10. Tribology of Machine Elements and Manufacturing Systems

Introduction and Context

Tribology becomes most consequential when the general principles of friction, wear, and lubrication are translated into the performance of complete machine elements. A bearing does not fail merely because its coefficient of friction is high; it fails because a duty cycle, load spectrum, lubricant, temperature, geometry, contamination level, and maintenance practice combine to produce an unacceptable damage mechanism. Similarly, a gear, seal, piston ring, brake, clutch, cutting tool, or forming interface must be designed as a tribological system rather than as an isolated component.

Machine-element tribology therefore requires simultaneous attention to contact mechanics, lubrication regimes, material compatibility, surface engineering, thermal behavior, manufacturing tolerances, reliability, safety, and cost. The designer must determine not only whether a component works under nominal conditions, but also how it behaves during start-up, overload, transient speed changes, contamination, misalignment, shock, temperature excursions, and progressive wear.

This chapter develops an application-oriented framework for selecting, designing, troubleshooting, and improving tribological machine elements and manufacturing interfaces. It emphasizes duty-cycle analysis, failure diagnosis, standards-based practice, safety factors, and the use of monitoring and digital tools.

Learning Objectives

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

1. Relate contact conditions, lubrication regimes, materials, and surface finish to machine-element performance.

2. Select and assess bearings, gears, seals, piston rings, brakes, clutches, cutting tools, and forming interfaces.

3. Apply load spectra, duty cycles, safety factors, and reliability considerations to tribological design.

4. Diagnose common friction, wear, lubrication, thermal, and alignment failures.

5. Interpret the role of standards, testing, condition monitoring, and digital analytics.

6. Evaluate emerging surface treatments, lubricants, sensors, and sustainable manufacturing practices.

A Systems View of Tribological Design

A tribological interface is defined by four interacting elements: the contacting bodies, the intervening lubricant or environment, the operating conditions, and the surrounding machine system. The commonly used “tribosystem” concept is useful because it prevents the designer from treating wear as an intrinsic property of one material. Wear depends on the counterface, contact pressure, sliding or rolling kinematics, temperature, debris, chemistry, and duration.

The basic engineering quantities are friction force, wear rate, contact stress, temperature, and lubricant-film thickness. A useful dimensionless indicator of lubrication is the film parameter

λ = h_min / σ_c

where h_min is the minimum lubricant-film thickness and σ_c is the composite root-mean-square surface roughness. Large λ values generally indicate full-film separation; intermediate values indicate mixed lubrication; and small values indicate boundary or severe mixed contact. These divisions are not absolute design limits, but they help organize failure analysis.

The Stribeck relationship expresses the broad influence of viscosity, speed, and load through a parameter proportional to ηU/W, where η is viscosity, U is entrainment speed, and W is load. Increasing viscosity or speed tends to increase film thickness, whereas increasing load reduces the film parameter. In practice, this relationship must be modified for temperature-dependent viscosity, non-Newtonian lubricants, surface texturing, starvation, elastic deformation, and transient operation.

Contact mechanics provides the second foundation. Hertzian contact theory estimates pressure distribution and elastic deformation for rolling and nonconformal contacts. For a spherical or cylindrical contact, the maximum pressure increases with load and decreases with effective contact radius and material compliance. In gears and rolling bearings, subsurface shear stresses may initiate fatigue cracks even when the visible surface appears smooth. In sliding interfaces, asperity pressure can be many times higher than the nominal pressure, explaining why surface chemistry and roughness are critical.

A successful design follows a chain of decisions: define the duty cycle, identify the dominant damage mechanisms, establish acceptable friction and life, select geometry and materials, choose lubrication and sealing, introduce safety factors, validate by testing, and establish monitoring and maintenance limits.

Duty Cycles, Load Spectra, and Design Criteria

Nominal load is rarely sufficient for tribological design. A bearing in an electric motor may experience steady radial load, whereas a robot joint may undergo alternating loads, reversals, dwell periods, acceleration, and impact. A gear transmission may spend most of its life at partial load but occasionally experience torque peaks. These histories determine both fatigue damage and lubricant-film behavior.

A duty cycle should record speed, torque or load, temperature, direction, duration, start-stop frequency, environmental contamination, and lubrication state. For rolling bearings, an equivalent dynamic load is commonly used to combine radial and axial components. Bearing life is often represented by a relation of the form

L_10 = (C/P)^p

where C is the dynamic load rating, P is equivalent dynamic load, and p is commonly 3 for ball bearings and 10/3 for roller bearings. L_10 denotes the life at which 90 percent of an identical population is expected to survive under defined conditions. Actual reliability requires consideration of lubrication, contamination, material cleanliness, alignment, and installation.

