CLS Lubrication Fundamentals & Applications 2 — Questions and Answers
Question 1: Which lubrication regime is characterized by complete separation of surfaces by a full fluid film?
- Boundary lubrication
- Mixed lubrication
- Hydrodynamic lubrication (Correct answer)
- Elastohydrodynamic lubrication
Correct answer: Hydrodynamic lubrication
Hydrodynamic lubrication occurs when a full fluid film completely separates the mating surfaces, eliminating metal-to-metal contact.
Question 2: What does the Stribeck curve illustrate?
- The relationship between lubricant viscosity and temperature
- The friction coefficient across different lubrication regimes as a function of speed and viscosity (Correct answer)
- The load-carrying capacity of different base oil types
- The oxidation rate of lubricants at elevated temperatures
Correct answer: The friction coefficient across different lubrication regimes as a function of speed and viscosity
The Stribeck curve plots friction coefficient against the ratio of viscosity × speed / load, showing transitions between boundary, mixed, and full-film lubrication regimes.
Question 3: What is the primary purpose of an EP (Extreme Pressure) additive in a lubricant?
- To lower the pour point of the lubricant
- To chemically react with metal surfaces under high pressure to prevent welding (Correct answer)
- To increase the viscosity index of the base oil
- To neutralize acidic combustion products
Correct answer: To chemically react with metal surfaces under high pressure to prevent welding
EP additives form sacrificial chemical films on metal surfaces at high contact pressures, preventing scuffing and welding of asperities.
Question 4: Which property of a grease describes its resistance to flow and is measured by a cone penetration test?
- Viscosity
- Drop point
- Consistency (NLGI grade) (Correct answer)
- Oil separation rate
Correct answer: Consistency (NLGI grade)
NLGI consistency grade, determined by cone penetration depth, classifies grease firmness from 000 (very fluid) to 6 (very hard).
Question 5: What is meant by the 'viscosity index' (VI) of a lubricant?
- The lubricant's resistance to oxidation over time
- A measure of how much viscosity changes with temperature (Correct answer)
- The minimum viscosity required for full-film lubrication
- The ratio of kinematic to dynamic viscosity
Correct answer: A measure of how much viscosity changes with temperature
Viscosity index indicates the rate of viscosity change with temperature; a higher VI means less viscosity change over a given temperature range.
Question 6: In boundary lubrication, what primarily prevents surface damage?
- A thick hydrodynamic fluid film
- Lubricant pressure generated by surface motion
- Adsorbed or chemically reacted films on the asperities (Correct answer)
- High lubricant flow rate keeping surfaces cool
Correct answer: Adsorbed or chemically reacted films on the asperities
In boundary lubrication, thin molecular films (adsorbed layers or reaction products from additives) on asperity tips carry the load and prevent welding.
Question 7: Which base oil group, per API classification, is produced by the hydrocracking process and offers improved VI and oxidation stability over Group I oils?
- Group I
- Group II (Correct answer)
- Group III
- Group IV
Correct answer: Group II
API Group II base oils are produced via hydrocracking, resulting in higher VI and lower sulfur content than solvent-refined Group I oils.
CLS Lubrication Fundamentals & Applications 2
Which lubrication regime is characterized by complete separation of surfaces by a full fluid film?