Chapter 8 Machines
245
Gears are used in groups of two or more. A group of gears is called
a gear train. The gears in a train are arranged so that their teeth closely
interlock, or mesh.
When two gears are of the same size, they act as a simple torque
transmitter. They both turn at the same speed, but in opposite directions,
as shown in Figure 8-39. The input motion and force of the drive gear are
applied to the driven gear. The driven gear transmits the output motion
and forces.
When two gears are of different sizes, as shown in Figure 8-40, they
act as torque converters. The larger gear is called a wheel. The smaller gear
is called a pinion. The pinion gear revolves faster, but the wheel delivers
more force.
Gear Calculations
The amount of torque delivered by a gear is described as a ratio. For
example, suppose a gear that has 10 teeth meshes with a gear that has
30 teeth. The smaller gear will make three revolutions for each revolution
of the larger gear. As the small gear makes one revolution, its 10 teeth
will have meshed with 10 teeth on the larger gear. The large gear will
have turned through ten-thirtieths or one-third of a revolution. The small
gear has to make three revolutions to turn the large gear through one full
revolution. The gear ratio is therefore 3:1. See Figure 8-41.
Gear trains are either simple or compound. In a simple gear train,
there is only one meshed gear on each shaft. Figure 8-42 shows an idler
gear placed between the drive gear and the driven gear. The driver gear
and the driven gear now rotate in the same direction. The idler gear does
not change the gear ratio between the driver gear and the driven gear.
A compound gear train also has a drive gear and a driven gear.
However, the intermediate gears are fixed together on one common shaft,
as shown in Figure 8-43. The gear wheels on the intermediate shaft are
not idlers, because one is a driven gear and the other is a drive gear. They
do affect the ratio of the gear train.
gure 8-39.
moves, its torque is transmitted to another gear.
gure 8-40. Two gears act like levers to
convert torque.