Industrial lubrication dictionary
More than 64 technical terms explained clearly. From viscosity and NLGI to DIN, AGMA standards and tribology concepts.
Physical properties
Viscosity
Resistance of a fluid to flow. The most important property of a lubricant: it determines the oil's ability to form a lubricating film between moving surfaces. Measured in mm²/s (cSt) or mPa·s (cP).
Kinematic viscosity
Dynamic viscosity divided by fluid density. Measured in mm²/s (cSt) at a reference temperature (normally 40°C or 100°C). It is the main classification parameter for industrial oils.
Dynamic viscosity
Force required to move one layer of fluid over another at a defined speed. Measured in mPa·s or cP. Relevant for evaluating lubricant behaviour at cold start.
Viscosity index (VI)
Dimensionless parameter indicating how viscosity varies with temperature. A high VI (>120) means viscosity changes little with temperature. PAO synthetic oils typically have VI 140-160. Conventional mineral oils have VI 90-100.
Classifications
ISO VG (Viscosity Grade)
Viscosity classification system for industrial oils defined by ISO 3448. Establishes 20 grades (ISO VG 2 to 1500). The number indicates nominal kinematic viscosity at 40°C in mm²/s.
SAE (Society of Automotive Engineers)
Viscosity classification system for engine oils and vehicle transmissions. For engine oils: SAE 0W to 60. 'W' indicates winter (cold) behaviour. SAE 15W-40 is multigrade with good cold start and operating performance.
NLGI (National Lubricating Grease Institute)
Consistency classification system for lubricating greases. Defines 9 grades (000 to 6). NLGI 2 is the most common for general-purpose bearings. NLGI 0-1 is used in low-temperature or centralised systems.
Lubricant bases
Mineral base oil
Oil obtained by distillation and refining of crude petroleum. Group I and II mineral bases are the most common and economical. Used in most conventional industrial applications. Has lower thermal stability and lower viscosity index than synthetic bases.
Synthetic base oil
Lubricating oil manufactured by chemical synthesis, not by petroleum distillation. Offers superior properties: higher viscosity index, better low-temperature behaviour, greater thermal stability and longer service life. Common types: PAO, esters, PAG and silicone.
PAO (Polyalphaolefin)
Group IV synthetic base obtained by polymerisation of olefins. The most widely used synthetic base in premium industrial lubricants. Excellent viscosity index (140-160), very low pour point and high thermal/oxidative stability. Compatible with most elastomers and additives.
PAG (Polyalkylene glycol)
Water-soluble synthetic base with excellent lubricating and cooling properties. Used in screw compressors (low gas absorption), cutting fluids and applications where water solubility is required. WARNING: incompatible with mineral and PAO bases.
Synthetic ester
Synthetic base obtained by reaction of a carboxylic acid with an alcohol. Excellent lubricity, high thermal stability and good biodegradability. Used in compressors, aviation turbines, fire-resistant hydraulic fluids and applications requiring biodegradability.
Hydrocracking mineral base (Group III)
Base obtained by petroleum distillation followed by severe hydrocracking. Has properties intermediate between conventional mineral and PAO synthetic. Viscosity index above 120. Often marketed as 'synthetic' or 'semi-synthetic' although technically of mineral origin.
Lubricating greases
Thickener
Solid component of grease that forms a three-dimensional structure retaining the base oil. Most common thickeners: lithium, lithium complex, calcium, calcium sulphonate complex, polyurea, bentonite and PTFE. Type determines drop point, water resistance and compatibility.
Drop point
Temperature at which a grease loses its structure and flows like a liquid. Indicates the maximum use temperature of the grease. Lithium greases ~180°C; lithium complex >260°C; bentonite and polyurea have no conventional drop point.
Penetration
Measure of grease consistency, determined by the standardised cone test (ASTM D217). Expressed in tenths of a millimetre of penetration depth. Higher penetration means lower consistency. Directly related to NLGI classification.
Oil separation (bleeding)
Release of base oil from the grease thickener structure. A certain bleeding level is necessary for lubrication. Excessive bleeding indicates degradation or incompatibility. Measured per ASTM D1742 or IP 121.
Standards
DIN 51517
German standard for industrial gear oils. Defines three categories: C (oils without special additives), CC (with anticorrosion and antioxidant additives) and CLP (with EP additives). CLP oils are most used in industrial gearboxes.
DIN 51524
German standard for mineral hydraulic oils. Three parts: HLP (with anti-wear and anticorrosion additives), HVLP (HLP with high viscosity index) and HVLP-D (with additional demulsifier additives). The most important technical reference for industrial hydraulic oils in Europe.
ISO 6743
ISO standard for lubricant classification. Organised in several parts by lubricant family: ISO 6743-4 (hydraulics), 6743-6 (gears), 6743-3A (compressors), etc. Defines the lubricant type designation with letters identifying its characteristics.
AGMA (American Gear Manufacturers Association)
American standard for gear oils. Defines specific viscosity grades (AGMA 0 EP to AGMA 13 EP). Used mainly in American transmissions and some special applications (AGMA 9005-F16 for open gear oils).
