Unlike microprocessors and electronic components, mechanisms have been in existence, in their more complex form, certainly for the last 300 years and, in their more basic form, since the dawn of mankind. The major changes over the last 150 years concern the materials used and the power source for mechanisms.
The most basic form of mechanism is a lever. A lever is a device which increases (or decreases) the amount of force you (or a motor) can exert.
The earliest version of a lever was probably a branch wedged under a small rock to lift a much heavier boulder.  A lever has three key points. A load, a fulcrum and an effort.
The load is the point where the thing that you’re trying to move (or hold, or cut) goes. The fulcrum is the pivoting part of the lever and the effort is where you (or a motor) holds/lifts/pulls/squeezes etc.
The illustration on the right is an example of a class 1 lever.  On the left I have shown the arrangement for a class 1, 2 and 3 lever. One of the things to note with levers is that when the fulcrum is in between the load and the effort, the direction is reversed (look at the illustration above). The important thing is that the distance between the effort and the fulcrum, compared to the load and the fulcrum, determines how much the force is magnified. Now I’ll show you how to calculate this.
In the illustration on the right I have shown a simple class 1 lever. I have stated that the effort applied is 200N and that the distance that this is applied is 4m from the fulcrum. The load on the other end of my lever is 2m away from the fulcrum. I am now going to calculate the moment (unit Nm) or turning force exerted by the load.
Moment = Force x Distance = 200 x 4 = 800Nm.
That same moment has to be balanced by an opposing moment on the other side of the fulcrum.
800Nm = L x 2 so therefore L = 400N
So by halving the distance between the load and the fulcrum, and the effort and the fulcrum, we have doubled the force available. This means that if you were to push down with a force of 200N you would be able to lift a load of 400N.
 By combining levers we can form linkages to control the motion and forces required. Without even thinking about it you have probably used or benefitted from several linkages already today. Below is an illustrartion showing three common linkages.
Whilst levers and linkages are undoubtedly useful, more advanced mechanisms can convert different types of motion and either speed up or slow down the output. Notice that word ‘output’ again because just as a lever can be broken down into input, process and output, so to can motion conversion mechanisms. Let’s first look at the different types of motion.
Linear motion is motion in a straight line. It doesn’t have to be absolutely straight; this train wouldn’t be described as rotary motion if it went around the Circle line.
Rotary motion is the motion of something that spins round and round. The example I have given is a washing machine drum (which spins round!)
Oscillating motion is the name given to something which swings in an arc such as this clock pendulum.
Motion back and forth in a straight line is classed as reciprocating motion.
When tackling a question which asks you to design a mechanism for a particular application, you need to ask yourself what type of output is required given the input. A change of speed will require a gear/pulley/chain mechanism (which I will talk about later) whilst a change of motion type will require one of the following devices.
Linear-Rotary
Look at the illustration of the train (above) and you will realise that the only reason it is able to move at all is because of the wheels which are in contact with the track. The wheels are an example of rotary motion and, by being in contact with the track, the output is linear motion.
The same is true of any wheel. The illustration to the right shows a Rack and Pinion mechanism which also converts rotary-linear and vice versa.
Reciprocating-Rotary
The next time you are on your bicycle, think about what your knees are doing.  They are simply moving up and down. They act as a pivot with the rest of your leg and there is a similar pivot (your ankle) between your leg and your foot. Take a look at your feet and you will see that your feet move in circles. The pedals form what is called a crank, which converts reciprocating motion to rotary. The same mechanism will convert rotary motion to reciprocating and there are other ways of doing this too.
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The mechanism shown below left is a cam. (You are probably familiar with it from the automata project). The shape of the cam will determine how quickly and how far the follower moves.
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Rotary-Oscillating
This type of motion conversion can be achieved using a cam with what is known as a radial arm follower. This is shown below along with the more conventional way of achieving the same result, a crank and slider.
All of the devices covered so far can be combined with levers and linkages to form complex mechanisms. We will now look at the mechanisms that allow power to be transmitted and change the speed.
The first thing you need to realise is that you don’t get something for nothing. What I mean is that you can use a gear mechanism to enable you to climb up a steep hill on a bicycle but the flip side is that you won’t be going very fast. Similarly, you can change gears so that you can speed up along a flat stretch, but the effort required will soon have your legs aching.
 Gears
On the left is a simple gear mechanism. Every tooth on one gear meshes with a tooth on the other. This means that when the gear with 30 teeth rotates once, the gear with only 24 teeth will have to rotate 1 1/4 times to keep up.It is not often that a gear will rotate just the once so now is a good time to talk of the speed in terms of rpm or rotations per minute.
Click here to see an animation
It is also necessary to define the input and the output gear. The input gear will be the one connected to the power source, either a motor or pedals or even the sails of a windmill. This gear is called the driver gear. The output gear is the one at the end of the system, connected to the wheel of a bike or the chuck of drill and it is called the driven gear.
The exam board may well ask you to calculate the Gear Ratio of the system. The formula is given below and you should notice how it differs from the previous formula.
Look at the arrows on the gear wheels and you will notice that the direction of rotation is reversed. You can overcome this problem by using an idler gear. This is sandwiched between the driver and driven gears and has no effect upon the speed reduction/increase of the system ~ weird but true ~ which is determined only by the gear ratio.
The great thing about gears is that because you have the two surfaces of the teeth pressing together, the power transmission is excellent. The disadvantage is that this causes friction, so gears either need to be lubricated or made of a low friction material such as brass or nylon, and also noise.
 There are limits to how much you can reduce or speed up a mechanism in one go. The illustration on the left shows an example of what is not possible. The strain placed upon the teeth of the small gear would be too great. There are various ways of getting around the problem and these are shown below. The gear train features two gears of different sizes fixed together so that they turn at the same speed. In contact with this set of gears is a driver and driven gear wheel.
Click here to see an animation
An alternative to this is the worm gear and worm wheel.  The worm gear has a screw thread on it which, as it rotates one revolution, causes the worm wheel to move on tooths worth.  The worm gear/wheel causes the rotation to turn through 90º. Another device which does this is the bevel gear which is shown to the right. This is used on hand whisks and drills.
Chain and Sprocket
When you talk about changing gear on a bicycle, you are in fact changing sprocket. A sprocket is a toothed wheel, similar to a gear wheel, but the teeth mesh with a chain. The formula for calculating the speed change remains the same as before as does the gear ratio. The big change is that the driver and driven wheels rotate in the same direction. The chain is series of flexible links and this is both a strength and a weakness. The plus side is that the chain can be made to ‘jump’ from one sprocket to another (as happens on a bicycle) and the transmission of power is good. The down-side is that the chain needs oiling to keep the system running smoothly and even then the chain can snap.
Pulley and Belt
This mechanism is very similar to the sprocket and chain mechanism, but relies upon the friction between a pulley wheel and a rubber belt. The speed change is determined by the diameters of the driver and driven pulley wheels. Use the same formula that is used for gears and sprockets but substitute pulley wheel diameter. Pulley wheels have the advantage of being very smooth running and are therefore quiet. The belt can be easily swapped from one set of pulley wheels to another and no maintenance or lubrication is required. It is possible for the belt to fray or snap, and more significantly, the belt can slip. This is bad news where transmission of power is concerned although it can act as a safety feature if a mechanism jams.
Pulley belts can appear in different forms such as flat, vee and toothed. The toothed belt needs a pulley wheel with matching grooves and has many of the same advantages of a regular pulley and belt mechanism with better power transmission.
Click here to see an animation
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