Page 195 - Analysis and Design of Machine Elements

Subscripts H contact stress Gear Drives 173
n normal
1 pinion r radial
2 gear t transverse
F bending stress

8.1 Introduction

8.1.1 Applications, Characteristics and Structures

Gears are toothed wheels used for transmitting motion and power from one rotating
shaft to another. A gear drive, or gearing, is an assembly of two or more gears. Gear
drives are the most widely used and most important form of mechanical drives. They
are used to transmit torque and power in a wide variety of applications, from delicate
instruments to the heaviest and most powerful machinery. The power transmitted var-
ious from negligibly small values in watches to megawatts in wind turbines. The gear
diameters can vary from a fraction of a millimetre to 10 and more metres [1–5].
Compared with other mechanical power transmission, such as belt drives and chain
drives, gear drives have advantages of high efficiency and reliability, compact design,
long service life, constant transmission ratio and a wide range of speed and speed
ratio transmission capabilities. However, as precision machine elements, gears must
be designed, manufactured and installed with great care to ensure proper function.
Otherwise, noise, vibration and dynamic loads will generate during operation. As such,
the manufacturing and assembly cost of gears is high. Besides, they are not suitable for
long distance power transmission.
A typical gear drive composes of a driving gear, usually the small gear called pinion,
and a driven gear, usually the larger one, called a gear. Both of them are installed on
shafts supported by bearings. When a single pair of gears cannot achieve desired output
speed because of speed ratio limitation, a combination of interconnected gears, called
the gear train, can be used. A gear reducer is a kind of gear train. They are desirable
because they can accomplish a large speed reduction in a rather small package, having
the possibility to be attached to driven machines with a wide range of torques, speeds
and speed ratios.
This chapter discusses the fundamentals, kinematics, strength analysis and design of
several types of gears, with emphasis on spur gears, as many fundamental principles of
these are applicable to other gear types.

8.1.2 Types of Gear Drives
Gear drives can be classified into three types according to relative shaft positions. Three
shaft orientations, that is, parallel, intersect and neither parallel nor intersect, account
for the three basic types of gear drive, namely parallel-shaft gearings, intersecting-shaft
gearings and crossed gearings, as presented in Table 8.1. Each of these can be studied
by observing a single pair of gears.
Parallel-shaft gearings are the simplest and most popular type of drives. They use spur
gears, helical gears, herringbone gears, pinion and rack and internal gears, to connect
parallel shafts and transmit large power with high efficiency.