A technical questrion about M DCT.

[goldminer]

Back in the early 70's, when I learned to drive, one of the things I was often told about driving a manual transmission was not to keep the clutch peddle depressed for prolonged periods of time while the engine was running.

This was supposedly to protect the throw out (or clutch release) bearing and keep it from wearing out prematurely. This throw out bearing is attached to the clutch release fork. It allows the fork (which levers against the bell housing and can't spin) to push against the clutch (which does spin) and release it for shifting or coming to a stop.

I had a clutch go in my old 69 Dodge Dart and replaced it myself. I was told to replace the throw out bearing with the clutch as they typically have similar service lives.

My question has to do with the DCT. While the engine is running it MUST ALWAYS have at least one of the clutches disengaged. This is a practice I was warned against but which is required for a dual clutch transmission. So how does Getrag and ZF get away with this and have a decent service life? Have they replaced the throw out bearing with a different device or does the oil immersion of the wet clutch protect the bearing?

[mkoesel]

Well for one thing, there is no clutchfork. There's just a hydraulic cylinder that attaches to the clutch and opens/closes it as necessary. And there is probably no tension on the clutch assembly when its open. Most likely, pressure is only applied to the clutch when it is engaged.

[BMWguy206]

Yes on the DCT there is no clutch fork therefore no throwout bearing. It's a wet dual clutch so the on/off operation is all hydraulic.

[masmole]

Like the others said, the DCT is a complete different design and quite a departure from conventional single clutch actuation. The mechanical operation is far more efficient and the packaging more compact, and thanks to a wet clutch, less wear overall than a conventual manual gearbox. A picture is worth a thousand words so to shed more light on how it operates, check out the video below that posted last week. It is representative of Porsche's 7-speed dual-clutch "PDK" system made by ZF, but the Borg-Warner/getrag system in the M3 is basically the same design.

http://www.zf.com/zfXmlServlet?resul...e=EN#toggleBox

[goldminer]

Originally Posted by mkoesel

Well for one thing, there is no clutchfork. There's just a hydraulic cylinder that attaches to the clutch and opens/closes it as necessary. And there is probably no tension on the clutch assembly when its open. Most likely, pressure is only applied to the clutch when it is engaged.

I am still confused. While there clearly is no clutch fork in the DCT, I believe that whatever clutch release mechanism is used it must be fixed relative to the transmission case doesn't it? (Or a portion of it does.) You have to attach the hoses from the hydraulic pump to it. If it is not fixed the hoses/tubes to the cylinder would get wrapped around the transmission input shaft.

Having thought about this some more I still don’t see how the surface of this cylinder, which isn’t spinning, can thrust against a clutch which is spinning unless there is some sort of bearing there. That is unless there is no cylinder and the hydraulic pressure acts directly on the clutch.

[BMWguy206]

Originally Posted by goldminer

I am still confused. While there clearly is no clutch fork in the DCT, whatever clutch release mechanism is used has to be fixed to the transmission case and lever against it.

If a hydraulic cylinder is used it (or a portion of it) must be fixed relative to the case doesn't it? You have to attach the hoses from the hydraulic pump to it. If it is not fixed the hoses/tubes to the cylinder would get wrapped around the transmission input shaft.

Having though about this some more I still don’t see how the surface of this cylinder, which isn’t spinning, can thrust against a clutch which is spinning unless there is some sort of bearing there. That is unless there is no cylinder and the hydraulic pressure acts directly on the clutch.

Hi Goldminer!

Check out this link How Dual Clutch Transmissions Work

The 2nd page shows pictures of the wet dual clutch setup. Just read it slowly and look at the pictures. I learned a lot on this website.

I can't find the BMW tech training manual on the DCT transmissions. It's more informative and has a lot of technical terms but it's one of those things where you have to see it to understand it rather than reading and looking at pictures.

[mkoesel]

goldiminer, check out the video. My description was somewhat off as I was describing more liek what happens with SMG. With M-DCT the hydraulics are interal to clutch itself. From what I can see it appears the clutch is engaged/disengaged by varying the pressure of the fluid on either side of it.

[mike3141]

The hydraulic clutches in the M-DCT (and VAG's DSG) are very similar to the clutch packs that are in "standard" automatic transmissions. Hydraulic pressure forces multiple discs together or apart as required by the TCU. What makes the M-DCT and DSG different from a regular AT is that they lack a torque converter (which steals power) and their gear trains are more like a manual transmission's rather than an automatic's planetary gears.

[goldminer]

Okay. I'm guess I'm starting to get it finally. The hydraulic pressure acts directly on the the springs and clutch pack themslves. So there is no mechanical or direct disconnecting of the clutch.