For variable loading, damage accumulation concepts such as Miner’s rule may provide a first approximation: the sum of the fractions of life consumed in different load blocks should remain below an allowable damage fraction. This approach is imperfect for tribological systems because lubrication and temperature history affect subsequent damage, but it is valuable for preliminary duty-cycle screening.

Safety factors should not be applied blindly. A larger load factor may protect against overload, but it can also produce unnecessary size, friction, heat, and cost. Separate factors may be appropriate for uncertainty in load, material strength, fatigue life, contamination, lubrication, thermal capacity, and human error. A high consequence of failure—such as loss of braking or leakage of hazardous fluid—requires a more conservative design and often redundancy or fail-safe behavior.

Bearings: From Load Capacity to Failure Diagnosis

Bearings support relative motion while controlling friction, stiffness, accuracy, and heat generation. Rolling-element bearings use balls or rollers; hydrodynamic and hydrostatic bearings support surfaces through a fluid film; magnetic and compliant bearings use specialized support mechanisms.

Rolling-element bearings

Rolling bearings are selected using load rating, life, speed, stiffness, allowable temperature, space, misalignment, and sealing requirements. Ball bearings are generally suitable for moderate loads and high speed, while cylindrical, tapered, and spherical roller bearings provide greater radial or combined-load capacity. Tapered bearings are particularly useful when axial and radial loads must be carried together.

The nominal life calculation is only a starting point. False brinelling can arise from vibration while stationary. True brinelling results from overload or impact. Smearing occurs when sliding accompanies inadequate lubrication or high acceleration. Spalling is commonly associated with rolling-contact fatigue, contamination dents, poor mounting, or excessive preload. Electrical current can produce fluting and premature surface damage; insulated bearings, grounding arrangements, or electrically appropriate designs may be required.

Bearing fits must account for which ring rotates relative to the load. A rotating load generally requires an interference fit to prevent creep, whereas a stationary load may permit a looser fit. Excessive interference reduces internal clearance and can generate heat. Misalignment produces edge loading, uneven raceway stress, and abnormal cage forces. Installation should use the correct mounting force path; transmitting pressing force through rolling elements can damage raceways before operation begins.

Fluid-film bearings

Hydrodynamic bearings develop pressure through relative motion and wedge geometry. During start-up and shutdown, however, the surfaces may pass through boundary or mixed lubrication. Design therefore requires adequate journal speed, viscosity, clearance ratio, bearing length, alignment, and thermal management. Hydrostatic bearings maintain a supporting film through externally pressurized lubricant and can operate at zero speed, but they require pumps, restrictors, control systems, and protection against supply failure.

Bearing troubleshooting should begin with evidence rather than replacement. Inspect load direction, shaft alignment, fits, lubricant condition, sealing, temperature history, vibration spectra, and contamination. A single discolored raceway may indicate overheating, while evenly distributed fatigue damage suggests a different mechanism from localized electrical erosion or mounting damage.

Gears: Rolling, Sliding, and Scuffing in Combination

Gear contacts combine rolling and sliding. The sliding component changes direction near the pitch point and is especially severe near the tooth tip and root. Gear tribology must therefore address pitting, micropitting, scuffing, abrasive wear, corrosive wear, tooth breakage, and plastic deformation.

Pitting is a rolling-contact fatigue process associated with repeated subsurface stress. Micropitting involves fine-scale surface distress, often promoted by inadequate film thickness, rough surfaces, unfavorable roughness direction, and particular additive or temperature conditions. Scuffing is a rapid adhesive damage process caused by local welding and tearing when the lubricant film and chemical protection are insufficient. It is promoted by high sliding speed, high temperature, overload, low viscosity, poor lubricant supply, and inadequate running-in.

Gear design involves tooth geometry, material hardness, heat treatment, surface finish, alignment, backlash, lubrication, and housing stiffness. Standards such as ISO and AGMA provide methods for rating bending strength, contact stress, service factors, and lubrication-related performance. The designer must ensure that the selected standard, quality grade, load spectrum, and safety factors correspond to the actual application.

