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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 ?

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  • $\begingroup$ can you draw a picture? what's the motivation/ application? $\endgroup$
    – Pete W
    May 11 at 14:51
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    $\begingroup$ Picture/Sketch added $\endgroup$
    – janherich
    May 11 at 20:04
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    $\begingroup$ 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. $\endgroup$
    – Pete W
    May 11 at 21:16
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    $\begingroup$ Thanks for all the advices, I will try to build the thing and share the results. Regarding alternatives: $\endgroup$
    – janherich
    May 11 at 21:53
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    $\begingroup$ 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. $\endgroup$
    – carloc
    May 12 at 5:41

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