Revision - Mechanisms

Types of motion

• Linear - movement in a straight line - i.e. a train
• Rotational - movement in a circle - i.e. the wheels of the train
• Reciprocating - back and forth movement in a straight line - i.e. a pneumatic drill
• Oscillating - back and forth movement along an arc - i.e. a pendulum

Levers
Levers use distance to magnify force. The further away from the pivot the force is applied, the more it is magnified, therefore levers can be used to lift heavy loads with smaller forces.

So, in this example, the force we are trying to lift is 30N, it is magnified by 5 (5cm away from the pivot) the input force we need to put in is magnified by 15cm, therefore we have to apply an input force of 10N.

Pulleys

Pulleys are another mechanism used to magnify a force by a distance to make something easier to lift.  By increasing the distance of rope pulled through the mechanism less force can be used.  This only works with more than one pulley.

The mechanical advantage is a way of describing how much easier the load is to move with the pulley.  You can work out the mechanical advantage by using the equation:
                        load
             MA = Effort

The velocity ratio concerns the distances covered by both the load and the effort.  You can work out the velocity ratio by using the equation:
                       distance moved by load
            VR = distance moved by effort
You could also count the ropes (not including effort) to find the velocity ratio.

No mechanical system is 100% efficient due to friction in the moving parts, so you will need to put in more force in real life than in calculation to move the load.  The efficiency can be calculated using the equation:
                               Mechanical Advantage
          Efficiency =       Velocity Ratio

Rotary Mechanisms

All rotary mechanisms are used to increase or decrease speed, and/or change the rotation through different angels (most commonly 90)

You need to know about:

• Spur Gears (Gear trains plus using an idler)
• Chain and Sprocket
• Belt and Pully
• Worm and Wheel
• Face Gears
• Bevel Edge Gears

The velocity ratio of all rotary systems can be found using the generic equation:
                       the amount of input motion
            VR = the amount of output motion

However we can use simpler equations when we know the size/number of teeth on a gear.  This can apply to chain and sprocket, meashed gears, bevel edged gears and face gears.  If you want to find the Velocity Ratio of a belt and pulley, you can follow the same theory but you will have to use the diameter of the pulley as it will have no teeth.

A smaller gear will always turn faster, therefore is a small gear is driving a bigger gear you will get a speed reduction so you are looking for the Velocity Ratio to be a fraction.

So if gear A is the driver, and gear B is the driven, you can find the VR = A/B = 12/18 or simplified to 2/3.  So if gear A turned at 300 rev/min, gear B would turn at 2/3 of that speed = 200 rev/min.

If a larger gear is driving a smaller gear you will have an increase in speed and are therefore looking for a whole number as your velocity ratio.  So if gear B was the driver and gear A the driven the VR = B/A = 18/12 = 1.5.  So if gear B turned at 300 rev/min, gear A would turn at 1.5 x 300 = 450 rev/min.

When deciding between a chain and sprocket and a toothed belt you need to consider the function of the mechanism.  A chain and sprocket is very good for eliminating slip, but if something jams in the mechanism and a motor is being used as your input, the motor will be trying to turn but can't so will burn out, a belt would allow the motor to still turn as it would slip over the pulleys and so the motor is protected.  Something with manual input like a bike is very suited to a chain drive to ensure that every rotation of the pedals results in a turn of the wheel.

You can try to increase friction between the belt and the pulley by changing the type of belt to a V-belt or a toothed belt.

To make sure that the belt does not stretch so that it falls off the pulley, a tensioner or jockey wheel may have to be used.

Worm and Wheel
A worm and wheel is a special rotary system as it only allows rotation in one direction - i.e. the worm can only ever be the driver so acts as a brake.  The worm and wheel also produces a big reduction in speed as the worm only has one tooth.  So VR = 1/number of teeth on worm.

Bevel Edge Gears
Bevel edge gears often do not have a difference in size as their main purpose is to transmit the rotation through 90.

Face Gears
Face gears transmit rotation through 90 and can also have a velocity ratio by changing the size of the gears used.

Compound Gear Systems
Compound gear systems combine mechanisms to produce a greater velocity ratio.  This is done by adding two gears on the same axle.  As these must be turning at the same speed, there is no velocity ratio between them, but now the driver of the second "pair" is turning at the same speed as the driven of the first "pair."

The above picture and graph shows a motor turning at 120 rev/min.  There is a great reduction in speed with the worm and wheel - VR = 1/10 (the blue line shows the speed of the wheel) a further reduction with the first chain drive - VR = 10/20 (the green line) and a last reduction with the second chain drive - VR = 15/25 (the purple line).

