6. Lubricants, Additives and Rheology

Introduction and Context

Tribology is concerned not only with the surfaces that interact, but also with the material placed between them. A lubricant separates surfaces, reduces friction and wear, transports heat and contaminants, protects against corrosion, and can transmit power. In advanced engineering systems, lubricant selection is therefore a design decision rather than a routine maintenance activity. The lubricant must remain effective under changing load, speed, temperature, pressure, contamination, and chemical environment.

Lubricants occur as liquids, greases, solids, and gases. Their behaviour depends on viscosity, pressure, temperature, molecular structure, shear history, and the presence of additives. A fluid that performs well in a lightly loaded journal bearing may fail in a high-temperature gear contact, while a grease suitable for an electric motor may be unsuitable for a high-speed spindle. The central engineering task is to match lubricant chemistry and rheology with the contact mechanics, operating environment, materials, sealing system, and maintenance strategy.

This chapter develops that decision framework. It examines mineral, synthetic, bio-based, solid, gaseous, and grease lubricants; explains viscosity–temperature–pressure relationships; introduces non-Newtonian behaviour; analyses additive chemistry and lubricant degradation; and presents methods for selection, compatibility assessment, monitoring, and failure prevention.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Distinguish the principal classes of lubricants and relate their molecular characteristics to performance.

2. Explain the effects of temperature, pressure, shear, and time on lubricant rheology.

3. Interpret viscosity indices, pressure–viscosity behaviour, and film-forming requirements.

4. Describe the functions and possible interactions of major lubricant additives.

5. Evaluate oxidation, contamination, additive depletion, and lubricant compatibility.

6. Select a suitable lubricant for a given machine element and operating environment.

7. Use condition-monitoring and analytical methods to support lubricant decisions.

8. Recognise environmental, safety, regulatory, and sustainability implications of lubricant use.

Lubricants as Engineering Materials

A lubricant is commonly described as a substance that reduces friction and wear, but this definition is incomplete. In a machine, the lubricant may also remove heat, damp vibration, prevent rust, seal clearances, carry debris to a filter, and enable hydraulic control. Its performance is consequently determined by a system of interactions among the lubricant, surfaces, load, motion, and environment.

The most important bulk property of a liquid lubricant is dynamic viscosity, \(\eta\), defined as the ratio of shear stress to shear rate for a Newtonian fluid:

\[

\tau = \eta \dot{\gamma}

\]

where \(\tau\) is shear stress and \(\dot{\gamma}\) is shear rate. Kinematic viscosity is \(\nu=\eta/\rho\), where \(\rho\) is density. Industrial lubricant specifications commonly use kinematic viscosity at a stated temperature, often expressed in mm²/s (cSt). Viscosity determines resistance to flow and contributes strongly to hydrodynamic film thickness. Excessively low viscosity can produce metal-to-metal contact; excessively high viscosity increases churning losses, temperature, and starting torque.

Viscosity is not the same as lubricity. Lubricity refers more broadly to the ability of a lubricant to reduce friction and wear under conditions where bulk fluid viscosity alone cannot maintain separation. Boundary films formed by polar molecules or chemically active additives may protect a surface even when the liquid film is very thin.

Mineral oils

Mineral oils are refined fractions of petroleum. Their properties depend on the distribution of paraffinic, naphthenic, and aromatic hydrocarbons and on the refining process. They are relatively economical, widely available, and compatible with many conventional seals and machine designs. Modern refining can remove undesirable sulphur, nitrogen, wax, and aromatic components and can produce high-quality base oils.

Their limitations include comparatively restricted temperature range, oxidation at elevated temperature, wax-related low-temperature problems in some grades, and variation in composition. Mineral oils remain dominant in industrial lubrication because the complete system cost includes not only the fluid but also seals, filters, storage, and established maintenance practice.

Synthetic oils

Synthetic lubricants are manufactured or highly modified to obtain controlled molecular structures. Important families include polyalphaolefins (PAOs), esters, polyalkylene glycols (PAGs), silicones, phosphate esters, and selected fluorinated fluids.

