7. Tribology, Bearings, Lubrication and Sealing

Introduction and Context

Tribology is the study and engineering application of friction, wear, and lubrication between interacting surfaces. Although these phenomena occur at interfaces that may appear small relative to a machine, their effects determine power loss, temperature, noise, reliability, maintenance cost, and service life. A bearing that fails by fatigue, a gear that develops scuffing, a hydraulic actuator that leaks, and a seal that hardens in service are all tribological problems.

In mechanical-system design, tribology cannot be treated as an isolated materials topic. Surface roughness, contact geometry, load, speed, temperature, lubricant chemistry, contamination, manufacturing accuracy, alignment, and maintenance practice interact continuously. Bearing and seal selection therefore requires more than choosing a catalogue number. The designer must translate operating conditions into a contact condition, identify the dominant failure mechanism, select suitable materials and lubricants, and verify performance over the expected lifecycle.

This chapter develops a design-oriented framework for evaluating friction and wear, identifying lubrication regimes, selecting journal and rolling bearings, choosing lubricants and seals, and diagnosing failures using operating and lifecycle data.

Learning Objectives

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

1. Explain the mechanisms of friction and wear at engineering contacts.

2. Distinguish boundary, mixed, hydrodynamic, hydrostatic, elastohydrodynamic, and solid-lubrication regimes.

3. Select journal and rolling-element bearings using load, speed, life, stiffness, temperature, and environmental criteria.

4. Specify lubricant type, viscosity, additives, delivery method, and replenishment strategy.

5. Select seals according to pressure, speed, temperature, fluid compatibility, contamination, and leakage requirements.

6. Diagnose common tribological failures from symptoms, inspection evidence, and operating data.

7. Evaluate lifecycle, environmental, safety, and maintenance implications of tribological design decisions.

The Tribological Contact

Friction and the real area of contact

Friction is the resistance to relative motion between surfaces. The classical dry-friction approximation is

\[F_f = \mu N\]

where \(F_f\) is friction force, \(N\) is normal load, and \(\mu\) is the coefficient of friction. This relation is useful for preliminary calculations, but it does not explain the physical origin of friction or remain constant across all operating conditions.

Engineering surfaces that appear smooth contain asperities. The apparent contact area may be large, while the actual contact occurs at a collection of microscopic asperity junctions. Friction results from adhesion at these junctions, ploughing by harder asperities or particles, deformation, and resistance generated by an intervening lubricant. Consequently, surface finish, hardness, oxide films, temperature, lubricant chemistry, and sliding speed all influence friction.

The friction coefficient is not a material constant in the broad sense. It depends on the pair of materials, surface condition, contact pressure, relative motion, lubricant, temperature, and time. A low friction coefficient is desirable in many power-transmission applications, but it is not the only design objective. A slightly higher-friction material may provide superior wear resistance, sealing, damping, or contamination tolerance.

Wear mechanisms

Wear is progressive material loss or surface damage caused by relative motion, loading, or interaction with a contacting medium. Several mechanisms frequently act together:

• Adhesive wear occurs when asperity junctions form and tear material from one surface to the other. It is promoted by high contact stress, inadequate lubrication, and chemically compatible surfaces that weld locally.
• Abrasive wear occurs when a hard surface or loose particle cuts, ploughs, or deforms a softer surface. Two-body abrasion involves a hard counterface; three-body abrasion involves particles trapped between surfaces.
• 疲劳 wear results from repeated subsurface stress cycles. Pitting and spalling in rolling bearings are typical examples.
• Tribochemical or corrosive wear results from chemical reactions between the surface, lubricant, moisture, and environment. Corrosion can weaken a surface and accelerate mechanical removal.
• Erosive and cavitation wear occur when high-velocity particles or collapsing vapour bubbles strike a surface.
• Fretting wear occurs under small-amplitude oscillatory motion, often at a nominally stationary joint. Oxidized debris and distinctive reddish-brown deposits may be produced.

The frequently used Archard relationship provides a first estimate for adhesive or abrasive wear:

\[V = K\frac{WL}{H}\]

where \(V\) is wear volume, \(W\) is normal load, \(L\) is sliding distance, \(H\) is hardness, and \(K\) is an empirical wear coefficient. The equation emphasizes that wear is affected by load, distance, hardness, and contact condition, but it should not be applied blindly to fatigue, corrosive, or lubricated contacts without experimental validation.

