The workings of a manual gearbox

Interesting, Others | April 20th, 2015 by 4
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The perfectly-calibrated BMW manual transmission is particularly popular with sporty drivers due to its precise handling. Here is a guide on manual transmission

I was talking to friend who has a 1985 VW Golf he’s managed to eek almost a half million miles out of. Recently the manual transmission has been grumbling, it would appear that one of the shaft bearings may be failing. The repair to the tranny will cost more than the car is worth. Amazing that he’s been able to put that many miles on the car – but it is pretty much the last of the ‘old school’ manual-everything cars, so repairs have been pretty straightforward.

He was wondering what bearings may be causing the problems and I knew that the shafts were suspended in bearings but was unsure what other bearings floated around inside the gearbox and what would be involved in changing gears. So it was time to dig into the workings of a manual gearbox.

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First, in a longitudinal layout – like the ZF S6-45 in my 135i, there is an input shaft and concentric output shaft (they are coupled but are free to spin independently), and also a layshaft (sometimes called a countershaft).

There is a gear on the input shaft which is in constant mesh with a gear that turns the layshaft – the input shaft and layshaft turn at the same speed (but in opposite directions).

There are a number of gears attached directly to the layshaft and with the exception of the layshaft gear directly meshed to the input shaft, the remainder of the layshaft gears are in constant mesh with gears on the output shaft.

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The gears on the output shaft spin freely on needle bearings and have a corresponding fixed gear on the layshaft. The gears are helical cut (for NVH purposes) and are stronger than straight cut gears (used in racing gearboxes). But the helical gears have one major drawback, they introduce axial loads, a load which has to be absorbed by the shaft bearings and the gearbox case – racing gearboxes use straight cut gears because they don’t present axial loads and racing gearboxes don’t have to be ‘quiet’. Racing gearboxes are noisy in ways that road going cars would find intolerable.

So far we have a series of helical cut gears, one directly coupled to the input shaft that’s in constant mesh with a gear directly connected to the layshaft. There are other gears directly coupled to the layshaft that are in constant mesh with gears that are freely spinning, in roller bearings, which are placed on the output shaft. And then placed between the gears on the output shaft are a series of selectors (which are made up of blocker rings, a synchronizer hub, a shift sleeve and some smaller parts).

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transmission-synchronizer

The selectors move fore and aft on splines that are on the output shaft, they are constantly engaged and are turning at road speed (the speed of the output shaft). To allow the selector to couple a gear to itself there are synchro (baulk) rings and teeth on the inner edge of both sides of the selector.

The British use the term ‘baulk rings’ to describe the synchros, and they are onto something because ‘baulk’ means ‘to brake’ – and that’s exactly what the synchros do; they brake the quicker of either the selector or output shaft gear allowing those side mounted teeth to engage without significant clashing.

The synchros resemble metal cones which are engaged and whose friction synchronizes the speed of the requested gear (which is turning at the same speed as the layshaft – engine speed) and the selector which is directly connected to the output shaft (turning at road speed). Remember, the layshaft is in constant connection to the input shaft.

The teeth used by the selectors and gears resemble incisors. They are triangular in shape and that shape allows them to engage without grinding. The synchros work their magic by braking the faster of the selector/gear pair and then the teeth find a good mesh based on their shape – not unlike a pointed toe boot finds the stirrup on a saddle.

So the selector couples a gear on the output shaft, which is freely floating on needle bearings, via its and the gears side mounted teeth, to the output shaft – driving the output shaft at a reduced or higher speed than the input shaft based on the gear selected.

The selector is moved on the output shaft by shift forks which sit in a grove on the outer surface (around the diameter) of the selector. The shift forks are pushed or pulled by rods which move back through the transmission case and are terminated near the gearshift. Moving the gearshift out of neutral engages one of the rods and moving the shifter fore or aft moves the rod which moves the selector (in the opposite direction the shifter is moved).

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In many manual gearboxes the gear on the input shaft can be selected which directly couples the input shaft to the output shaft – in that case the input shaft and output will turn at the same speed – the 1 to 1 gear ratio. In this case the gear on the input shaft serves not as a reduction or overdrive, as it continues to turn the layshaft and its gears, but the selector itself couples the input and output shaft together

Reverse is the one gear set that is not in constant mesh (it is un-synchronized) and the vehicle should be at a dead stop for it to be engaged. Often reverse gears are straight cut (ever wonder why the transmission whines in reverse?).

With the engine running, the clutch pedal out (not depressed), and the gearshift lever in neutral, the input shaft is spinning at engine speed and since the layshaft is directly connected to the input shaft, it too is spinning at engine speed. The selectors are on the output shaft not spinning – this is because none are engaging a gear at the moment.

Depress the clutch and the input shaft is disconnected from the engine and starts to lose momentum (spins down). Press the shift lever forward for first, in the case of the 135i, and the far right shift rod moves back and engages a selector on first gear in the transmission. The synchros do their magic, the teeth on the selector and first gear bite together and the output shaft is now engaged to the layshaft (through the selector). Release the clutch and give the engine some gas and the input shaft spins up – that in tern spins the layshaft.

The gear is selected, in this case first, and through the selector it spins the output shaft at the appropriate speed and transmits the engine torque to the output shaft. The output shaft spins the propshaft (drive shaft) and that spins the differential which spins the axles which spins the wheels.

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Where can things go wrong? Well, over time the axial loads of helical cut gears will reek havoc on the shaft bearings – which I would suspect is the issue with my friend’s transmission. Nothing short of rebuild will resolve that. The transmission can ‘pop out of gear’, often a failure of the linkage to hold the coupled selector to the selected gear. Keep in mind that the selected gear is not directly coupled to the output shaft – it spins freely on roller bearings, it’s the selector (whose teeth are biting together with the teeth on the gear) which drives the output shaft.

And that brings me to an admonition. Do not rest your hand on the gearshift. The transmission vibrates in sympathy with the rest of the drivetrain. Resting your hand on the gearshift dampens those vibrations. And damping those vibrations forces the shift fork to wear away the outer edges of the grooves on the selectors. Eventually you’ll be looking at a rebuild – and probably before half a million miles.

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