PAOs offer good low-temperature fluidity, oxidation resistance, and relatively low volatility, but their additive response and seal compatibility must be checked. Esters possess strong polarity, good solvency, and high lubricity; however, some are sensitive to hydrolysis or may interact with paints, seals, and other base oils. PAGs can provide favourable friction and high viscosity indices, especially in particular gear or compressor applications, but many are immiscible with mineral oils and PAOs. Silicones retain useful viscosity over a broad temperature range and have low surface tension, although their load-carrying and additive characteristics may not suit heavily loaded gears. Phosphate esters provide fire resistance but require careful material compatibility assessment.

A synthetic base oil is not automatically superior. Its value arises when its particular thermal, chemical, volatility, or low-temperature properties solve a defined engineering problem.

Bio-based lubricants

Bio-based lubricants are derived partly or wholly from renewable resources, commonly vegetable oils, modified esters, or other biological feedstocks. Their polar molecular structure can provide high lubricity and good film adherence. They are attractive where biodegradability, low toxicity, or renewable sourcing is important, such as agricultural equipment, forestry machinery, marine systems, and environmentally sensitive locations.

Unmodified vegetable oils can suffer from oxidation, hydrolysis, and poor low-temperature flow. Chemical modification and antioxidant packages improve performance, but biodegradability and toxicity must be assessed for the complete formulation rather than inferred from the feedstock alone. Storage stability, seal compatibility, and transition procedures from mineral oil also require attention.

Greases

Grease is a semi-solid lubricant consisting principally of a base oil immobilised by a thickener, together with additives. The base oil may be mineral or synthetic. Common thickeners include lithium soaps, lithium complex, calcium sulphonate complex, aluminium complex, polyurea, and clay-based systems. The thickener creates a structure that holds lubricant in place, resists leakage, and supports lubrication where an oil system would drain away.

Grease consistency is commonly classified by worked penetration grades, while dropping point indicates the temperature at which a grease loses its structural integrity under a specified test. Neither property alone defines service temperature. Bearing speed, relubrication interval, oil viscosity, mechanical stability, water resistance, and compatibility are equally important.

Greases should not be mixed merely because they share the same nominal grade. Thickener incompatibility can cause softening, hardening, oil separation, or loss of mechanical stability. Compatibility tests and manufacturer guidance are required, particularly when changing from one thickener family to another.

Solid and gaseous lubricants

Solid lubricants are useful where liquid or grease lubrication is impossible, undesirable, or short-lived. Graphite, molybdenum disulphide (MoS₂), boron nitride, PTFE, and certain soft metals operate through low-shear mechanisms or protective transfer films. Graphite generally benefits from an appropriate atmosphere containing adsorbed species, whereas MoS₂ is particularly effective in vacuum and high-load conditions. Solid lubricants may be applied as powders, bonded coatings, composite fillers, or surface treatments. Their limitations include oxidation, sensitivity to humidity or atmosphere, limited heat removal, and possible wear or failure of the supporting binder.

Gases, especially air, can act as lubricants in gas bearings. Gas viscosity is low, so friction and power loss can be extremely small, and contamination of the product is avoided. However, gas films have low load capacity and stiffness compared with liquid films, requiring precise clearances, high rotational speed or externally supplied pressure, and careful control of instability and damping.

Viscosity, Pressure, Temperature and Film Formation

Temperature dependence

Liquid viscosity generally decreases strongly as temperature increases. The relationship is nonlinear and is often represented empirically by ASTM viscosity–temperature correlations or equations such as the Walther relation. The viscosity index (VI) provides a comparative measure of how much viscosity changes with temperature: a higher VI indicates a smaller change over the specified temperature range. VI is useful, but it does not describe oxidation life, shear stability, pressure response, or boundary lubrication.