Oil-bath, splash, forced-circulation, and grease lubrication each have appropriate operating ranges. Excessive oil level can cause churning losses and temperature rise; insufficient level causes starvation. Filters, breathers, seals, and cleanliness control are as important as the lubricant grade itself. In high-speed systems, synthetic lubricants may improve thermal stability, but compatibility with seals, coatings, and additives must be verified.

Seals and the Control of Leakage

Seals perform two opposing functions: they retain lubricant and exclude contaminants. A seal is therefore a tribological component, not merely a barrier. Its lip, shaft, lubricant, pressure, temperature, surface finish, speed, and installation determine friction and life.

Radial lip seals depend on a controlled contact band and a thin lubricating film. Excessive interference increases friction and heat; inadequate interference causes leakage. Shaft lead, roughness, hardness, runout, and damage at the seal path are decisive. Spiral machining marks can act as pumping features and drive lubricant outward. Elastomer selection must consider temperature, fluid chemistry, swelling, compression set, and compatibility with additives.

Mechanical face seals use controlled face contact and often a fluid film only a few micrometres thick. Face flatness, waviness, spring loading, cooling, and clean assembly are essential. Dry running, vaporization, abrasive particles, or loss of spring force can rapidly cause leakage. Labyrinth and magnetic seals reduce contact but may require greater space or careful control of pressure and contamination.

Leakage should not be treated automatically as a seal-quality problem. Misalignment, excessive pressure, shaft vibration, blocked vents, thermal expansion, bearing play, and incorrect lubricant level may be the root cause. A durable solution addresses the system condition that created the leakage.

Piston Rings, Cylinder Liners, Brakes, and Clutches

Piston-ring tribology is governed by reciprocating motion, changing speed, gas pressure, temperature, lubrication starvation, and surface texturing. Ring-pack friction can represent a significant portion of engine mechanical losses. The top ring experiences high temperature and combustion pressure; oil-control rings regulate lubricant distribution. Ring tension, end gap, face coating, liner honing, oil viscosity, and cylinder distortion must be considered together.

Wear mechanisms include adhesive wear, abrasive wear from particles, scuffing under lubricant starvation, polishing, corrosion, and deposit-induced sticking. Honing creates crosshatch features that retain lubricant, but excessive roughness increases wear and friction. Modern rings may use chromium, molybdenum, ceramic, or diamond-like carbon-based coatings, selected according to temperature, load, fuel or oil chemistry, and compatibility with the liner.

Brakes and clutches convert mechanical energy into heat through friction. Their design requires friction stability, wear resistance, thermal capacity, pressure distribution, fade resistance, noise control, and safe release behavior. The friction coefficient is not constant; it depends on temperature, sliding speed, pressure, surface condition, humidity, and material transfer. Brake fade occurs when temperature reduces friction or causes changes in the friction material. Judder may result from nonuniform contact, thermal distortion, thickness variation, or transfer-layer instability.

A clutch must transmit torque without excessive slip but engage progressively enough to avoid shock. Torque capacity depends on friction coefficient, normal force, effective radius, number of friction surfaces, and allowable pressure. Repeated engagement requires an energy-based thermal analysis rather than a static torque calculation. Ventilation, heat sinks, friction-material compatibility, and controlled bedding-in are important design measures.

Manufacturing Tribology: Cutting and Forming Interfaces

In manufacturing, tribology influences tool life, dimensional accuracy, surface integrity, energy consumption, and production cost. In cutting, the tool–chip interface experiences high pressure, sliding, adhesion, abrasion, diffusion, and severe temperature gradients. Tool wear may appear as flank wear, crater wear, notch wear, built-up edge, chipping, or plastic deformation.

Cutting-tool selection involves substrate, coating, geometry, cutting speed, feed, depth of cut, workpiece material, and coolant strategy. Cemented carbides, ceramics, cermets, polycrystalline diamond, and cubic boron nitride occupy different application ranges. Coatings can reduce adhesion, act as thermal barriers, improve hardness, or provide chemical stability. However, a coating is not universally beneficial; interrupted cutting, edge preparation, residual stress, and substrate toughness may control failure.

The tool-life relationship often takes the empirical form

V T^n = C

where V is cutting speed, T is tool life, and n and C depend on the tool–workpiece system. The relation is useful within a defined range but should not be extrapolated across changes in cooling, tool geometry, or wear mechanism. Minimum-cost cutting speed differs from maximum-productivity speed because tool-change time, insert cost, machine utilization, and rejected parts must be included.

