TRANSMISSION SYSTEM OF CAR
Transmission, or gearbox?
Latest blog entry
| 09/23/2013 07:00 AM |
| Finding the right part in a hurry |
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If you tinker with your own car, you'll know how
difficult it can be sometimes to find the right parts quickly. Do you go
to the local store? Do you look online? When you're online, how do you
know that company A has a better price than company B? Who actually has
it in stock? If you go the internet route, it's easy to waste half an hour or more banging through search sites and dealers trying to get to the bottom of that particular can of worms. But I recently came across a site that's a bit more clever than most. It's like an aggregated list of many online vendors. You put in the part you want (from the list of available vehicles and parts) and the site then shows you a whole load of online vendors who have it in stock, with the price. So you can click through and buy the part pretty easily from there. There might be other sites out there that do the same thing but I've not see one before, so this is still a novelty for me. It's American-market only right now. The site has an odd name but I think it's worth checking out - OEMcats.com |
That question depends on which side of the Atlantic you're on. To the
Europeans, it's a gearbox. To the Americans, it's a transmission.
Although to be truthful, the transmission is the entire assembly that
sits behind the flywheel and clutch - the gearbox is really a subset of
the transmission if you want to split hairs.
Either way, this page aims to deal with the whole idea of getting the power from your engine to the ground in order to move your car (or bike) forwards.
Either way, this page aims to deal with the whole idea of getting the power from your engine to the ground in order to move your car (or bike) forwards.
Manual gearboxes - what, why and how?
From the Fuel & Engine Bible
you know that the pistons drive the main crank in your engine so that
it spins. Idling, it spins around 900rpm. At speed it can be anything up
to 7,500rpm. You can't simply connect a set of wheels to the end of the
crank because the speed is too high and too variable, and you'd need to
stall the engine every time you wanted to stand still. Instead you need
to reduce the revolutions of the crank down to a usable value. This is
known as gearing down - the mechanical process of using interlocking gears to reduce the number of revolutions of something that is spinning.
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A quick primer on how gears work
In this case I'm talking about gears meaning 'toothed
wheel' as oppose to gears as in 'my car has 5 gears'. A gear (or cog, or
sprocket) in its most basic form is a flat circular object that has
teeth cut into the edge of it. The most basic type of gear is called a
spur gear, and it has straight-cut teeth, where the angle of the teeth
is parallel to the axis of the gear. Wider gears and those that are cut
for smoother meshing are often cut with the teeth at an angle, and these
are called helical gears. Because of the angle of cut, helical gear
teeth have a much more gradual engagement with each other, and as such
they operate a lot more smoothly and quietly than spur gears. Gearboxes
for cars and motorbikes almost always use helical gears because of this.
A side effect of helical gears is that if the teeth are cut at the
correct angle - 45 degrees - a pair of gears can be meshed together
perpendicular to each other. This is a useful method of changing the
direction of movement or thrust in a mechanical system. Another method
would be to use bevel gears.
The
number of teeth cut into the edge of a gear determines its scalar
relative to other gears in a mechanical system. For example, if you mesh
together a 20-tooth gear and a 10-tooth gear, then drive the 20-tooth
gear for one rotation, it will cause the 10-tooth gear to turn twice.
Gear ratios are calculated by divinding the number of teeth on the
output gear by the number of teeth on the input gear. So the gear ratio
here is output/input, 10/20 = 1/2 = 1:2. Gear ratios are often
simplified to represent the number of times the output gear has to turn
once. In this example, 1:2 is 0.5:1 - "point five to one". Meaning the input gear has to spin half a revolution to drive the output gear once. This is known as gearing up.
Gearing
down is exactly the same only the input gear is now the one with the
least number of teeth. In this case, driving the 10-tooth gear as the
input gear gives us output/input of 20/10 = 2/1 = 2:1 - "two to one". Meaning the input gear has to spin twice to drive the output gear once.
By
meshing many gears together of different sizes, you can create a
mechanical system to gear up or gear down the number of rotations very
quickly. As a final example, imagine an input gear with 10 teeth, a
secondary gear with 20 teeth and a final gear with 30 teeth. From the
input gear to the secondary gear, the ratio is 20/10 = 2:1. From the
second gear to the final gear, the ratio is 30/20 = 1.5:1. The total
gear ratio for this system is (2 * 1.5):1, or 3:1. ie. to turn the
output gear once, the input gear has to turn three times.