A lubricant selected only by its viscosity at room temperature may fail during cold start or at operating temperature. Cold viscosity affects pumpability, starting torque, and starvation. Hot viscosity affects film thickness, leakage, friction, and bearing temperature. Multigrade oils use viscosity-index improvers to provide an acceptable viscosity range, but these polymeric additives may suffer temporary or permanent shear loss.

Pressure dependence

Liquid viscosity increases with pressure. A common approximation is the Barus equation:

\[

\eta(p)=\eta_0\exp(\alpha p)

\]

where \(\alpha\) is the pressure–viscosity coefficient. This effect is essential in elastohydrodynamic lubrication (EHL), where pressures may be sufficiently high to increase viscosity dramatically within the contact. Pressure-induced viscosity growth allows a lubricant film to develop between rolling or heavily loaded conformal surfaces.

The pressure–viscosity coefficient depends on base-oil chemistry and temperature. It should not be treated as a universal constant. In high-speed contacts, thermal effects may reduce viscosity, while pressure effects increase it; the actual film depends on the balance of these mechanisms.

Rheological regimes

A Newtonian fluid has constant viscosity at a given temperature and pressure, independent of shear rate. Many simple mineral oils approximate Newtonian behaviour over ordinary machine operating ranges. However, formulated lubricants frequently contain polymers, particles, soaps, or structured additives and may be non-Newtonian.

For a power-law fluid,

\[

\tau=K\dot{\gamma}^{n}

\]

where \(K\) is the consistency index and \(n\) is the flow behaviour index. When \(n<1\), the fluid is shear-thinning; apparent viscosity decreases as shear rate rises. When \(n>1\), it is shear-thickening. A Bingham plastic requires a yield stress before flow, while a Herschel–Bulkley model combines yield stress with power-law behaviour.

Grease is commonly described as thixotropic: its apparent viscosity decreases under sustained shear and partially rebuilds when at rest. This behaviour assists pumping and churning control, while allowing the grease to remain in place during idle periods. The rebuilding rate and degree of structural recovery influence channel formation, bearing torque, and relubrication practice.

Shear stability is especially important for viscosity-index-improved oils and greases. Temporary viscosity loss occurs when polymer coils are aligned by flow; permanent loss occurs when polymer molecules are mechanically degraded. A lubricant that meets a new-oil viscosity specification may therefore perform differently after service.

Film thickness and operating maps

The dimensionless film parameter, often written as

\[

\lambda=\frac{h_{min}}{\sqrt{R_{q1}^{2}+R_{q2}^{2}}},

\]

relates minimum film thickness to the combined root-mean-square surface roughness. Large values indicate stronger full-film separation, while small values indicate increasing asperity interaction. The parameter is a useful guide rather than a complete failure criterion because surface chemistry, roughness direction, contaminants, transient load, and additive films also matter.

Stribeck-type analysis relates friction and lubrication regime to viscosity, speed, and load. At low speed or high load, boundary or mixed lubrication dominates. As speed or viscosity rises, a full fluid film develops, friction may decrease, and at still higher viscosity or speed viscous losses increase. Practical selection therefore seeks an operating region that provides sufficient film without unnecessary fluid friction and heat generation.

Additive Chemistry and Function

Base oils rarely provide all required performance characteristics. Additives are deliberately selected chemicals that improve existing properties, suppress undesirable reactions, or create protective surface films. Their concentration, interaction, solubility, and decomposition products are as important as their individual functions.

Antioxidants interrupt oxidation pathways by decomposing hydroperoxides or scavenging reactive radicals. Without control, oxidation produces acids, varnish, sludge, viscosity change, deposits, and corrosive species. Detergents and dispersants keep acidic or particulate products suspended or chemically neutralised and help control deposits. In engine oils, detergents are often metal-containing compounds, whereas ashless dispersants are organic molecules that stabilise contaminants.

Anti-wear additives react under boundary conditions to form sacrificial films. Zinc dialkyldithiophosphate (ZDDP) is a widely used example in engine lubrication, although its ash, phosphorus, catalytic-system, and environmental implications require management. Extreme-pressure additives such as sulphur-, phosphorus-, or chlorine-containing compounds react more aggressively under high local temperature and pressure to form low-shear protective layers. Their suitability depends on surface metallurgy; chemically active additives can stain or corrode yellow metals and other materials.

