Friction

Unit: Friction

Chapter 6: Friction

Chapter 7: Clutches

Chapter 8: Belt and Rope Drives

Chapter 9: Brakes

Chapter: 6

Friction

Learning Objectives

While reading and after studying this chapter, you will be able to:

• Describe the various types of friction experienced by a body.

• Understand the fundamental concepts of friction.

• Explain the principle of working of a screw jack and derive an expression for its mechanical efficiency.

• Derive the expression for the effort required to raise a load with a screw jack taking Fiction into consideration.

• Understand the meaning of overhauling and self-locking screws.

• Appreciate why square threads are preferred to V-threads for lifting loads.

• Find the frictional torque of a flat pivot, a conical pivot and a trapezoidal conical pivot bearing.

SYLLABUS

Types of friction - Equilibrium of a body on a horizontal and an inclined planes – Friction of screw and nut - Screw jack - Friction of a V-thread Pivot and collar friction.

1. INTRODUCTION

Friction is a force that opposes the motion or tendency of a movement. Friction resists the relative motion of solid surface, fluid layers or material elements moving against each other. It is the phenomenon as just opposite to slipperiness.

1. Nature of Friction

As we readily see that friction is ever-present in our daily activities and in many engineering systems. For example, consider the heat generated by rubbing dry hands together, the skid marks made by automobile tires sliding on dry pavements, and the action of bicycle brakes. It is clearly understood from these phenomena when there is a tendency towards relative motion between two bodies in contact, the reaction between them will have component acting along the common tangent at the point of contact. This component is commonly referred to as a friction force, and is always in the direction that opposes the tendency of stationary bodies to move or slows down moving bodies.

2. Effects of Friction

The effects of friction are both beneficial and detrimental.

• Beneficial effects: We cannot walk, run, or drive automobiles without the beneficial effects of friction. It makes the tractive forces possible. In some applications such as tires, clutches, brakes and belt drives, higher friction is beneficial. That is why sometimes friction is said to be 'necessary evil'.

• Detrimental effects: In contrast, the friction is undesirable in moving parts of machines. In machine elements such as bearings and gears, friction must be minimized to avoid power losses. The tremendous amount of money spent for lubricants to reduce the friction between rubbing surfaces indicates a disadvantage of friction. The friction causes the loss of energy and hence the mechanical efficiency is reduced.