Lubrication Regimes and the Stribeck Concept

The purpose of lubrication is not merely to reduce friction. A lubricant separates surfaces, removes heat, prevents corrosion, carries away debris, damps vibration, and sometimes transmits power. The operating regime depends on viscosity, speed, load, geometry, surface roughness, and temperature.

Boundary lubrication

In boundary lubrication, the lubricant film is too thin to separate surface asperities fully. Friction and wear are controlled largely by adsorbed or chemically reacted molecular films and by the properties of the surfaces. Boundary conditions occur during start-up and shutdown, at low speed or high load, and in contacts with insufficient lubricant supply. Anti-wear and extreme-pressure additives are particularly important in this regime, although additive compatibility with materials and seals must be verified.

Mixed lubrication

Mixed lubrication combines partial hydrodynamic or elastohydrodynamic separation with asperity contact. It is common during acceleration, deceleration, fluctuating load, and moderate-speed operation. Surface finish and additive chemistry strongly affect performance because a small change in film thickness can change the proportion of load carried by asperities.

Hydrodynamic lubrication

In hydrodynamic lubrication, relative motion drags lubricant into a converging wedge and generates pressure sufficient to support the load. The surfaces are separated by a full fluid film. A simplified journal-bearing analysis uses the Sommerfeld number, which relates viscosity and speed to load and bearing geometry. Increasing viscosity or speed generally increases film thickness, while increasing load reduces it. Excessive viscosity, however, raises viscous drag and temperature.

A journal bearing requires a stable lubricant supply and suitable clearance. At start-up, the shaft may contact the bearing surface before a hydrodynamic film develops. This explains why starting and stopping can be more damaging than steady running.

Hydrostatic lubrication

Hydrostatic bearings receive pressurized fluid from an external pump, allowing a load-carrying film even when the shaft is stationary. They provide high stiffness, low starting friction, and excellent precision, but require a pump, restrictors, filtration, controls, and protection against pressure loss. They are used in precision machine tools, test rigs, and high-value rotating systems.

Elastohydrodynamic lubrication

Rolling-element contacts and gear teeth operate mainly under elastohydrodynamic lubrication (EHL). High pressure causes elastic deformation of the contacting bodies and increases lubricant viscosity locally. The resulting film can separate surfaces despite very small contact dimensions. Film thickness depends on speed, viscosity, material elasticity, load, and geometry. Surface roughness, contamination, and lubricant starvation can reduce the effective film ratio and lead to micropitting or fatigue.

Solid lubrication

Graphite, molybdenum disulfide, polytetrafluoroethylene, and certain engineered coatings may provide lubrication where liquid lubricants evaporate, oxidize, contaminate a product, or cannot remain in place. Solid lubricants are valuable in vacuum, high-temperature, clean-room, and oscillatory applications. Their load capacity, oxidation resistance, transfer behaviour, and compatibility with the substrate must be established for the actual environment.

Film parameter and Stribeck interpretation

A useful indicator of separation is the film parameter:

\[\Lambda = \frac{h_{min}}{\sigma}\]

where \(h_{min}\) is minimum film thickness and \(\sigma\) is the composite root-mean-square surface roughness. A low value indicates substantial asperity interaction; a high value indicates improved separation. The boundaries between regimes are not universal because material, lubricant, surface texture, and transient conditions differ.

The Stribeck curve relates friction to a viscosity-speed-load parameter. As the parameter increases from a low value, friction may initially decline because lubricant increasingly supports the load. At still higher values, viscous shear causes friction to rise. The curve provides a conceptual basis for selecting viscosity and operating conditions, rather than a substitute for detailed bearing analysis.

Bearings as Designed Interfaces

A bearing constrains relative motion while carrying radial, axial, or combined loads. Selection must reflect not only nominal load and speed but also stiffness, alignment, shock, temperature, lubrication, installation, contamination, and consequences of failure.

Journal bearings

Journal bearings use sliding contact between a rotating journal and a bearing surface. Common forms include plain cylindrical bearings, flanged bearings, tilting-pad bearings, and hydrostatic bearings. Materials include bronzes, babbitt alloys, aluminium-based linings, polymer composites, sintered materials, and engineered ceramics.