This also neatly shows how you can do the calculation and miss the middle gear ratios - ultimately you need the ratio of input to output. In this example, the final output is 30 and the original input is 10. 30/10 = 3/1 = 3:1.
This also neatly shows how you can do the calculation and miss the middle gear ratios - ultimately you need the ratio of input to output. In this example, the final output is 30 and the original input is 10. 30/10 = 3/1 = 3:1.
Collections of helical gears in a gearbox are what give the gearing
down of the speed of the engine crank to the final speed of the output
shaft from the gearbox. The table below shows some example gear ratios
for a 5-speed manual gearbox (in this case a Subaru Impreza).
| Gear | Ratio | RPM of gearbox output shaft when the engine is at 3000rpm |
|---|---|---|
| 1st | 3.166:1 | 947 |
| 2nd | 1.882:1 | 1594 |
| 3rd | 1.296:1 | 2314 |
| 4th | 0.972:1 | 3086 |
| 5th | 0.738:1 | 4065 |
Final drive - calculating speed from gearbox ratios.
It's important to note that in almost all vehicles there is also a
final reduction gear. This is also called a final drive or a rear- or
front-axle gear reduction and it's done in the differential with a small
pinion gear and a large ring gear (see the section on differentials
lower down the page). In the Subaru example above, it is 4.444:1. This
is the final reduction from the output shaft of the gearbox to the
driveshafts coming out of the differential to the wheels. So using the
example above, in 5th gear, at 3000rpm, the gearbox output shaft spins
at 4065rpm. This goes through a 4.444:1 reduction in the differential to
give a wheel driveshaft rotation of 914rpm. For a Subaru, assume a
wheel and tyre combo of 205/55R16 giving a circumference of 1.985m or
6.512ft (see The Wheel & Tyre Bible).
Each minute, the wheel spins 914 times meaning it moves the car (914 x
6.512ft) = 5951ft along the ground, or 1.127 miles. In an hour, that's
(60minutes x 1.127miles) = 67.62. In other words, knowing the gearbox
ratios and tyre sizes, you can figure out that at 3000rpm, this car will
be doing 67mph in 5th gear.
Making those gears work together to make a gearbox
If you look at the image here you'll see a the
internals of a generic gearbox. You can see the helical gears meshing
with each other. The lower shaft in this image is called the layshaft -
it's the one connected to the clutch - the one driven directly by the
engine. The output shaft is the upper shaft in this image. To the
uneducated eye, this looks like a mechanical nightmare. Once you get
done with this section, you'll be able to look at this image and say
with some authority, "Ah yes, that's a 5-speed gearbox".
So how can you tell? Well look at the output shaft. You
can see 5 helical gears and 3 sets of selector forks. At the most basic
level, that tells you this is a 5-speed box (note that my example has
no reverse gear) But how does it work? It's actually a lot simpler than
most people think although after reading the following explanation you
might be in need of a brain massage.
With the clutch engaged (see the section on clutches below), the layshaft is always turning. All the helical gears on the layshaft are permanently attached to it so they all turn at the same rate. They mesh with a series of gears on the output shaft that are mounted on sliprings so they actually spin around the output shaft without turning it. Look closely at the selector forks; you'll see they are slipped around a series of collars with teeth on the inside. Those are the dog gears and the teeth are the dog teeth. The dog gears are mounted to the output shaft on a splined section which allows them to slide back and forth. When you move the gear stick, a series of mechanical pushrod connections move the various selector forks, sliding the dog gears back and forth.
With the clutch engaged (see the section on clutches below), the layshaft is always turning. All the helical gears on the layshaft are permanently attached to it so they all turn at the same rate. They mesh with a series of gears on the output shaft that are mounted on sliprings so they actually spin around the output shaft without turning it. Look closely at the selector forks; you'll see they are slipped around a series of collars with teeth on the inside. Those are the dog gears and the teeth are the dog teeth. The dog gears are mounted to the output shaft on a splined section which allows them to slide back and forth. When you move the gear stick, a series of mechanical pushrod connections move the various selector forks, sliding the dog gears back and forth.
In the image to the left, I've rendered a close-up of
the area between third and fourth gear. When the gearstick is moved to
select fourth gear, the selector fork slides backwards. This slides the
dog gear backwards on the splined shaft and the dog teeth engage with
the teeth on the front of the helical fourth gear. This locks it to the
dog gear which itself is locked to the output shaft with the splines.
