Lubrication Fundamentals

Six Factors Affecting the Life of a Lubricant
by Suzi Wirtz

Editor’s Note: Some of the materials in this article is based on content originally published in Tribology & Lubrication Technology (TLT), STLE’s official monthly magazine.



What is it?

How is it threatening?

What Do I Look For?


The chemical combination of oil or grease with oxygen.

Oxidation is the most limiting factor to a lubricant's useful life. Oil possibly may gel and become unpumpable, and eventually cause severe and wear and seizure. Varnish and sludge (polymerized products) increase oil viscosity, decrease viscosity index, reduce heat transfer abilities, block oil ways and promote foaming and emulsification.

Severely oxidized oils tend to become very viscous at low temperatures. Volatile and non-volativle acids attack white-metal bearings, can be water-soluble and are more aggressive when the lubricant is wet. Sludge, varnish, emulsification, poor air release.

Thermal Degradation

Cracking at high temperatures, in the absence of oxygen.

Safety hazard due to lowered flash points of the oil. Rapidly forming deposits on metal surfaces are not able to function as lubricants.

Thermally degraded oils form carbonaceous residues and volatile gases. Heat built-up.


Most common contaminants of oils or greases are:  water, fluid-soluble materials, fluid-insolubale materials erroneous fluid additives and fluid degradation.

First of all, contamination is the most common cause of oil failure or rejection. It affects aeration, foaming, air release and demulsibility.

Aeration can cause reduced compressibility of hydraulic fluids;reduced volumetric efficiency of hydraulic system pumps; loss of power transmission efficiency; cavitation damage in pump suctions and servo-valves; inadequate response times for turbine over-speed systems; localized oil oxidation in highly loaded regions; interference to oil flow through filters.


The action of frothy bubbles being formed in the fluid due to excess air.

Foam is not a good lubricant. Air or oil foam can accumulate in the headspace of reservoirs, gearboxes, crankcases, sumps and other components with vapor spaces.

Excessive foam may be forced out of the reservoir through the breather cap. May be ingested ito the circulation pump. May interfere with the effective lubrication of gears and bearings.

Air Release

Letting air out of bubbles in the oil. This should occur quickly.

Significantly affected by oil viscosity and temperature. Poor air release can contribute to oil foaming.

High oil viscosity. Low oil temperature. Contamination by diesel engine oils, greases and corrosion preventives. Presence of rust particles. Contact with very hard water.


The ability to release or shed water.

Undesirable if water is not separating rapidly from the oil (especially in turbine and gear oils or hydraulic fluids).

Poor oil or grease demulsibility can cause corrosion of ferrous metals, significant reduction in the fatigue life of ball bearings, roller bearings and gears; and the removal of rust inhibitors and some antiwear and lubricity additives from oils.