The bearing clearance must be sufficient to accommodate thermal expansion and permit lubricant flow, but excessive clearance reduces stiffness, increases vibration, and may cause impact or instability. The bearing surface should often be softer or more conformable than the journal so that small particles embed rather than score the shaft. This is a deliberate design compromise: a sacrificial lining may protect an expensive shaft.

Advantages of journal bearings include high damping, low noise, compact radial dimensions, and good suitability for very high speeds or large shafts. Limitations include start-up wear, sensitivity to lubricant supply, possible oil-whirl or oil-whip instability, and the need for accurate alignment and clearance control.

Rolling-element bearings

Rolling bearings use balls or rollers between races. Ball bearings are suited to relatively high speed and moderate loads; cylindrical, spherical, tapered, and needle rollers provide different combinations of radial load, axial load, conformity, and stiffness.

The basic rating life is commonly expressed as

\[L_{10} = \left(\frac{C}{P}\right)^p\]

where \(L_{10}\) is the life at which 10 percent of a sufficiently large group may have failed, \(C\) is the basic dynamic load rating, \(P\) is equivalent dynamic bearing load, and \(p=3\) for ball bearings and \(p=10/3\) for roller bearings. Catalogue life is only a statistical reference. Real service life is strongly affected by contamination, lubrication, misalignment, electrical current, installation damage, static overload, and corrosion.

Equivalent load converts combined radial and axial loading into a single reference load. For a particular bearing family, it is typically evaluated from manufacturer-specific factors of the form

\[P = XF_r + YF_a\]

where \(F_r\) and \(F_a\) are radial and axial loads. The correct factors depend on bearing type, internal clearance, contact angle, and load ratio.

Static load rating must be checked where shock, oscillation, slow rotation, or stationary loading occurs. Excessive permanent deformation can produce noise, vibration, and premature fatigue even if the calculated dynamic life is adequate.

Selection workflow

A defensible bearing-selection process normally follows these stages:

1. Define radial and axial loads, load spectra, speed range, acceleration, shock, and duty cycle.

2. Establish temperature limits, contamination level, moisture, corrosive agents, electrical conditions, and available space.

3. Choose the bearing family according to load direction, speed, stiffness, misalignment, and required motion accuracy.

4. Calculate equivalent load, static safety, rating life, lubrication condition, and thermal behaviour.

5. Specify internal clearance, preload, fit, seals or shields, cage material, and mounting arrangement.

6. Check installation, removal, inspection, relubrication, and replacement requirements.

7. Validate the selection through testing, condition monitoring, or supplier engineering analysis.

Preload can increase stiffness and reduce internal clearance, but excessive preload increases friction, temperature, and fatigue risk. Fits must prevent creep without generating excessive raceway stress. Axial locating and floating arrangements are often used to accommodate shaft thermal expansion.

Lubricants: Selection, Delivery, and Control

Lubricants may be mineral oils, synthetic oils, greases, water-based fluids, or solid films. Selection begins with the operating requirements rather than with brand preference.

Viscosity and viscosity index

Viscosity governs film formation, drag, leakage, and heat generation. A low-viscosity oil may provide inadequate film thickness under heavy load, while a high-viscosity oil can cause churning and excessive temperature. Viscosity changes strongly with temperature; the viscosity index describes the relative sensitivity of viscosity to temperature change. For high or widely varying temperatures, synthetic fluids may offer improved viscosity-temperature behaviour, oxidation resistance, or low-temperature flow.

Additives

Additives provide functions that the base oil cannot provide adequately. Anti-oxidants slow chemical degradation; anti-wear additives form protective films; extreme-pressure additives react under severe contact conditions; detergents and dispersants control deposits; rust inhibitors protect ferrous surfaces; anti-foam agents limit entrained air; and pour-point depressants improve low-temperature flow. Additives can interact with yellow metals, coatings, elastomers, filters, or other lubricants. Mixing products solely because their nominal viscosity is similar is unsafe.

