ITI Mechanic Tractor (AITT Theory) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Fire triangle

Three conditions needed for combustion: fuel, oxygen and heat. Removing any one extinguishes fire. Extinguisher choice depends on which fuel class is burning.

Acetylene regulator limit

Acetylene delivery pressure must not exceed about 1.0 bar (15 psi). Higher pressure risks decomposition and explosion, and copper fittings must be avoided in acetylene service.

First-angle projection

The object is placed between the observer and the projection plane. The view drawn to the right of the front view therefore shows the left side of the object.

Surface roughness Ra

Arithmetic mean deviation of the profile from its mean line, expressed in micrometres. A drawing tick with a value sets the maximum permitted roughness.

Case hardening

Adds a hard, wear-resistant surface layer to low-carbon steel while the core stays tough. Used for gears, pins and shafts that need wear resistance plus shock strength.

Hacksaw blade pitch

Teeth per inch. Use 18 for thick soft sections, 24 for general steel, and 32 for thin tube and sheet so teeth do not snag or break.

Hole-basis fit system

The hole is made at basic size with an H tolerance. Shaft letters below h give clearance fits, above h give interference fits, and adjacent letters give transition fits.

Tap drill size rule

For metric coarse threads, subtract the pitch from the nominal diameter. An M10 × 1.5 thread therefore needs an 8.5 mm tap drill.

Vernier least count

Smallest main-scale division divided by the number of vernier divisions. A 0.5 mm main scale with 50 vernier divisions gives a least count of 0.01 mm.

Micrometer least count

Screw pitch divided by the number of thimble divisions. A 0.5 mm pitch with 50 divisions on the thimble gives a least count of 0.01 mm.

Compression ratio

Swept volume plus clearance volume, divided by clearance volume. Diesel tractor engines typically run 16:1 to 22:1, which gives ignition by compression heat.

Four-stroke cycle

Induction, compression, power and exhaust strokes, completed in two crankshaft revolutions. A two-stroke engine completes the same sequence in one revolution.

Injector opening pressure

Pressure at which the injector nozzle lifts and sprays fuel. Too low gives poor atomisation, black smoke, rough running and hard starting.

Thermostat function

Valve that stays closed until coolant reaches about 80–90 °C, then opens to the radiator. A stuck-open thermostat causes slow warm-up and poor heater output.

Clutch pedal free play

Small gap that ensures the release bearing is unloaded when the clutch is engaged. Too little free play causes slip; too much causes difficult gear engagement.

Synchromesh

Cone clutch that equalises shaft speeds before the dog clutch engages. Wrong or low gear oil causes baulking and crunching when changing gear.

Differential function

Lets the outer wheel turn faster than the inner wheel in a turn while still splitting torque. A locked differential causes tyre scrub and heavy steering.

Ackermann steering geometry

Arrangement giving the inner wheel a larger steer angle than the outer, so both wheels turn about a common centre and tyre scrub is reduced.

Toe-in

Small difference in track measured at the front and rear of the tyres. Correct toe-in makes wheels parallel when rolling; wrong toe causes rapid shoulder wear.

Tyre size 13.6-38

Section width 13.6 inches, bias construction, rim diameter 38 inches. An R in place of the dash, as in 18.4R38, means radial construction.

Draft control

Linkage mode that senses pulling load and adjusts working depth automatically. Position control holds a fixed depth regardless of load, as needed for mowers.

Hydraulic relief valve

Limits maximum circuit pressure to protect pump and hoses. If it sticks closed, pressure rises until a hose bursts or the pump is damaged.

Battery cold cranking amps

Current a battery can deliver at minus 18 °C for 30 seconds while keeping a usable terminal voltage. It indicates starting capability in cold weather.

Alternator charging voltage

Regulated output measured at the battery with the engine running, normally about 13.8 to 14.4 volts. A reading below about 13 volts indicates a charging fault.

Ohm's law

Voltage equals current multiplied by resistance, and power equals voltage multiplied by current. A 12 V load drawing 60 W therefore takes 5 A.

Black exhaust smoke

Sign of incomplete combustion from over-fuelling or air restriction. Check the air filter, intake hoses and injection pump setting before dismantling the engine.

Blue exhaust smoke

Indicates engine oil is being burned. Common causes are worn piston rings, worn valve guides or seals, and oil entering the intake through a breather.

Compression test

Measures cylinder pressure to assess ring and valve condition. If a wet test with oil raises compression, worn rings are the likely cause rather than valves.

PTO standard speeds

Independent power take-off speeds are standardised at 540 rpm and 1000 rpm at rated engine speed. Shaft guards and the master shield must always be fitted.

Battery open-circuit voltage

A fully charged 12 V lead-acid battery reads about 12.6 volts at rest. About 12.0 volts indicates roughly half charge and needs recharging.

Tolerance versus allowance

Tolerance is the difference between upper and lower limits of one part. Allowance is the intentional difference between the shaft and hole at basic size.

Torque formula

Torque equals force multiplied by perpendicular distance from the pivot. A 40 N force at 0.5 m produces 20 newton metres of turning effort.