234
Technology: Engineering Our World
Moments and Levers
Imagine a lever with a fulcrum in the middle. On one side is an effort
and on the other a resistance. When at rest, this lever is said to be balanced.
If the effort is increased, the lever turns in a counterclockwise direction. If
the resistance is increased, the lever turns in a clockwise direction, as shown
in Figure 8-13. The turning force is called a moment.
The moment depends on two things: the effort and the distance of the
effort from the fulcrum.
Moment = Effort × Distance
If a beam is in balance, the clockwise moments are equal to the
counterclockwise moments.
4 × 50 = 8 × 25
The levers discussed so far have been used to increase force, distance
moved, or speed. Levers can also be used to reverse the direction of motion.
Think of a lever with a fulcrum in the center. If it pivots about its
fulcrum, the ends move in opposite directions. One end moves down,
and the other end moves up. See Figure 8-14. A single lever with a pivot
in the center reverses an input motion.
This idea is used in linkages. A linkage is a system of levers used to
transmit motion. Figure 8-15 illustrates a reverse motion linkage. The input
force and output force are equal.
If the pivot is not at the center, the input force is increased or decreased
at the output. This is shown in Figure 8-16.
The Pulley
The pulley is a special kind of Class 1 lever. See Figure 8-17. Its action
is continuous. The resistance arm is the same length as the effort arm. The
length of each arm is the radius of the pulley. See Figure 8-18.
4 m 8 m
Effort
Resistance
Fulcrum
Counter-
clockwise
moment
Clockwise
moment
50 N 25 N
gure 8-13. Forces acting on a lever are
called moments.
Upward
motion
Fixed pivot
(fulcrum)
Downward
motion
Force
gure 8-14. Some levers are designed to
change motion.