ITI Mechanic Tractor (AITT Theory) cheat sheet

Safety colour code: red = fire-fighting equipment and prohibition; yellow = caution and physical hazards; blue = mandatory action; green = safe condition, first aid, exits.

Fire triangle legs are fuel, oxygen and heat; extinguisher choice follows fuel class - Class A water/foam, Class B foam/CO2/dry powder, Class C dry powder/CO2, electrical CO2 or dry powder only.

PPE order for tractor workshop: safety footwear, eye protection, hearing protection above 85 dB(A) continuous exposure, gloves matched to chemical or mechanical risk.

Jack and axle-stand rule: never work under a load held only by a hydraulic jack; cribbing or rated stands carry the load after lifting.

Oxy-acetylene set: acetylene regulator maximum 1.0 bar (15 psi) delivery, no copper fittings, flashback arresters at both torch and regulator ends.

Lifting limit guideline: manual single-person lift near 25 kg at waist height; team lift or mechanical aid above this, load close to the body.

Battery hazard: lead-acid electrolyte is sulphuric acid; charging releases hydrogen, so no flame or spark and ventilate the area.

Environmental rule: drain oil, coolant and hydraulic fluid into labelled containers; used oil is recyclable, never poured to ground or drain.

First-angle projection: the object sits between observer and plane, so the view on the right of the front view is the left-hand side view. Third-angle reverses this.

Scale notation 1:5 means drawing dimension = actual dimension ÷ 5; 5:1 means the drawing is five times full size.

Line convention: continuous thick = visible outline; short-dash = hidden edge; chain thin = centre line and pitch circle; continuous thin = dimension and projection lines.

Geometric tolerance symbols: flatness is a parallelogram, perpendicularity a T-square symbol, position a crosshair circle, concentricity/coaxiality two concentric circles.

Surface roughness Ra is the arithmetic mean deviation in micrometres; a tick with a number sets the maximum Ra, a tick with a circle means no machining allowed.

Section view: hatching at 45° indicates cut material; adjacent parts hatch in opposite directions; a half-section shows one half in section.

Dimensioning rule: never repeat a dimension; each feature is dimensioned once on the view that shows it best, with the smallest number of dimensions.

Thread callout M20 × 2.5 means metric thread, 20 mm nominal major diameter, 2.5 mm pitch; a fine series would show a smaller pitch.

Steel carbon content: low carbon (mild) up to about 0.3% C, medium 0.3–0.6%, high carbon 0.6–1.4%; hardness and wear resistance rise with carbon.

Cast iron is brittle in tension but strong in compression and damps vibration; grey cast iron is used for engine blocks and housings.

Case hardening adds a hard, wear-resistant skin to a tough low-carbon core; used for gears, pins and shafts needing wear plus shock resistance.

File grade rule: rough, bastard, second cut, smooth - coarser grades for stock removal, smooth for finishing; file only on the forward stroke.

Hacksaw blade selection: 18 teeth per inch for thick soft sections, 24 for general steel, 32 for thin tube and sheet to avoid tooth snagging.

Spanner rule: use the ring end for breaking tight fasteners, the open end for speed; never extend a spanner with a pipe or add a cheater bar.

Torque wrench is a measuring instrument: return it to zero after use, do not use it to loosen fasteners, and recalibrate at defined intervals.

Chisel and punch safety: mushroomed heads must be ground off; hold with a guard, and wear eye protection when striking.

Fits: clearance fit always has a gap, interference fit always has overlap, transition fit may give either depending on actual sizes.

Hole-basis system: the hole is the basic size with H tolerance; shaft letters below h give clearance, above h give interference (for example H7/g6 clearance, H7/p6 interference).

Tolerance is the difference between upper and lower limits; allowance is the intentional difference between shaft and hole at the basic size.

Tap drill size for metric coarse thread ≈ nominal diameter − pitch; for M10 × 1.5 use 8.5 mm.

Thread identification: measure pitch over several threads with a thread gauge; BSP is denoted by nominal bore, not by measured outside diameter.

Scraping and lapping produce flatness and fit by removing fine material; lapping uses abrasive paste between tool and work for high accuracy and finish.

Doweling and pinning locate mating parts before bolting; reamed holes give closer location than drilled holes alone.