In forming operations, friction governs material flow, forming force, die wear, galling, surface defects, and dimensional accuracy. Drawing, extrusion, forging, and sheet forming require lubricants that survive high pressure and temperature while remaining compatible with cleaning, coating, welding, and environmental requirements. Boundary films, solid lubricants, phosphate layers, polymer films, and engineered textures may be used. Excessive lubrication can cause wrinkling or poor process control, while insufficient lubrication causes seizure and scoring.

A Practical Selection and Troubleshooting Framework

A robust engineering workflow begins with a tribological specification. It should state loads, speeds, motion, temperature range, atmosphere, contamination, required life, allowable friction, leakage limits, maintenance interval, safety consequence, and manufacturing constraints.

The next step is to map the dominant regimes and damage mechanisms. A high-speed bearing may be film-limited; a start-stop actuator may be boundary-limited; a gear may be fatigue-limited; a seal may be chemically limited; and a forming die may be galling-limited. The design should then select materials, surface treatments, lubricant, geometry, and sealing as a compatible package.

Validation should include representative duty-cycle testing, not only steady-state bench tests. Instrumentation may measure temperature, torque, friction, vibration, acoustic emission, lubricant debris, pressure, and leakage. Acceptance criteria should distinguish normal running-in from progressive damage.

When a failure occurs, preserve evidence. Record operating history, lubricant identity and condition, assembly method, photographs, damage location, and failed-part orientation. Inspect the counterface and lubricant as carefully as the failed component. A useful fault tree separates overload, misalignment, contamination, lubrication, material or heat-treatment error, installation error, design inadequacy, and abnormal operation.

Case Study: Premature Failure of a Conveyor Gearbox Bearing

Background

This illustrative case concerns a helical gearbox driving a bulk-material conveyor. The gearbox was specified for continuous operation with periodic overload during conveyor start-up. Its output shaft used a tapered roller bearing arrangement, lubricated by splash oil.

Situation and problem

Bearing temperature increased gradually over several weeks, followed by vibration at the rotational frequency and its harmonics. Replacement bearings showed discoloration on the large end of the rollers and localized raceway distress. The initial response was to install a bearing of greater load rating.

Analysis

The operating review showed that the conveyor was frequently started while heavily loaded, producing torque peaks well above the nominal rating. More importantly, an inspection found that the oil level was maintained at the upper inspection mark recommended for a different gearbox orientation. The excessive oil level generated churning and raised oil temperature. Viscosity consequently fell during operation, reducing film thickness. The bearing also had excessive preload because the assembly procedure used a fixed nut position rather than the specified end-play or torque method.

The combined mechanism was thermal: overload increased contact stress, excessive preload increased friction, and churning increased lubricant temperature. Reduced viscosity then promoted mixed lubrication and surface distress. Simply increasing the bearing capacity would not remove these causes.

Approach and solution

The gearbox was reassembled using the manufacturer’s prescribed preload-setting procedure. Oil level was corrected for the actual mounting orientation, and an oil with appropriate viscosity–temperature behavior was selected. A controlled start sequence reduced peak torque, and a temperature alarm was added. Vibration and oil-particle measurements were incorporated into the maintenance program.

Outcome

The bearing temperature returned to a stable level, vibration decreased, and no progressive distress was observed during subsequent representative duty-cycle testing. The case demonstrates that tribological failure is often a system interaction rather than a simple strength deficiency.

Learning points

1. Load rating cannot compensate for incorrect preload or lubrication.

2. Duty-cycle peaks and start-up conditions must be included in design.

3. Oil level depends on housing geometry and operating speed.

4. Failure evidence should be interpreted with thermal and assembly history.

5. Monitoring is most useful when alarm limits are linked to known failure mechanisms.

Risks, Limitations, and Professional Considerations

Tribological calculations contain uncertainty because surface condition, lubricant chemistry, contamination, and transient events are difficult to represent exactly. Laboratory wear rates may not transfer directly to field conditions. Accelerated tests can change the mechanism being studied by increasing temperature, pressure, or speed beyond realistic values.

Common mistakes include selecting lubricant solely by viscosity, ignoring compatibility between additives and seals, treating surface roughness as a single numerical value, using nominal rather than peak loads, neglecting installation quality, and assuming that a low friction coefficient always means superior performance. A component with slightly higher friction may be preferable if it provides stable friction, reliable heat rejection, and longer life.

