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Author Subject: Gearbox test thread
Welly Forum Admin

Location: Brizzle

Registered: 11 Jan 2004

Posts: 21,797

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Post #1
Clutch
A clutch is a mechanism for transmitting rotation, which can be engaged and disengaged. Clutches are useful in devices that have two rotating shafts. In these devices, one shaft is typically driven by a motor or pulley, and the other shaft drives another device.
The clutch connects the two shafts so that they can either be locked together and spin at the same speed, or be decoupled and spin at different speeds.
In simple layman terms, the clutch allows a break in the engine rotation to the gearbox.
The flywheel is fixed to the crankshaft.


Onto this is a clutch kit. In effect it's another flywheel with a friction plate sandwhiched between the 2.
The cover plate (the bigger bit in the picture below) has a sort of free floating flywheel on the other side and a section sprung fingers with a bearing in the middle. The fingers push the cover plate and friction plate apart to seperate the flywheel and clutch. On the gti6 it actually pull's the fingrs allowing the cover plate to move apart enough for the friction plate to free spin. As this spinning it needs a thrurst bearing to allow the push/pull movement whilst allowing it to spin. The thrust bearing is the bit in the middle of the fingers of the cover plate.

The friction plate is just that. A plate with a friction matrail rivited to it to "grab" the flywheel and cover plate and make them spin at the same time. The friction plate sits on the layshaft of the gearbox but between the flywheel and the coverplate. It has normally got springs in the centre to take away the intially shock of the clutch engaging.




Normal 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.
With the clutch engaged 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 below

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.
Synchromesh
If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at different speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter together. For this reason, a modern dog clutch in an automobile has a synchronizer mechanism or synchromesh, where before the teeth can engage, a cone clutch is engaged which brings the selector and gear to the same speed.

Moreover, until synchronization occurs, the teeth are prevented from making contact, because further motion of the selector is prevented by a blocker (or "baulk" ) ring. When synchronization occurs, friction on the blocker ring is relieved and it twists slightly, bringing into alignment certain grooves and notches that allow further passage of the selector which brings the teeth together. Of course, the exact design of the synchronizer varies from manufacturer to manufacturer.

The synchronizer has to change the momentum of the entire input shaft and clutch disk. Additionally, it can be abused by exposure to the momentum and power of the engine itself, which is what happens when attempts are made to select a gear without fully disengaging the clutch. This causes extra wear on the rings and sleeves, reducing their service life. When an experimenting driver tries to "match the revs" on a synchronized transmission and force it into gear without using the clutch, the synchronizer will make up for any discrepancy in RPM. The success in engaging the gear without clutching can deceive the driver into thinking that the RPM of the layshaft and transmission were actually exactly matched. Nevertheless, approximate "rev-matching" with clutching can decrease the general delta between layshaft and transmission and decrease synchro wear



Sequential manual transmission

A sequential gearbox is just like a manual gearbox but the selector system is different. The manual gearbox example at the top of the page showed a series of selector forks which were moved by the physical position of the gearstick. In a sequential box, those selector forks are connected to a single shaft that has corkscrew-type grooves in it. The collar that fits around this selection shaft has a ballbearing in it which sits in a recess in the collar as well as in one of the corkscrew grooves. When the gearstick is moved forwards or backwards, the selector shaft is mechanically turned by some number of degrees. That twisting motion rotates the corkscrew groove which in turn interacts with the ballbearings and the selector fork collars, forcing them to slide back and forth. Each click of the gearstick rotates the shaft another number of degrees and all the selector forks change position in one go.
That's why it's called a sequential gearbox - the gears are always selected in sequence. You can't jump from first to third, you have to go via second. Often, sequential gearboxes have a "double-click for neutral" option and when you do this, it disengages the clutch and rotates the selector shaft back around to the neutral position, just before first gear.



Dog engagment "dogbox"

Dog engagment is no differnent to a normal box, aside from the fact it doesn't use synchromesh.
Instead it has a certain amount of teeth on the side of the gear.
When the gear is selected the teeth on the side of the gear mesh together locking the 2 together.

