---
title: "Lubricant functions; grease and solid lubricant use"
subject: "Engineering Chemistry"
chapter: 8
key: "semester_2/engineering_chemistry:lubricant-functions-grease-solid"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/lubricant-functions-grease-solid
site: BCT Engineering
---

# Lubricant functions; grease and solid lubricant use

Engineering Chemistry · Chapter 8: Lubricants and Paints

- Asked in: 2069 Poush, 2 marks
- Video: [Lubricants || Lubricant Important Functions And Classification ||Lubrication ||Engineering Chemistry](https://www.youtube.com/watch?v=Kuj2X6ncPBc) — Chemistry by Dr. Anjali Ssaxena
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/lubricant-functions-grease-solid
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-8

## Question

Mention the main functions of lubricants. In which situation, the grease and solid lubricants are used.

## Short answer

Lubricants reduce friction, wear and heat, seal, prevent corrosion and clean, saving energy. Grease is used where oil cannot be retained (slow, heavy loads, sealing). Solid lubricants like graphite are used at high temperatures, heavy loads, vacuum, or where oil would contaminate.

## Answer

#### Main functions of lubricants
1. **Reduces friction** between moving surfaces by forming a thin film that separates them.
2. **Reduces wear and tear**, which increases the life of machine parts.
3. **Acts as a coolant:** carries away heat produced by friction.
4. **Acts as a seal:** prevents leakage of gases, e.g. between piston rings and cylinder.
5. **Prevents corrosion** by coating metal surfaces from air and moisture.
6. **Cleans** by washing away dirt, metal particles and carbon deposits.
7. **Saves energy:** smoother motion means less power loss and higher efficiency; also reduces noise and vibration.

#### Where grease is used
Grease is a semi-solid lubricant (oil thickened with metal soap). It is used where oil cannot be retained or supplied regularly: slow-speed, heavily loaded or intermittently working bearings, open gears, chassis and axles, and machines where leakage and dust must be avoided (semi-solid grease stays in place and seals).

#### Where solid lubricants are used
**Solid lubricants** are solids with a layered structure that slide easily over each other, such as graphite, molybdenum disulfide (MoS₂), talc and soapstone. They are used where oils and greases fail:
- at very high temperatures (graphite up to about 450 °C in air) where oil burns or evaporates;
- under extremely heavy loads and very slow speeds where an oil film would be squeezed out;
- in vacuum or at very low temperature;
- where oil would contaminate the product (food, textile, paper machines) or collect dust;
- as an additive or dry film on parts that cannot be lubricated often.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is a function of lubricants?
   A. Washing away dirt, metal particles and carbon deposits
   B. Producing metal particles
   C. Increasing noise
   D. Removing the need for any cooling
   Answer: A. Lubricants clean, cool, seal, and reduce wear. (Key point: Cleaning)

2. In which situation is grease used?
   A. High-speed light instruments
   B. Very high temperature above 450 °C
   C. Slow-speed, heavily loaded or intermittent bearings where oil cannot be retained
   D. In vacuum only
   Answer: C. Grease is used where oil cannot stay or be supplied regularly. (Key point: Grease use)

3. What is grease?
   A. A semi-solid made of oil thickened with metal soap
   B. Pure graphite powder
   C. A synthetic fluid without soap
   D. A vegetable oil only
   Answer: A. Grease is oil thickened with metallic soap. (Key point: Grease composition)

4. Why are solid lubricants used at very high temperature?
   A. Oil becomes thicker
   B. Graphite melts to flow
   C. Oil burns or evaporates while graphite remains effective
   D. Grease works better
   Answer: C. Oil fails at high temperature; graphite works up to about 450 °C in air. (Key point: Solid use)

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