High Torque CVT - Engineering Explained

[bobdoo]

Originally Posted by KrisZ
Originally Posted by DriveHard
Originally Posted by KrisZ
It is still metal on metal contact. The difference is that this new arrangement can essentially have two cones dividing the torque between them. Throughout the majority of the video I kept thinking how they will cope with the friction forces as the contact area is even less than that of a CVT belt and that metal on metal contact, until he mentioned the special fluid. Basically it all rides on this fluid. I can only guess that Nissan will claim this a lifetime fluid as well.



No it not actually. It is s special fluid that transmits the torque. There is no metal to metal contact in a well executed CVT transmission. The "tech" is all in the traction fluid.


Hence my last mention that it all rides on the fluid. Otherwise it's full metal to metal contact, just like with the steel belt CVTs. The moment that fluid overheats and shears down, the transmission is toast in quick order.



NO.

There is *zero* metal to metal contact in CVT's. Period. There is always an oil layer. In toroidal, and in 'belt' (chain) drives.

If actual metal contact occurs, you have a CVT failure. My two subies with CVT, and countless 100's of thousands of others, have never had metal to metal contact.

Yes, the fluid is critical. So are all the other fluids in engines. Cylinder to ring contact? Failure. Cam to lifter contact? Failure. Crank to journal contact? Failure.

If you understand this, your wording cleverly hides that fact. Please be more clear and concise in your replies, so the rest of us have a chance to understand what you mean, instead of what you say.

(And please, god - get my wife to understand this point!!!!)

 

[KrisZ]

Originally Posted by bobdoo
NO.

There is *zero* metal to metal contact in CVT's. Period. There is always an oil layer. In toroidal, and in 'belt' (chain) drives.

If actual metal contact occurs, you have a CVT failure. My two subies with CVT, and countless 100's of thousands of others, have never had metal to metal contact.

Yes, the fluid is critical. So are all the other fluids in engines. Cylinder to ring contact? Failure. Cam to lifter contact? Failure. Crank to journal contact? Failure.

If you understand this, your wording cleverly hides that fact. Please be more clear and concise in your replies, so the rest of us have a chance to understand what you mean, instead of what you say.

(And please, god - get my wife to understand this point!!!!)


Let me clarify, hopefully.

The fluid film in a CVT, either steel belt/pulley or torodial type is not just there to provide hydrodynamic film to prevent metal to metal contact. Unlike oil film in the engine that reduces friction, the oil film in this application actually creates friction between the two materials so that one can drive the other. The oil, which is heavily friction modified, becomes the coupling.

That is a huge difference because while in an engine, as long as oil film is maintained the protection is there, plus engines are designed to tolerate certain amount of metal to metal contact. With CVT, once the polymer chains fail to provide the required frictional forces, for whatever reason, the oil film is not enough to prevent slippage. And once slippage starts to occur, the oil film will degrade very rapidly since the contact area is so small and the pressure to keep the drive engaged extremely high. That is why CVTs are so sensitive to overheating and fluid degradation.

Have you seen a timken machine in action? Basically, once these cones start to slip, that's exactly the effect it will create and the oil film will break down pretty quickly.

 

[JTK]

As far as I know, conventional automatic transmissions also rely on friction modifiers in the fluid or they go kaboom in short order as well. I just don't understand why this is such a frightening concept.

 

[KrisZ]

Originally Posted by JTK
As far as I know, conventional automatic transmissions also rely on friction modifiers in the fluid or they go kaboom in short order as well. I just don't understand why this is such a frightening concept.

Well, that's like saying all cars use four wheels, so I don't understand why using four wheels is so frightening of a concept in a Chrysler or VW, but perfectly fine in a comparable Honda or Toyota.

 

[JTK]

Originally Posted by KrisZ
Originally Posted by JTK
As far as I know, conventional automatic transmissions also rely on friction modifiers in the fluid or they go kaboom in short order as well. I just don't understand why this is such a frightening concept.

Well, that's like saying all cars use four wheels, so I don't understand why using four wheels is so frightening of a concept in a Chrysler or VW, but perfectly fine in a comparable Honda or Toyota.

Kris, I hear you. I just don't agree with the 'everything is riding on the proper fluid' argument as being a detriment to CVTs, when conventional step-shift automatics are exactly the same in that regard.

 

[KrisZ]

Originally Posted by JTK
Originally Posted by KrisZ
Originally Posted by JTK
As far as I know, conventional automatic transmissions also rely on friction modifiers in the fluid or they go kaboom in short order as well. I just don't understand why this is such a frightening concept.

Well, that's like saying all cars use four wheels, so I don't understand why using four wheels is so frightening of a concept in a Chrysler or VW, but perfectly fine in a comparable Honda or Toyota.

Kris, I hear you. I just don't agree with the 'everything is riding on the proper fluid' argument as being a detriment to CVTs, when conventional step-shift automatics are exactly the same in that regard.