Grease

Grease is a semisolid lubricant consisting of a base oil, thickener, and performance additives. It remains in place and can simplify sealing, but it may generate more heat at high speed. Grease selection considers base-oil viscosity, thickener type, consistency grade, operating temperature, speed factor, water resistance, load, and compatibility. Overgreasing is a common failure cause because churning raises temperature and forces grease through seals.

Lubricant delivery

Oil may be delivered by bath, splash, ring, circulating system, mist, jet, or metered injection. Grease may be applied manually, through single-point devices, or by automatic centralized systems. Delivery must provide adequate quantity without aeration, starvation, contamination, or unnecessary consumption. Filtration, breathers, reservoirs, coolers, and sampling ports are part of the lubrication system, not optional accessories.

Oil analysis can reveal viscosity change, oxidation, water, fuel or process-fluid dilution, wear metals, particles, and additive depletion. Trend analysis is more valuable than a single result. Sampling location and method must be repeatable; a sample taken after a filter may conceal the condition of the contact.

Sealing Mechanical Systems

A seal controls the movement of fluid or prevents ingress of contaminants across a relative-motion or stationary interface. Perfect zero leakage is rarely physically achievable in dynamic seals; the design objective is controlled leakage compatible with safety, product quality, emissions, lubrication, and equipment life.

Seal types

Static joints commonly use O-rings, gaskets, bonded seals, and labyrinth arrangements. Dynamic applications use lip seals, mechanical face seals, packing, piston seals, rod seals, magnetic seals, and non-contact labyrinth or gap seals.

An elastomeric lip seal relies on a controlled contact band and a thin lubricant film. Too little lubrication produces heat and wear; too much leakage may indicate excessive film, shaft damage, wrong lip orientation, or pressure overload. A mechanical face seal uses two highly finished faces pressed together by springs, bellows, or system pressure. Its performance depends on face flatness, cooling, fluid film, installation, and resistance to dry running.

Non-contact seals have low friction and long life at high speed but may permit more leakage. Labyrinth seals are valuable where contamination exclusion is important and a small clearance can be tolerated. Seal selection must account for pressure differential, shaft speed, eccentricity, runout, temperature, fluid chemistry, surface hardness, roughness, and installation damage.

Elastomer and material compatibility

Nitrile rubber is widely used with mineral oils but has temperature and chemical limitations. Fluoroelastomers provide improved temperature and chemical resistance in many applications. Ethylene-propylene materials suit some water, steam, and glycol environments but are incompatible with many petroleum oils. Polytetrafluoroethylene offers low friction and broad chemical resistance, while polyurethane provides high abrasion resistance in selected applications. These are general tendencies, not substitutes for supplier compatibility data.

A seal groove must provide correct squeeze, stretch, clearance, and compression control. Sharp edges should be removed, assembly tools should avoid cutting the seal, and shaft surfaces should be free of burrs and damaging lead patterns. Spiral machining marks can pump fluid past a seal.

Failure Diagnosis Through Evidence

Tribological diagnosis begins with a symptom and proceeds toward a mechanism. “Bearing failed” is not a diagnosis. The useful questions are: Where is the damage? What is its pattern and direction? What changed before failure? Was lubrication adequate in quantity and quality? Did load, speed, alignment, temperature, contamination, or installation differ from the design assumptions?

A practical investigation includes operating history, vibration and temperature trends, lubricant analysis, particle inspection, dimensional checks, photographs, and examination of mating components. Failure surfaces should not be cleaned or struck before evidence is documented.

Typical clues include:

• Uniform raceway flaking or spalling suggests rolling-contact fatigue, often associated with accumulated stress cycles, excessive load, or inadequate film.
• Dents at regularly spaced intervals suggest static overload or impact during installation or handling.
• Smearing and discoloration indicate sliding, skidding, inadequate lubrication, or excessive heat.
• Axial bands or fluting may indicate electrical current passing through a bearing.
• Red-brown fretting debris suggests small-amplitude movement at a fit or contact.
• Longitudinal scoring commonly indicates abrasive contamination or shaft damage.
• Seal lip hardening, cracking, or swelling points toward thermal ageing, chemical incompatibility, ozone, or excessive pressure.
• Foamy or milky oil may indicate air entrainment or water contamination.

Corrective action should address the initiating cause, not merely replace the failed component. For example, replacing a damaged bearing without correcting contamination or misalignment creates recurrence.

