Lubrication: Definition

What Is Lubrication?

When two metal surfaces move against each other under load without a lubricant between them, the microscopic asperities (surface peaks) on each surface interlock and shear, generating heat and tearing metal from both surfaces. This process, called adhesive wear, is rapid and self-accelerating: as wear products accumulate in the contact zone, they act as abrasives that accelerate further wear. A bearing that completely loses its lubricant film does not gradually degrade; it progresses from normal operation to accelerating wear and seizure within minutes under typical industrial loads and speeds.

The purpose of a lubricant is to interpose a film of material between those surfaces so that the surfaces themselves never make direct contact. In hydrodynamic lubrication, the lubricant film is built up by the wedge geometry and relative motion of the surfaces: the film is thick enough that the surfaces float on a continuous fluid layer and no metal-to-metal contact occurs. In boundary lubrication, which occurs at low speeds or under very high loads, the hydrodynamic film is too thin to prevent occasional contact, and the lubricant's additive chemistry (anti-wear and extreme pressure additives) provides chemical protection at contact points. Most industrial applications operate somewhere between these extremes, in mixed lubrication, where the film separates the surfaces most of the time but boundary conditions occur intermittently.

Effective lubrication management is considerably broader than simply applying grease to a fitting on a schedule. It encompasses lubricant selection (matching the correct product to each application's speed, load, temperature, and environmental conditions), correct application (right quantity, right method, right interval), contamination control (preventing ingress of water, dirt, and incompatible lubricants), condition monitoring (detecting lubricant degradation and wear before they cause failure), and drain and replenishment intervals calibrated to actual lubricant service life rather than generic recommendations.

How Lubricants Work: The Stribeck Curve

The relationship between lubrication regime, viscosity, speed, and load is described by the Stribeck curve, a fundamental concept in tribology (the science of friction, lubrication, and wear). The Stribeck curve plots the coefficient of friction against a combined parameter that includes viscosity, speed, and contact pressure:

• Hydrodynamic (full film) lubrication: The lubricant film completely separates the two surfaces. Friction is low and comes entirely from the viscous shear of the lubricant itself, not from metal contact. This regime is maintained when the film thickness is greater than the combined surface roughness of the two surfaces. Most well-lubricated journal bearings and rolling element bearings operate in or near this regime.
• Mixed lubrication: The film thickness is comparable to the surface roughness. Some asperity contact occurs between the surfaces, supplemented by the lubricant film. Friction is higher than in the full-film regime and includes both viscous and asperity-contact components. Anti-wear additives provide chemical protection at contact points.
• Boundary lubrication: The lubricant film is too thin to separate the surfaces. Almost all load is carried by asperity contacts, and the lubricant's role is primarily chemical: adsorbed layers and reaction films from extreme pressure and anti-wear additives prevent adhesive welding and reduce shear strength at contact points. Friction is highest in this regime and wear rates are significant.

The practical implication is that correct viscosity selection moves the application as far into the hydrodynamic regime as the speed and load conditions allow, minimizing both friction and wear. This is why a lubricant that works well in a high-speed, lightly loaded bearing will fail in a slow-speed, heavily loaded gear, even if everything else about the application looks similar.

Types of Lubricants

Liquid lubricants (oils)

Oils are classified primarily by viscosity grade. Industrial oils use the ISO viscosity grade (VG) system, which specifies kinematic viscosity in centistokes at 40°C; grades range from ISO VG 2 (very light, similar to water) to ISO VG 3200 (very heavy). Engine and transmission oils use the SAE viscosity classification. The correct ISO VG grade for a gearbox, circulating oil system, or hydraulic circuit is specified by the equipment manufacturer and is based on operating speed, load, temperature, and clearance dimensions.

Beyond viscosity, oil performance is determined by its additive package: anti-wear agents (zinc dialkyldithiophosphate, ZDDP, is the most common), extreme pressure (EP) additives for heavily loaded gears, rust and corrosion inhibitors, oxidation inhibitors that extend service life, detergents and dispersants that keep contamination in suspension for filtration, and viscosity index improvers that reduce viscosity change with temperature. Synthetic base oils (PAO, PAG, esters) offer better thermal stability, lower pour points, and longer service life than conventional mineral oils for demanding applications.