When the clutch is let out and the engine drives the layhshaft, all the
gears turn as before but now the second helical gear is locked to the
output shaft and voila - fourth gear.
Grinding gears.
In the above example, to engage fourth gear, the dog gear is disengaged
from the third helical gear and slides backwards to engage with the
fourth helical gear. This is why you need a clutch and it's also the
cause of the grinding noise from a gearbox when someone is cocking up
their gearchange. The common misconception is that this grinding noise
is the teeth of the gears grinding together. It isn't. Rather it's the
sound of the teeth on the dog gears skipping across the dog teeth of the
helical output gears and not managing to engage properly. This
typically happens when the clutch is let out too soon and the gearbox is
attempting to engage at the same time as it's trying to drive. Doesn't
work. In older cars, it's the reason you needed to do something called
double-clutching.
Double-clutching, or double-de-clutching (I've heard it called both) was a process that needed to happen on older gearboxes to avoid grinding the gears. First, you'd press the clutch to take the pressure off the dog teeth and allow the gear selector forks and dog gears to slide into neutral, away from the engaged helical gear. With the clutch pedal released, you'd 'blip' the engine to bring the revs up to the speed needed to engage the next gear, clutch-in and move the gear stick to slide the selector forks and dog gear to engage with the next helical gear.
Double-clutching, or double-de-clutching (I've heard it called both) was a process that needed to happen on older gearboxes to avoid grinding the gears. First, you'd press the clutch to take the pressure off the dog teeth and allow the gear selector forks and dog gears to slide into neutral, away from the engaged helical gear. With the clutch pedal released, you'd 'blip' the engine to bring the revs up to the speed needed to engage the next gear, clutch-in and move the gear stick to slide the selector forks and dog gear to engage with the next helical gear.
The synchromesh - why you don't need to double-clutch.
Synchros, synchro gears and synchromeshes - they're all
basically the same thing. A synchro is a device that allows the dog
gear to come to a speed matching the helical gear before the dog teeth
attempt to engage. In this way, you don't need to 'blip' the throttle
and double-clutch to change gears because the synchro does the job of
matching the speeds of the various gearbox components for you. To the
left is a colour-coded cutaway part of my example gearbox. The green
cone-shaped area is the syncho collar. It's attached to the red dog gear
and slides with it. As it approaches the helical gear, it makes
friction contact with the conical hole. The more contact it makes, the
more the speed of the output shaft and free-spinning helical gear are
equalised before the teeth engage. If the car is moving, the output
shaft is always turning (because ultimately it is connected to the
wheels). The layshaft is usually connected to the engine, but
it is free-spinning once the clutch has been operated. Because the gears
are meshed all the time, the synchro brings the layshaft to the right
speed for the dog gear to mesh. This means that the layshaft is now
spinning at a different speed to the engine, but that's OK because the
clutch gently equalises the speed of the engine and the layshaft, either
bringing the engine to the same speed as the layshaft or vice versa
depending on engine torque and vehicle speed.
So to sum up that very long-winded description, I've
rendered up an animation - when you see parts of a gearbox moving in an
animation, it'll make more sense to you. What we have here is a single
gear being engaged. The layshaft the blue shaft with the smaller helical
gear attached to it. To start with, the larger helical gear is
free-spinning on its slip ring around the red output shaft - which is
turning at a different speed because it's connected to the wheels. As
the gear stick is moved, the gold selector collar begins to slide the
dog gear along the splines on the output shaft. As the synchromesh
begins to engage with the large helical gear, the helical gear starts to
spin up to speed to match the output shaft. Because it is meshed with
the gear on the layshaft, it in turn starts to bring the layshaft up to
speed too. Once the speed of everything is matched, the dog gear locks
in place with the output helical gear and the clutch can be engaged to
connect the engine to the wheels again.
What about reverse?
Reverse gear is normally an extension of everything
you've learned above but with one extra gear involved. Typically, there
will be three gears that mesh together at one point in the gearbox
instead of the customary two. There will be a gear each on the layshaft
and output shaft, but there will be a small gear in between them called
the idler gear. The inclusion of this extra mini gear causes the last
helical gear on the output shaft to spin in the opposite direction to
all the others. The principle of engaging reverse is the same as for any
other gear - a dog gear is slid into place with a selector fork.
Because the reverse gear is spinning in the opposite direction, when you
let the clutch out, the gearbox output shaft spins the other way - in
reverse. Simple. The image on the left here shows the same gearbox as
above modified to have a reverse gear.