Professional practice requires compliance with applicable bearing, gear, pressure, brake, machinery-safety, lubricant-handling, and environmental standards. Safety-critical brakes, lifting systems, aircraft components, and hazardous-fluid seals require documented verification, traceability, inspection, and controlled change management. Used oils, contaminated wipes, coatings, and forming lubricants must be handled in accordance with environmental and occupational-health requirements.

Digital Tribology and Emerging Directions

Condition monitoring increasingly combines vibration analysis, thermography, torque measurement, lubricant spectroscopy, ferrous-debris sensing, acoustic emission, and operating data. Machine-learning models can identify patterns associated with bearing or gear degradation, but they require representative training data and careful control of false alarms. A model should support engineering judgment rather than replace inspection or safety procedures.

Digital twins can link a component’s calculated life with actual load, temperature, speed, and maintenance history. Their value depends on sensor quality, model validity, and transparent uncertainty estimates. Physics-informed analytics are especially promising because they connect observed changes to mechanisms such as lubrication loss, misalignment, or fatigue.

Emerging developments include low-global-warming lubricants, biodegradable forming fluids, additive-manufactured textures, advanced carbon and ceramic coatings, solid-lubricant films, electrically insulating bearing components, and embedded sensors. Sustainability assessment must include component life, lubricant consumption, energy loss, recyclability, and the environmental consequences of coatings and disposal. Reducing friction is valuable, but extending service life and preventing catastrophic leakage may produce greater overall benefit.

Best-Practice Design Checklist

A professional tribological design should:

• define the complete duty cycle rather than only nominal conditions;
• identify likely wear and failure mechanisms before selecting materials;
• evaluate film thickness, surface roughness, contact stress, and temperature together;
• select lubricant, seal, coating, and counterface as a compatible system;
• apply appropriate reliability and consequence-based safety factors;
• account for contamination, alignment, installation, and maintenance;
• validate under representative transient and steady conditions;
• establish measurable inspection and alarm criteria;
• preserve failure evidence and use root-cause analysis;
• review environmental, safety, regulatory, and lifecycle implications.

Chapter Summary

Tribology of machine elements and manufacturing systems requires the integration of contact mechanics, lubrication, materials, surface engineering, thermal analysis, duty-cycle evaluation, and reliability practice. Bearings must be designed for load, life, preload, alignment, lubrication, contamination, and installation. Gears require control of rolling–sliding contact, fatigue, scuffing, surface finish, and lubricant supply. Seals manage both leakage and contamination exclusion. Piston rings, brakes, and clutches operate under severe thermal and transient conditions, while cutting and forming interfaces determine tool life, energy use, product quality, and die performance.

The most reliable approach is systems-based: define actual operating histories, identify dominant damage mechanisms, apply standards and suitable safety factors, validate under representative conditions, and monitor the variables that reveal degradation. Digital analytics and advanced surfaces are expanding capability, but they remain effective only when grounded in sound tribological reasoning and responsible professional practice.

Key Terms

Tribosystem: A contacting system comprising surfaces, lubricant or environment, operating conditions, and surrounding machine elements.

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

Mixed lubrication: A regime in which part of the load is carried by lubricant film and part by asperity contact.

Boundary lubrication: A regime in which surface chemistry and adsorbed films carry much of the load.

Hertzian contact: Elastic contact between curved bodies with a characteristic pressure distribution.

Duty cycle: The time-dependent sequence of loads, speeds, temperatures, motions, and operating states.

L10 life: The life at which 90 percent of an identical bearing population is expected to survive.

Preload: An intentional initial force imposed on a bearing or contact assembly.

False brinelling: Localized damage caused by oscillatory motion or vibration without normal rolling travel.

Micropitting: Fine-scale fatigue or surface distress associated with inadequate film separation and roughness effects.

Scuffing: Severe adhesive damage caused by local welding and tearing of sliding surfaces.

Churning loss: Power and heat loss caused by excessive lubricant agitation.

Transfer layer: A deposited layer of friction material or counterface material that influences subsequent sliding behavior.

Fade: Reduction in brake or clutch friction performance, commonly due to temperature or surface changes.

Flank wear: Progressive wear on the clearance face of a cutting tool.

Built-up edge: Workpiece material adhered to the cutting edge during machining.

Galling: Severe adhesive wear involving material transfer and surface tearing.

Safety factor: A design margin accounting for uncertainty, variability, and consequences of failure.

Condition monitoring: Systematic measurement used to detect changes associated with component degradation.

Root-cause analysis: A structured investigation intended to identify the physical and systemic causes of failure.

