ITI Mechanic Tractor (AITT Theory) key terms

A physical shield over a moving part plus a device that stops the machine or prevents operation when the shield is removed or opened, protecting the operator from entanglement or crushing.

A procedure in which energy sources are isolated, locked and marked with a warning tag before maintenance begins, so that an engine or implement cannot be started or released while a person is working on it.

A strong frame fitted around the operator station that keeps a crushed zone for the driver if the tractor overturns, and which must be paired with a correctly worn seat belt to be effective.

A sensing arrangement on a tractor with a roll-over protective structure that warns or prevents starting unless the operator is seated and the belt is fastened, reducing ejection risk during overturn.

A cover over the power take-off stub and the attached shaft, usually a master shield plus a rotating telescopic guard, which prevents clothing or limbs from being caught by the turning shaft.

A drawing in which a cutting plane is imagined through the object and the material in front of the plane removed, with the cut surface shown hatched so internal features such as bores and oil passages are clearly visible.

A thick chain line with arrows at the ends that shows where an imaginary cut is taken and in which direction the remaining part is viewed, often labelled with letters matching the section title.

Evenly spaced thin parallel lines used to fill the cut areas of a sectional drawing, with spacing and direction varied between adjacent parts so separate components remain distinguishable.

A symbolic frame on a drawing stating the permitted deviation of form, orientation, location or runout of a feature, together with the datum references against which that deviation is measured.

A theoretically exact reference plane, axis or point on a part from which dimensions and geometric tolerances are measured, established by identified datum features on the drawing.

The classification of mating parts by the gap or interference between shaft and hole, such as clearance, transition and interference fits, used to specify whether a bearing or bush must slide or be pressed on.

A precision measuring instrument using a calibrated screw and thimble to measure outside or inside dimensions to hundredths of a millimetre, read by combining the barrel scale with the thimble division that aligns with the reference line.

A measuring tool with a fixed main scale and a sliding vernier scale that reads outside, inside and depth dimensions, where the vernier graduation coinciding with a main-scale line gives the fractional reading.

A set of thin steel blades of marked thickness used to check small gaps such as valve clearances or spark plug gaps by finding the blade that just fits the space.

A wrench that slips, clicks or signals when a preset twisting moment is reached, used to tighten fasteners to the manufacturer's specification so joints are neither loose nor overstressed.

The specified order and number of stages in which fasteners of a joint, such as a cylinder head, are tightened, usually working from the centre outwards to distribute clamping load evenly and avoid distortion.

The sequence of intake, compression, power and exhaust strokes, each occupying one piston stroke and two crankshaft revolutions, used by most tractor engines to convert fuel energy into rotary work.

The ratio of the cylinder volume above the piston at bottom dead centre to the volume at top dead centre, a key figure that sets an engine's thermal efficiency and its suitability for diesel or petrol fuel.

The small gap left between the valve stem and rocker or tappet when the valve is closed, allowing for thermal expansion and measured with a feeler gauge during adjustment.

The timed delivery of atomised diesel under high pressure into the combustion chamber near the end of compression, where the resulting heat ignites the charge without a spark plug.

The exact crankshaft position at which fuel delivery begins, set by pump-to-engine alignment marks; too early or too late timing causes harsh knocking, poor power or excessive exhaust smoke.

A filter assembly with a settling bowl and drain that removes dirt and condensed water from diesel before it reaches the injection pump, protecting precision plungers and nozzles from wear and corrosion.

A temperature-sensitive valve in the cooling circuit that stays closed while the engine warms and opens at a set temperature to route coolant through the radiator, keeping operating temperature within design limits.

A spring-loaded cap that holds the cooling system above atmospheric pressure, raising the coolant boiling point, and contains a relief valve that vents excess pressure and admits air on cooling.

The path taken by oil from the sump through the pump, filter and galleries to bearings, rocker gear and cylinder walls, then draining back to the sump, with a relief valve limiting maximum pressure.

