(SEM VI) THEORY EXAMINATION 2023-24 TRIBOLOGY

TRIBOLOGY – KME063

Section-wise Important Questions & Ready Answers

SECTION A

(Attempt all – 2 marks each)

(a) Definition of Tribology

Tribology is the science and engineering of interacting surfaces in relative motion. It deals with the study of friction, wear, and lubrication and their practical applications in engineering systems.

(b) Functions of Lubricants

Lubricants reduce friction between moving surfaces, minimize wear, remove heat generated during operation, protect surfaces from corrosion, and ensure smooth and efficient machine operation.

(c) Causes of Friction

Friction occurs due to surface roughness, adhesion between contacting asperities, deformation of surface layers, and presence of contaminants between sliding surfaces.

(d) Fatigue Wear

Fatigue wear occurs due to repeated cyclic stresses on surfaces in rolling or sliding contact. Over time, these stresses cause crack initiation and material removal in the form of pits or flakes.

(e) Jet Lubrication

Jet lubrication is a method where lubricant is supplied under pressure in the form of a jet directly onto the moving surfaces. It is commonly used in high-speed and heavily loaded machine components.

(f) Elasto-Hydrodynamic Lubrication (EHL)

EHL occurs when elastic deformation of surfaces and pressure-dependent viscosity of lubricant become significant. It is commonly observed in rolling element bearings and gears.

(g) Solid Lubricants

Solid lubricants are materials that reduce friction without requiring liquid lubricants. Examples include graphite, molybdenum disulfide (MoS₂), PTFE, and boron nitride.

(h) Characteristics of Wear-Resistant Coatings

Wear-resistant coatings possess high hardness, strong adhesion to substrate, resistance to abrasion and corrosion, low coefficient of friction, and good thermal stability.

(i) Friction Stability

Friction stability refers to the ability of a system to maintain a consistent coefficient of friction under varying operating conditions such as load, speed, and temperature.

(j) Hysteresis Losses

Hysteresis losses occur due to energy dissipation during repeated loading and unloading cycles, especially in elastomeric and viscoelastic materials, contributing to friction and wear.

SECTION B

(Attempt any three – 10 marks each)

1. Classification of Lubricants

Lubricants can be classified as liquid lubricants (mineral and synthetic oils), semi-solid lubricants (greases), solid lubricants, and gaseous lubricants. Each type is selected based on operating conditions such as load, temperature, and speed.

2. Adhesion Theory of Friction

According to adhesion theory, friction arises due to microscopic junctions formed between contacting asperities. When motion occurs, these junctions must be sheared, requiring force. The theory explains why friction depends on real area of contact rather than apparent area.

3. Hydrodynamic Lubrication Theory

In hydrodynamic lubrication, a continuous lubricant film completely separates the moving surfaces. Pressure is generated due to wedge-shaped geometry and relative motion, preventing direct metal-to-metal contact and minimizing wear.

4. Properties of Solid Lubricants

Solid lubricants have layered crystal structures that allow easy shear, low friction coefficient, high temperature resistance, and ability to function in vacuum or extreme environments.

5. Concept of Friction Angle

Friction angle is defined as the angle whose tangent is equal to the coefficient of friction. It represents the limiting condition before sliding begins and is useful in analyzing frictional systems.

SECTION C

Q3(a) Lubrication Regimes

Lubrication regimes include boundary lubrication, mixed lubrication, hydrodynamic lubrication, elastohydrodynamic lubrication, and hydrostatic lubrication. Each regime depends on load, speed, viscosity, and surface roughness.

Q3(b) Viscosity and Its Measurement

Viscosity is the resistance of a fluid to flow. It determines the load-carrying capacity of lubricants. Viscosity can be measured using methods such as capillary viscometers or rotational viscometers.

Q4(a) Types of Wear

Common types of wear include adhesive wear, abrasive wear, fatigue wear, corrosive wear, erosive wear, and fretting wear. Each type occurs due to different operating and environmental conditions.

Q4(b) Friction Control and Wear Prevention

Friction and wear can be controlled by proper lubrication, surface coatings, material selection, surface treatments, design modification, and controlling operating conditions.

Q5(a) Journal Bearing and Minimum Oil Film Thickness

A journal bearing supports a rotating shaft using a lubricant film. Minimum oil film thickness depends on load, speed, clearance, and lubricant viscosity and is crucial for preventing metal contact.

Q5(b) Hydrostatic and Squeeze Film Lubrication

Hydrostatic lubrication uses externally pressurized lubricant to separate surfaces, while squeeze film lubrication occurs when lubricant pressure is generated due to squeezing motion between surfaces.

Q6(a) Techniques for Wear-Resistant Coatings

Wear-resistant coatings are produced using techniques such as thermal spraying, physical vapor deposition, chemical vapor deposition, hardfacing, and electroplating.

Q6(b) Friction and Wear of Lamellar Solids

Lamellar solids like graphite and MoS₂ have layered structures that slide easily over each other, resulting in low friction and excellent wear resistance.

Q7(a) Maximum Efficiency of Square Thread Screw

The maximum efficiency of a square thread screw is derived by equating frictional and lifting forces. Efficiency depends on lead angle and coefficient of friction.

Q7(b) Centrifugal Friction Clutch (Numerical)

The required shoe weight is calculated using power transmitted, speed, friction coefficient, and geometry of the clutch. The clutch engages when centrifugal force overcomes spring force.