Illustrative Case Study: Repeated Failure of a Conveyor Drive Bearing

Background

A mineral-processing plant experienced repeated failure of a spherical roller bearing supporting a conveyor drive shaft. The bearing was rated for the nominal radial load and had a calculated catalogue life exceeding the planned maintenance interval.

Situation and problem

Failures occurred after three to five months rather than the expected service period. The symptoms were rising housing temperature, increasing vibration at the bearing rotational frequency, and eventual spalling of the raceway. Maintenance records showed that grease was added frequently because operators believed more grease would protect the bearing.

Analysis

Inspection found darkened, softened grease and evidence of churning. The housing seal had been damaged, allowing fine mineral dust to enter. Vibration data showed a gradual increase rather than an abrupt impact event. Raceway examination revealed fatigue spalling with polishing and fine abrasive marks. Shaft alignment measurements indicated a small but significant angular misalignment introduced after a coupling replacement.

The original rating-life calculation assumed clean lubrication, correct alignment, and a stable load. It did not represent the actual contamination and lubrication condition. Overgreasing increased temperature, reduced grease consistency, and promoted seal leakage. Dust then entered through the compromised seal, reducing film quality and initiating abrasive and fatigue damage.

Approach and solution

The plant adopted a combined corrective plan. The bearing housing seal was replaced with a contamination-resistant arrangement; shaft and housing fits were inspected; coupling alignment was corrected; and grease quantity was calculated from bearing free volume and operating speed. A compatible grease was selected after checking thickener and base-oil compatibility. Automatic relubrication was set to deliver small, scheduled quantities rather than occasional large additions. Temperature and vibration trends were added to the maintenance system, and grease samples were periodically inspected for contamination.

Outcome

The bearing operated beyond the previous replacement interval, while grease consumption and unplanned shutdowns declined. The important improvement did not come from selecting a larger bearing. It came from aligning the real operating environment with the assumptions used in selection, lubrication, sealing, and maintenance.

Learning points

The case illustrates four principles: catalogue life is not actual service life; lubrication quantity is as important as lubricant grade; sealing and contamination control are part of bearing design; and trend data are most useful when interpreted alongside physical inspection and maintenance history.

Design Risks, Professional Responsibility, and Sustainability

Tribological design involves safety and environmental responsibilities. A failed bearing in a high-speed rotor can release stored energy, while a leaking seal may expose workers to toxic, flammable, or biologically active fluids. Designers must apply relevant machinery, pressure, hazardous-area, emissions, and occupational-safety requirements and must document assumptions used in calculations.

Lubricant leakage can contaminate soil and water, create slip hazards, and cause product contamination. Lifecycle decisions should consider lubricant consumption, energy lost to friction, component replacement, remanufacturing, disposal, and the environmental cost of maintenance. Reduced friction is not automatically sustainable if it requires an incompatible or hazardous formulation. Condition-based maintenance can reduce unnecessary oil changes and premature replacements, provided monitoring is reliable.

Common professional mistakes include selecting by load rating alone, ignoring transient conditions, using an unsuitable viscosity, mixing greases, overgreasing, fitting bearings by impact, neglecting shaft finish, assuming a seal is independent of the lubricant, and accepting a failure report without examining the failed component. A disciplined design review should challenge these assumptions.

Digital Monitoring and Emerging Practice

Sensors increasingly support tribological management. Accelerometers identify bearing defect frequencies; temperature sensors indicate friction, overload, and lubrication problems; torque measurement reveals power loss; and online oil sensors estimate particle concentration, dielectric change, or water content. Machine-learning tools can detect abnormal trends, but their predictions depend on representative training data, stable sensor installation, and meaningful failure labels. An algorithm should support engineering judgement rather than conceal uncertainty.

Digital records can connect bearing identity, installation method, lubricant batch, operating load, vibration history, and failure evidence. Such traceability enables reliability-centred maintenance and improves future design. Emerging developments include low-friction coatings, self-lubricating polymers, additive manufacturing of textured surfaces, biodegradable lubricants, electrically insulated bearings, and advanced seal materials. Each innovation must be evaluated for manufacturability, compatibility, repairability, and lifecycle impact rather than judged only by laboratory friction values.