Crash gearboxes or dog boxes.
Having gone through all of that business about
synchromeshes, it's worth mentioning what goes on in racing gearboxes.
These are also known as crash boxes, or dog boxes, and use straight-cut
gears instead of helical gears. Straight-cut gears have less surface
area where the gears contact each other, which means less friction,
which means less loss of power. That's why people who make racing boxes
like to use them.
Normally, straight-cut gears are mostly submerged in oil rather than relying on it sloshing around like it does in a normal gearbox. So the extra noise that is generated is reduced to a (pleasing?) whine by the sound-deadening effects of the oil.
Normally, straight-cut gears are mostly submerged in oil rather than relying on it sloshing around like it does in a normal gearbox. So the extra noise that is generated is reduced to a (pleasing?) whine by the sound-deadening effects of the oil.
But what is a dog box? Well - motorbikes have been
using them since the dawn of time. Beefing the system up for cars was
the brainchild of a racing mechanic who wanted to provide teams with a
quick method of altering gear ratios in the pits without having to play
"chase the syncro hub ball bearings" as they fell out on to the garage
floor.
Normal synchro gearboxes run at full engine speed as the clutch directly connects the input shaft to the engine crank. Dog boxes run at a half to a third the speed of the engine because there is a step-down gear before the gearbox. The dog gears in a dog box also have less teeth on them than those in a synchro box and the teeth are spaced further apart. So rather than having an exact dog-tooth to dog-hole match, the dog teeth can have as much as 60° "free space" between them. This means that instead of needing an exact 1-to-1 match to get them to engage, you have up to 1/6th of a rotation to get the dog teeth pressed together before they touch each other and engage. The picture on the right shows the difference between synchro dog gears and crash box dog gears.
So the combination of less, but larger dog teeth spaced further apart, and a slower spinning gearbox, allegedly make for an easier-to-engage crash box. In reality, it's still quite difficult to engage a crash box because you need exactly the right rpm for each gear or you'll just end up grinding the dog teeth together or having them bounce over each other. That results in metal filings in your transmission fluid, which ultimately results in an expensive and untimely gearbox rebuild.
But it is more mechanically reliable - it's stronger and able to deal with a lot more power and torque which is why it's used in racing.
So in essence, a dog box relies entirely on the driver to get the gearchange right. Well - sort of. Nowadays the gearboxes have ignition interrupters connected to them. As you go to change gear, the ignition system in the engine is cut for a fraction of a second as you come to the point where the dog teeth are about to engage. This momentarily removes all the drive input from the gearbox making it a hell of a lot easier to engage the gears. And when I say 'momentary' I mean milliseconds. Because of this, it is entirely possible to upshift and downshift without using the clutch (except from a standstill). Pull the gear out of first, and as you blip the throttle to get the engine to about the right speed, the ignition is cut just as the gears engage.
Even the blip of the throttle isn't necessary now either - advanced dog boxes can also attempt to modify the engine speed by adjusting the throttle input to get the revs to the right range first.
Of course even with all this cleverness, you still get nasty mechanical wear from cocked up gear changes, but in racing that doesn't matter - the gearbox is stripped down and rebuilt after each race.
Normal synchro gearboxes run at full engine speed as the clutch directly connects the input shaft to the engine crank. Dog boxes run at a half to a third the speed of the engine because there is a step-down gear before the gearbox. The dog gears in a dog box also have less teeth on them than those in a synchro box and the teeth are spaced further apart. So rather than having an exact dog-tooth to dog-hole match, the dog teeth can have as much as 60° "free space" between them. This means that instead of needing an exact 1-to-1 match to get them to engage, you have up to 1/6th of a rotation to get the dog teeth pressed together before they touch each other and engage. The picture on the right shows the difference between synchro dog gears and crash box dog gears.
So the combination of less, but larger dog teeth spaced further apart, and a slower spinning gearbox, allegedly make for an easier-to-engage crash box. In reality, it's still quite difficult to engage a crash box because you need exactly the right rpm for each gear or you'll just end up grinding the dog teeth together or having them bounce over each other. That results in metal filings in your transmission fluid, which ultimately results in an expensive and untimely gearbox rebuild.
But it is more mechanically reliable - it's stronger and able to deal with a lot more power and torque which is why it's used in racing.
