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I have been thinking about small hydrostatic transmission to directly translate movement of stepper motor (NEMA 17-23 size, geared with 3:1 or 5:1 ratio).

My design currently involves 2 identical radial piston motors/pumps with 3 cylinders all connected to central crankshaft and individual hydraulic lines connected pair of cylinders (pump<->motor), so there are no extra valves, reservoirs, etc.

Each hydraulic line (3 in case of 3 cylinder pump/motor combo) is filled and purged separately, after that it should be (theoretically) sealed maintenance-free system able to precisely translate bidirectional movement of pump-driving stepper motor to actuator.

There will be obviously friction losses, but given that continuous power requirements for the whole system are quite small (4-5W) and the driving motor has safe max operating power level at least ~5 of that, it should be ok.

Rough sketch of the concept (pump/motor internals like conrods/crank not shown): Hydro-transmission

Do you see something obviously wrong with the design, why it won't work as envisioned ?

janherich
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  • can you draw a picture? what's the motivation/ application? – Pete W May 11 '21 at 14:51
  • Sure, the intended purpose is to drive extruder gears (extruder screw in the future) in 3D printer hot-end - the moving extruder/hot-end head has to be light so rapid movement/acceleration is possible which means that mounting ~400g stepper motor directly on the head is not desirable (the whole print head is so far ~80g including hot-end melting the filament). The printer currently uses bowden tube and extruder gears with stepper motor mounted on rigidly on printer frame, but as the bowden tube has to be quite long (almost 1m), it has obvious limits. – janherich May 11 '21 at 19:41
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    Picture/Sketch added – janherich May 11 '21 at 20:04
  • ok, so it's something like [this](https://d1n63dwz6yw5wy.cloudfront.net/cache/catalog/he3d/he3d-k280-large-delta-1-750x930.jpg), and the extruder head is translating in 3 axes in space? – Pete W May 11 '21 at 20:59
  • First some alternatives, just in case: (1) You may be able to get a stepper that is more efficient in terms of weight, with a "pancake" type that has bigger diameter and less length. Different gears also, if you have not already considered. (2) You may be able to use a flexible shaft to transmit the rotational motion to the 3-axis translating stage, and control any error from the flexing by adding an optical encoder at the point of application. (3) Not sure what the limits of the bowden tube are, but if it is control, then you could maybe again improve it by feedback vs the "working end". – Pete W May 11 '21 at 21:06
  • (4) more similar to what is drawn, you could maybe use bicycle brake cables instead of hydraulic lines, to get the 3 "pistons" moving together. They could only pull of course. – Pete W May 11 '21 at 21:09
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    For the hydraulics, I suppose it is worth trying! If the "motor" shown is small, it could be quite elegant. The fluid dissolves more air, and esp. CO2 than water, so it could be helpful if there were a way to periodically release that. Look at bleeder valves on auto brakes. Alternatively you can reduce gas in solution before filling by a combination of heating, vacuum, and agitation. – Pete W May 11 '21 at 21:16
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    Thanks for all the advices, I will try to build the thing and share the results. Regarding alternatives: – janherich May 11 '21 at 21:53
  • 1) Steppers are quite heavy, even the pancake one weighs significantly more then 100g (that's pure motor without any extruding gears attached) and I want to make this future-proof, eq when I decide to drive remotely mounted miniature plasticising screw instead of traditional hot-end and power requirement will jump from 4-5W to 30-50W, I want to be able to provide that by just swapping the motor driving the pump for something more powerful -> as that's rigidly mounted and not moving, I don't care (within limits) how much does that weigh. – janherich May 11 '21 at 21:54
  • 2) Remote drive via flexible shaft is actually common solution to the problem, but I think that hydraulic lines can be both lighter and more flexible -> I have quite some experience with bicycle brake/shifting mechanics and hydraulic line are always able to cope with tight turns inside frame tubing much better then cables, it doesn't matter how you route them (as long as the tubes are not pinched) and shifting/braking is flawless in correctly set-up system, the same can't be said about cable operated shifting/brakes at all, they are very sensitive to proper routing, avoiding tight bends, etc. – janherich May 11 '21 at 21:54
  • 3) The firmware I'm using to control the 3d printer (Klipper) already has something called "pressure advance", which is tuning parameter attempting to correct for slack of filament inside bowden tube, but it's quite finicky, dependant on exact diameter of the filament (which is sometimes not constant even on one roll), etc. – janherich May 11 '21 at 21:54
  • 4) Yep, I'm aiming for very, very low weight, for prototype just proving that it works, I will use cylinders cut from extruded acrylic tubes + 3d printed parts from PLA, for "real" stuff, I plan to mold the cylinders/pistons from braided CFRP tube and use carbon sandwich construction for crank/crankcase/conrods + light kevlar wrapped hydraulic lines. – janherich May 11 '21 at 21:54
  • This is a cool idea. – Drew May 11 '21 at 22:33
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    Just a report on my experience, I saw something of the kind done ages ago on industrial scale. That was 2 double effect cylinders interlinked by hydraulic oil filled hoses. The shift of one ram had to be copied onto the other. The big issue was the micro air bubbles in the oil, starting from zero relative pressure, they turned oil in a compressible fluid and the result was a very slacky behaviour. Each change of direction exploited a heavy deadband but, in my guess, you just need unidirectional movement and this should help. – carloc May 12 '21 at 05:41

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