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How does an automatic transmission's one way clutch/clutches work?

Hi, I’ve been learning about car mechanics recently and I would like to know how the automatic transmission’s one way clutches work. I know how the normal grab clutches work that are splined to the ring gear, and how they are actuated and what they do, but I can’t seem to quite understand just exactly how the one way clutches work. I think what I’m confused about is what they actually do, I know how a one way clutch works (it allows rotation in one direction but not the other) but I don’t know how such a one way clutch would allow a reverse gear. I think I’m missing something here. By the way, I’ve watched all the videos I can to see how the automatic transmission fully works, but none of them fully explain how it actually works.

Picture roller bearings around a ring in a cage to support them.turning one way, they roll like a bearing. Turning the reverse direction, the cage has ramps cut in it thst wedge the rollers against the shaft they surround holding it instead of acting as a bearing.

Hard to describe but you must have found a picture somewhere. Reread while looking at the picture. Search “one way roller sprage clutch”

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I know how it works, about the oblong bearing idea and all that, but I don’t know exactly what it’s supposed to do, or where it is. Like I said, I know how the clutch packs and the bands work and how they are actuated and how it shifts the transmission, but the one way clutch is what gets me. I don’t know what it’s supposed to do and where it’s supposed to be, I watched a video on it a little while ago and it sort of scrambled my brain because the guy explaining it had a 2d block graph, which is VERY hard to understand when you have basically no context where the parts are. I don’t understand how a reverse gear would work with a one way clutch.

I know it allows the car to “freewheel” to reduce fuel consumption. Removes all engine braking allowing the car to coast. It is also used to redirect motion inside the trans but I have never studied automatics in any detail. Sorry, can’t help with that. While I have rebuilt manual transmissions, I have avoided automatics!

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I think I heard about the freewheeling. There’s also the brake clutches which, when shifting with the gear lever in an automatic, makes an engine braking effect, I don’t think that happens in normal automatic mode though, only at higher speeds or something

I think it happens in higher gears… highway driving. The 6 speed auto un my car actually does allow some engine braking in gears 1 thru 4. That may only be in manual mode, I can’t evaluate automatic mode since it up shifts to 5th and 6th.

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I think in manual mode it never disengages the transmission ever even when coasting, but when in automatic mode I do think it does. I could be wrong but that’s what I’ve seen in auto’s with a manual mode.

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I have my custom TH400 set up with a reverse ordered manual valve body. It freewheels in all gears except final because it is manual VB and set up that way. It’s set up for drag racing, to reduce the possibility of blowing up the engine. All I have to do is let off the gas and it freewheels, the engine goes to idle no matter the speed the car is travelling. When you step on the gas and the input shaft turns fast enough for the selected gear, it reactivates and power is again transmitted through the trans. The advantage on the street is I can go into stealth mode immediately if I see the cops eyeing my ride. The downside is no compression braking at all and that’s kind of a bummer now that I no longer race it…

This is a more simple mechanical version of a transmission. Modern trannies are much more complex in how they mix automatic and manual modes. We had this discussion previously, maybe even more than once…

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Most of the time, modern transmissions (like in the new Honda Accord) have manual mode with paddleshifters. Other modern vehicles have the DCT’s, but paddleshift automatics basically act like the older automatics with the 1st and 2nd gear selectors, but just with all the gears selectable instead of only a few. That does sound like it would be nice to have a reverse ordered valve body… would be really nice in racing I suppose!

The Bands are holding the outer casing of a planetary gear set… which by design will provide different ratios…by either letting the outer case spin…or holding it in place and with clutches in the mix it can forcing the planetary to supply a gear reduction…or not…

I’m not the trans expert but I do understand planetary gear sets… this and the links within the site may help you.


Ok, I think I get this. So the sprag clutch only works with the ring gear and nothing else? And also. from what it seems, the outer case is only there because it houses the clutch pack. So when that would stop spinning, the clutch packs that were on the casing would stop also, which would force the planetary to make a downshift/upshift. Do I have this right?

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@zaverxexperiments_166654 You got it man…

I think my question now is (maybe I don’t know too much about clutch packs) how exactly are the clutch packs mounted to the ring gear carrier… and how do they slow the planet carrier/ring gear down? I think I’m still missing something here lol, none of the videos I watch give any context behind anything in the transmission…

If you follow the links I sent it should help.

However in the meantime… simply study any picture or cutaway drawing of a clutch on a dirt bike… Say a 90’s Honda CR250 or 125… You will see how the multi clutch pack all fits inside of the carrier…and the springs that hold the clutch together…the rod that releases them to allow the trans to uncouple…etc.

Its all right there for you to study…

Ok wait, I think I might get this a little bit. So, the metal disks in the clutch packs are splined to the drum but not to the planet carrier, and then the friction material disks are ‘splined’ to the planet carrier but not splined to the drum… correct?

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Ok, yes, I watched a video on the topic a few minutes ago and yes, the friction material disks are splined to the planet carrier.

Absolutely correct… you got it.

In the dirt bike the clutch cage is driven by the crank… the inside is the shaft to the tranny. The clutch plates have little tabs or ears that interface with the cage the inside radius of the plates interface with the cog that drives the tranny shaft… You really need a picture…they are available. The pressure plate in the middle holds the clutch pack together…and the clutch actuating rod uncouples the shaft from the cage by letting the clutch plates slip.

In a dirt bike…if someone complains of a “grabby” clutch that cannot be let out smoothly and accurately… it is from repeated hard hits…like whoop dee doos…or big jumps with the clutch out and bike in gear. The tremendous shock encountered by the clutch plates ears or tabs…actually dig grooves in the cage…and thus the plates cannot be uncoupled smoothly… Its like an on off switch when that happens. Only way to fix it is to file down the grooves in the cage (creating clutch plate slack or slop)…or replace the cage. I used to race in the dirt…and was and still am my own tech. lol

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Thanks a lot for the help!

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No problemo… If you need a pic of the bike clutch let me know… I can find some for you… Its much easier with a picture

Here look at this new clutch for a CR… I apologize…the clutch plates tabs interface with the cage…and the steel “slip plates” interface with the inner cog that drives the tranny shaft… The outer cage is not included in this picture or link below.

If you need more info…just look up dirt bike clutch animation or something… use the internet…it is your friend…lol

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I have seen dirt bike clutches before, didn’t think about the fact that they look similar. Thanks for the link btw!

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