A sensor that closes or opens an electrical contact according to lubricating oil pressure, illuminating a warning lamp or sounding an alarm when pressure falls below a safe level.

The small movement of the pedal or release bearing before the release mechanism begins to act, which must be set correctly so the clutch engages fully yet still disengages cleanly without slipping.

A fault in which the friction disc fails to transmit full engine torque, causing engine speed to rise without matching vehicle speed, often from worn lining, oil contamination or insufficient release bearing clearance.

A transmission in which all gear pairs stay permanently in mesh and are engaged by sliding dog clutches, allowing helical gears and synchromesh rings to be used for quieter, smoother gear changes.

A cone-and-friction ring device that brings the speed of the gear to be engaged close to that of the shaft before the dog teeth mesh, preventing gear clash during a shift.

A mechanism that can rigidly connect the two axle half shafts so both wheels turn together, used to regain traction when one wheel loses grip, and which must be disengaged before travelling on firm ground or turning.

The arrangement of steering arms that makes the inner front wheel turn through a larger angle than the outer one, so both wheels follow circles about a common centre and tyres scrub as little as possible.

The amount by which the front edges of the front tyres are closer together than the rear edges, measured with a gauge and set to limit uneven tyre wear and wandering of the steering.

Camber is the inward or outward tilt of the wheel in the vertical plane, while caster is the backward or forward tilt of the steering axis; both affect steering effort, self-centring and tyre wear.

The loss of braking force when friction surfaces overheat, caused by a falling coefficient of friction or vaporised fluid, typically after repeated heavy applications or prolonged dragging of the brakes.

Gas trapped in the brake fluid circuit that compresses under pedal force, giving a spongy pedal and reduced braking, removed by bleeding each wheel cylinder in the specified order.

A load-carrying index on a tyre sidewall based on the number of cotton plies the casing is equivalent to, used with inflation pressure to determine the permitted axle load.

The air pressure maintained inside a tyre, which must follow the load and speed recommendations for the tractor; under-inflation causes sidewall flexing and heat, over-inflation reduces the ground contact area.

The difference between the distance a driven wheel would travel unloaded and the distance it actually travels, expressed as a percentage; moderate slip gives best pull, excessive slip wastes fuel and damages soil.

A hydraulic linkage sensing mode that measures the pull between tractor and implement and raises or lowers the arms to hold a steady draft load, useful for ground-engaging implements on varying soil.

A linkage mode in which the lift arms move to and hold a set height regardless of load, used for implements carried above the ground such as sprayers or mowers.

The standard mounting of two lower lift arms and one upper link that attaches an implement to the tractor, controls its depth and tilt, and transfers draft forces into the tractor chassis.

A splined shaft on the tractor that drives implement machinery at a standard rotational speed, engaged by a lever or clutch and protected against overload by a shear bolt or slip clutch.

A spring-loaded valve in the tractor hydraulic circuit that opens to divert oil back to the reservoir when pressure exceeds a set value, protecting the pump, seals and linkages from overload.

A belt-driven generator with a rectifier that converts the engine's rotation into direct current for charging the battery and running electrical loads, controlled to hold a regulated system voltage.

The control device, built into or mounted with the alternator, that limits charging output so system voltage stays near its design value, preventing battery overcharge and electrical component damage.

The density of battery electrolyte compared with water, measured with a hydrometer; it falls as the cell discharges and rises on charging, so it indicates state of charge when corrected for temperature.

A heavy-duty relay that, when the key switch energises its coil, closes contacts to send battery current to the starter motor and usually also throws the drive pinion into mesh with the flywheel ring gear.

A planned list of service tasks such as oil and filter changes, coolant checks and lubrication, performed at stated hourly intervals to prevent breakdowns rather than to repair failures after they occur.

A short visual and manual check before starting work covering fluid levels, tyre condition and pressure, guards, belts, lights, leaks and loose fasteners, so faults are found before they cause damage.

A logical sequence that moves from the simplest and most likely causes to complex ones, using symptoms, tests and elimination to identify a fault without unnecessary dismantling of components.