They are not and I tried to explain that. CVTs do not have clutch plates with friction materials on them, that work together with friction modifiers in the ATF. That is why so many automatic transmissions work perfectly fine with universal ATFs.
It is pure metal to metal with nothing else in between but the fluid with highly specialized friction modifiers and polymers in CVTs
Now, I'm not saying that these CVTs will start failing left and right, I'm just saying that the margin of error, or call it the safety factor, is that much slimmer with these transmissions.

 

[Surestick]

Originally Posted by KrisZ
Originally Posted by JTK
Originally Posted by KrisZ
Originally Posted by JTK
As far as I know, conventional automatic transmissions also rely on friction modifiers in the fluid or they go kaboom in short order as well. I just don't understand why this is such a frightening concept.

Well, that's like saying all cars use four wheels, so I don't understand why using four wheels is so frightening of a concept in a Chrysler or VW, but perfectly fine in a comparable Honda or Toyota.

Kris, I hear you. I just don't agree with the 'everything is riding on the proper fluid' argument as being a detriment to CVTs, when conventional step-shift automatics are exactly the same in that regard.



They are not and I tried to explain that. CVTs do not have clutch plates with friction materials on them, that work together with friction modifiers in the ATF. That is why so many automatic transmissions work perfectly fine with universal ATFs.
It is pure metal to metal with nothing else in between but the fluid with highly specialized friction modifiers and polymers in CVTs
Now, I'm not saying that these CVTs will start failing left and right, I'm just saying that the margin of error, or call it the safety factor, is that much slimmer with these transmissions.


It's not like we don't have the technology to monitor transmission temperature and adjust engine power or transmission settings to cope with excess heat and protect the transmission.
As long as the transmission is matched to the vehicle's power output and intended use there shouldn't be an issue.

 

[JTK]

Originally Posted by KrisZ
[ ..That is why so many automatic transmissions work perfectly fine with universal ATFs.
It is pure metal to metal with nothing else in between but the fluid with highly specialized friction modifiers and polymers in CVTs
Now, I'm not saying that these CVTs will start failing left and right, I'm just saying that the margin of error, or call it the safety factor, is that much slimmer with these transmissions.



What about the various "universal" CVT fluids? I've not heard of a single failure associated with the proper use of one of those.

I know step ATs have clutch plates that essentially clamp together against metallic rings, etc and those require specialized fluids between them as well. Given there's multiples of them in one unit, I suppose that gives this setup more of a margin of error.

 

[Exhaustgases]

With all this talk about no metal to metal contact in a CVT because of the magic snake oil. Then there should be no reason to have hardened parts in the V pulley belt system, since yall say they never make contact. And this magic fluid should be the new top of the line lube for all mechanical / manual transmission and differential gear systems, since its not desireable to have metal to metal contact there either, and while at it then there should be no need to heat treat and harden the gears anymore because the magic snake oil will do that job of separation. Same goes for rolling element bearings needing to be so hard, the snake oil will keep them separated. And that CVT fluid should make it so that the extreme pressure test machine never locks up. Has anyone ever seen it tested on one?

 

[StevieC]

CVT's use extremely high oil pressure in the hundreds of PSI versus a standard gear/clutch transmission. This is why the parts are very hard because you can't have distortion from the pressure especially when a couple millimeters difference can make all the difference in the gear ratio as explained in the video above.

 

[KrisZ]

Metal hardening is done to improve component rigidity and gears in regular transmissions are hardened as well. This has nothing to do with oil pressure or antiwear additives in the oil. Oil is there to minimize the wear, as metal to metal contact will wear down the components very fast, wether they were hardened or not. Of course hardened metal will resist wear better in those situations where oil film failed when compared to unhardened metal.

This one is like reading the oil section about thinner oils and less wear protection safety margin. No matter what evidence is provided, it is taken as an attack, instead of an opportunity to learn something new.

 

[Exhaustgases]

I know about the reasons for heat treat. I'm just showing the fallacy of thinking there is some magic fluid added to a CVT which many here were on board with.
CVT is a transmission that is in constant slippage mode. The belt has to be pulled out of a tightly pinched V pulley, it doesn't just ride in the pulley like normal NON pinched pulleys are. Also when the transmission is doing acceleration and deceleration the belt and pulley interface is in constant slippage mode as well and the slipping from transitioning out of or into the pulley's. In a normal clutch pack automatic transmission after the shift the slipping stops, and at normal highway speed or sustained minimum TC lockup speed, the converter is locked up and there is no slippage at all, and is why you will never find a CVT in any kind of heavy haul vehicle.

 

[PandaBear]

Originally Posted by JTK
I remember reading about these toroidal CVTs years ago for the automotive realm. It's got to be a high cost situation at present or they would have taken over the belt/chain CVTs by now.



You need a lot more space for toroidal, very difficult to put in a FWD V6.