Why a multidisc clutch pack doesn't necessarily hold more than a single disc clutch

Let's begin by asking a simple question: what determines how much a clutch will hold?

Well, clutches hold because of friction. So basic physics tells us that the force due to friction is simply the "normal force" times the coefficient of friction. The normal force in this case is the amount of clutch apply pressure.

Let's begin with a little mental exercise. Imagine that I have two blocks of material, one is a 2" cube (2" on each side) and the other is a 4" cube. Now the two blocks are made of different materials so they happen to weigh the exact same amount-- 10# in our case. Moreover, the have the exact same coefficient of static friction, which we will arbitrarily assign as being .15

So, one block is a 2x2x2 cube and weighs 10# and has a static friction coefficient of .15. The other block is 4x4x4 cube, but otherwise identical in terms of weight and friction coefficient.

So, the question is: if I attach a string to each block, which one will be harder to drag across the floor? In other words, which block has more "holding power" with the floor?

The formula to determine the force of static friction is simply Normal force (the weight of the block) times the coefficient (.15). Thus, the SIZE DOESN'T MATTER. Both blocks will have an IDENTICAL "holding power" of 1.5 pounds, or .15 times the 10# weight of each block.

How can they be the same? The larger block's increased frictional area is canceled out by the fact that the force is acting on a larger area, which reduces pressure. Just like in hydraulics: 100psi acting on 10sq inches is the same as 10psi acting on 100 sq inches.


How does this relate to torque converters? Well, the TC clutch apply pressure determines the "normal force" when it acts on the surface area of the clutch. Remember, we learned from the blocks that surface area has nothing to do with breakaway (static) friction-- because at the same apply pressure, the increased area "dilutes" the pressure.

So let's say I swap out a stock TC for a single disc with double the clutch area, but I don't mod the VB, so the apply pressure is exactly the same. What happened to my holding power? NOTHING--- it's EXACTLY THE SAME AS BEFORE, assuming the same friction material is used. My TC clutch will last longer because there is now more bearing surface, but it has NOT increased its holding power. My new TC with double the clutch area is see HALF the holding force applied to that clutch, thus canceling it out and maintaining the status quo.

Ahh, you say-- but what about a triple disc clutch?

Well, if I add 5 identical links to a chain of 10 links, is it any stronger? Clearly not, because a chain is only as strong as its weakest link, right?

Similarly, if I add more clutch discs to and make a clutch pack out of it, all that's required for the pack to slip is for one individual clutch to slip. A clutch pack only has the holding power of it's weakest clutch plate or friction. One a single member of that clutch pack is slipping, the pack is slipping, just as a single broken link breaks the chain.

A clutch pack can, and often will, last LONGER because it's rare that the same disc will always slip. Thus, the total clutch wear is distributed across the pack, and each individual friction disc will experience less wear because not only is it slipping less often, but it's usually not slipping as far. Total wear is reduced greatly.

By now you can probably see that the ONLY way to increase the holding power of a TC lockup clutch is to increase the clutch apply pressure. The increased pressure is REQUIRED for a TC with larger clutch area if increased holding power is sought. Simply increasing surface area only increases bearing surface-- it actually reduces the force acting to push the clutch into the TC and lock it up.

Sorry so long, but I'm doing all this as a first draft-- no special writeup tonight.

Justin

 

Well said Hohn. Makes sense. The only thing I've always wanted to know, do triple disc TCs just have 3 clutches pancaked against each other? Or is there a stationary plate (stationary to the TC housing) inbetween each one? Because if there's no stationary plate then you would think that a triple disc would be prone to more slippage than a single disc given the same pressure. If there are stationary plates then it would outperform a single disc.

From what you said it appears that the 3 clutch packs are just pancaked against each other. Now why do they seem to perform better than singles? I think you got it right on when you said:

Having said all that, I still feel like we're missing a few more specific facts on triple disc TCs.

 

Thanks Hohn the reason I wasn't seeing it like that is I was thinking that a single disc would be gripping with so much pressure and a tripple disc would be able to grip that much pressure per disc, but I guess The transmission isn't going to be pumping the transmission fluid at anyhigher pressure just becouse you go from a single disc to a triple disc. It would get more gripping power from a vb the pushes more tranny fluid right? Sorry for not wording that real well.

