Retrotechtacular: Fluid Coupling


We realize the transmission fluid of an automobile’s automatic transmission is used to transfer the power from the engine to the drive shaft. But after watching this Department of Defense video from 1954 we now have a full understanding of the principles involved in fluid coupling. Like us, you probably have seen a diagram of a transmission which shows the fan-like blades that are affected by the moving fluid. But it’s worth watching the 12-minute clip after the break to understand how that liquid is moving and why that matters so much in the design. The motion of the rotors, along with the design of the enclosure, causes the fluid to move in a continual corkscrew —  the shape of slinky whose ends have been attached to each other. This type of illustration leads to an intuitive understanding of how it’s possible to facilitate an efficient power transfer using a liquid.

Check out some of the comments left in the Reddit thread regarding this film. We agree with [Runxctry]; there’s something about the format of the presentation that makes these informative and engaging to an almost addictive level. But maybe it’s just the engineering geek deep inside that’s cause these feelings?


  1. defaultex says:

    I understood how they worked from a textbook definition. But the way this video presents it makes it feel so simple that I could build a fluid coupling in my kitchen out of ordinary materials. Really helps to see something without all the ridiculous jargon that’s gotten attached to it over the years.

    • This reminds me of the mechanical computer videos from the Navy and the old movie about how a differential works. They make things seem so intuitively simple.

      I think what makes them work is that they take the time to go step by step. Sure, that 10 seconds of watching fluid in a bowl spinning around may seem “slow”, but when they show several other steps in the same manner and then put them together, it just clicks.

      I’d love to see an intro engineering course in a college that was totally built around these types of videos and actual physical models for students to play with.

      • Mr Name Required says:

        You beat me to it! If you liked this film then look on YouTube for the one on analog computers called ‘Fire Control Computers’. It is as good as this one – simple graphics, logical steps and no background music combine to make informative viewing.

    • AllThatJazz says:

      Actually, you can build a demo out of ordinary kitchen materials:
      1. Fill a deep bowl 3/4 full with water
      2. Stir with a spoon in one direction to create a ‘whirlpool’
      3. Drop in another spoon and watch as it revolves, too. Voila! Fluid coupling to transfer mechanical energy!

    • Alex Rossie says:

      I genuinely thought automatic/fluid transmissions still used clutches!!! Never driven an auto so I forgive my self

      • They usually do, but only above a certain road speed. That’s the only way to be 100% efficient.

        • To clarify: some more modern and efficient automatics implement a locking system that locks the input shaft to the output such that there’s no slippage. Sometimes that looks like a clutch, sometimes not.

          More generally, “automatic” means a transmission has a fluid coupling (torque converter). There ARE, however, a number of transmissions which appear to be automatic but, by definition, are not. For example, a DCT (double clutch transmission). This style has two clutch packs (one for even, one for odd gears) which are actuated electronically, but in the same sort of style that a traditional manual is.

      • Joe Cool says:

        The fluid coupling/torque converter is used to transfer power/torque from the motor to the transmission. There is a (usually hydraulically or electrically operated) gearbox attached to the output shaft. The gearbox component does use clutches and or drums/bands to engage and disengage the various gears sets.This is done by an extension of the hydraulics and appears automatic to the operator.
        The more modern systems also utilise computers and in some cases a (almost) conventional gearbox. That is the ‘simple’ explanation! All the above is in a single enclosure.

  2. Tom the Brat says:

    Always wondered how that worked. A very clear explanation. I’ve always prefered a clutch myself, but I’ll drive my automagic with more respect from now on.

  3. enlight22 says:

    love these DOD videos… Department of defence is so awesome

  4. Hirudinea says:

    I haven’t watched the video yet but it looks like an army video and since the army has to teach things to soldiers who can be anything from geniuses to slightly retarded it would have to explain it in the simplest terms possible. (That’s not a shot, its challenging to make instructions that EVERYONE can understand.)

    • barry99705 says:

      Most military technical manuals are written at an 8th grade reading level for the same reason.