Friction modifiers reduce shear strength at the interface, often through polar molecules that orient on surfaces. Rust and corrosion inhibitors form protective films or neutralise corrosive species. Pour-point depressants modify wax-crystal growth and improve low-temperature flow in suitable base oils. Viscosity-index improvers reduce the rate of viscosity loss with temperature. Antifoam agents destabilise foam, but excessive or poorly dispersed antifoam can cause air-release problems or filter plugging.

Additive interactions may be synergistic, antagonistic, or neutral. A detergent may compete with an anti-wear additive for surface sites; a dispersant may influence seal swelling; an antifoam may be removed by fine filtration. Consequently, additive packages should not be assembled by simply adding the maximum amount of each component. Formulation is a controlled balance supported by bench tests, component tests, and field validation.

Degradation, Contamination and Compatibility

Lubricant degradation is a progressive change in chemical or physical condition. Oxidation accelerates with temperature, oxygen availability, catalytic metals, and residence time. A useful engineering rule is that small temperature increases can substantially accelerate oxidation, but the precise rate depends on formulation and system design. Thermal degradation, hydrolysis, photochemical reactions, additive depletion, and mechanical shear may also occur.

Common contaminants include water, dust, wear metals, process chemicals, fuel, refrigerant, cleaning agents, and other lubricants. Water can reduce film strength, promote rust, hydrolyse susceptible fluids, create hydrogen embrittlement risk in some systems, and cause bearing surface distress. Solid particles produce abrasive wear and can catalyse oxidation. Contamination control through seals, breathers, filtration, desiccation, and clean handling is often more effective than attempting to compensate with additives.

Compatibility has several dimensions. Two lubricants may be chemically miscible but produce an unacceptable viscosity or additive balance. They may be immiscible yet individually suitable. Elastomer compatibility concerns swelling, shrinkage, hardening, and loss of sealing force. Paints, coatings, cables, adhesives, and plastic cages may also be affected. When changing lubricant, the engineer should identify the existing fluid, inspect the system, drain or flush where necessary, replace filters, and verify the new lubricant with supplier data or compatibility testing.

Oil analysis provides evidence for decisions. Viscosity measurement detects dilution, thickening, or shear loss. Infrared spectroscopy identifies oxidation, nitration, water, and fuel-related changes. Elemental analysis detects wear metals and additive elements. Particle counting evaluates cleanliness, while ferrography or analytical ferrography examines ferrous debris morphology. Karl Fischer titration quantifies water. These techniques are most valuable when interpreted against a baseline and a trend rather than against a single universal alarm limit.

A Selection Framework for Engineers

Lubricant selection should begin with the contact and machine function, not with a brand name. The following sequence is effective:

1. Define the contact: bearing, gear, seal, slideway, hydraulic component, compressor, cutting interface, or other element.

2. Establish operating conditions: load, speed, temperature range, pressure, duty cycle, start–stop frequency, vibration, and shock.

3. Identify the lubrication regime and required viscosity at operating temperature.

4. Assess environmental exposure: water, dust, chemicals, oxygen, radiation, food contact, fire risk, or outdoor conditions.

5. Check material compatibility, including metals, elastomers, plastics, coatings, cages, and filters.

6. Determine required additive functions and restrictions, such as yellow-metal compatibility, ash limits, catalytic-system compatibility, or electrical insulation.

7. Compare oil, grease, solid, and gaseous options, including delivery method and heat removal.

8. Evaluate availability, storage life, environmental profile, disposal requirements, and total cost.

9. Establish a monitoring and relubrication plan.

For a rolling bearing, the selected base-oil viscosity must be high enough to provide a suitable film at operating temperature, while speed and heat generation limit excessive viscosity. For a heavily loaded gear, pressure–viscosity response, anti-scuff performance, micropitting resistance, and material compatibility become prominent. For a food-processing machine, incidental-contact approval, cleanability, odour, and contamination consequences may govern the choice. In a fire-prone hydraulic system, fire resistance may outweigh conventional mineral-oil economics.

