Download Levers

A mechanism is simply a device which takes an input motion and force, and outputs a different motion and force. The point of a
mechanism is to make the job easier to do. The mechanisms most commonly used in mechanical systems are levers, linkages, cams,
gears, and and pulleys.
Levers
Levers: 1
You need to know how to calculate the mechanical advantage obtained by using levers, the velocity ratio in levers and pulley
systems, and gear ratio and output speed when using gears.
A lever is the simplest kind of mechanism. There are three different types of lever. Common examples of each type are the crowbar,
the wheelbarrow and the pair of tweezers.
All levers are one of three types, usually called classes. The class of a lever depends on the relative position of the load, effort and
fulcrum:
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The load is the object you are trying to move.
The effort is the force applied to move the load.
The fulcrum (or pivot) is the point where the load is pivoted.
Class 1 levers
A class 1 lever has the load and the effort on opposite
sides of the fulcrum, like a seesaw. Examples of a
class-one lever are a pair of pliers and a crowbar.
For example, it would take a force of 500N to lift the
load in the animation below. But using a lever - a rod
with the fulcrum placed closer to the load than the
point of effort - it only requires a force of 100N.
Class 2 levers
A class 2 lever has the load and the effort on the same side of the fulcrum, with the
load nearer the fulcrum. Examples of a class-two lever are a pair of nutcrackers or
a wheelbarrow.
In the diagram, the wheel or fulcrum on the wheelbarrow is helping to share the
weight of the load. This means that it takes less effort to move a load in a
wheelbarrow than to carry it.
Mechanical advantage and velocity ratio
Class 1 and class 2 levers both provide mechanical advantage. This means that
they allow you to move a large output load with a small effort. Load and effort are
forces and are measured in Newtons (N). Mechanical advantage is calculated as
follows:
Mechanical advantage = load ÷ effort
In the example above, where the load=500N and the effort=100N, the mechanical advantage would be:
500N ÷ 100N = 5
Velocity ratio
The mechanical advantage gained with class-one levers and class-two levers makes it seem like you are getting something for
nothing: moving a large load with a small effort. The catch is that to make the effort smaller, you have to move a greater distance. In
the first diagram the trade-off is that you need to push the lever down further to move the load up a smaller distance. This trade-off is
calculated by the velocity ratio:
Velocity ratio = distance moved by effort ÷ distance moved by load
A class 3 lever does not have the mechanical advantage of class-one levers and
class-two levers, so examples are less common. The effort and the load are both on
the same side of the fulcrum, but the effort is closer to the fulcrum than the load, so
more force is put in the effort than is applied to the load. These levers are good for
grabbing something small, fiddly or dirty, or picking up something that could be
squashed or broken if too much pressure is applied. The common example of class
3 levers is a pair of barbeque tongs or a pair of tweezers.