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Glossary of terms you will come across during your Phase 2 training.



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Assorted relaease bearings

Also known as the 'thrust bearing'.

This is the component that allows us to exert a force on the centre of the pressure plate, thereby releasing the clutch from the flywheel. Doing this allows us to change gear easily, or to slip the clutch in order to ensure the gradual take up of drive.

The reason it is called a 'release bearing' is because when we press on it, the bearing acts on the centre of the pressure plate and causes the clutch to be released from the flywheel - we are disengaging the clutch.

In operation, the release bearing sustains large thrust (or axial) loads in operation, and this explains why the component is sometimes referred to as the thrust bearing.

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Do not confuse 'air gap' with 'spark gap', they are two entirely different things.

Air gap between a flywheel and an ignition coil.Air gap normally refers to the clearance between a revolving object and a stationary coil or pick-up sensor.

In the diagram on the right, we can see a magneto ignition coil and a flywheel. The correct air gap between them is 0.4 mm, with a tolerance of + or - 0.2 mm. This means that an air gap of between 0.2 mm and 0.6 mm is acceptable.

In magneto ignition systems, the air gap is the clearance between the laminated iron core of the ignition coil and the revolving flywheel which has a permanent magnet mounted on it.

In this case, the ignition coil is able to produce a HT spark because the magnetic lines of force from the permanent magnet on the flywheel move rapidly across the coil windings. This induces a current in the primary windings. When we suddenly open the circuit in the primary windings, the magnetic field associated with the current in the primary windings collapses, inducing the HT spark in the secondary windings.

The air gap between the coil and the magnet on the flywheel is vital if we are to produce a good quality HT spark.

If the air gap is too small, the flywheel can physically contact and damage the laminated iron core of the ignition coil. This is a common fault in small plant petrol engines. Always check this gap during service.

If the air gap is too big, the magnetic lines of force (magnetic flux) tend to splay out into space, and do not act on the windings of the ignition coil, thus causing the coil to produce a weak spark.

Other components where air gaps are essential are items such as inductive ABS wheel sensors. If the air gap is too large, the sensor is incapable of producing a signal of sufficient amplitude, and this will cause the control system to log a fault code and to display a warning lamp.

See also: 'spark gap'

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Centrifugal Force, Centripetal Force

One of Newton's laws of motion tells us that when a body is moving, it wants to do so in a straight line. If a body is moving on a curved path (say in a circle), there must be some force acting on the body to make it move in a curved path. This force is called centripetal force.

Newton also tells us that for every action, there is an equal and opposite reaction, and the opposite to centripedal force is centrifugal force. This force acts in the opposite direction to centripedal, it acts radially outwards.

Read more here about Newton's three laws of motion, and how they affect small plant vibrating machinery.

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(You can read about float chamber type carburetors here)

Diaphragm CarburettorThese type of carburettors are used on machinery that moves about and operates at varied angles. For example, concrete saws and chainsaws use diaphragm carburetors.

  • Fuel is drawn into the carburettor because the pule tube from the engine crankcase causes the diaphragm to move rapidly p and down.
    The needle controls the entry of fuel to the unit.
  • When the area above the diaphragm, and the fuel channel is full of fuel, the needle is forced closed, preventing the diaphragm drawing any more fuel in from the tank.
  • When the engine is cranked by the pull chord, air is induced through the inlet. This air must speed up to get through the venturi. When it speeds up, there is a corresponding drop in pressure.
  • The small drilling at the base of the carburettor ensures there is always atmospheric pressure below the diaphragm.
  • Now we have a case of less than atmospheric pressure in the venturi, and atmospheric pressure below the diaphragm.
  • This forces fuel to issue out the orifice.
  • As the fuel empties from above the diaphragm, this unseats the needle valve, allowing more fuel in above the diaphragm.

Below, we can see the diaphragm carburetor off the Stihl concrete saw disassembled:

Typical diaphragm carburetor disassembled.

Click the image for a larger diagram:

Exploded view of a diaphragm carburetor, click to download larger image

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This is content over and above what is on the Phase 2 syllabus for CP Fitters.
It is sometimes posted on the site in order to help apprentices who may wish to find out a little more about certain topics or equipment that is covered during Phase 2.
Additional content will not appear in any Phase 2 examinations.
You need only view additional content if you want to broaden or deepen your knowledge about specific topics or pieces of equipment.

Example of Additional Content

During the Electrical Module, there is a learning unit called 'Lighting Circuits'. During this unit, you will build various wiring circuits. One such circuit is an indicator (turn signal) circuit.
This type of circuit originally used what was known as a 'hot-wire' flasher unit. Nowadays, these older type flasher units have been replaced by electronic version.
If you would like to learn more about these more modern units, there is a page of additional information with an overview, links to data sheets and a video of the operation of a typical electronic flasher unit:
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In DC electricity, a single source of that energy is known as a cell.

When we join cells together in series, parallel or series parallel, we then have a battery of cells, or just simply a battery.

