(SEM VI) THEORY EXAMINATION 2021-22 TRIBOLOGY

TRIBOLOGY (KME063) – SECTION-WISE EXAM NOTES

SECTION A – Short Answer Notes (2 Marks Each)

1. Industrial Importance of Tribology

Tribology deals with friction, wear, and lubrication between interacting surfaces. It plays a vital role in increasing machine life, reducing energy losses, minimizing maintenance cost, and improving efficiency of mechanical systems in industries like automotive, aerospace, and manufacturing.

2. Friction and Types of Friction

Friction is the resisting force that opposes relative motion between two surfaces in contact.
Main types are static friction, kinetic friction (sliding), rolling friction, and fluid friction.

3. Delamination Theory of Wear

Delamination theory explains wear due to subsurface crack formation caused by repeated cyclic stresses. These cracks propagate parallel to the surface and eventually lead to material separation in thin layers.

4. Properties of an Ideal Lubricant

An ideal lubricant should have suitable viscosity, high viscosity index, good thermal stability, oxidation resistance, non-corrosiveness, and strong oil film strength.

5. Measurement of Viscosity

Viscosity can be measured using capillary viscometers, falling sphere viscometers, rotational viscometers, and Redwood or Saybolt viscometers.

6. Hysteresis Losses

Hysteresis loss occurs due to internal energy dissipation when materials undergo cyclic deformation, especially in rolling contacts like tyres and bearings.

7. Surface Engineering

Surface engineering involves modifying surface properties of materials through coatings, heat treatment, or surface modification techniques to improve wear, corrosion, and fatigue resistance.

8. Line Contact vs Point Contact Lubrication

Line contact lubrication occurs between cylinders (e.g., roller bearings), while point contact lubrication occurs between spheres (e.g., ball bearings). Pressure is higher in point contact.

9. Bearing Material – Advantages and Disadvantages

Bearing materials should have low friction, good wear resistance, high load capacity, and compatibility. Disadvantages may include cost, limited temperature resistance, or lower fatigue strength.

10. Cone Clutch vs Centrifugal Clutch

Cone clutch operates by axial force applied manually, whereas centrifugal clutch engages automatically due to centrifugal force as speed increases.

SECTION B – Long Answer Notes (10 Marks)

1. Methods of Measuring Frictional Force

Various methods include inclined plane method, belt friction method, pin-on-disc test, and friction dynamometers.
Pin-on-disc method is commonly used, where frictional force is measured as a pin slides over a rotating disc under controlled load.

2. Petroff’s Equation for Journal Bearing

Petroff’s equation gives frictional torque for lightly loaded journal bearings assuming uniform pressure, concentric journal and bearing, and laminar flow of lubricant.

T=2π2μNr3LcT = \frac{2\pi^2 \mu N r^3 L}{c}T=c2π2μNr3L​

where μ = viscosity, N = speed, r = radius, L = length, c = clearance.

3. Newton’s Law of Viscosity

Newton’s law states that shear stress is proportional to velocity gradient.

τ=μdudy\tau = \mu \frac{du}{dy}τ=μdydu​

It applies to Newtonian fluids where viscosity remains constant.

4. Falling Sphere and Flower’s Viscometer

In falling sphere viscometer, viscosity is calculated by measuring the terminal velocity of a sphere falling through the fluid.
Flower’s viscometer measures viscosity based on flow rate under gravity through a calibrated orifice.

5. Bowden and Tabor Adhesion Theory

This theory states that friction arises due to adhesive junctions formed at microscopic contact points. Shearing these junctions requires force, causing friction.

SECTION C – Descriptive Answers (10 Marks)

1. Surface Hardening Processes

Common surface hardening processes include carburizing, nitriding, cyaniding, flame hardening, and induction hardening. These processes improve surface hardness while retaining core toughness.

2. Requirements for Surface Coating Selection

Surface coating selection depends on operating temperature, load, wear environment, substrate compatibility, adhesion strength, and cost effectiveness.

3. Wear Mechanisms

Major wear mechanisms include adhesive wear, abrasive wear, corrosive wear, fatigue wear, and erosive wear.
Adhesive wear occurs when material transfers from one surface to another due to junction formation.

4. Wear Testing Methods

Common wear tests include pin-on-disc test (ASTM G99), block-on-ring test, four-ball wear test, and abrasion tests.

5. Bearing Materials and Alloys

Common bearing alloys include Babbitt metal, bronze, aluminium alloys, and copper-lead alloys.
Babbitt alloys provide excellent conformability and embeddability, while bronze alloys offer higher load capacity.