High-performance synthetic lubricants are formulated to meet specific application requirements. Proper selection depends, to some extent, on the lubrication goals, such as optimize friction, reduce wear, extend service intervals or simply increase reliability.
Yet, identifying the tribological system itself is the single most important factor in beginning a selection process. This involves the types of motion, components, surface materials, and lubrication or friction regimes involved. Critical variables can be summarized by the acronym “L.E.T.S.” – Load, Environment, Temperature and Speed.
Load, the first application variable, identifies the amount of force or stress placed on the component or surface. In general, loads can be classified as heavy, moderate or light.
- Heavy loads may require boundary lubrication with anti-seize pastes or anti-friction coatings (AFCs), which contain high levels of solid lubricants.
- Moderate loads may require greases or pastes with lower amounts of solid lubricants.
- Light loads can be handled by oils, greases and silicone compounds.
- Shock, vibration and frequent stop-start cycles are considered loads requiring pastes and AFCs.
Another variable in the tribological system is the operating environment. Atmosphere pressures; high humidity; water immersion; the presence of chemicals or fuels; or airborne contaminants such as dust, dirt, fly ash or salt spray can impact the choice of the proper lubricant.
- Full or partial vacuums may require an anti-seize paste or AFC with MoS2 solid lubricants.
- Applications with high humidity may require the use of graphite or PTFE solid lubricants.
- Water-rich environments require lubricants with good water-washout resistance.
- Exposure to chemicals and fuels may call for a fluorosilicone grease or compound.
- Dusty, dirty conditions may need a dry-film bonded lubricant.
- Inaccessible components may require an oxidation-resistant grease, paste or AFC.
- Damping greases and compounds are often used to eliminate noise issues.
The third key variable is temperature, which must take into account the actual operating temperature, the "soak" temperature at standstill and the ambient temperature. Viscosity is the most important property of liquid-based lubricants, and the viscosity will thicken in cold and thin out in heat.
- Synthetic lubricants generally have a much wider service-temperature range than conventional oils and greases.
- Silicone-based lubricants are noted for their excellent cold-temperature properties, as are low-viscosity polyalphaolefin (PAO) lubricants.
- Anti-seize pastes and AFCs with solid lubricants perform well in extreme heat and cold.
- Conventional high-temperature oils and greases are typically used in moderate-temperature applications; they can stiffen in extreme cold and oxidize or dry out in extreme heat.
Speed is a variable that runs from high to low, and it involves rotational and reciprocating speeds as well as frequency of motion. The contact speed can significantly affect lubricant performance.
- Static conditions are harsh on liquid-based lubricants and can lead to increased evaporation and oil separation from gravity.
- Slower speeds can require use of higher-viscosity oils or, with heavier loads, anti-seize pastes or AFCs with solid lubricants.
- Higher speeds can be served best with lower-viscosity lubricants to prevent problems with shear.
- Solid lubricants typically are not used in high-speed applications, yet they are excellent for stop-start cycling and oscillating speeds.
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L.E.T.S. can help end-users in industrial assembly and maintenance make the right decisions selecting effective lubricants for specific applications. As a general rule, conventional oils and greases work well in most hydrodynamic lubrication regimes with light to moderate loads, typical operating environments, normal temperatures and moderate to high speeds. Synthetic lubricants, especially those with solids content, are often for elastohydrodynamic (EHD) and boundary lubrication. They are especially well suited to provide effective lubrication under heavy loads, in harsh environments, with extreme heat or cold, and at low to moderate speeds.