Last reviewed 1 Oct 2026 · 9 min read
Power flow in a tractor
Engine → clutch → gearbox (transmission) → differential → final drives (reduction gears) → rear wheels, with a branch from the gearbox/clutch to the power take-off (PTO) and another to the hydraulic pump.
The transmission transmits and modifies the engine's power: it connects and disconnects the drive (clutch), changes the speed and torque ratio (gearbox), allows the wheels to turn at different speeds on curves (differential), and gives the final speed reduction (final drive).
Clutch
The clutch connects and disconnects the engine from the transmission, to allow starting, gear changing and stopping without stalling the engine.
Principle
A friction clutch has a flywheel, a clutch disc (friction plate) on the transmission input shaft, and a pressure plate pressed by springs. When the pedal is released, the springs clamp the disc between the flywheel and the pressure plate, and the drive is transmitted by friction. When the pedal is pressed, the release (throw-out) bearing pushes the levers and pulls the plate back, freeing the disc.
Torque capacity
where = coefficient of friction (about 0.25–0.4), = spring clamping force (N), = mean friction radius (m), = number of friction surfaces.
A single-plate dry clutch (two friction faces, ) has a mean radius of 0.12 m, a clamping force of 6000 N and .
— adequate for an engine delivering up to about 300 N·m with a safety factor of 1.4.
Types
| Type | Description |
|---|---|
| Single-plate dry clutch | The simplest; common in small and medium tractors |
| Dual (double) clutch | Two clutch plates: the first for the transmission, the second for the PTO (for independent PTO operation, two-stage pedal) |
| Multi-plate wet clutch | Several discs running in oil; smoother, cooler, longer life; used for PTO and hydraulic engagement |
| Live PTO (independent PTO) | The PTO has its own clutch so it keeps rotating when the tractor clutch is pressed — essential for PTO-driven implements (mowers, balers) |
| Centrifugal and electromagnetic | Special |
Clutch pedal free play (about 15–25 mm) must be maintained — too little causes slip and wear; too much causes incomplete disengagement and hard gear change. Slip is shown by a burning smell and loss of drawbar power.
Gearbox (transmission)
The gearbox gives several speed ratios, so the engine can work at its best speed while the tractor moves at different speeds and pulls; it also provides reverse.
Types
| Type | Features |
|---|---|
| Sliding-mesh | Gears slide along the shaft to mesh; simple, gear change needs the tractor to stop (or double-declutching); noisy (spur gears); older and small tractors |
| Constant-mesh | All the gears always in mesh; selection by dog clutches (collars); helical gears (quiet, strong); a smoother change |
| Synchromesh | Constant-mesh with synchronisers (cone clutches) that match the speed of the gear and the shaft before engaging — easy shifting while moving; popular in many tractors |
| Collar-shift | A form of constant mesh with sliding collar |
| Power-shift (full) | Hydraulic wet clutches change gears under load without clutching; large tractors |
| Hydrostatic (CVT) | A pump-and-motor system gives infinitely variable speed (garden and compact tractors) |
Gear and ranges: tractors usually have 8–12 forward gears and 2–4 reverse (with high–low range and creeper gears), covering speeds from 1.5 km/h to about 30 km/h — low speeds for tillage and sowing and high speeds for transport.
Gear ratio and speed
where , , are the reductions in the gearbox, differential (bevel pinion/crown) and final drive.
The theoretical forward speed is (m/s) with = rolling radius of the driving wheel (m), = engine rpm and the overall ratio; an actual speed is lower because of wheel slip:
A tractor has an engine speed of 2000 rpm, an overall reduction of 62 : 1 in a given gear and a rear-wheel rolling radius of 0.72 m.
. If the measured speed under load is 7.9 km/h, the slip is — acceptable (the optimum is about 8–15 % on firm soil and 10–15 % on tilled).