Fits on shafts: keyed and splined joints transmit torque; a key fails in shear, so key length and width must match the torque load.

Vernier least count = smallest main-scale division ÷ number of vernier divisions; a 0.5 mm main scale with 50 vernier divisions gives 0.01 mm.

Micrometer least count = pitch ÷ number of thimble divisions; a 0.5 mm pitch with 50 divisions gives 0.01 mm.

Read a vernier by adding the main-scale value before the zero to the vernier division that coincides with a main-scale line.

Zero error: if the jaws close and the vernier zero is to the right of the main zero, subtract that error from every reading; to the left, add it.

Vernier calliper measures outside, inside and depth; a micrometer is for outside diameters only and needs a slip-gauge check for accuracy.

Dial gauge (plunger type) measures runout, flatness and end play; it must be mounted rigidly and preloaded about one-third of its travel.

Torque = force × perpendicular distance; a 40 N force at 0.5 m gives 20 N·m, and torque is the basis of fastener tightening.

Workshop practice: mark out from a datum, centre-punch before drilling, and deburr holes; drill speed falls as drill diameter and material hardness rise.

Four-stroke cycle order: induction, compression, power (expansion), exhaust - two crankshaft revolutions per cycle; a two-stroke completes it in one revolution.

Compression ratio = (swept volume + clearance volume) ÷ clearance volume; diesel tractors typically run 16:1 to 22:1, petrol engines 8:1 to 12:1.

Diesel ignition is by compression heat; petrol needs a spark. Diesel therefore has no carburettor or spark plug and is safer with fuel vapour.

Torque is the turning effort; power = torque × angular speed, so at constant power torque falls as engine speed rises.

Volumetric efficiency falls as speed rises because of flow losses; turbocharging raises the mass of air trapped per cycle and therefore power.

Valve timing: inlet opens before TDC and closes after BDC to use inertia; overlap is the period both valves are open near TDC.

Firing order spreads power strokes; a four-cylinder engine commonly uses 1-3-4-2 to balance loads on the crankshaft.

Knock (detonation) is uncontrolled rapid burning; causes include over-advanced timing, low cetane fuel, overheating and carbon deposits.

Diesel fuel system order: tank, pre-filter, lift pump, fine filter, injection pump, injectors, return line; air in the low-pressure side causes hard starting and misfire.

Injector opening pressure is set by shims/spring; a low pop pressure gives poor atomisation, smoke and hard starting.

Bleeding is essential after filter change or tank run-dry: bleed low-pressure side first, then high-pressure lines by cranking with injectors cracked.

Cooling: thermostat opens near 80–90 °C; a stuck-open thermostat causes slow warm-up and poor heater output, a stuck-closed one causes overheating.

Coolant is water plus antifreeze/corrosion inhibitor; a 50/50 mix gives frost protection near −35 °C and raises boiling point.

Lubrication: pump draws from sump, passes oil through the filter (full-flow) to main bearings, big ends, camshaft and rockers; oil pressure warning means stop the engine.

Oil grade SAE 15W-40 means a multigrade: 15 viscosity grade cold, 40 hot; API or ACEA service category must meet the manufacturer's requirement.

Overheating causes: low coolant, slipping fan belt, blocked radiator fins, failed thermostat, late injection timing, or a blown head gasket.

Clutch function: transmits torque by friction between flywheel, disc and pressure plate; free play at the pedal is needed so the release bearing is unloaded when engaged.

Clutch slip symptoms: engine speed rises without ground speed, burning smell; causes include worn disc, weak diaphragm spring, oil contamination, no free play.

Clutch drag (not releasing) causes difficult gear engagement and creeping; causes include air in the hydraulic release circuit or a warped disc.

Constant-mesh gearbox: gears stay in mesh, dog clutches or synchromesh rings lock the selected gear to the shaft.

Synchromesh uses a cone clutch to equalise shaft speeds before the dog engages; wrong or low oil grade causes baulking and gear crunch.

Final drive ratio = ring gear teeth ÷ pinion teeth; a lower numerical ratio gives higher road speed for the same engine speed.

Differential allows the outer wheel to turn faster in a turn while splitting torque; a locked or welded differential causes tyre scrub and steering fight.