Best-Practice Design Framework

A robust tribological design review should answer the following questions:

1. What relative motions, loads, speeds, temperatures, and transients occur?

2. What are the expected friction, wear, leakage, and heat-generation mechanisms?

3. Which lubrication regime dominates during start-up, steady operation, overload, and shutdown?

4. Are bearing life, static capacity, stiffness, clearance, preload, and alignment adequate?

5. Is the lubricant compatible with the materials, seals, temperature, and contamination environment?

6. How will lubricant be supplied, filtered, sampled, replenished, and disposed of?

7. What sealing performance is required, and what leakage is acceptable and safe?

8. Which measurements will detect degradation before functional failure?

9. How will installation, maintenance, and failure evidence be controlled?

10. What are the lifecycle cost, energy, environmental, and safety consequences?

This framework integrates design calculations with operational reality. It also creates a clear link between component selection and the broader mechanical system.

Chapter Summary

Tribology governs the performance and life of mechanical systems through the interacting effects of friction, wear, lubrication, materials, geometry, environment, and maintenance. Friction arises from asperity interaction, deformation, adhesion, ploughing, and lubricant shear. Wear may be adhesive, abrasive, fatigue-based, corrosive, erosive, or fretting in nature.

Lubrication regimes range from boundary and mixed conditions to hydrodynamic, hydrostatic, elastohydrodynamic, and solid lubrication. The Stribeck concept and film parameter help interpret the transition between regimes. Journal bearings provide damping and high-speed capability, whereas rolling bearings offer standardized capacity and predictable rating life; both require correct load, clearance, alignment, lubrication, and contamination control.

Lubricant selection depends on viscosity, temperature, speed, load, additives, compatibility, delivery, and monitoring. Seals must be selected as part of the tribological system, with attention to pressure, speed, material compatibility, surface condition, and acceptable leakage. Failure diagnosis should combine damage morphology with vibration, temperature, lubricant, alignment, and maintenance data. Effective design is therefore lifecycle-oriented: it addresses installation, operation, condition monitoring, safety, environmental impact, and root-cause elimination.

Key Terms

Tribology: Engineering study of friction, wear, lubrication, and interacting surfaces.

Asperity: Microscopic peak or irregularity on an apparently contacting surface.

Adhesive wear: Material loss caused by local adhesion and tearing at contacting asperities.

Abrasive wear: Material removal by hard asperities or particles cutting or ploughing a surface.

Fretting: Wear and surface damage caused by small-amplitude oscillatory motion.

Boundary lubrication: Regime in which surface films carry much of the load and asperity contact remains significant.

Mixed lubrication: Regime combining fluid-film support with partial asperity contact.

Hydrodynamic lubrication: Full-film lubrication produced by viscous flow through a converging wedge.

Hydrostatic lubrication: Full-film lubrication maintained by externally pressurized fluid.

Elastohydrodynamic lubrication: Lubrication in highly loaded contacts where elastic deformation and pressure-dependent viscosity form the film.

Film parameter: Ratio of minimum lubricant-film thickness to composite surface roughness.

Journal bearing: Sliding bearing in which a shaft journal runs within a bearing surface.

Rolling-element bearing: Bearing using balls or rollers between races.

Basic dynamic load rating: Reference load associated with the statistical rating life of a rolling bearing.

Preload: Intentional internal bearing load used to control clearance and increase stiffness.

Viscosity: Resistance of a fluid to shear flow.

Grease: Semisolid lubricant consisting of base oil, thickener, and additives.

Seal: Device that limits fluid leakage or prevents contaminant entry across an interface.

Condition monitoring: Measurement and interpretation of operating data to detect degradation.

Lubrication starvation: Condition in which insufficient lubricant reaches the tribological contact.