So we can work out the total VR:

VR = 1/10 x 10/20 x 15/25
      = 0.03

The output speed will be the motor speed x VR so:

Output speed = 120 x 0.03
                      = 3.6 rev/min

If an idler mechanism was used, the effect of the speed reduction would be greatly reduced as the idler does not affect the velocity ratio, only the driven and the driver.

Rotary to Linear

The rack and pinion turns the rotary motion of the pinion into the linear motion of the rack.  You can do calculations based on the number of teeth per metre on the rack and the speed of the rack.

i.e. if the rack has 100 teeth per metre and the pinion has 10 teeth, it has to turn at 10 revolutions a second for the rack to move 1m/s.

Rotary to Reciprocating


Crank and Slider: A crank is motion off centre of a circle.  This could be a single member, or a wheel.  The off centre motion pushes the slider back and forth, thus creating reciprocating motion.

Cam and Follower: The raised section of a Cam pushes a follower up and the dewll makes it fall again.  There are many shapes of Cam, the most common of these being a pear shaped cam.

Click here for more information about cam and follower

Safety Mechanisms
Sometimes it is necessary to ensure rotation in one direction only.  As mentioned before, a worm and wheel could be used for this purpose.  Alternatively a Ratchet and Pawl could be used.  This works by shaping the teeth on the ratchet so that in one direction the pawl slides over them and in the other direction gets stuck, stopping the mechnism.

Revision - Logic

You need to remember each type of logic gate - its symbol and truth table.

You then need to be able to go between English descriptions, to complex truth tables, to Boolean expressions and circuits.  You could be given any of the previous and be asked to produce a different form.

Your Exam

On the 10th May you will sit your S3 exam.

You need to know about:

Mechanisms:

• Different types of mechanisms
• Velocity Ratios
• Speed
• Uses, Advantages and disadvantages of different types of mechanism

Logic:

• AND, OR, NOT, NOR, NAND, XOR
• Truth tables
• Boolean expressions
• Logic circuits
• You need to be able to go between any of the above i.e. get a circuit from a Boolean expression or get a Boolean expression from a truth table.

Pneumatics:

• Name valves
• How to wire up different valves to control cylinders or produce air signals
• Cylinder calculations - F = PA and how to find area or diameter.

Programmable Control:

• Parts of a microprocessor
• Flowcharts
• PBASIC

Electronics Basics:

• Series Circuits
• Ohm's Law

Keep checking the blog in the second week of the holidays, more information will arrive when I am back from holiday!

Electronics

This week we have started to look at electronics.

An electrical circuit is a closed loop network of different components and a power supply.

The main properties of an electrical circuit are:

Current: the flow of electrons around a circuit.  It flows from positive to negative, or from a high point to a low point - like water.  Current is measured in Amps (A)

Voltage: the "push" - voltage drives the current around the circuit.  Voltage is measured in volts (V)

Resistance: a material's reluctance to allow current to flow.  All materials can conduct electricity, but they all have different resistance - those who let current flow easily - conductors - and those which make current flow difficult - insulators.  The more resistance a material has, the more voltage is required to make current flow.  Resistance is measured in Ohms (Ω) (Though sometimes people are lazy and write R instead of Ω)

First we looked at resistors as components.  Resistors are important to gain some control over the voltage and current in a circuit as some components are sensitive and too much current could destroy them.  The following picture explains how to read resistor colour code. 

This website can also be used to check, but in real life you may not have this luxury!

Remember your prefixes. You already use Kilo (k) and Milli (m) on a daily basis i.e. kg and mm.  So kilo is a thousand - 1000 or x103 and milli is a thousandth - 0.001 or x10-3.  To help you organise your numbers you may be able to find the engineering mode on your calculator which will put your number in the correct scientific notation to quickly convert to the correct prefix.  This table should help you make conversions.

The first type of circuit we looked at were series circuits - where component are connected end to end.  Be built a circuit with a resistor and an LED in series and measured the voltage over the resistor for different resistor values. 

To measure voltage we had to place one probe on one leg of the resistor and the other probe on the other leg.

We found that as the resistance increased, the voltage dropped over it increased.

We also found, through circuit simulation and using Kirchoff's law, that all the voltages dropped in the circuit must equal the supply voltage i.e. VR + VLED = VS

In this circuit the switch has no resistance as it is a very good conductor when closed.  You can see that the voltage over the resistor (4.1v) and the voltage over the LED (1.9V) equal the supply (6v)

We then used simulation software to measure the current in a series circuit.  The flow of current is the same throughout the loop and therefore current is the same at all points.  Also the larger the resistor, the less current can flow in the circuit.

The relationship between voltage, current and resistance is known as Ohm's law and can be described using the equation V = IR where V = voltage, I = Current and R = Resistance.

Other things that you should be aware of:


Types of Switch
Ohm's Law