API (American Petroleum Institute)
Classifies lubricant bases into groups (I to V) and defines quality specifications for engine oils. API SP is the current spec for petrol (2020); API CK-4 for heavy duty diesel. Also classifies vehicle gear oils (API GL-4, GL-5).
ACEA (Association des Constructeurs Européens d'Automobiles)
European quality specification system for engine oils. Defines categories for petrol passenger cars (A), diesel passenger cars (B), low-SAPS oils (C) and heavy duty (E). ACEA E9 and E11 are the references for modern truck engines with after-treatment.
NSF H1
National Sanitation Foundation certification for lubricants suitable for use in food processing machinery with incidental contact risk. All ingredients must be FDA-approved (21 CFR §178.3570). Required standard in industries under IFS Food, BRC and FSSC 22000.
NSF H2
NSF certification for lubricants used in food processing machinery where there is no contact risk with the product. Allows more technical formulation than H1, without the FDA-approved ingredient restrictions.
Additives
EP (extreme pressure)
Additives activated at high temperature (generated by high pressure or shock load) to form a protective layer on metal surfaces and prevent welding or scuffing. Most common: sulphur-phosphorus or barium based. Used in CLP gear oils and greases for high-load applications.
AW (anti-wear)
Additives that form a protective layer on metal surfaces at normal operating temperatures, reducing wear under mixed lubrication conditions. ZDDP (zinc dialkyldithiophosphate) is the most widespread AW in hydraulic and engine oils.
Antioxidant
Additive that retards the oxidation of lubricating oil, extending lubricant service life. Oxidation produces acids and sludges that degrade oil and damage components. Antioxidants are consumed during service; their depletion indicates end of oil life.
Anticorrosion
Additive that protects metal surfaces from corrosion by water, oxidation acids or process products. Distinguishes between rust inhibitors (protect steel) and non-ferrous metal corrosion inhibitors (copper, aluminium, etc.).
Detergent / Dispersant
Additives mainly present in engine oils. Detergents (organic metallic salts) neutralise acids from the combustion process. Dispersants keep soot particles and impurities in suspension, preventing engine deposits.
Viscosity modifier (VM)
Polymer that improves viscosity index of an oil, allowing multigrade oils with good cold and hot properties. Main components of 5W-40, 10W-40 etc. Their mechanical degradation is the usual cause of engine oil viscosity loss.
Antifoam
Additive that reduces foam formation and persistence in lubricant. Foam impairs the lubricant film, causes cavitation and noise in hydraulic systems and reduces cooling efficiency. Most common antifoams are silicones and fluorine compounds.
Technical concepts
Tribology
Science studying friction, wear and lubrication between surfaces in contact and relative motion. Industrial tribology aims to reduce energy losses through friction and component wear by correct lubricant selection and application.
Hydrodynamic lubrication (EHD)
Lubrication regime in which moving surfaces are completely separated by a continuous lubricant film of sufficient pressure. The ideal regime: minimum wear and maximum component life. Achieved with adequate viscosity, speed and geometry.
Mixed lubrication
Lubrication regime in which the oil film is partially sufficient: there is sporadic metallic contact between surface asperities. Regime where AW and EP additives are most active. Occurs at start-up, shutdown and low-speed/high-load conditions.
Boundary lubrication
Lubrication regime in which the oil film is insufficient and metallic contact between surfaces is widespread. Occurs under extreme load, low speed or high temperature. EP additives react with surfaces to prevent scuffing.
Friction coefficient
Ratio between friction force and normal force between two surfaces. A lubricant reduces the friction coefficient, reducing energy losses and heat generated. Friction modifier (FM) additives further reduce the coefficient under mixed lubrication.
Degradation phenomena
Lubricant oxidation
Chemical reaction of lubricant with air oxygen, accelerated by temperature and presence of catalytic metals (copper, iron). Produces organic acids, sludges and varnishes that degrade oil and damage components. Main cause of industrial oil deterioration.
Cavitation
Violent formation and collapse of vapour or gas bubbles in lubricant, caused by sudden pressure drops. Produces noise, vibration and erosion in hydraulic pumps, bearings and other components. Prevented with low-foam oils and properly designed suction systems.
Scuffing
Catastrophic lubrication failure where heat from metallic contact between surfaces causes their welding and breakage. Occurs when lubricant film completely fails and EP additives are insufficient. Destroys components in seconds.
Surface scuffing
Severe surface deterioration by adhesion and material transfer between contact surfaces, prior to total scuffing. Manifests as longitudinal scratches in the sliding direction. Indicative of insufficient EP lubrication or lubricant contamination.
Stick-slip
Irregular sliding phenomenon (alternating grip-slide) on machine tool slideways. Causes vibration, poor surface finish and imprecise positioning. Prevented with CGLP slideway oils containing specific friction modifier additives (CINCINNATI P-47).
Predictive maintenance
In-service oil analysis
Predictive maintenance technique analysing oil samples from in-service equipment to assess lubricant condition and detect component wear or contamination. Optimises change intervals, anticipates failures and reduces unscheduled downtime.
