[Sketch notes: the gearbox consists of a cast steel/iron casing, an input shaft (from the engine, via a flexible coupling), a large output shaft (to the propeller shaft), and a set of reduction gears (a pinion on the input shaft meshing with a large gear on the output shaft). It may have a thrust bearing, a lubrication system, and a reversing arrangement (for controllable-pitch or reversing gearboxes).]

Important features:

1. Reduction gearing: a pinion (small gear) on the input shaft meshing with a large gear on the output shaft, giving the required speed reduction (e.g. 3:1 to 5:1) so the medium-speed engine (e.g. 500-1000 rpm) drives the propeller at a lower speed (e.g. 100-200 rpm).
2. Flexible coupling: a flexible coupling between the engine and the gearbox input to absorb torsional vibration and misalignment.
3. Thrust bearing: a thrust bearing in the gearbox (or separate) to take the propeller thrust.
4. Lubrication system: a forced lubrication system (oil pump, filter, cooler) to lubricate and cool the gears and bearings.
5. Casing: a rigid, oil-tight casing to support the gears and bearings and to contain the oil.
6. Reversing arrangement (if required): for a fixed-pitch propeller, the gearbox may include a reversing mechanism (e.g. a reversing gear or a clutch) to allow astern running; alternatively a controllable-pitch propeller is used.
7. Access covers and inspection ports for maintenance.

Gears are made of high-strength alloy steel (e.g. case-hardened nickel-chromium or chromium-molybdenum steel) for the pinion and gear, or of cast steel/forged steel. The gear teeth are manufactured by:

1. Forging/casting the gear blank.
2. Machining the teeth by hobbing (a hob cutter generates the tooth profile) or by shaping (a pinion-shaped cutter), or by broaching.
3. Case hardening (carburising) the teeth to give a hard, wear-resistant surface, followed by hardening and tempering.
4. Grinding the teeth to the final profile and finish (for high accuracy and to remove distortion from heat treatment).
5. Inspection (e.g. by gear testing) to confirm the tooth profile and accuracy.

The gears are made of hardened alloy steel with ground teeth to give high strength, wear resistance, and quiet, accurate operation.

As Second Engineer, I would conduct the inspection by:

1. Isolating the gearbox (engine stopped, turning gear engaged, permit-to-work), draining the oil and cleaning the casing.
2. Removing the inspection covers and visually inspecting the gear teeth for wear, pitting, scuffing, cracking, and chipping.
3. Checking the tooth contact pattern (using marking compound) to confirm correct meshing.
4. Checking the bearing clearances and the gear backlash.
5. Checking the oil for metal particles (indicating wear) and the oil filter.
6. Checking the coupling and the shaft alignment.

Instruction to the cadet on the type of damage most likely to be found:

• Pitting: small pits on the tooth flanks from fatigue (overload or poor lubrication).
• Scuffing/welding: scoring of the tooth flanks from boundary lubrication (overload, high temperature, or poor oil).
• Wear: gradual loss of material from the tooth flanks (abrasive wear from contaminated oil).
• Cracking/chipping: cracks or broken teeth from fatigue, overload, or impact.
• Tooth breakage: complete failure of a tooth from severe overload or fatigue.
• Bearing damage: wiping or wear of the gearbox bearings.

The cadet should be taught to look for these and to report any abnormality, and to understand that metal particles in the oil and abnormal noise indicate gear damage.