Gearbox and Transmission Systems Questions and Answers

Answer: A

A petrol or diesel engine develops no torque below its idling speed and useful torque over perhaps a three to one speed range, whereas a vehicle must operate from rest to its maximum speed, a range of twenty to one or more. The gearbox bridges that gap by trading speed for torque. An electric motor develops full torque from zero and needs no such device, which is why electric vehicles use a single reduction ratio. Reverse rotation is a secondary requirement met by an idler gear rather than the reason the gearbox exists.

Answer: B

Neglecting losses, power is conserved, so reducing the speed by a factor multiplies the torque by the same factor. A ratio of three to one gives three times the torque at one third the speed. Real transmissions lose a few per cent to friction. The order in which the effects act is what decides the outcome, so it is worth establishing first. Naming what changes and what stays the same is the reliable route through questions of this shape.

Answer: C

The earliest gearboxes slid straight cut gears along splined shafts into engagement, which required the driver to match the speeds by double declutching. Any mismatch clashed the teeth and damaged them. Constant mesh and synchromesh designs superseded it. Checking which component actually carries the load settles most questions of this kind at once. Working the distinction through on a single concrete case fixes it more firmly than any rule of thumb.

Answer: D

Double declutching releases the clutch, selects neutral, engages the clutch and blips the throttle to bring the layshaft to the right speed, then releases the clutch again to select the gear. It is still taught for heavy vehicles with unsynchronised boxes. The skill disappeared from ordinary motoring once synchromesh became universal. Naming what changes and what stays the same is the reliable route through questions of this shape.

Answer: A

Because the gear pairs remain permanently meshed and selection is made by a dog clutch, the teeth never have to slide into engagement and can be helical for quiet running. Sliding mesh required straight cut teeth. This is the principal reason constant mesh superseded the earlier arrangement, quite apart from the ease of selection. Working the distinction through on a single concrete case fixes it more firmly than any rule of thumb.

Answer: B

The gears run free on the output shaft until a splined sleeve slides across and engages dog teeth on the gear, locking it to the shaft. The synchroniser matches the speeds first but transmits no drive itself. The selector fork moves the sleeve but does not carry torque. Tracing the requirement back to the physical constraint that produces it makes the answer follow. Checking which component actually carries the load settles most questions of this kind at once.

Answer: C

A conical friction surface between the gear and the synchroniser ring accelerates or decelerates the gear until it matches the sleeve, and only then does a blocking ring allow the dog teeth to engage. The whole operation takes a fraction of a second. It is the reason a modern gearbox requires no skill to use. The shared behaviour explains why these belong together and the differences explain why both exist. Tracing the requirement back to the physical constraint that produces it makes the answer follow.

Answer: D

The baulk ring is indexed so that its dog teeth are misaligned with those of the sleeve while a speed difference persists, physically blocking the sleeve until the cone has done its work. Once synchronised the ring rotates slightly and the teeth align. Forcing the lever defeats the mechanism and causes the characteristic crunch. Checking which component actually carries the load settles most questions of this kind at once.

Answer: A

Brass rings with fine threads cut into the cone break through the oil film and grip quickly, and coated or carbon lined steel rings are used where higher capacity is needed. A polished low friction surface would be exactly wrong, since the ring must grip. Multi cone synchronisers are used for the lower gears where the inertia to be overcome is greatest. Tracing the requirement back to the physical constraint that produces it makes the answer follow.

Answer: B

Second is the gear most often selected on a downchange while the vehicle is still moving quickly, so its synchroniser does the most work. First is usually selected from rest, when little synchronisation is needed. A crunch into second on a downchange is the classic first symptom of a worn gearbox. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision. Most errors here come from carrying across a result from a neighbouring case where it does not hold.