Grease Technology & Application Flashcards

For a high-speed electric motor bearing (ndm > 300,000), which NLGI grade and base oil viscosity combination is most appropriate?

Answer: NLGI 2, ISO VG 32 base oil

High-speed bearings require lower viscosity base oils (such as ISO VG 32) for adequate film formation at high speeds, and NLGI 2 provides a good balance of consistency and pumpability for typical electric motor applications.

What is the primary mechanism by which antioxidant additives in grease extend its service life?

Answer: They interrupt oxidation chain reactions by donating hydrogen atoms to free radicals

Antioxidant additives (typically hindered phenols or aromatic amines) act as radical scavengers, donating hydrogen atoms to interrupt the chain reaction of oxidative degradation.

Which factor most significantly increases the re-lubrication interval for a grease-lubricated bearing?

Answer: Reducing operating temperature and speed

Lower operating temperature slows oxidation and degradation of the grease, and lower speed reduces mechanical shear degradation, both of which significantly extend the re-lubrication interval.

What condition is indicated when a grease sample taken from a bearing housing shows a significant decrease in worked penetration (hardening) compared to fresh grease?

Answer: Churning and working hardening or evaporation of lighter base oil fractions

Hardening of in-service grease typically indicates thickener concentration increase due to evaporation of light base oil fractions, or oxidative thickening, often signaling the grease has exceeded its useful service life.

What is the purpose of the 'purge and replace' method when switching to a new incompatible grease type?

Answer: To completely remove the old grease and prevent incompatible mixing that could cause degradation

The purge and replace method ensures that incompatible old grease is completely removed from the bearing housing before introducing the new grease type, preventing softening, hardening, or oil separation caused by incompatible thickener chemistry.

In evaluating grease for a food-grade application, which certification or standard is most relevant?

Answer: NSF H1 registration for incidental food contact

NSF H1 registration is required for lubricants used in food processing equipment where there may be incidental contact with food products, ensuring they meet safety requirements for human health.

What does the Four-Ball Weld Load test (ASTM D2596) specifically evaluate in a grease?

Answer: The maximum load at which the lubricant film fails and metal-to-metal welding occurs

The Four-Ball Weld Load test (ASTM D2596) determines the load in kilograms at which the lubricating film completely breaks down and the four steel balls weld together, indicating the grease's ultimate EP performance limit.