Illustrative Case Study: Grease Failure in a High-Speed Electric-Motor Bearing

Background

A manufacturing plant operated several medium-sized electric motors driving centrifugal pumps. The bearings had historically been lubricated with a lithium-complex mineral-oil grease. After a maintenance shutdown, a synthetic polyurea grease was introduced because it was advertised as offering longer life and higher temperature capability.

Situation and problem

Within six weeks, two motors developed elevated bearing temperature and audible roughness. Initial inspection showed darkened grease and localized cage wear. The maintenance team suspected inadequate relubrication, but the grease quantity and interval had not changed.

Analysis

The investigation identified three interacting issues. First, the previous grease and the replacement grease had not been compatibility-tested; their thickener systems could produce an unstable mixture. Second, the maintenance procedure added the new grease without adequately removing the old material. Third, the motor bearings operated at relatively high speed, so the new grease quantity was excessive for the bearing free volume. The resulting churning increased temperature, while mixed grease structure reduced oil release and distribution.

Vibration data showed a developing bearing fault rather than an immediate catastrophic defect. Grease examination indicated thickener separation and oxidation, but no evidence that the base oil itself lacked temperature capability.

Approach and solution

The plant isolated the affected motors, cleaned the bearing housings, replaced damaged bearings, and adopted one approved grease for the motor population. The selection considered base-oil viscosity, speed factor, oxidation resistance, noise performance, thickener compatibility, and relubrication quantity. The maintenance procedure specified measured quantities, clean grease guns, identification labels, and a controlled transition process. Temperature and vibration trends were added to the condition-monitoring programme.

Outcome

Bearing temperatures returned to their previous operating range, and the abnormal vibration trend disappeared after bearing replacement and correct application of the selected grease. The plant also reduced unnecessary grease addition and improved storage control.

Learning points

The case demonstrates that a technically advanced lubricant can fail when the delivery quantity, compatibility, or transition procedure is wrong. Lubricant selection must include the complete tribological system, and “higher temperature rating” is not a substitute for speed, quantity, compatibility, and monitoring analysis.

Professional Practice, Risks and Governance

Common mistakes include selecting viscosity from ambient rather than operating temperature, assuming all synthetic oils mix, treating grease grade as a complete specification, overgreasing bearings, using contaminated transfer equipment, and replacing oil solely because it has reached a calendar date. Another error is interpreting a high viscosity as proof of adequate film; oxidation, contamination, aeration, and additive depletion may still cause failure.

Lubricant management is also a governance issue. Facilities should maintain an approved-lubricant list, safety data sheets, storage and labelling procedures, contamination-control standards, waste records, and change-control documentation. Regulatory requirements may apply to fire-resistant fluids, food-contact lubricants, transport, chemical classification, and disposal. Environmental claims should be based on recognised biodegradability and toxicity criteria for the complete product.

Personnel must understand that lubricant handling involves chemical exposure, pressurised systems, hot surfaces, slips, and environmental release risks. Used oil may contain hazardous contaminants and should be collected and processed through authorised channels. Ethical engineering practice requires accurate reporting of lubricant condition and failure causes rather than attributing every breakdown to operator error.

Digital Monitoring and Emerging Developments

Modern lubrication programmes increasingly combine laboratory analysis with sensors and maintenance software. Online viscosity, dielectric, moisture, particle, temperature, and ferrous-debris sensors can provide continuous information in critical assets. Vibration, acoustic emission, motor-current analysis, and lubricant data can be fused to distinguish lubrication starvation from bearing damage or misalignment.

Data analytics can establish asset-specific baselines and detect trends earlier than fixed calendar intervals. Machine-learning systems may classify abnormal particle signatures or predict remaining useful life, but their output depends on representative training data, sensor quality, explainability, and competent human review. Automation should support, not replace, physical inspection and engineering judgement.

