10 November 2018 Farm Collector
The class 1 lever has the weight at one end and the applied
force at the other, while the fulcrum is placed somewhere
between. Many levers used for moving and prying objects
fall into this class.
A nail lifter and a pair of shears are examples of first class
levers. The claws of a nail lifter form the weight arm and the
nail to be pulled is the weight. The crook in the nail lifter
bears against the board and is the fulcrum, while the long
handle is the force arm.
A pair of shears consists of two levers sharing a single ful-
crum: the bolt that holds the shearing blades together. The
handles are the force arms, the blades are the weight arms,
and the material to be cut is the weight.
In that case, the length of the weight arms is measured
from the fulcrum to the
point where the cut is being
made. For that reason, a ma-
terial that’s too tough to be
cut at the blade tips can be
easily cut if it’s moved near-
er the fulcrum bolt, making
the handles, or force arms,
longer than the weight
arms.
Class 2 levers
A class 2 lever has the ap-
plied force at one end of the
lever and the fulcrum at the
other, with the weight be-
tween them. A wheelbarrow
is a good example of this
class of lever. The wheel and
axle of a wheelbarrow pro-
vides a fulcrum, while the
handles are the force arms.
The load in the wheelbar-
row is the weight. Because
the load is closer to the ful-
crum than the handles, a
heavy load can be lifted.
Class 3 levers
If the fulcrum is at one
end of the lever and the weight at the other, with the ap-
plied force in between, it is a class 3 lever. A pair of tweezers
is an example of a third class lever. To pluck an errant eye-
brow hair, milady must first grasp it firmly with a tweezers.
The two tweezer halves are joined at one end, forming the
fulcrum. The open ends must be brought together to firmly
grasp the offending hair, which is the weight. The pressure
of the fingers is the force and is applied somewhere between
the fulcrum and the weight.
Variations on a theme
A lever isn’t always a straight bar. The force arm and the
weight arm can be at an angle to each other, forming an
angular or bell-crank lever as shown in drawing No. 3 (pre-
vious page). When a claw hammer is used to pull a nail, the
claws that slip under the nail head become the weight arm,
while the hammer handle is the force arm. A hammer used
in this manner becomes an angular lever and a horizontal
pull on the handle produces a vertical lift on the nail.
In many machines, two or more levers are connected,
with the force arm of one being linked to the weight arm of
another as in drawing No. 4 (previous page). This increases
the lifting force at the weight, along with keeping the length
of the lever within compact limits.
A McCormick-Deering No. 9 mowing machine, a device chock-full
of levers. Even the double tree is actually a second class lever with
the weight being the load (mower) connected to the center hole of
the evener. The force is the pull applied by one horse on the single-
tree pin at one end, while the fulcrum is the single-tree pin at the
other end. This concept may be a little confusing until you consider
that the pull of each horse in the team provides the fulcrum against
which the other horse applies force to the weight in the center of
the evener. Photo by Sam Moore.This compounding can go on indefinitely and is found in
many variations. One example is the platform scale, where a
heavy load on the platform is balanced by a small weight on
the scale beam. In the drawing, the weight arm of the upper
lever becomes, through the vertical link, the force operating
on the force arm of the lower lever.
If we look at the wheels of a ground-driven mowing ma-
chine, we find that the wheel rim, spokes and axle form a
continuous lever as in drawing No. 5 (previous page). The
wheel rim transmits the force of the forward movement
of the mower to the spokes, which are the force arms. The
spokes are attached to the wheel hub, which is keyed to the
axle. The center of the axle is the fulcrum, and the radius of
the axle is the weight arm.
In the case of the
mower, the weight arm
(or axle radius) is ex-
tended by the addition
of a gear, which is also
keyed to the axle. The
radius of the drive gear
becomes the weight
arm, while the teeth on
the circumference pull
the weight. In a mower,
the weight is the system
of intermediate gears
and the pitman that
causes the cutter bar
knife to reciprocate and
cut hay.Making work
easier
If the wheel is driven
by the gear and axle,
as in the case of a trac-
tor drive wheel, the
situation is reversed. Al-
though the center of the
axle is still the fulcrum,
the spokes and attached
wheel rim become the
weight arm, while the
radius of the axle and the gear become the force arm.
While it may seem that a lever allows a lot of work to
be accomplished on the one side of a fulcrum, and a lot
less on the other, the “Law of Machines” prevails. This law
states that “the force multiplied by the distance it moves,
equals the weight multiplied by the distance it moves.”
What that means, is that, while a pound of pressure on
the force arm can be made to lift 10 or 100 times as many
pounds on the weight arm by varying the relative lengths
of these arms, the force arm then has to move 10 or 100
times as far as the weight arm. Thus the work done on one
side of the fulcrum is exactly equal to that done on the
other side, provided there is no friction involved.
Thanks to old Archimedes, we’ve learned to combine le-
vers in many forms to build machines that make our work
easier and our lives more enjoyable. FCSam Moore grew up on a farm in western Pennsylvania. He
now lives in Salem, Ohio, and collects antique tractors, imple-
ments and related items. Contact Sam by email at letstalkrus-
[email protected].