Greases

Grease is a dispersion of base oil in a thickener matrix. The thickener (most commonly a metallic soap: lithium, calcium, aluminum, or sodium, or a non-soap thickener such as polyurea or bentonite clay) holds the base oil in place and releases it slowly to the contact zone. The NLGI consistency grade (0 through 6, with higher numbers indicating firmer consistency) describes the grease's hardness; most bearing applications use NLGI grade 2, which has a smooth, butter-like consistency.

Thickener compatibility is critical when changing greases or topping up partially filled bearings. Many combinations of different thickener types are incompatible: when mixed, they undergo a chemical reaction that breaks down the thickener matrix and causes the grease to soften or liquefy, losing its ability to stay in the bearing. Compatibility charts from lubricant suppliers identify which combinations are safe. When compatibility is unknown, the bearing should be purged of the old grease completely before the new lubricant is applied.

The single most common cause of grease-related bearing failure in industrial facilities is over-greasing: applying too much grease too frequently. Excess grease in a bearing cannot escape from the cavity. The rolling elements churn through the excess, generating heat. Elevated temperature accelerates oxidation of the base oil, producing acidic degradation products that attack the bearing steel. In motors, over-greasing forces grease through the labyrinth seal into the winding cavity, where it can short-circuit insulation and cause winding failure. The correct quantity, typically filling 30 to 50 percent of the bearing cavity, is critical to avoid these outcomes.

Dry and solid lubricants

Molybdenum disulfide (MoS2), graphite, and PTFE (polytetrafluoroethylene) provide lubrication in applications where oils and greases are not suitable. They work by shearing along crystalline planes at very low force, providing a low-friction sliding surface without a liquid or semi-solid film. Applications include high-temperature environments above the operating range of conventional lubricants, vacuum or cleanroom environments where liquid lubricants would outgas or contaminate, food processing where incidental food contact makes petroleum-based lubricants unacceptable, and surfaces in assembly where a dry, non-migrating coating is required.

Lubricant Properties and Selection

Lubrication Failure Modes

Insufficient lubrication

The lubricant film collapses when lubricant quantity is too low, replenishment intervals are too long, or the lubricant has not reached the contact zone. Without a film, metal asperities interlock, heat is generated rapidly, and wear accelerates exponentially. Early-stage insufficient lubrication is often detectable by elevated bearing temperature and increasing vibration amplitude at frequencies characteristic of surface fatigue. Advanced stages produce audible noise, visible discoloration of the bearing housing from heat, and ultimately bearing seizure.

Over-lubrication

Over-lubrication in grease-lubricated bearings is one of the most common self-inflicted failures in industrial maintenance. When grease is over-applied, the excess cannot drain and churns under the rolling elements, converting kinetic energy to heat. The elevated temperature accelerates oxidation and thickener breakdown. Extended over-temperature operation damages the bearing cage and races and can cause grease to migrate through bearing seals into motor windings, where it degrades insulation and causes winding failures. In motors, it is common to find that the proximate cause of a winding failure is a bearing that was over-greased over months or years, not a primary electrical fault.

Wrong lubricant

Using the wrong viscosity grade is the most common wrong-lubricant failure: too low a viscosity for the load and speed allows the film to collapse; too high a viscosity prevents the lubricant from flowing into tight clearances, starving the contact zone. Thickener incompatibility in greases is a related hazard: mixing incompatible greases causes thickener breakdown and loss of consistency. Using mineral oil in an application that requires synthetic base oil for high-temperature stability results in premature oxidation and deposit formation.