Oil analysis
TAN (Total Acid Number)
Total acid number. Indicates concentration of acidic compounds in oil, expressed in mg KOH/g. An increase in TAN versus new oil indicates lubricant oxidation. Control parameter in turbine, hydraulic and compressor oils.
TBN (Total Base Number)
Total base number. Indicates oil's alkaline reserve to neutralise acids generated in combustion. Expressed in mg KOH/g. Key control parameter for diesel engine oils. Change recommended when TBN drops to 50% of initial value.
Ferrography
Oil analysis technique that separates and analyses magnetic wear particles present in oil. Identifies wear mechanism (abrasive, adhesive, fatigue) and affected materials, giving very precise information on lubricated component condition.
Process fluids
Cutting emulsion
Emulsifiable cutting fluid used in machining (drilling, milling, turning, grinding). Diluted in water at 3-10% to form a milky emulsion that cools, lubricates and evacuates chips. Mineral, semi-synthetic and synthetic types exist.
Cutting fluid
Generic term for all fluids used in machining operations. Includes cutting emulsions, pure cutting oils, MQL oils and synthetic fluids. Function: cool tool and workpiece, lubricate cutting zone and evacuate chips.
Emulsion
Stable mixture of two immiscible liquids (normally oil and water) thanks to emulsifiers. In lubrication, cutting emulsions are prepared by adding concentrate to water (never the reverse). Emulsion stability is critical for performance and fluid life.
Sustainability
Biodegradable
Lubricant that decomposes naturally into non-toxic products by microorganism action. Biodegradability measured per OECD 301B (ultimate aerobic biodegradability). Biodegradable lubricants mandatory in sensitive environments (water, soil) and sectors like marine or forestry.
Equipment and systems
Hydraulic system
System transmitting mechanical energy through a pressurised fluid (hydraulic oil). HLP and HVLP hydraulic oils are specific lubricants for these systems. Viscosity, cleanliness and oil quality are decisive for pump and valve life.
Hydraulic oils
HLP
DIN 51524-2 classification for mineral hydraulic oils with anti-wear (AW), anticorrosion and antioxidant additives. Standard hydraulic oil for most industrial systems. Available in ISO VG 22, 32, 46, 68 and 100.
HVLP
HLP hydraulic oil with high viscosity index (VI ≥ 140). Formulated for systems operating in wide temperature range. Mandatory in construction machinery, agricultural machinery and outdoor hydraulic systems. Reduces energy consumption and improves cold-system response.
Gear oils
CLP
Closed gear oil (gearboxes) with EP (extreme pressure) additives. DIN 51517-3 classification. Available in ISO VG 68 to 1500. Standard oil for industrial gearboxes (Rossi, Bonfiglioli, SEW, etc.). Usual change point: 4,000-8,000 service hours.
Applications
Compressor oil
Lubricating oil specifically formulated for air compressors. In screw compressors, oil lubricates, seals and cools the rotor. Must have low carbon deposit tendency and high oxidation stability. Synthetic oils allow 6,000-8,000 hour intervals.
Turbine oil
Highly refined oil for steam, gas and hydraulic turbines. Requires excellent oxidation stability (typical service life 25,000-50,000 hours), good water separation and absence of foam tendency. Reference standard: ISO 6743-5 and DIN 51515.
Thermal oil
Heat transfer fluid used in indirect industrial heating systems (between 150°C and 320°C). Not a conventional lubricating oil but a process fluid. Must have high thermal stability, low flash point and form no coke or deposits.
Chain oil
Oil with high adherence (adhesive additives) for transmission chains. In high-temperature applications (>180°C), synthetic ester or white-base oils are used. In food applications H1 certification is required. Correct chain lubrication multiplies their service life by 3-5 times.
Maintenance
Relubrication
Periodic addition of grease to a bearing or lubrication point to replace consumed or expelled lubricant. Relubrication interval depends on bearing type, speed, temperature and ambient conditions. Incorrect relubrication (excess or deficiency) is a common cause of failure.
Grease compatibility
Ability of two greases to mix without loss of properties. Changing from one grease to another with different thickener without fully purging the system can cause incompatibility: the mixture loses consistency and fails catastrophically. Always consult compatibility chart before changing grease.
Grease purge
Process of complete grease replacement in a bearing or joint, normally injecting new grease until the old is expelled through the outlet. Necessary when changing thickener type or when grease is highly degraded.
Lubrication audit
Systematic evaluation of all lubrication points in a facility: identification of lubricants used, verification of correct application, leak detection, over-lubrication and critical points. Result is an optimised lubrication plan with reduced references and costs.
Engine oils
SAPS (Sulphate, Ash, Phosphorus and Sulphur)
Sulphated ash, phosphorus and sulphur content in engine oils. High SAPS levels can damage after-treatment systems (DPF, catalyst). Low-SAPS oils (ACEA C1-C5) are mandatory in Euro 5 and 6 vehicles with DPF.
Bearings
K factor (bearings)
Speed factor used to calculate bearing relubrication interval. K = n × √dm, where n is speed in rpm and dm bearing mean diameter in mm. The higher the K factor, the more frequent relubrication should be.
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