      • lwatcdr says:

        “Most military technical manuals are written at an 8th grade reading level for the same reason.”
        Actually I believe they may be written at a lower level than that. What most people do not get is that when writing technical manuals and documention that the goal is the write it at the lowest gradelevel possible. Any idiot can make a simple concept complicated it takes reall tallent to make the complex simple.

        • cmholm says:

          I think Iwatcdr is on the right track. Consider that today – and say what you will about social promotions in public schools – over 80% of Americans are high school graduates. In the 1930’s, less than half. They were working with really raw material.

  5. Fantastic video — incredibly low tech filming of course, but it gets the point across wonderfully.

    • I think that’s the reason why it works. They didn’t have fancy videos or computer animation, so they had to actually think about how to explain things. That pipe cleaner bit is a prime example.

      • n0lkk says:

        Tools are tools. A person picking up a hammer today has to think about how they will use it on a project like a person had to 150 years ago. A person using modern animation tools still have to think about how they are going to use them in a project, just like those who made this film had to think about how they where going to use the tools they had. That pipe cleaner was a tool. Due to modern tools there are so many animations available to us. Yea some are better than others.

  6. lukefabis says:

    A bit more math would’ve been nice — at least a brief segment on adding vectors to explain why the two directions of fluid motion add up the way they do, if not all the equations you’d need to figure out how to build a decent coupling yourself.

    That said, I do appreciate the way these videos go step-by-step through the theory behind these devices and principles, with ample demonstrations, illustrations, and analogies. You just don’t see videos like that anymore.

    • Brian Neeley says:

      The math isn’t really necessary. If you already know vectors, then that gives you added depth, but mentioning vectors and phasors (do remember this is pre-Star Trek) would definately be over some peoples heads, and that would tend to make them loose interest.

      Actually, when I was in the Navy, they taught us (Naval Nuclear Power School) integeration and differentation of graphs. Now, 90%+ of those in the nuclear power program (enlisted, the officers had already had enough calculus in college to understand the principles at a numeric level) could have handled calculus, but in wasn’t necessary for us to be able to do that level of math. We simply needed to understand rates of change, and how an increase or decrease in one thing effected another. And trust me when I say that we already had enough things to understand. Teaching calculus would have added several weeks to the school, and wouldn’t have given enough benefit to warrent it’s inclusion.

      And if the military does understand one thing, that would be cost/benefit analysis of training.

  7. James says:

    Ok. Every time I see one of these style videos on here, I have to watch it. I don’t know quite when they went totally and utterly wrong making informational videos, but they have done. I have very, very little interest in cars (Nice bits of kit, I appreciate their complexity, but I’m a electronics guy) and I sat at 2am and watched this.

    Everything in it is simple and clear and actually informative. Not crammed full of marketing BS, no attempt to be hip or ‘with it’, just the facts, a simple, clear demonstration and an explanation. Bonus points for making the engine and transmission a pair of black boxes. Don’t distract the learner from the actual lesson with more complex stuff and moving parts they can watch.

    Seriously. Make it a weekly feature. Find one of these videos, stick it up and tag em all so they can be easily found. I’d watch the hell out of them. Really doesn’t matter on the subject at all.

    • masterburner says:

      If you’d like to see more, check out the “publicresourceorg” channel on youtube. They seem to have a bunch of these interesting videos with great explanation. I watch a few of those every night. This stuff is better than watching the “science” channels on TV! :)

  8. Hitek146 says:

    “We realize the transmission fluid of an automobile’s automatic transmission is used to transfer the power from the engine to the drive shaft”…

    Uhh, no. The transmission fluid in an automatic transmission is used for lubrication of the gears that transmit power from the torque converter to the drive shaft, and is also utilized in the valve body, which acts as a sort of hydraulic logic circuit. The torque converter, which is what the guy is pointing to in the photo, is sealed and uses it’s own fluid to buffer the output of the crankshaft to the input of the transmission. The torque converter is usually located in the bell housing of the transmission or transaxle, with the exception of some odd configurations, such as some Porsches, which place the transmission at the rear differential, and leave the torque converter mounted to the back of the engine in the front of the auto.