When deciding to join single cells in series or in parallel, we consider what we are trying to achieve:

  • If we want to increase the available capacity of a cell, we add in more positive and negative plates in parallel with each other into the cell.
    Two 12 volt batteries connected together in parallel to give an overall 12 volts, but also adding the capacity of one battery to the otherOne example of this arrangement would be where we are trying to start a vehicle with a flat battery by 'jump starting' the vehicle. What we do here is that we get a vehicle with a charged battery and connect the two vehicle batteries together in parallel (positive to positive, negative to negative). By doing this, we don't change the overall voltage of either vehicle, we are simply adding the capacity of the 'good' battery to that of the 'flat' battery, and using the additional capacity to start the second engine.
    Another example of this arrangement
    is where we want to increase the capacity of a 12V lead/acid battery. Instead of having just one positive and one negative plate, we commonly put 10 or more negative and positive plates in parallel with each other into the cell. This has no effect on the cell voltage, but does significantly increase cell capacity.
  • If we want to increase the available voltage, we join cells in series. The overall capacity of each cell joined in series remains the same.
    Six individual two volt cells arranged in series with each other to give an overall 12 voltsAn example of this series arrangement would be in a 6-cell lead/acid battery. Each individual cell has a nominal voltage of 2V, and when they are joined together in series, these 6 cells in the battery give an overall nominal voltage of 12V.
    Another example of this series arrangement would be connecting four 6 volt batteries together in series, giving us an overall voltage of 24 volts.
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To induce something is to cause something to happen without any physical contact.

For example, in an ignition coil, we induce a HT spark. There is no physical contact between the various components in the coil, yet when magnetic lines of force around the primary winding suddenly collapse (because we open the primary circuit), this induces a very high voltage in the secondary windings.

Another example would be an alternator. By spinning a current carrying rotor in close proximity to a stator, we can induce electricity in that stator, and thus supply all electrical loads in a plant vehicle. Once again, there is no physical contact between the rotor and the stator.

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Typical crankshaft speed position sensorAn inductive sensor, for example, a crankshaft position sensor, is one that uses an induced electrical current to signal the ECU. For example, the crankshaft speed/position sensor uses induction to produce a sine wave that is sent back to the ECU, which then analyses this data and determines the speed and position of the crankshaft at any given instant.

In order to induce electricity (generate electricity), we need 3 things:

  1. A conductor
  2. Magnetism
  3. Relative movement between the two

Crankshaft with reluctor wheelAn inductive sensor normally has a permanent magnet, surrounded by a coil of wire, wound into many turns around, but not touching the magnet. Then, some sort of toothed wheel (known as a reluctor wheel) is mounted on to the shaft whose speed and position we wish to sense. The toothed wheel is made of ferrous material, and as the wheel moves past the sensor, it 'traps' and 'releases' the flux from the permanent magnet in the sensor. This movement of the magnetic field generates a sine wave across the coil of wire, and this signal is fed back to the ECU.

Cutaway view of an inductive sensor and reluctor wheel

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Reluctor wheel on its ownReluctor wheel fitted to a crankshaft

Inductive crankshaft sensorAn inductive crankshaft sensor normally has a permanent magnet, surrounded by a coil of wire, wound into many turns around, but not touching the magnet.

Then, a toothed wheel known as a reluctor wheel is mounted on to the crankshaft whose speed and position we wish to sense.

The toothed wheel (the reluctor) is made of ferrous material, and as the wheel moves past the sensor, it 'traps' and 'releases' the flux from the permanent magnet in the sensor.

This movement of the magnetic field generates a sine wave across the coil of wire, and this signal is fed back to the ECU.

As this is an analogue signal, it cannot be processed by the ECU, so it must first be converted from a sine wave to a square wave. Once this happens, the ECU can now deduce the rotational speed of the crankshaft (because of the 'missing tooth' on the reluctor wheel), and the position of the crankshaft.

Note that while the ECU can know the crankshaft is, say 10o BTDC, it cannot deduce what stroke any individual piston is on. In order to do that, we fit a camshaft sensor


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Two different contexts here:

  1. Induction week on the CP Fitters' course. This refers to the first week on your phase 2 course where all the final administration details (like pay, bank details, next-of-kin details and tool kit inspections) are sorted out. During this week, a lot of the health and safety and manual handling training take place.
  2. The ability to produce voltage or current, without physical contact.

For example, in an alternator, passing current through the rotor and spinning it in close proximity to the stator induces, or produces both a voltage across and a current through that stator.

Note: there is no physical contact between the rotor and the stator, they are just very close to each other.

Once this happens, we use the voltage to push the current out to all vehicle electrical loads in order to operate them.

Of course, we can't do this directly, first we must rectify the produced AC to DC and then we must take steps to limit the maximum voltage to around 14.7V.


Inductive sensors like the crankshaft sensor work in a similar way.

Inductive crankshaft sensorWe have a coil of wire wound around a permanent magnet. There is no physical contact between the two, but when the magnetic lines of force from the permanent magnet are made to move, they interact with the windings to produce a voltage across either end of the coil of wire, and a small amount of electricity to flow through the coil of wire.

We make the magnetic lines of force move by spinning a reluctor wheel (mounted on the crankshaft) in close proximity to the permanent magnet.

The reluctor wheel and the permanent magnet are only separated by a very small air gap. As the reluctor wheel spins (because it is mounted on the crankshaft), the soft iron material in the reluctor wheel 'traps' the magnetic lines of force whenever a tooth lines up with the magnet, and 'releases' the magnetic lines of force whenever the tooth moves away and a gap is lined up with the magnet.

Reluctor wheel mounted on a crankshaftAs the crankshaft, and the reluctor wheel which is mounted on it, spins past the sensor, it causes the sensor to produce a sinusoidal wave which is sent back to the engine ECU to be processed. From this electrical information, the ECU can deduce the rotational position of the crankshaft, and the rotational speed of the crankshaft

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