Power Transmission
Domain focus
Power Transmission covers clutch, gearbox, driveshafts, final drive, differential, and automatic transmission operation. The exam tests torque multiplication, speed reduction, and diagnosis of slip, noise, and ratio faults.
Core rules
- A clutch transmits torque through friction and must slip during engagement, then lock without slip under load.
- Manual gearboxes use constant-mesh gears; ratio = driven teeth ÷ driving teeth.
- Overall ratio = gearbox ratio × final-drive ratio. Larger overall ratio gives more torque at the wheels and lower road speed.
- A differential allows inner and outer wheels to turn at different speeds while dividing torque.
- CV joints tolerate steering and suspension movement; worn joints click or vibrate under load.
Worked example
A vehicle has a 3.5:1 gearbox ratio, 4.1:1 final drive, and tyre rolling circumference of 2.0 m. Engine speed is 3,000 rpm. Overall ratio = 3.5 × 4.1 = 14.35:1. Wheel speed = 3,000 ÷ 14.35 = 209 rpm. Road speed = 209 × 2.0 × 60 ÷ 1,000 = 25.1 km/h. The calculation shows why lower gearing improves gradeability but raises engine speed at a given road speed.
Traps
Adding ratios instead of multiplying them, using tyre diameter instead of rolling circumference, and assuming both drive wheels always turn at differential casing speed. A noisy transmission under one condition may be a bearing, not a gear ratio fault.