Doctor Diesel

Manual gearboxes

Image converted using ifftoany

Sixty years back, most cars had simple three-speed manual change gearboxes, and you needed a fair bit of skill to change gear without sickening crunches of clashing gears.

Today’s five and six-speed gearboxes have synchromesh on all gears, and anyone can quickly master them, but the basic principles and internal layout are much the same now as they were back then.

Any manual gearbox basically contains three shafts; usually a short input shaft connected to the clutch that drives a countershaft, or layshaft, through a pair of gears, on which are fixed a number of gearwheels, or pinions, of different sizes.

In parallel with the layshaft is the output shaft, on which are clusters of pinions, of different diameter and tooth pitch, each paired with a layshaft pinion to achieve the different gearing ratios that engines need to deliver torque and power at the wheels over a wide speed range.

The five or six pairs of gears in the box, plus another pair for reverse gear, remain constantly in mesh with each other, with the countershaft gears permanently fixed to their shaft, those on the output shaft being free to rotate on their shafts when the gearbox is in neutral.

Gear selection involves engaging a dog clutch by sliding a dog gear along the output shaft by a selector fork operated by the gear lever, and connected to it either by cable, or rods and levers. Pushing the gear lever forwards, say into first gear, makes the fork slide a dog gear along grooved splines that the shaft, until its dog engages with matching slots machined on the first gear pinion, thus locking it to the output shaft and rotating it at the shaft speed.

2010 Acura TL

The drive line, through the pair of now engaged pinions, creates the gearing effect, according to the pinion sizes and their teeth numbers of the two pinions engaged. For reverse gear, the opposite rotation of the output shaft is achieved by inserting a reverse idler gear between countershaft and output pinions.

The gear changing system involves the engagement of the dog gear into the first gear pinion, and the process of engagement is where the crunch might come in an older gearbox. Today, all gear ratios have synchromesh, which involves an item called a synchroniser and baulk ring that initiate an advance sliding contact between the dog gear and the pinion, allowing them to synchronise their speeds before the actual engagement of the teeth.

With some effort at matching engine speeds with the accelerator and clutch, smooth and silent gear selection is thus easily achieved. When a gear change is required, the disengagement of the clutch allows the dog gear to be withdrawn by the selector fork, whilst another selector fork will almost simultaneously engage another dog gear to lock the engagement of another pair of pinions.

Pinions in modern production cars are rarely straight cut simple spur gears, but helical gears where the teeth are cut diagonally, such that the process of engagement and power transmission is smoother and quieter.

In practice, in most gearboxes where the engine and driven wheels are located together, at either front or rear, the gearbox will form part of a combined transaxle unit where gearbox, differential, and final drive share a common outer casing, the internal layout being depending on engine orientation – longitudinal or transverse.

Doc Diesel



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