    • drewlssix says:

      Well that would suprise not only me but my drivetrain instructors back in school then entire aoutomotive industry and the racing community because every single auto trans I have ever seen, rebuilt or heard of lacks this magical sealed torque converter you speak of!

      Plain and simple you are wrong and anyone with a search engine can become better educated than you seem to be in about 5 minutes.

      • Hitek146 says:

        Do you mean that they lack torque converters or that the torque converters were not “sealed”? Because the example I pointed out(the photo) and Wikipedia(I know, not the best source) would indicate that the torque converters were sealed, and the few units I have held in my hands were sealed. Surely you are not arguing that a torque converter is somehow integral to a transmission…

        • bagetu says:

          The DoD video shows a fluid coupling which is different to a torque converter. The fluid coupling shown has two elements, the impellor and the turbine whereas a torque converter has three elements, the impellor, the turbine and the stator.

          Fluid couplings come in both sealed and unsealed versions depending on the application.

          Automotive Torque Converters are not sealed, they share the same oil as the transmission. The oil enters and exits the torque converter through the hub. This is similar to the cooler setup shown 10 minutes into the DoD video except the oil enters and exits on the same side..

          • spike says:

            The device in the video *IS* a torque converter.

          • Joe Cool says:

            Spike: The device in the army video is a fluid coupling. Torque converters are very similar but contain extra internal components that improve the efficiency of the device and can actually allow a higher relative output speed than the fluid coupling.

            To other posters – the older fluid couplings/torque converters usually had a common fluid reservoir and pump to the rest of the transmission and were ‘open’ (if you pull a converter/coupling from its transmission you will almost invariably gel transmission fluid leaking!). I believe that there ARE modern converters that are hydraulically isolated from the rest of the transmission. An example could be viscous couplings or ferro/magnetic couplings. I think even these would still need plumbing for cooling and to maintain pressure. Both of which are mentioned in the video!

        • krazeecain says:

          Yes it is, I know this because I did lots of research into my old crown vics transmission (ended up finding out it was toast.) In order for me to do a full transmission fluid flush, it was necessary to remove a drain plug from the bell housing, and align the torque converter so a small drain hole in the torque converter itself could allow the fluid to drain properly. I’m not sure if this is how all torque converters are, but I know for sure that’s the way it is on my old car (and therefore, nearly every other rwd ford that shares the same basic transmission design.)

          It just makes sense to share the fluid with the torque converter, because it needs to be cooled anyways. Isolating 2 different fluids would be costly and complicated.

      • pcf11 says:

        The torque converters look separate, and outside of transmissions to me too. But all automatics seem to have them.

    • alig says:

      On my forklift it uses the same fluid and it is not sealed.

  9. Nikita says:

    Interesting…I always thought these were filled with silicon fluid that heats up and thickens as the the unit spins!

  10. Nunya Bidness says:

    …and hackaday fully joins the list of sites that just repost crap from Reddit.

    • Whatnot says:

      Hackaday takes lots of stuff from various sources, there is nothing wrong with that if you pick the right stuff and mix it with submissions.

      And incidentally reddits also often enough uses HaD as source material.

  11. Herbööö says:

    Happy new year hack a day it has been a pleasure to read your posts this year and yea im drunk so thats all!

  12. Toaster_Chicken says:

    “The smooth application of the clutch plates to allow the engine to take hole slowly and smoothly takes considerable skill on part of the driver.” IT WAS THESE JERK HOLES! They’re responsible for all these crappy drivers on the road! If it doesn’t take considerable skill on part of the driver then any old moron can get behind the wheel!

  13. Dr DFTBA says:

    Mike Szczys seems to treat this site like it’s his own blog.

    • Whatnot says:

      I think that’s the big thing for 2013: Bitching about everything and everybody regardless if it’s justified or relevant.

      (When I say everything and everybody I of course don’t mean the politicians you should bitch about, those get a free pass.)