Review Questions

Short Answer Questions

1. Why is the coefficient of friction not a universal material constant?

2. Distinguish adhesive wear from abrasive wear.

3. What is the purpose of bearing preload?

4. State two functions of a lubricant other than friction reduction.

5. Why can overgreasing cause bearing failure?

Long Answer Questions

1. Explain the principal lubrication regimes and relate them to speed, load, viscosity, and surface roughness.

2. Describe a systematic procedure for selecting a rolling-element bearing for combined radial and axial loading.

3. Discuss journal-bearing clearance, hydrodynamic film formation, start-up behaviour, and instability.

4. Explain the factors governing dynamic seal selection in a rotating shaft application.

5. Develop a tribological failure-investigation procedure using physical evidence and operating data.

Analytical / Application Questions

1. A high-speed bearing shows increasing temperature after a lubrication change, although vibration remains initially stable. Analyse possible causes and identify the data required before deciding whether to reduce viscosity or lubricant quantity.

2. A gearbox develops pitting after operation in a dusty environment. Distinguish fatigue, abrasive, and corrosive contributions, and propose a corrective plan covering lubricant, filtration, sealing, alignment, and monitoring.

3. A machine-tool spindle requires low friction, high stiffness, accurate rotation, and operation during frequent starts and stops. Compare a rolling bearing, hydrodynamic bearing, and hydrostatic bearing, and justify a selection framework rather than a single choice.

22. Multiple Choice Questions with Answers

Question 1

Which condition most directly characterizes boundary lubrication?

A. The surfaces are completely separated by a thick fluid film

B. Externally pressurized fluid supports the load at zero speed

C. Chemically or physically formed surface films carry much of the load while asperity contact remains

D. The contact is lubricated only by a solid coating in vacuum

Correct Answer: C. Chemically or physically formed surface films carry much of the load while asperity contact remains

Question 2

In the simplified Archard wear relationship, increasing surface hardness while other variables remain constant will generally:

A. Increase wear volume

B. Reduce wear volume

C. Have no effect on wear volume

D. Reverse the sliding direction

Correct Answer: B. Reduce wear volume

Question 3

A rolling bearing has a catalogue life calculation based on clean lubrication, but operates with abrasive dust ingress. The most appropriate interpretation is that:

A. The catalogue life remains guaranteed

B. Contamination can substantially reduce actual service life

C. Only axial load affects life

D. Grease quantity is irrelevant

Correct Answer: B. Contamination can substantially reduce actual service life

Question 4

For the basic rating-life equation of a ball bearing, the exponent p is normally:

A. 1

B. 2

C. 3

D. 10/3

Correct Answer: C. 3

Question 5

What is a likely consequence of excessive bearing preload?

A. Lower friction and lower temperature in all cases

B. Increased stiffness accompanied by possible heat generation and fatigue

C. Elimination of the need for lubrication

D. Guaranteed compensation for severe misalignment

Correct Answer: B. Increased stiffness accompanied by possible heat generation and fatigue

Question 6

A journal bearing is most likely to experience direct surface contact during:

A. Stable operation with a fully developed hydrodynamic film

B. Start-up before the lubricant film has developed

C. Hydrostatic operation with maintained supply pressure

D. Operation with zero load and high speed

Correct Answer: B. Start-up before the lubricant film has developed

Question 7

Which observation most strongly suggests electrical current damage in a rolling bearing?

A. Regularly spaced dents from impact

B. Axial fluting or electrical erosion marks

C. Milky oil caused by water contamination

D. A softened elastomeric seal

Correct Answer: B. Axial fluting or electrical erosion marks

Question 8

Why can a high-viscosity lubricant be unsuitable even when it provides a thick film?

A. It always evaporates more rapidly

B. It may increase churning, viscous drag, and operating temperature

C. It cannot carry any load

D. It prevents all additive reactions

Correct Answer: B. It may increase churning, viscous drag, and operating temperature

Question 9

Which design factor is especially important for a dynamic elastomeric lip seal?

A. Only the nominal shaft diameter

B. Shaft surface condition, pressure, speed, temperature, and fluid compatibility

C. Only the colour of the elastomer

D. The bearing static rating alone

Correct Answer: B. Shaft surface condition, pressure, speed, temperature, and fluid compatibility

Question 10

In the conveyor-bearing case, the most effective corrective strategy was to:

A. Install a larger bearing without changing operating practice

B. Add grease more frequently in large quantities

C. Correct alignment, control contamination, optimize grease delivery, and monitor condition

D. Remove the housing seal to allow heat to escape

Correct Answer: C. Correct alignment, control contamination, optimize grease delivery, and monitor condition