Figure 3-9.-A pointer’s handwheel.
Figure 3-10.-Developing a torque.
Look at figure 3-9. When this gun pointer pulls on one
handle and pushes on the other, he’s producing a couple.
If he cranks only with his right hand, he no longer has a
couple—just a simple first-class lever! And he’d have
to push twice as hard with one hand.
A system of gears-a gear train-transmits the
motion to the barrel. A look at figure 3-10 will help you
to figure the forces involved. The radius of the wheel is
6 inches—1/2 foot-and turns each handle with a force
of 20 pounds. The moment on the top that rotates the
wheel in a clockwise direction is equal to 20 x 1/2 = 10
ft-lb. The bottom handle also rotates the wheel in the
same direction with an equal moment. Thus, the total
twist or torque on the wheel is 10 + 10 = 20 ft-lb. To get
the same moment with one hand, apply a 20-pound
force. The radius of the wheel would have to be twice
as much—12 inches—or one foot. The couple is a
convenient arrangement of the wheel-and-axle
machine.
SUMMARY
Here is a quick review of the wheel and axle-facts
you should have straight in your mind:
A wheel-and-axle machine has the wheel fixed
rigidly to the axle. The wheel and the axle turn
together.
Use the wheel and axle to magnify your effort or to
speed it up.
You call the effect of a force rotating an object
around an axis or fulcrum a moment of force,
or simply a moment.
When an object is at rest or is moving steadily, the
clockwise moments are just equal and opposite
to the counterclockwise moments.
Moments of force depend upon two factors: (1) the
amount of the force and (2) the distance from
the fulcrum or axis to the point where the force
is applied.
When you apply two equal forces at equal distances
on opposite sides of a fulcrum and move those
forces in opposite directions so they both tend
to cause rotation about the fulcrum, you have a
couple.
3-6
