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How do one way clutches in an auto transmission help gears shift?

YriYri Member Posts: 3
edited April 2020 in General
Hi,

I've been researching cars recently, and when I got to the automatic transmission, I stumbled across the one way clutch.
Keep in mind, everything else about an automatic transmission makes sense to me except for the one way clutch. I know how it works (sprag or roller types) but I can't seem to understand exactly where it is and what it does, and if the outer race is held stationary (in a sprag clutch). It doesn't make any sense to add it. I know it isn't actuated by hydraulics like the clutch packs and that it is purely mechanical. I think what I'm confused by is now it can be actuated on and off depending if you are in low gear or in drive. If anyone can help, it would be appreciated!
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  • thecardoc3thecardoc3 Member Posts: 5,745
    edited April 2020
    With a planetary assembly, input power into one of the members, hold one of the members and you get some type of a reaction from the third one. If you hold two of the members then the assembly locks in place. If you drive two of the members then the assembly drives the third member. Think of the output as something that "tries" to hold one of the members.

    Now depending on which member(s) are being driven and which are being held you can get in increase or a decrease in the gear ratio, you can also reverse the direction of rotation. In some situations it would add a lot of size and weight to add a regular clutch to try and hold an assembly from rotating, especially if we only need to hold it to produce a given gear in one particular direction, enter the one way clutch. If we use a clutch to hold one of the members to the case it cannot move at all, but with a one way clutch it can be held in one direction but allowed to release in the other.

    Here is a link to a PDF manual for troubleshooting and rebuild of a Ford C6 transmission. I'll note some specifc pages especially related to the one way clutch. It's going to take you some time to read through it and attempt to visualize just what is occurring but this is where you need to start.

    https://shop.ukrtrans.biz/wp-content/uploads/catalogs/C6.pdf

    PG 30 Low / Reverse clutch hub (includes one way clutch)

    PG 32 Clutch and Band apply chart

    PG 44 Clutch and Band Application Summary Includes AOD
  • thecardoc3thecardoc3 Member Posts: 5,745
    Watch this video too. That will help you see how the clutch connects to the other members. https://www.youtube.com/watch?v=qgs3MNiLJrM
  • YriYri Member Posts: 3
    edited April 2020
    thecardoc3 said:

    Watch this video too. That will help you see how the clutch connects to the other members. https://www.youtube.com/watch?v=qgs3MNiLJrM

    Thank you for replying. From what I understand now, the one way clutch is splined to the transmission casing, so the outer race never spins, but the inner race does. In this case, the inner race connects to the intermediate shaft, locking when going in the opposite direction of the engine output. I'm still confused as to how it actuates in low/reverse gear
  • YriYri Member Posts: 3
    OH WAIT, I think I get it now. So a one way clutch is splined to the transmission case. Now, that one way clutch is attached to a planet carrier, which has a clutch pack on it. When that clutch pack is applied, that one way clutch is also applied, which stops the planet carrier that the one way clutch's inner race is slipped on to . So, in other words, the one way clutch is activated by a clutch pack, and when that clutch pack is deactivated, the one way clutch is also deactivated. This makes some sense now.
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