Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!
Print

Design & Technology

Mechanisms

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: 1

You need to know how to calculate the mechanical advantage [mechanical advantage: The ratio of the output force to the input force in a mechanism - in other words, the factor by which the mechanism multiplies the force put into it. ] obtained by using levers, the velocity ratio [velocity ratio: The ratio of the distance travelled by the effort to the distance travelled by the load. Also sometimes called the distance ratio. ] in levers and pulley systems, and gear ratio [gear ratio: The ratio of the number of teeth on a driving gear to the number of teeth on the driven gear. A form of velocity ratio, the gear ratio determines the number of revolutions made by each gear. ] 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:

  • 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. Press play to see a demonstration.

Class 2 levers

A wheelbarrow's Fulcrum is the wheel. The Load sits in the middle, and the Effort is applied on the handles

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

Back to Systems and control index

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.