Emerging lubricant development focuses on lower toxicity, improved biodegradability, reduced energy loss, ash-reduced additive systems, advanced surface-active molecules, ionic or deep-eutectic fluids, nanostructured additives, and lubricants designed for electrified powertrains. These materials may offer benefits but also introduce questions about long-term compatibility, disposal, standardisation, particle release, and life-cycle impact. Laboratory performance must therefore be confirmed under realistic contacts and duty cycles.

Best-Practice Checklist

A robust lubrication programme should:

• specify viscosity at relevant operating temperatures, not only at room temperature;
• match additive chemistry to load, metallurgy, emissions systems, seals, and environment;
• prevent contamination during storage, transfer, and application;
• control grease quantity, speed, relubrication interval, and compatibility;
• use condition monitoring and trend analysis for critical assets;
• validate lubricant changes through controlled trials;
• document product identity, batch, location, date, quantity, and responsible person;
• review environmental, safety, disposal, and regulatory requirements;
• investigate failures through evidence from lubricant, surfaces, machine condition, and operating history.

Chapter Summary

Lubricants are functional engineering materials whose performance depends on chemistry, rheology, contact mechanics, and operating environment. Mineral oils provide economical and versatile service, while synthetic, bio-based, solid, gaseous, and grease lubricants address specialised requirements. Viscosity changes with temperature, pressure, shear, and degradation; pressure–viscosity behaviour is particularly important in EHL contacts, while non-Newtonian and thixotropic behaviour must be considered in formulated oils and greases.

Additives control oxidation, wear, friction, corrosion, deposits, low-temperature flow, and foam, but their interactions require formulation discipline. Water, particles, incompatible fluids, thermal stress, and additive depletion can reduce performance. Selection should integrate viscosity, load, speed, temperature, materials, delivery method, compatibility, environmental conditions, monitoring, and total life-cycle cost. Digital condition monitoring and emerging sustainable formulations offer significant opportunities, provided their limitations and validation requirements are recognised.

Key Terms

Viscosity: A fluid’s resistance to shear flow.

Kinematic viscosity: Dynamic viscosity divided by density, commonly reported in mm²/s.

Viscosity index: A comparative measure of viscosity change with temperature.

Pressure–viscosity coefficient: A parameter describing the increase of viscosity with pressure.

Lubricity: The ability of a lubricant to reduce friction and wear, especially under boundary conditions.

Newtonian fluid: A fluid whose viscosity is independent of shear rate at specified conditions.

Non-Newtonian fluid: A fluid whose apparent viscosity changes with shear rate or time.

Shear thinning: Reduction in apparent viscosity as shear rate increases.

Thixotropy: Time-dependent reduction of viscosity under shear with structural recovery at rest.

Elastohydrodynamic lubrication: Lubrication in which elastic deformation and pressure-dependent viscosity form a separating film.

Film parameter: The ratio of lubricant-film thickness to combined surface roughness.

Base oil: The principal liquid component of a formulated lubricant.

Additive: A chemical component added to modify or improve lubricant performance.

Anti-wear additive: An additive that forms a protective film under moderate boundary conditions.

Extreme-pressure additive: A chemically active additive that protects surfaces under severe load and temperature.

Grease: A semi-solid lubricant containing base oil, thickener, and usually additives.

Oxidation: Chemical reaction of lubricant constituents with oxygen, producing degradation products.

Compatibility: The ability of lubricants and contacting materials to coexist without unacceptable chemical or physical change.

Lubricant condition monitoring: Measurement and interpretation of lubricant and machine data to assess serviceability.