  14. sandlapper says:

    Say what you will, the 1950’s DoD spent a lot of time on education. I learned basic Electronics from a set of books from this same time frame. Still have them, and use them when teaching young people the first steps. I wish our modern educators would study these products, imagine the what is possible if we had training tools this simple and understandable explaining networks, databases, ad your technology here….

    • n0lkk says:

      Not sure if your wish for simple training tools if from the viewpoint of an educator, or a student. Respectfully if it’s from the viewpoint of an educators why not step up and create them or help create them in a cooperative effort? Not that I think it would be easy. Anyway no doubt such teaching aids are already out there on the web, and you ain’t found them yet.

    • lwatcdr says:

      They pretty much had too. The US pretty much had a draft from 1940 to 1973. Once you where a bit over 18 and male you had a good chance to end up in the military unless you where in college, married, worked in a needed job and so on. In many ways I think this was why the US excelled in so many ways during that time. If you where a clueless, lazy student you where going to be put in the military and they would have to find something for you to do. If you didn’t want to spend two years mopping floors and carring a rife you better learn a skill. If you really didn’t want to go then you better get good grades and become a sciene teacher or be involved in one of the sciences or enginering. The military was flooded with a bunch of people that they had to do something with so they spend a lot of effort teaching them. And of course they had the GI bill when they got out so they could go on with there education. Worked well but todays culture would never put up with it.

  15. John says:

    I love it when Hackaday digs up these old, but very informative videos!

  16. Matt says:

    I wish there were more videos like this in the world. I love the low tech nature of it.

  17. Adobe/Flash hater. says:

    I Inherited this version of the Army technical manual
    ” Principal of Automotive Vehicles TM9-2700 ”
    Amazing how well it explained so much.
    Even covered Turbochargers,
    40 years before Detroits big 3 acted it was some new idea.

    No on board computers ,of course, but it covers most all of the end pieces that do the work
    and still are fundamentally applicable to understanding a vehicles operation.

  18. JD says:

    That was the Army of our grandfathers…. Too bad the current Army is but a shadow of it’s former self.

  19. n0lkk says:

    My cousin’s husband done a lot of restoration, and mild custom on early ’50s GM cars. I can’t member what make it was it had a torque converter that was garage serviceable. Assemble with a lot of short bolts holding it together. As I recall he was able to find new parts except for gasket that was in good shape, so he hand to make that himself.

  20. Kian Mehr says:

    Good video, but it would have been better if it said something about how the coupling “disengages”.

    • Joe Cool says:

      Fluid couplings of the type demonstrated don’t disengage. They do have a state called decoupled where the input and output shafts are rotating at the same speed with no load e.g. coasting slowly down a hill . This simply means that there is no effective energy transfer between the two sides. At low speed the inherent inefficiency means that they appear to be disconnected.
      Any disengagement would occur in the gearbox component. I think that’s about right!!

  21. Marvin says:

    I simply love the “mode of speaking” they use in these old videos… I wonder why this was lost…. It’s almost hynotizing :)

    No wonder there are so many vocal samples from that time im all kinds of chill music…

  22. Ted R says:

    These kinds of videos are amazing. No flashy images, no high-tempo background music, no flashing images or zooming in and out to get your attention, no commercial breaks. Just the essentials, with pauses to let you digest the information.

  23. Bojangles says:

    So how about DIY project to build a water brake absorber for a dyno?

    Have you ever felt/seen/heard a 650 HP race engine tested on a SuperFlow dyno at 8000 rpm? It’ll make the hair on the back of your neck stand up!

    The tiny water brake is about 1 foot quare and about 6″ thick.

  24. david says:

    thanks for the post. i love the straightforwardness of this type of video. if you want more information, you can always go out and find it, but it does such a great job of covering the main points.

  25. Peter Lewis says:

    In that part with the set-up on the table (TRANSMISSION box on the left, ENGINE box on the right), why is the housing of the torque converter also turning?

  26. Peter Lewis says:

    I read (pictorial) books on this 45 years ago. But till today I still wonder about the (low) coupling efficiency. What is the efficiency when the turbine “catches up to speed”?

  27. For those who will say if it will blend, it will blend :)

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