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

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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 Nowadays, these older type flasher units have been replaced by electronic version.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:

(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


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.


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'


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.


In the SI, there are 7 base units of measurement. The ones we come across in the phase 2 course are:

Other units of measurement such as those for torque, speed, power etc. are all based on SI units of measurement, so they are said to be 'base derived' units of measurement.


James Watt

The Watt is the measurement of power, and gets its name from James Watt. This is the modern metric way of measuring power.

The old way of doing it was to measure the power of something relative to the power of a horse:

  • HP: Throughout the United Kingdom, and in many other parts of the world - horsepower
  • CV: In France and other areas of the world under French influence, 'cheval vapeur', or 'steam horse'
  • PS: Often referred to as 'the metric horsepower', this is a German version of measuring power and means 'the strength of a horse'

If a force of 1 Newton moves an object through a distance of 1 metre in 1 second, then 1 Watt of power is said to have been expended.


A shunt resistor is a usually small value resistor placed in series with a load.

You will come across shunt resistors in DMM ammeters, or in electronic chips that need to monitor how much electricity is flowing in a particular circuit.

Shunt resistors are normally small value resistors, say 30mΩ (30 milliohms or 0.03 Ohms).

The reason they have such small values is that we don't want to interfere with the circuits in which we place them. What we do is we measure the voltage drop across the small resistance. Then, using Ohm's Law, we know the resistance and we've measured the voltage drop, therefore we can calculate the current flowing through the resistor:


This allows us to calculate the current flowing through a circuit, without having to pass that current through an ammeter - we just measure the voltage drop across the resistor and divide that by the value of that resistor (in this case, 30 milliohms, 0.03 ohm).


Electromagnetic compatibility (EMC) refers to the unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage in operational equipment.

EMC is the correct operation of different  electronic equipment in a common electromagnetic environment.

You can read more about EMC on Wikipedia


Provider of microcontroller chips. They are now part of the Atmel group and Microchip Technology Inc.

You can read more about these companies here.


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