The other thing I wanted to here some more about was stall speeds and also what exactly determines how tight or loose the tc is?

 

So I went down to DTT today and talked to them they seemed pretty friendly and helpfull. I told them what I have and what my plans are. They said there super single tc would be good. They told me the prices on the tc and the triple disc is only $25 more then a single disc. Is the main benefit of a triple disc that it is goign to last longer? For $25 if it will last longer that seems like a good idea. They also gave me Stephan's phone numder so I can call him if I have any questions or when I want to order it. They also said the wait time is about 2 months.

 

As Hohn explained its all in the pressures
That said do yourself a favor and go see Rob at Robs automatic down in Langley. He can install a Dunrite VB and converter for less money than you think PM me and I can give you his # Do a search on Tonys (Dunrite) stuff, its first rate and pretty pioneering in VB technology. Give me a shout and Ill take you for a ride in mine one day.

 

ratsun you get the ride working again bud? You going to the mission races this year? I was thinking about trying it and see how much damage I can do.

 

Yeah shes seems to be holding so far
Still need one more re-torque before I try and break it again but preliminary tests seem optimistic. If its running on a race weekend that I'm not at work I will be there, I guess the first race of the season is coming up soon? Hows that new tranny workin for you? Like it better than the PDR one?

 

Honestly, so far I like the PDR unit better it shifted better but to be fair to DTT they haven't had a chance to tweak it up and work the way it should. The truck seems to vibrate more with their flex plate than the BD unit I had. The DTT converter is better than the old TCS converter that Harry put in. The truck seems to launch better than the old PDR unit but so far I'm not impressed with the DTT OD shift. I'm used to the hard launch/kick in the pants that the old PDR unit did. I have crossing my fingers that I can make it. I'm still in shock that I can drive my truck. I just pulled a hard 50psi run, and I still have forward and reverse gears. I want to put in a new injection pump if I go. I'm even thinking about doing the BD dyno day and try out the mission track. Who knows I could pop a head gasket. Next weekend we have a DTR met in Edmonton and there some tuff truck to run against. The last met I only got fourth or fifth place I think. This year I have to do better or I will have to build a better mouse trap. It would cool to kick open Peaks door and see why those guys are laughing about all the time I call them. Either too much diesel fumes or leaky NOS bottles.

 

Hohn -
Just curious when it comes to sled pulling and Double disk CLUTCHES not torques.

I understand and grasp the theory/fact(s) you explain but why is everyone making a killing off of double disk clutches if you could get a single disk w/ the same apply pressure to hold?

Just curious, I may be stupid on this one.

 

Pedal pressure and engagement.

 

I can easly slip my stock TC with the mods in my sig, and towing heavy is out of the question

 

Dual disc manual clutches are very different than TC clutches.

A manual trans clutch will hold more at higher plate loads, which is roughly equivalent to the TC lockup apply pressure.

But a key difference is that the plate load is a FORCE, while the lockup clutch apply effort is a PRESSURE.

The difference of course, is that the force exerted by the pressure is a function of area. So for example, you could increase TC lockup force by making the cylinder larger, OR increasing line pressure or (ideally) both.


Anyway, back on point. A dual disc clutch works well because a single disc clutch built with the same plate load and frictional coefficients would be almost undriveable. Yes, you could theoretically built a single disc Iron Giant with 4200# plate load and 12 feramic buttons. But it would be essentially a ZERO slip clutch, with next to nothing for engagement.

The dual disc doesn't increase holding power, per se. It makes a given amount of holding power more progressive and drivable.

If you built a single disc I.G. and took it to the pulling track, it would probably bog your truck, or blast away the tires or break something badly.

The Con FE is about is grabby as you'd want a clutch to be in a street oriented truck. Going to a dual disc setup allows a higher plate load (and more holding) for a given amount of "progressiveness"-- or the transition between slipping and gripping.

So, a Dual disc is either smoother engaging at the same level of holding power, or holds more at the same level of smoothness.

Of course, there's no free lunch and the higher weight of the dual disc makes shifting a little less responsive as the synchros have more weight to manage.

jmo