Review Questions

Short Answer Questions

1. Distinguish dynamic viscosity from kinematic viscosity.

2. What information does viscosity index provide, and what does it not provide?

3. Why can two greases with the same consistency grade be incompatible?

4. State two functions of lubricant additives.

5. What is meant by shear thinning?

Long Answer Questions

1. Compare mineral oils, PAOs, esters, PAGs, and bio-based lubricants with respect to performance and compatibility.

2. Explain the effects of temperature and pressure on lubricant viscosity and film formation.

3. Describe the principal classes of lubricant additives and discuss possible additive interactions.

4. Explain the causes and consequences of lubricant oxidation, water contamination, and particulate contamination.

5. Develop a systematic lubricant-selection procedure for a heavily loaded industrial gear system.

Analytical / Application Questions

1. A bearing operates at low start-up temperature but reaches a high steady-state temperature. Explain how viscosity, viscosity index, speed, and delivery method should influence selection.

2. A plant changes from lithium-complex grease to polyurea grease without cleaning the housings, and bearing temperature rises. Analyse the likely mechanisms and propose a corrective plan.

3. Oil analysis shows falling viscosity, increased iron particles, and rising water content. Distinguish possible failure mechanisms and recommend an investigation and monitoring response.

22. Multiple Choice Questions with Answers

Question 1

Which property most directly describes the increase in lubricant viscosity with pressure in an elastohydrodynamic contact?

A. Viscosity index

B. Pressure–viscosity coefficient

C. Dropping point

D. Pour point

Correct Answer: B. Pressure–viscosity coefficient

Question 2

A lubricant whose apparent viscosity decreases as shear rate increases is described as:

A. Shear-thickening

B. Newtonian

C. Shear-thinning

D. Perfectly elastic

Correct Answer: C. Shear-thinning

Question 3

Which statement best describes the function of a grease thickener?

A. It converts all boundary lubrication into gas lubrication

B. It immobilises base oil and helps retain lubricant at the contact

C. It always increases the base-oil viscosity index

D. It eliminates the need for additives

Correct Answer: B. It immobilises base oil and helps retain lubricant at the contact

Question 4

Why is a high viscosity index desirable in many applications?

A. It ensures zero oxidation at all temperatures

B. It reduces the change in viscosity over temperature variation

C. It proves that the oil is compatible with every seal

D. It measures the oil’s water content

Correct Answer: B. It reduces the change in viscosity over temperature variation

Question 5

Which situation most strongly indicates a need to assess lubricant compatibility before a change?

A. Changing the colour of a machine label

B. Changing from one grease thickener family to another

C. Increasing the accuracy of a thermometer

D. Replacing a clean filter with an identical new filter

Correct Answer: B. Changing from one grease thickener family to another

Question 6

A bearing becomes hot immediately after relubrication with a new grease. Which explanation is most consistent with the case discussed?

A. Excess grease caused churning losses

B. The bearing necessarily had too little load

C. Higher viscosity always lowers temperature

D. The thickener cannot influence grease behaviour

Correct Answer: A. Excess grease caused churning losses

Question 7

Which additive is primarily intended to interrupt lubricant oxidation reactions?

A. Antioxidant

B. Pour-point depressant

C. Antifoam agent

D. Viscosity-index improver

Correct Answer: A. Antioxidant

Question 8

What is a principal advantage of gaseous lubrication?

A. Very high load capacity at all speeds

B. Very low friction and no liquid contamination

C. Complete insensitivity to clearance errors

D. No requirement for pressure or speed control

Correct Answer: B. Very low friction and no liquid contamination

Question 9

Which oil-analysis result most directly indicates the presence of solid wear debris?

A. Particle count or elemental iron increase

B. A higher viscosity index

C. A lower grease dropping point

D. A change in lubricant colour alone

Correct Answer: A. Particle count or elemental iron increase

Question 10

Why should lubricant selection consider seals, coatings, and plastics as well as metallic surfaces?

A. They determine the lubricant’s pressure–viscosity coefficient exclusively

B. They may swell, shrink, soften, crack, or otherwise lose function through incompatibility

C. They always increase the lubricant’s biodegradability

D. They eliminate the need for contamination control

Correct Answer: B. They may swell, shrink, soften, crack, or otherwise lose function through incompatibility