This means clutchless gearchanges but does mean you have to match the rotation of the gears to the engine speed and do it quick to gain any advantage.
It really is unsuitable for the road as you can't do low rpm gearchanges smoothly.



Straight cut gears

Straight cut where the gears have straight cuts as opposed to helical.
Straight cut gears are stronger and allow more tourque to be used. However they are bloody noisey. Ever heard a mini from the inside? How about reverse?

For the above 2 check out this video.
Youtube link to PPG

DSG

Thanks to the use of a twin multi-plate clutch with electro-hydraulic control, two gears can be engaged at the same time.
During dynamic operation of the car, one gear is engaged. When the next gearshift point is approached, the appropriate gear is pre-selected but its clutch kept disengaged.
The gearshift process opens the clutch of the activated gear and closes the other clutch at the same time with a certain overlap. The gear change consequently takes place under load, with the result that a permanent flow of power is maintained.

The DSG/S-Tronic all but eliminates the lag inherent in SMTs (seqeuntial manual transmission).
The DSG is essentially two 3-speed gearboxes with a pair of clutches. When the driver starts out, transmission #1 is in first gear and transmission #2 is in second. The clutch engages and the car starts out in first.
When it's time to change gears, the DSG uses the clutches to swap transmissions. The #1 transmission immediately shifts to third gear. At the next change the DSG swaps transmissions again, and #2 shifts to fourth.
The DSG's computerized controller calculates the next likely gearchange and shifts the "idle" transmission into that gear.
The advantage is the speed of gearchanges: The DSG takes about 8 milliseconds to upshift. Compare that to the SMT in the Ferrari Enzo, which takes 150 ms to upshift. It's also significantly faster than a human: According to Audi, the A3 runs 0-60 in 6.9 seconds with a 6-speed manual and 6.7 seconds with the 6-speed DSG.
Like the SMT, the DSG performs double-clutch downshifts and can skip gears (i.e. downshifting from 6th directly to 4th, 3rd, etc).


copyrights
As most of this was collated from all different sites it's only fair to link to the original sites.
The carbible
Wikipedia

________________________________________

Life is a lie. The only certainty of which is death. Go live and enjoy yourself. The clock is ticking

Vrs power!!
Posted 21st Dec 2007 at 08:23
Welly Forum Admin

Location: Brizzle

Registered: 11 Jan 2004

Posts: 21,797

Status: Offline

Post #2
Note to self:
Diff
LSD
ATB

Note to others:
Please don't delete or fiddle

________________________________________

Life is a lie. The only certainty of which is death. Go live and enjoy yourself. The clock is ticking

Vrs power!!
Posted 21st Dec 2007 at 08:25
khj100 Forum Admin

Location: York

Registered: 30 Mar 2003

Posts: 11,156

Status: Offline

Post #3
No fiddling

Note to Paul:
2.25am is a time for sleeping (or occasionally drunken crawling in gutters), not writing about gearboxes

________________________________________

Trackday, trackday, trackday Hyper
Posted 21st Dec 2007 at 12:46
Welly Forum Admin

Location: Brizzle

Registered: 11 Jan 2004

Posts: 21,797

Status: Offline

Post #4
and so is 6.46!

________________________________________

Life is a lie. The only certainty of which is death. Go live and enjoy yourself. The clock is ticking

Vrs power!!
Posted 21st Dec 2007 at 21:27
Welly Forum Admin

Location: Brizzle

Registered: 11 Jan 2004

Posts: 21,797

Status: Offline

Post #5
Note to others, i did this a while ago and evr got round to finishing it.


Unlocked it outta reference to the sequential gearbox thread.

Never know, you might learn something

________________________________________

Life is a lie. The only certainty of which is death. Go live and enjoy yourself. The clock is ticking

Vrs power!!
Posted 4th Jan 2010 at 05:48

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