Mechanical Systems

Tools and Machines have enabled humans to adapt and control their environment.

From the earliest times, humans have used tools to extend their capabilities.

Some tools have remained virtually unchanged over a several centuries, whilst others have been modified and developed in order to exploit the energy resources available

Some of the earliest known tools were simple mechanisms.

The technological advances humans have made through the ages, have been closely linked with their ability to harness energy and use it to perform mechanical work.

Machines have become so important in our lives today that we have a machine to do almost every task imaginable. Because we have so many machines around us we often do not notice them or even realize that some of these devices are machines.

To understand machines we don’t have to look at exotic or expensive devices. We all use a variety of machines every day.

We may call these things many different names but they are really very simple machines.

They use one of three basic movements, either singularly or combined. The three basic movements are :

	Movement in a straight line (linear or reciprocating)
	Movement in an arc (oscillating)
	Movement about a centre

These movements are made by mechanisms such as screw-threads, lever arms, wheels, pivots, etc.

Although humans are extremely versatile creatures, our size, structure and limited muscle power put severe limitations on our capabilities. These physiological limitations stop us doing many tasks unaided. When situations arise which are beyond our normal capabilities, humans have always turned to technology for a solution.

Some early structures such as the Pyramids, the Easter Island Statues and Stonehenge were designed and built by early civilizations with very little technology

These and many other monuments were not only very large but were constructed of very heavy materials. To this day we still don’t know precisely how the stones they used were moved or lifted, but many attempts have been made to duplicate these efforts with little success.

The only sensible answer is that they must have made use of early machines and mechanisms

All mechanisms are based on what ancient philosophers called the Five Basic Machines. The five basic machines are the five simplest machines which form the basis of all the other machines developed throughout human history. They are :

The Wheel

The Lever

The Inclined Plane

The Wedge

The Screw

The five basic machines mentioned above were intended to supplement human muscle; they enhanced early human capabilities and allowed them do do things which would otherwise have been impossible. The history of technology mirrors the history of human achievement.

Naturally-occurring forms of energy can be used to do work, and humans soon discovered how to tap these resources. They harnessed animals such as horses and oxen, they developed machines such as windmills for converting a naturally-occurring form of energy into a more directly useful form of work and then developed engines to harness the energy locked into fossil fuels. You will find that engines are often referred to as prime movers because they provide the energy that initiates the action of machines.

The purpose of any machine is to do mechanical work. A machine is used to convert (change) or transmit (pass on) energy. Machines are made from a number of working parts (components) which we collectively call mechanisms.

A mechanism or mechanical control system changes an input force and movement into a desired output force and movement.

Mechanical Control Systems are used in anything that controls movement. They can be used to:

Change the Velocity or Speed of movement

Change the Distance or Direction of movement

Change the type of movement (from one type of motion to a different type of motion)

Make it easier to move a load.

All mechanisms do at least one of these things.

The Lever

All levers have three things in common: the effort (where you do the pushing), the load (the thing that resists the pushing) and the pivot or fulcrum (the point round which the first two revolve).

Levers were probably the first kind of mechanism ever used, they were used to move large rocks or open shells in much the same way that we might use a crowbar to open a crate.

By changing the position of the load, effort and fulcrum, you find three different arrangements. These are called the three classes of lever.

Most levers that you will come across will be examples of class 1 or class 2 levers. They are more common than class 3 levers because both class 1 and class 2 levers give you a mechanical advantage (MA).

This means that you can move a large load using a relatively small effort. The mechanical advantage of the class two lever (wheel barrow) shown below is found by comparing the weight of the load with the effort needed to move it. It uses a formula which you should be familiar with.

The mechanical advantage of any mechanism can be calculated in the same way. The larger the number, the greater the mechanical advantage. Class 3 levers are used less often because their mechanical advantage is less than one this means that the force needed to use them is greater than the force they can move.

When calculating the M.A. it may seem that you are getting something for nothing, you can move a large load with a much smaller force!

As with most things however you rarely get something for nothing and this is certainly true for the diagram below!

Look at how far you would have to move effort in comparison to the the distance the load is moved.

By comparing the two distances you obtain the velocity ratio (V.R.) The velocity ratio is also a formula which you should be familiar with !

All this assumes that a mechanism is 100% efficient. In practice they never are !, parts twist and bend, surfaces rub and produce friction, energy is lost as waste heat etc. All this makes them less than 100% efficient.

The efficiency of a mechanism is found very easily by comparing its M.A. and V.R. using the formula shown opposite:

Now let us examine a number of common objects which use the principle of levers. Can you spot the load, effort and fulcrum points here ?

By connecting levers together, we can make some useful objects. Have you considered how the pedal bin works?

It is an example of simple linkage. The act of pressing your foot down on the pedal works a lever which pushes up a rod at the back of the bin. This rod is connected to the lid, which is pivoted at the back, thus causing the lid to rise.

Linkages are very important in mechanical systems, because they allow forces and movement to be transmitted to where it is needed. They can change the direction of a movement, the size of a force, or make things move in a particular (constrained) way. They often do several of these at once.

Bell cranks and reverse motion linkages can be used to change the direction of motion.

A bell crank is useful for taking motion round a corner.

A good example of this type of linkage can be found on the brakes of a bicycle. The caliper brakes use two bell cranks which pivot on the same bolt.

The forces needed to stop a mountain bike are often greater than a normal bicycle because of the degree of control required.

By changing the position of the fulcrum or extending the length of one side of the lever, the amount of movement or the size of the force produced can be changed.

The brakes used on a mountain bike have much longer levers on the effort side of the fulcrum than those on the load (brake block) side. This allows far greater forces to be applied to the wheel when braking.

A reverse motion linkage is useful for changing a "push" to "pull"

Parallel Linkages.

Linkages based on a parallelogram can be used to make two or more parts move together or apart whilst keeping them parallel to one another. The important feature of this linkage is that opposite sides remain parallel to each other as they move.

Linkages based on this idea can be found in use on tool boxes, keeping the trays level, on safety gates, allowing the gates to fold back into a smaller space, and on scissor lift tables where the table top is kept parallel with the ground.

Treadle Linkage.

This linkage involves changes between rotary and oscillating motion. It can be used to power a rotary machine by working the oscillating treadle (as in an old - fashioned foot operated sewing machine for instance) It can also be used in the other way to change rotary motion into oscillating motion. Some car windscreen wipers work in this way when combined with a parallel linkage.