So in essence, a dog box relies entirely on the driver to get the gearchange right. Well - sort of. Nowadays the gearboxes have ignition interrupters connected to them. As you go to change gear, the ignition system in the engine is cut for a fraction of a second as you come to the point where the dog teeth are about to engage. This momentarily removes all the drive input from the gearbox making it a hell of a lot easier to engage the gears. And when I say 'momentary' I mean milliseconds. Because of this, it is entirely possible to upshift and downshift without using the clutch (except from a standstill). Pull the gear out of first, and as you blip the throttle to get the engine to about the right speed, the ignition is cut just as the gears engage.
Even the blip of the throttle isn't necessary now either - advanced dog boxes can also attempt to modify the engine speed by adjusting the throttle input to get the revs to the right range first.
Of course even with all this cleverness, you still get nasty mechanical wear from cocked up gear changes, but in racing that doesn't matter - the gearbox is stripped down and rebuilt after each race.
Before the gearbox - the clutch
So now you have a basic idea of how gearing works there's a second item in your transmission
that you need to understand - the clutch. The clutch is what enables
you to change gears, and sit at traffic lights without stopping the
engine. You need a clutch because your engine is running all the time
which means the crank is spinning all the time. You need someway to
disconnect this constantly-spinning crank from the gearbox, both to
allow you to stand still as well as to allow you to change gears. The
clutch is composed of three basic elements; the flywheel, the pressure
plate and the clutch plate(s). The flywheel is attached to the end of
the main crank and the clutch plates are attached to the gearbox
layshaft using a spline. You'll need to look at my diagrams to
understand the next bit because there are some other items involved in
the basic operation of a clutch. (I've rendered the clutch cover in
cutaway in the first image so you can the inner components.) So here we
go.
In the diagram here, the clutch cover is bolted to the
flywheel so it turns with the flywheel. The diaphragm springs are
connected to the inside of the clutch cover with a bolt/pivot
arrangement that allows them to pivot about the attachment bolt. The
ends of the diaphragm springs are hooked under the lip of the pressure
plate. So as the engine turns, the flywheel, clutch cover, diaphragm
springs and pressure plate are all spinning together.
The clutch pedal is connected either mechanically or hydraulically to a fork mechanism which loops around the throw-out bearing. When you press on the clutch, the fork pushes on the throw-out bearing and it slides along the layshaft putting pressure on the innermost edges of the diaphragm springs. These in turn pivot on their pivot points against the inside of the clutch cover, pulling the pressure plate away from the back of the clutch plates. This release of pressure allows the clutch plates to disengage from the flywheel. The flywheel keeps spinning on the end of the engine crank but it no longer drives the gearbox because the clutch plates aren't pressed up against it.
As you start to release the clutch pedal, pressure is released on the throw-out bearing and the diaphragm springs begin to push the pressure plate back against the back of the clutch plates, in turn pushing them against the flywheel again. Springs inside the clutch plate absorb the initial shock of the clutch touching the flywheel and as you take your foot off the clutch pedal completely, the clutch is firmly pressed against it. The friction material on the clutch plate is what grips the back of the flywheel and causes the input shaft of the gearbox to spin at the same speed.
Burning your clutch
You might have heard people using the term 'burning your clutch'. This is when you hold the clutch pedal in a position such that the clutch plate is not totally engaged against the back of the flywheel. At this point, the flywheel is spinning and brushing past the friction material which heats it up in much the same was as brake pads heat up when pressed against a spinning brake rotor (see the Brake Bible). Do this for long enough and you'll smell it because you're burning off the friction material. This can also happen unintentionally if you rest your foot on the clutch pedal in the course of normal driving. That slight pressure can be just enough to release the diaphragm spring enough for the clutch to occasionally lose grip and burn.
A slipping clutch
The other term you might have heard is a 'slipping clutch'. This is a clutch that has a mechanical problem. Either the diaphragm spring has weakened and can't apply enough pressure, or more likely the friction material is wearing down on the clutch plates. In either case, the clutch is not properly engaging against the flywheel and under heavy load, like accelerating in a high gear or up a hill, the clutch will disengage slightly and spin at a different rate to the flywheel. You'll feel this as a loss of power, or you'll see it as the revs in the engine go up but you don't accelerate. Do this for long enough and you'll end up with the above - a burned out clutch.