Review Questions

Short Answer Questions

1. What variables determine the film parameter in a tribological contact?

2. Why is L10 bearing life not a guarantee of individual component life?

3. Distinguish pitting from scuffing in a gear contact.

4. Why can excessive lubricant level raise gearbox temperature?

5. State two functions performed by a machine seal.

Long Answer Questions

1. Explain a systems-based procedure for selecting a bearing for variable-load service.

2. Discuss the major tribological failure mechanisms in gears and the design measures used to control them.

3. Explain the interaction of ring tension, liner finish, lubricant, and temperature in piston-ring assemblies.

4. Compare the tribological design requirements of brakes and clutches.

5. Describe how friction and wear influence cutting and forming operations, including the role of lubricants and coatings.

Analytical / Application Questions

1. A gearbox bearing operates correctly at moderate load but overheats during frequent loaded starts. Develop a fault tree and identify the measurements needed to distinguish overload, preload, and lubrication causes.

2. A sealing system leaks after a shaft is refinished to a lower average roughness. Explain why leakage may nevertheless increase and propose an investigation plan.

3. A forming operation suffers galling only at high production speed. Analyze the likely changes in temperature, lubricant film, sliding, and material transfer, and recommend a validation strategy.

22. Multiple Choice Questions with Answers

Question 1

A bearing operates under a variable load spectrum with frequent overload during start-up. Which design approach is most appropriate?

A. Use only the nominal continuous load in the life calculation

B. Include peak loads, duty-cycle fractions, lubrication, and reliability requirements

C. Select the bearing solely from shaft diameter

D. Increase lubricant level above the housing specification

Correct Answer: B. Include peak loads, duty-cycle fractions, lubrication, and reliability requirements

Question 2

A low film parameter most directly indicates that the contact is likely to experience:

A. Complete elastic separation of surfaces

B. Only hydrostatic lubrication

C. Mixed or boundary lubrication with asperity interaction

D. No thermal generation

Correct Answer: C. Mixed or boundary lubrication with asperity interaction

Question 3

Which combination most strongly promotes gear scuffing?

A. Low sliding, low temperature, and abundant lubricant

B. High sliding speed, overload, high temperature, and inadequate film protection

C. Low contact stress and excellent alignment

D. Stationary operation with no contact pressure

Correct Answer: B. High sliding speed, overload, high temperature, and inadequate film protection

Question 4

Why can excessive oil level increase the temperature of a gearbox?

A. It eliminates all lubricant circulation

B. It increases churning losses and power dissipation

C. It always reduces lubricant viscosity at room temperature only

D. It prevents the gears from making contact

Correct Answer: B. It increases churning losses and power dissipation

Question 5

A rotating bearing ring generally requires an interference fit primarily to prevent:

A. Lubricant oxidation

B. Cage rotation

C. Ring creep relative to the loaded seat

D. Electrical insulation

Correct Answer: C. Ring creep relative to the loaded seat

Question 6

Which symptom is most consistent with a seal shaft having helical machining marks?

A. Improved exclusion of all contaminants

B. Possible pumping of lubricant along the shaft and leakage

C. Elimination of frictional heating

D. Automatic compensation for misalignment

Correct Answer: B. Possible pumping of lubricant along the shaft and leakage

Question 7

In a brake or clutch, fade refers to:

A. A permanent increase in friction at low temperature

B. A reduction in friction performance, commonly associated with heating

C. A reduction in normal force caused only by corrosion

D. The complete absence of sliding during engagement

Correct Answer: B. A reduction in friction performance, commonly associated with heating

Question 8

Which cutting-tool failure is characterized by wear on the clearance face?

A. Flank wear

B. Crater wear

C. Built-up edge

D. Thermal cracking only

Correct Answer: A. Flank wear

Question 9

The conveyor gearbox case demonstrates that increasing bearing load rating alone may fail because:

A. Bearing capacity is unrelated to contact stress

B. Preload, churning, temperature, and viscosity can jointly cause failure

C. All bearing failures are caused by material defects

D. Higher load rating always increases operating temperature

Correct Answer: B. Preload, churning, temperature, and viscosity can jointly cause failure

Question 10

Which practice best supports reliable tribological condition monitoring?

A. Collect vibration data without recording operating conditions

B. Use sensors and alarm limits linked to known failure mechanisms

C. Replace every component at the first minor temperature change

D. Rely exclusively on a machine-learning output without inspection

Correct Answer: B. Use sensors and alarm limits linked to known failure mechanisms