications.

Standards

ISO 3448 establishes a system used to classify industrial lubricants based on viscosity. It covers industrial lubricants and related fluids including mineral oils, hydraulic fluids, dielectric oils, and other industrial fluids, but may not cover pure chemicals and naturally occurring products.

SAE J306 is an automotive motor oil viscosity standard. It is used to depict limits by which lubricants are classified based on rheology. The number referenced, referred to as the weight of the oil, defines viscosity grade. The suffix "W" denotes winter grades and viscosity is measured at cold-start. Single-grade oils are classified by viscosity at a fixed temperature while multi-grade oils are tested at both low and high temperatures with a range of viscosity grades; e.g. 5W-30.

Resources

Basics of Lubrication

Characterization of Lubricants for Research and Development, Quality Control and Application Engineering (PDF file)

General Properties of a Lubricating Oil

Lubricating Grease Basics (PDF file)

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Dow Corning - Molykote® | Everlube Products | CIMCOOL