Power take-off (PTO) speeds are standardised at 540 rpm and 1000 rpm at rated engine speed; the PTO clutch and shaft guard must be intact.

Ackermann steering geometry gives the inner wheel a larger steer angle so both wheels turn about a common centre, reducing tyre scrub.

Toe-in is set so the wheels are parallel when rolling; incorrect toe causes rapid shoulder wear and wander.

Caster gives self-centring and straight-line stability; too little caster causes wander, too much causes heavy steering.

Hydraulic steering: check fluid level, pump belt and relief valve; hard steering at idle with good fluid points to a weak pump or a stuck relief.

Brake bleeding order for a tractor: start at the wheel cylinder furthest from the master cylinder, keep the reservoir topped up.

Air in the brake circuit gives a spongy pedal; a low pedal that pumps up indicates air, a pedal that sinks indicates a leak or worn seals.

Tyre marking 13.6-38 means section width 13.6 inches, bias construction, rim diameter 38 inches; the R suffix (for example 18.4R38) means radial.

Tyre pressure is set for load and soil; over-inflation reduces traction and increases compaction, under-inflation damages sidewalls and beads.

Hydraulic power = pressure × flow; a pump delivering 20 L/min at 150 bar gives about 5 kW of hydraulic power.

Relief valve protects the circuit by limiting maximum pressure; a stuck-closed relief causes hose burst or pump damage.

Three-point linkage categories I, II and III differ in pin diameter and spacing; category must match the implement to avoid overload.

Draft control senses pulling load and adjusts depth automatically; position control holds a fixed depth regardless of load - used for mowers and sprayers.

Lower link sensing versus top link sensing: the sensing point determines how the draft signal reaches the control valve.

Hydraulic oil cleanliness matters: contamination causes spool sticking, valve erosion and pump wear; filter and breather maintenance protects the system.

Quick-coupler rule: relieve pressure before connecting or disconnecting; trapped pressure makes coupling impossible and can inject fluid.

Fluid injection hazard: never search for a leak with a bare hand; pinhole leaks at high pressure penetrate skin and require immediate medical treatment.

Ohm's law: V = I × R; power = V × I. A 12 V system drawing 60 W uses 5 A.

Battery capacity is rated in ampere-hours at a 20-hour rate; cold cranking amps (CCA) is the current the battery can deliver at −18 °C.

Lead-acid cell gives about 2.1 V; six cells give a nominal 12 V. Open-circuit voltage near 12.6 V means fully charged, near 12.0 V about half charged.

Charging system: alternator output is regulated near 13.8–14.4 V; a reading below 13 V with the engine running indicates a charging fault.

Starting circuit: battery, heavy cable, solenoid, starter motor, earth return. Voltage drop above about 0.5 V on a cable indicates a bad connection.

Alternator has a built-in rectifier and regulator; never disconnect the battery while running, and disconnect the battery before arc welding on the tractor.

Diesel cold start uses glow plugs, an intake heater or excess-fuel device; long cranking without fuel causes battery drain and starter overheating.

Fuse and relay rule: replace a blown fuse with the same rating; a fuse that blows repeatedly indicates a short circuit to earth.

Service intervals follow engine hours, not calendar time, for oil and filters; daily checks cover oil level, coolant, tyre pressure and visible leaks.

Air filter restriction causes black smoke, power loss and high fuel consumption; clean or replace the element and check the dust bowl and intake hoses.

Black smoke = incomplete combustion (over-fuelling, air restriction); blue smoke = oil burning (worn rings or valve guides); white smoke = unburnt fuel or coolant.

Troubleshooting rule: verify the simplest causes first - fuel level, air in fuel, blocked filter, battery charge - before dismantling.

Preventive inspection: check brake and clutch adjustment, wheel nut torque, hydraulic hose condition, and PTO guard before each season.

A compression test on a diesel shows ring and valve condition; a wet test raising compression points to worn rings rather than valves.

Record keeping: hours, oil analysis and repair history support warranty claims and predict failures; oil analysis detects coolant or metal contamination.

Storage preparation: clean the machine, full fuel tank to limit condensation, charge the battery, block the clutch pedal and protect tyres from sunlight.