LEGO – Fluid Drive Madness

So…ever since I was a wee lad, I dreamt of building one of these little contraptions, and yet, here it is, achieved, it seems. I should be over the moon right?? Well, I am. :)

So, here it is…the big reveal…a LEGO Fluid Drive Coupling…which actually works!!


I’m actually surprised how quickly I got to this point…but as you’ll see from the video at the bottom of this post, the darn thing works!


What on earth could I use such a coupling for you may ask? Well…the venerable London Routemaster bus used one successfully, as did many vehicles of the Chrysler Corporation all those years ago. And let’s not forget, today’s far-advanced cousin, the torque converter –  as seen in countless automatic transmissions – no doubt owes its very existence to the humble fluid coupling.

The advantage this coupling offers over a conventional dry plate friction clutch is smoothness – to the extreme. And you will see this touted no doubt in many old-time promotional materials and videos concerning fluid drives…smooth this, smooth that, no jerks in the drive-line, less wear and tear, etc, etc.

I actually got the idea whilst building my Revell Routemaster London Bus plastic model kit (yes that’s right, I’ve started building a model kit and haven’t yet blogged about it – naughty me!). So now much of my energy is being spent refining this fluid coupling of mine.

So let’s delve further into the technical details of this shall we?

How It Works

The basic principle of a fluid coupling is as follows: you have two elements opposing each other…the input and the output elements. These elements are NOT physically connected in any way whatsoever. The input element is the “driving” element – or pump – which is powered by an engine or motor. The output element is connected to the transmission and thus to the wheels (for example) of a vehicle and serves as the “driven” element – or turbine. The key component in a fluid drive is, as the name states, FLUID. Without fluid, the coupling would not function. It’s hard to imagine that such a flighty liquid could actually transmit torque and driving force, but this is in effect what is exactly happening in a fluid drive. There is of course bound to be a certain degree of slippage in the unit as a matter of principle, but after seeing this thing work “in real”, in LEGO form, I am utterly convinced that I can use this coupling to power my next LEGO model vehicle.

The old-time image below illustrates the principle of operation of the fluid couple. Here it is likened to two electric fans opposing each other. One plugged-in and running, the other stationary and off. The running fan produces airflow which is directed towards the opposing stationary fan, which actually causes it to rotate also. The principle of a fluid drive is the same, however instead of air, a fluid medium is employed (fluid, being much less compressible, can impart a greater amount of force and influence than air ever could).


So, the LEGO fluid coupling I have doesn’t actually need too much oil, it’s actually kept about a quarter-full which is just as well, because having it too full would mean greater incidence of oil leakage. The rear output shaft seal on my prototype isn’t that great you see, and so I have to play things cautiously.

What is great is the ‘welds’ I’ve achieved around the coupling case you may have noticed in the pictures so far. Crude as they may seem, they are actually quite sturdy and serve the hold the two halves of the casing together even while it spins around at great speed, with the centrifugal force of the oil pressing ever outwards against the join.

If you haven’t already gathered, the coupling case – in the prototype version at least – is simply two bottle-caps joined together, permanently (or semi-permanently) sealing the LEGO components in an (almost) leak-proof casing. The input shaft is actually bonded to the couple casing, so that the unit spins as one. As it does, oil inside gets flung about with great force, significantly influencing the rotation of the output shaft turbine, which is free-wheeling. It works surprisingly well in practice (it’s greatly exceeded my expectations) and I’m quite excited about it all.

I won’ t go too far into the technicalities here and unfortunately I don’t have pictures of the unit disassembled, but the video at the end of this post will hopefully help to show and explain just how this thing works.

Suffice to say, it works, and I’ll be doing my best to implement it into a vehicle and get it off the test bed.


As always, here’s a gallery with images and video to do the rest of the talking. Enjoy, and stay tuned for more (hopefully).

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