Transmission, Clutch, Gearbox and Differential
What is tested
Transmission, Clutch, Gearbox and Differential tests power flow from engine to wheels, clutch operation and adjustment, gear ratios, differential action, final drive and PTO.
Core rules
- The clutch transmits torque through friction between the flywheel, friction plate and pressure plate. Correct pedal free play prevents release-bearing wear and clutch slip.
- A constant-mesh gearbox keeps gears in mesh and uses synchros or dog clutches to select ratios; a sliding-mesh gearbox moves gears into mesh. Always stop or match speed before shifting a non-synchronised tractor gearbox.
- The differential allows outer and inner wheels to turn at different speeds in a turn while delivering torque to both. The final drive provides the last speed reduction and torque multiplication.
- PTO speed is normally standardised at 540 or 1000 rpm at rated engine speed; check the guard and never wear loose clothing near it.
- Use the specified transmission/hydraulic oil; wrong oil can cause brake, clutch and gearbox problems in shared-sump tractors.
Worked example
An engine turns at 2000 rpm. The selected gearbox ratio is 4:1 and the final drive is 6:1. Wheel speed = 2000 ÷ (4 × 6) = 83.3 rpm. The reasoning is that each reduction divides speed and multiplies torque, so the combined ratio is the product of the stages. If the tyre rolling circumference is known, ground speed follows from wheel rpm × circumference.
Traps
Riding the clutch, which glazes the plate and burns the release bearing; engaging differential lock on a hard road, which causes wind-up and tyre scrub; and forcing a non-synchronised gear. Another error is checking oil level with the wrong dipstick or using engine oil in a transmission that requires a UTTO or gear oil.