Figure 10-21.--Gears can change the direction of applied motion

uses and advantages. But, it is better to understand

their operating theory before learning of their uses and

advantages.

Figure 10-20, view A, shows the action of a

single-threaded worm gear. For each revolution of the

worm gear, the worm wheel turns one tooth. Thus, if

the worm wheel has 25 teeth, the gear ratio is 25:1.

Figure 10-20, view B, shows the action of a

double-threaded worm. For each revolution of the

worm gear in this case, the worm wheel turns two

teeth. Thus, if the worm wheel has 25 teeth, the gear

ratio is 25:2.

Likewise, a triple-threaded worm gear would turn

the worm wheel three teeth per revolution of the worm

gear.

A worm gear-worm wheel is really a combina-

Figure 10-21.--Gears can change the direction of applied

tion of a screw and a spur gear. Tremendous

motion.

mechanical advantages (M.A.) can be obtained with

this arrangement. Worm drives can also be designed

so that only the worm gear is the driver--the spur gear

the telescopes to the right. Thus, with a simple system

cannot drive the worm gear. On a hoist, for example,

of gears, it is possible to keep the two telescopes

you can raise or lower the load by pulling on the chain

pointed at a moving target. In this and many other

that turns the worm gear. But, if you let go of the chain,

practical applications, gears serve one purpose-they

the load cannot drive the spur gear and will let the load

change the direction of motion.

drop to the deck. This is a nonreversing worm drive.

CHANGING SPEED WITH GEARS

CHANGING DIRECTION WITH GEARS

As you have already seen in the eggbeater, gears

can be used to change the speed of motion. Another

No doubt you know that the crankshaft in an

example of this use of gears is found in your clock or

automobile engine can turn in only one direction. If

watch. The mainspring slowly unwinds and causes the

you want the car to go backwards, the effect of the

hour hand to make one revolution in 12 hours.

engine's rotation must be reversed. This is done by a

Through a series, or train, of gears, the minute hand

reversing gear in the transmission, not by reversing the

makes one revolution each hour, while the second

direction in which the crankshaft turns.

hand goes around once per minute.

A study of figure 10-21 will show you how gears

Figure 10-22 will help you understand how speed

are used to change the direction of motion. This is a

changes are made possible. Wheel A has 10 teeth,

schematic diagram of the sight mounts on a Navy gun.

If you trunk the range-adjusting handle, A, in a

clockwise direction, the gear, B, directly above it, is

made to rotate in a counterclockwise direction. This

motion causes the two pinions, C and D, on the shaft

to turn in the same direction as gear B against the teeth

cut in the bottom of the table. The table is tipped in the

direction indicated by the arrow.

As you turn the deflection-adjusting handle, E, in

a clockwise direction, gear F, directly above it, turns

in the opposite direction. Since the two bevel gears, G

and H, are fixed on the shaft with F, they also turn.

These bevel gears, meshing with the horizontal bevel

gears, I and J, cause I and J to swing the front ends of

Figure 10-22.--Gears can change the speed of applied motion.

10-11