ITMC Lubrication 3 — Questions and Answers
Question 1: An electric motor bearing is running hotter than normal and emitting a grinding noise. After inspection, you find the grease has become dry and crumbly. What has most likely occurred?
- The grease was over-applied, causing churning heat
- The base oil has bled out of the thickener, leaving behind a dried soap structure (Correct answer)
- The bearing has been contaminated with water, washing out the grease
- The NLGI grade was too high for the operating speed
Correct answer: The base oil has bled out of the thickener, leaving behind a dried soap structure
When grease ages or overheats, the base oil separates (bleeds) from the thickener, leaving a dry soap structure that no longer lubricates.
Question 2: What does a low 'demulsibility' rating indicate about an oil?
- The oil resists breaking down at high temperatures
- The oil has difficulty separating from water once mixed (Correct answer)
- The oil provides poor film strength under pressure
- The oil will oxidize rapidly in the presence of moisture
Correct answer: The oil has difficulty separating from water once mixed
Demulsibility measures an oil's ability to shed water; poor demulsibility means the oil and water form a stable emulsion that is difficult to separate.
Question 3: Which lubrication regime is present when the lubricant film completely separates two surfaces and load is fully supported by fluid pressure?
- Boundary lubrication
- Mixed-film lubrication
- Hydrodynamic (full-film) lubrication (Correct answer)
- Elastohydrodynamic lubrication
Correct answer: Hydrodynamic (full-film) lubrication
Hydrodynamic lubrication occurs when a continuous pressurized fluid film fully separates the surfaces, resulting in very low friction and wear.
Question 4: What is 'over-greasing' a bearing likely to cause?
- Premature wear due to insufficient lubrication film
- Elevated operating temperature due to grease churning and increased resistance (Correct answer)
- Rust and corrosion on the bearing raceways
- Contamination of the bearing with metal particles
Correct answer: Elevated operating temperature due to grease churning and increased resistance
Excess grease causes the rolling elements to churn through the grease, generating heat that can degrade the lubricant and damage the bearing.
Question 5: In an oil mist lubrication system, what is the purpose of the 'reclassifier' fitting at the bearing?
- To filter out water and contaminants from the mist before it reaches the bearing
- To convert the dry mist into larger oil droplets that wet the bearing surfaces (Correct answer)
- To regulate the pressure of the oil mist entering the bearing
- To return excess oil back to the reservoir
Correct answer: To convert the dry mist into larger oil droplets that wet the bearing surfaces
A reclassifier fitting at the bearing point converts the fine dry mist into larger droplets ('wet mist') that actually deposit oil on the bearing surfaces.
Question 6: Which type of oil additive is used to prevent rust on metal surfaces in the presence of water?
- Anti-foam additive
- Corrosion inhibitor (rust inhibitor) (Correct answer)
- Pour point depressant
- Viscosity index improver
Correct answer: Corrosion inhibitor (rust inhibitor)
Rust or corrosion inhibitors adsorb onto metal surfaces to form a protective barrier that displaces water and prevents oxidative rust formation.
Question 7: A technician notices that oil drained from a gearbox has a milky white appearance. What does this indicate?
- The oil has oxidized and formed varnish deposits
- Water has emulsified with the oil (Correct answer)
- The oil has been contaminated with refrigerant
- The viscosity improver additive has sheared down
Correct answer: Water has emulsified with the oil
A milky or cloudy appearance in oil indicates that water has entered and emulsified with the oil, which significantly reduces its lubrication effectiveness.
ITMC Lubrication 3
An electric motor bearing is running hotter than normal and emitting a grinding noise.
After inspection, you find the grease has become dry and crumbly.
What has most likely occurred?