Gears and Gear Trains

Unit – 2

Gears and Gear Trains

Chapter 4: Gears

Chapter 5: Gear Trains

Gears

Learning Objectives

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

• List out the advantages and disadvantages of gear drive over other drives. 

• List the various types of gears and their applications.

• Define and understand the various terminologies used in the study of gears. 

• State and prove the fundamental law of gearing.

• Appreciate the advantages of involute tooth profile as compared to cycloidal profile.

• Understand the phenomenon of interference in gears and the various measures to eliminate it.

• Explain the term undercutting in gears.

• Derive an expression for minimum number of teeth required for the pinion in order to avoid interference.

SYLLABUS

Classification of gears – Spur gear nomenclature – Law of gearing – Conjugate profiles - Contact ratio - Interference and undercutting in involute gears - Non-standard gears.

INTRODUCTION

• Gears are toothed wheels used for transmitting motion and power from one shaft to another when they are not too far apart and when a constant velocity ratio is desired.

• In addition to transmitting the motion, gears are often used to increase or reduce speed, or change the direction of motion from one shaft to the other.

• In comparison with belt, chain and friction drives, gear drives are more compact, can operate at high speeds and can be used where precise timing is required

• Also gear drives are used when large power is to be transmitted.

1. Frictional Wheels Vs. Gear Drives

• The gear drives are basically evolved from frictional wheels.

• The frictional wheels/discs/cylinders are used in early days to transmit power from one shaft to another, as shown in Fig.4.1(a).

• The power which can be transmitted by frictional wheels depends on the friction between the contact surfaces. Due to slippage, the friction wheels cannot be used for transmitting higher power. Thus the friction wheels are non-positive drive.

• In order to provide a positive drive (i.e., drive without any slip), teeth are cut on the contacting wheels as shown in Fig.4.1(b). The friction wheels with teeth cut on them are known as gears. The gear drive is called positive drive, because there is no slip between the gear wheels.

• In any pair of gears, the smaller one is called pinion and the larger one is called gear or wheel, or bull gear, irrespective of which is driving the other.

• When pinion is the driver, it results in step down drive in which the output speed decreases and the torque increases.

• On the other hand, when the gear is the driver, it results in step up drive in which the output speed increases and the torque decreases.*

2. Advantages and Limitations of Gear Drive over Belt and Chain Drives 

1. Advantages of Gear Drive

1. Since there is no slip, so exact velocity ratio is obtained.

2. It is capable of transmitting larger power (than that of the belt and chain drives). 

3. It is more efficient (upto 99%) and effective means of power transmission.

4. It requires less space (as compared to belt and rope drives) and hence it is more compact.

2. Limitations of Gear Drive

1. The manufacture of gears require special tools and equipment and hence the manufacturing cost of gears is high.

2. The maintenance cost of gear drive is comparatively high.

3. The error in cutting teeth may cause vibrations and noise during operation.

4. The gear drive requires precise alignment of shafts for power transmission.

Do you know?: 

• Gears are most commonly used in transmissions to convert an electric motor's high speed and low torque to a shaft's requirements for low speed high torque. It may be noted that speed is easy to generate, because voltage is easy to generate; whereas torque is difficult to generate, because it requires large amounts of current.

• Gears do not depend on friction and work better when friction is minimized.

• Tooth Vs Teeth: Tooth is used in singular form while teeth is plural. i.e., if it is one then ‘tooth' and if it is more than one then 'teeth'.