CLS Grease Technology & Application 3

Question 1

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

Accepted Answer

NLGI 2, ISO VG 32 base oil

Answer

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.

Question 2

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

Accepted Answer

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

Answer

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

Question 3

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

Accepted Answer

Reducing operating temperature and speed

Answer

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.

Question 4

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?

Accepted Answer

Churning and working hardening or evaporation of lighter base oil fractions

Answer

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.

Question 5

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

Accepted Answer

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

Answer

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.

(CLS) Certified Lubrication Specialist Practice Test

(CLS) Certified Lubrication Specialist Practice Test

CLS Grease Technology & Application 3