UNIT 2 FRICTION
Structure
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 Introduction
Objectives

Friction

Types of Friction Laws of Dry Friction Static and Kinetic Friction Coefficient of Friction Angle of Repose Least Force Required to Drag a Body on a Rough Horizontal Plane Horizontal Force Required to Move the Body Screw and Nut Friction

2.10 Self-Locking Screws 2.11 Condition for Maximum Efficiency 2.12 Screw Jack 2.13 Pivot and Collar Friction
2.13.1 Flat Pivot 2.13.2 Conical Pivot 2.13.3 Collar Bearing

2.14 Clutch 2.15 Types of Clutches
2.15.1 Conical Clutch 2.15.2 Single Plate Clutch 2.15.3 Multi Plate Clutch

2.16 Journal Bearing 2.17 Rolling Friction 2.18 Ball and Roller Bearings 2.19 Summary 2.20 Key Words 2.21 Answers to SAQs

2.1 INTRODUCTION
When a body moves or tends to move on another body, a force appears between the surfaces. This force is called force of friction and it acts opposite to the direction of motion. Its line of action is tangential to the contacting surfaces. The magnitude of this force depends on the roughness of surfaces. In engineering applications friction is desirable and undesirable. We can walk on the ground because of friction. Friction is useful in power transmission by belts. It is useful in appliances like brakes, bolts, screw jack, etc. It is undesirable in bearing and moving machine parts where it results in loss of energy and, thereby, reduces efficiency of the machine. In this unit, you will study screw jack, clutches and different type of bearings. 51

Theory of Machines

Objectives
After studying this unit, you should be able to      know application of friction force, know theory background of screw jack, know different type of bearings, analyse bearings, and know function of cluthes.

2.2 TYPES OF FRICTION
There are two types of friction : (a) (b) Friction in un-lubricated surfaces or dry