Contaminated or degraded lubricant

Water ingress promotes corrosion of bearing raceways and rolling elements and reduces the oil film's load-carrying capacity. Hard particle contamination from external dust, wear debris, or improper servicing acts as an abrasive in the contact zone, accelerating surface fatigue. Oxidation produces acid and varnish that block oil passages, damage surfaces, and reduce additive effectiveness. Oil contamination analysis detects water content, particle count and size distribution, and acid number, giving the maintenance team a quantitative measure of lubricant condition before it causes equipment damage.

Oil Analysis as a Diagnostic Tool

Oil analysis is the practice of periodically sampling lubricant from operating equipment and analyzing it in a laboratory. The results give the maintenance team two categories of information: the condition of the lubricant (is it still fit for service?) and the condition of the equipment (are any internal components showing signs of abnormal wear?)

Wear metal analysis measures the concentration of metallic elements in the oil sample: iron from steel components, copper from bronze bushings or cooler tubes, aluminum from housings, chromium from chrome-plated components. Increasing wear metal concentration, particularly when the element pattern matches a specific component material, is one of the earliest detectable indicators of developing internal wear, often predating the vibration signals that vibration analysis can detect. Wear particle analysis extends this further by examining the size and morphology of particles: normal rubbing wear produces fine particles; fatigue wear produces plate-like particles; severe adhesive wear produces large, irregular particles.

Lubricant condition analysis measures viscosity (has it thinned through thermal cracking or dilution, or thickened through oxidation or contamination?), acid number (have oxidation products accumulated to the point of corrosive risk?), water content (is seal or cooler integrity intact?), and additive depletion (are anti-wear additives still present at effective concentrations?). Together these measurements determine whether the lubricant should be changed now, at the next scheduled interval, or can be extended further.

Oil analysis, combined with infrared analysis of bearing and gearbox temperature and vibration monitoring of bearing condition, provides a multi-technology picture of lubrication system health that supports predictive maintenance decisions. All lubrication routes, specifications, intervals, and oil analysis results should be managed within the CMMS, with tasks appearing on preventive maintenance schedules and results linked to asset history.

Building an Effective Lubrication Program

An effective industrial lubrication program addresses the full lifecycle of lubrication: selection, application, monitoring, and replenishment.

• Lubricant rationalization: Most facilities use far more lubricant grades and products than necessary. A rationalized lubrication program identifies the minimum number of approved products that cover all equipment requirements, reducing the risk of cross-contamination and simplifying procurement and storage.
• Lubrication routes and intervals: Each piece of equipment should have a documented lubrication specification stating the correct lubricant product, application point, quantity, and interval. Intervals should be based on operating hours, temperature, and contamination exposure, not on calendar time alone.
• Quantity discipline: Grease quantities should be specified in grams or strokes of a calibrated grease gun, not in vague terms like "add grease until it appears." Using a calibrated gun with a defined stroke volume and recording the number of strokes applied is the only way to consistently apply correct quantities.
• Contamination control: New lubricant from a supplier is not always clean. Lubricant should be filtered to the equipment's cleanliness target before use and stored in sealed containers that prevent water and particle ingress. Transfer equipment (pumps, hoses, dispensing guns) should be dedicated by lubricant type to prevent cross-contamination.
• Oil analysis sampling: Representative oil samples taken from the operating equipment (not from the drain plug, which produces a non-representative sample) at defined intervals provide the feedback needed to manage lubricant condition and detect developing wear.

Frequently Asked Questions

The Bottom Line

Lubrication is the single preventive maintenance activity with the highest impact on mechanical reliability in most industrial facilities. The majority of bearing, gear, and sliding-contact failures trace back to a lubrication deficiency that was either preventable or detectable before catastrophic damage occurred. A disciplined lubrication program covering correct lubricant selection, precise application quantities and intervals, contamination control, and regular condition monitoring through oil analysis delivers avoided replacement costs and reduced unplanned downtime that far exceed the cost of the program itself.

The integration of lubrication tasks with predictive maintenance programs creates the most complete defense against lubrication-related failure: preventive lubrication tasks prevent the majority of failures, while oil analysis and vibration monitoring catch the failures that occur despite correct lubrication, providing time for planned intervention before secondary damage occurs and production is affected.