The clutch pedal is connected either mechanically or hydraulically to a fork mechanism which loops around the throw-out bearing. When you press on the clutch, the fork pushes on the throw-out bearing and it slides along the layshaft putting pressure on the innermost edges of the diaphragm springs. These in turn pivot on their pivot points against the inside of the clutch cover, pulling the pressure plate away from the back of the clutch plates. This release of pressure allows the clutch plates to disengage from the flywheel. The flywheel keeps spinning on the end of the engine crank but it no longer drives the gearbox because the clutch plates aren't pressed up against it.
As you start to release the clutch pedal, pressure is released on the throw-out bearing and the diaphragm springs begin to push the pressure plate back against the back of the clutch plates, in turn pushing them against the flywheel again. Springs inside the clutch plate absorb the initial shock of the clutch touching the flywheel and as you take your foot off the clutch pedal completely, the clutch is firmly pressed against it. The friction material on the clutch plate is what grips the back of the flywheel and causes the input shaft of the gearbox to spin at the same speed.
Burning your clutch
You might have heard people using the term 'burning your clutch'. This is when you hold the clutch pedal in a position such that the clutch plate is not totally engaged against the back of the flywheel. At this point, the flywheel is spinning and brushing past the friction material which heats it up in much the same was as brake pads heat up when pressed against a spinning brake rotor (see the Brake Bible). Do this for long enough and you'll smell it because you're burning off the friction material. This can also happen unintentionally if you rest your foot on the clutch pedal in the course of normal driving. That slight pressure can be just enough to release the diaphragm spring enough for the clutch to occasionally lose grip and burn.
A slipping clutch
The other term you might have heard is a 'slipping clutch'. This is a clutch that has a mechanical problem. Either the diaphragm spring has weakened and can't apply enough pressure, or more likely the friction material is wearing down on the clutch plates. In either case, the clutch is not properly engaging against the flywheel and under heavy load, like accelerating in a high gear or up a hill, the clutch will disengage slightly and spin at a different rate to the flywheel. You'll feel this as a loss of power, or you'll see it as the revs in the engine go up but you don't accelerate. Do this for long enough and you'll end up with the above - a burned out clutch.
Motorcycle 'basket' clutches
It's worth spending a moment here to talk about basket
clutches as found on some Yamaha motorbikes. Even though the basic
principle is the same (sandwiching friction-bearing clutch plates
against a flywheel), the design is totally different. If nothing else, a
quick description of basket clutches will show you that there's more
than one way to decouple the a spinning crank from a gearbox.
Basket clutches need to be compact to fit in a motorbike frame so they can't have a lot of depth to them. They also need to be readily accessible for mechanics to be able to service them with the minimum amount of fuss, something that's near impossible with regular car clutches. A basket clutch has a splined clutch boss bolted to the shaft coming from the engine crank with strong springs. Metal pressure plates slide on to this shaft, in alternating sequence with friction material clutch plates. The clutch plates are splined around the outside edge, where they fit into slots in an outer basket - the clutch housing. The clutch housing is bolted on to the layshaft which runs back through the middle of the whole mechanism and into gearbox. Clever, but as usual, not much use without a picture, so here you go.
Basket clutches need to be compact to fit in a motorbike frame so they can't have a lot of depth to them. They also need to be readily accessible for mechanics to be able to service them with the minimum amount of fuss, something that's near impossible with regular car clutches. A basket clutch has a splined clutch boss bolted to the shaft coming from the engine crank with strong springs. Metal pressure plates slide on to this shaft, in alternating sequence with friction material clutch plates. The clutch plates are splined around the outside edge, where they fit into slots in an outer basket - the clutch housing. The clutch housing is bolted on to the layshaft which runs back through the middle of the whole mechanism and into gearbox. Clever, but as usual, not much use without a picture, so here you go.
In operation, a basket clutch is simplicity itself. A
throw-out bearing slides around the outside of the layshaft and when you
pull the clutch lever, the throw-out bearing pushes against the clutch
boss. The clutch boss compresses the clutch springs and removes pressure
from the whole assembly. The friction plates now spin freely in between
the pressure plates. When you let the clutch out, the springs push the
clutch boss in again and it re-asserts the pressure on the system,
crushing the friction and pressure plates together so they grip. And
there you have a second type of clutch.
You should now feel proud that with all your newfound (and somewhat geeky) understanding of clutches, you can go about your business safe in the knowledge that you sort of understand how all this spinning, geared-and-splined witchcraft works.
You should now feel proud that with all your newfound (and somewhat geeky) understanding of clutches, you can go about your business safe in the knowledge that you sort of understand how all this spinning, geared-and-splined witchcraft works.
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