(SEM VI) THEORY EXAMINATION 2024-25 TRIBOLOGY

BME063 – TRIBOLOGY

Section-Wise Solved Answers (2024–25)

SECTION A

Attempt all questions in brief (2 × 10 = 20 marks)

(a) Define viscosity index and its significance.

Viscosity index (VI) is a measure of how much the viscosity of a lubricant changes with temperature. A high viscosity index indicates that the lubricant’s viscosity remains relatively stable over a wide temperature range. This is significant because lubricants with high VI provide consistent lubrication performance in both cold start and high-temperature operating conditions.

(b) What are the primary causes of friction?

The primary causes of friction are surface roughness, adhesion between contacting surfaces, deformation of asperities, and ploughing action when a harder surface penetrates a softer one. These factors resist relative motion between surfaces.

(c) List the different modes of lubrication.

The different modes of lubrication include boundary lubrication, mixed lubrication, hydrodynamic lubrication, elasto-hydrodynamic lubrication, and solid lubrication.

(d) What is the role of lubricant additives?

Lubricant additives improve the base oil properties. They reduce wear and friction, prevent oxidation and corrosion, enhance viscosity-temperature behavior, and improve cleaning and anti-foaming characteristics.

(e) Differentiate between adhesive and abrasive wear.

Adhesive wear occurs when material transfers from one surface to another due to strong adhesive forces at contact points. Abrasive wear occurs when a harder surface or hard particles remove material from a softer surface through cutting or scratching action.

(f) State any two properties of greases as lubricants.

Greases have high consistency and stay in place under load. They also provide good sealing against dust and moisture, making them suitable for applications where frequent lubrication is difficult.

(g) Define minimum oil film thickness in hydrodynamic bearings.

Minimum oil film thickness is the smallest distance between the bearing surface and shaft during operation. It is critical to prevent metal-to-metal contact and ensure safe hydrodynamic lubrication.

(h) What are the functions of solid lubricants?

Solid lubricants reduce friction and wear in conditions where liquid lubricants fail, such as high temperature, vacuum, or extreme pressure. They also provide lubrication in dry and contaminated environments.

(i) Explain the term “friction angle” with respect to screw threads.

The friction angle is the angle whose tangent equals the coefficient of friction between screw threads. It represents resistance to motion and plays a key role in determining self-locking and efficiency of screw mechanisms.

(j) List two examples of tribo-design considerations in kinematic pairs.

Selection of suitable materials for mating surfaces and proper lubrication method are important tribo-design considerations in kinematic pairs such as gears and bearings.

SECTION B

Attempt any three (10 × 3 = 30 marks)

(a) Dependence of oil viscosity on temperature and pressure.

Oil viscosity decreases significantly with increase in temperature due to reduced intermolecular attraction. At higher temperatures, lubricants become thinner. Conversely, viscosity increases with increase in pressure because molecular spacing decreases. Graphically, viscosity-temperature curves show exponential decrease, while viscosity-pressure curves show a rising trend. This behavior affects bearing performance and film thickness.

(b) Different theories of dry friction.

The adhesion theory states that friction arises due to microscopic welding at contact points. The deformation theory attributes friction to energy loss due to deformation of surface asperities. The ploughing theory explains friction as resistance caused by hard asperities cutting into softer surfaces. In practice, dry friction is a combination of all these mechanisms.

(c) Mechanism of pressure development in hydrodynamic bearings.

Pressure in hydrodynamic bearings develops due to wedge-shaped oil film formed between the rotating shaft and bearing surface. As the shaft rotates, it drags lubricant into the converging clearance, generating pressure that supports the load. This pressure prevents direct contact between surfaces.

(d) Wear-resistant coatings for solid lubrication.

Wear-resistant coatings such as MoS₂, graphite, PTFE, and diamond-like carbon (DLC) reduce friction and wear. These coatings possess low shear strength and provide lubrication under extreme conditions like high temperature and vacuum.

(e) Tribological analysis of belt drives and cam-follower systems.

In belt drives, friction between belt and pulley determines power transmission and efficiency. Wear affects belt life. In cam-follower systems, lubrication reduces contact stress and wear. Proper material selection and lubrication are essential to reduce friction losses.

SECTION C

Q3. Attempt any one (10 marks)

(a) Properties of lubricants and their significance.

Important lubricant properties include viscosity, viscosity index, flash point, pour point, oiliness, and oxidation stability. Viscosity determines load-carrying capacity, flash point indicates safety, and oxidation stability affects lubricant life. Selecting proper lubricant ensures reduced wear and longer machine life.

(b) Mineral oil-based vs synthetic lubricants.

Mineral oils are derived from petroleum and are economical but have limited temperature range. Synthetic lubricants are chemically engineered and offer superior performance, higher thermal stability, and longer service life, though at higher cost.