CLS Lubrication Fundamentals & Applications 3 — Questions and Answers
Question 1: What is the significance of the 'film thickness ratio' (lambda, λ) in lubrication?
- It describes the ratio of oil viscosity to grease consistency
- It compares the minimum film thickness to the composite surface roughness (Correct answer)
- It measures the ratio of EP additive concentration to base oil volume
- It indicates the proportion of synthetic to mineral base oil in a blend
Correct answer: It compares the minimum film thickness to the composite surface roughness
Lambda ratio (λ = minimum film thickness / composite RMS surface roughness) determines the lubrication regime: λ>3 is full-film, 1–3 is mixed, and <1 is boundary.
Question 2: Which type of wear is caused by the formation and collapse of vapor bubbles in a lubricant?
- Abrasive wear
- Adhesive wear
- Cavitation erosion (Correct answer)
- Corrosive wear
Correct answer: Cavitation erosion
Cavitation erosion occurs when vapor bubbles form due to local pressure drops and then implode near metal surfaces, generating intense shock waves that pit the surface.
Question 3: What is the 'pour point' of a lubricant?
- The temperature at which a lubricant begins to oxidize rapidly
- The lowest temperature at which a lubricant will flow under gravity (Correct answer)
- The temperature at which a lubricant's viscosity reaches 40 cSt
- The point at which wax crystals fully dissolve in the base oil
Correct answer: The lowest temperature at which a lubricant will flow under gravity
Pour point is the lowest temperature at which a lubricant will still flow, typically 3°C above the temperature at which flow ceases.
Question 4: In elastohydrodynamic lubrication (EHL), what happens to the lubricant's viscosity under very high contact pressures?
- Viscosity decreases sharply due to shear thinning
- Viscosity increases dramatically, allowing it to support high loads (Correct answer)
- Viscosity remains constant because the film is so thin
- Viscosity drops to zero as the lubricant becomes a solid
Correct answer: Viscosity increases dramatically, allowing it to support high loads
In EHL contacts, pressures can reach several GPa, causing the lubricant's viscosity to increase by orders of magnitude (piezo-viscous effect), enabling it to support high loads.
Question 5: What does the term 'oil bleed' refer to in the context of grease?
- Oxidation products leaching out of the grease
- Separation of base oil from the thickener during storage or use (Correct answer)
- Contamination of grease by water ingress
- Loss of grease consistency due to mechanical working
Correct answer: Separation of base oil from the thickener during storage or use
Oil bleed (or oil separation) is the release of base oil from the thickener matrix; controlled bleed is necessary for grease to lubricate, but excessive bleed indicates an unstable product.
Question 6: Which additive type is specifically designed to prevent rust on ferrous metal surfaces in contact with water?
- Antioxidant
- Rust inhibitor (Correct answer)
- Metal deactivator
- Demulsifier
Correct answer: Rust inhibitor
Rust inhibitors (e.g., sulfonates, carboxylates) adsorb onto ferrous surfaces to displace water and form a protective barrier against rust formation.
Question 7: What is the role of a demulsifier additive in a lubricating oil?
- To keep water suspended as fine droplets for even distribution
- To promote rapid separation of water from the oil (Correct answer)
- To neutralize acidic water contamination
- To increase the oil's affinity for water to prevent dry spots
Correct answer: To promote rapid separation of water from the oil
Demulsifiers reduce interfacial tension between oil and water, encouraging water droplets to coalesce and settle out so they can be drained, protecting equipment.
CLS Lubrication Fundamentals & Applications 3
What is the significance of the 'film thickness ratio' (lambda, λ) in lubrication?