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prevent oil/liquid from getting through threads on actuator

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Instead of a linear piston arrangement have you considered small positive displacement pumps?

I guess peristaltic pumps would be too big.

Does it have to be oil? Blood pressure cuff air pumps are small and cheap - but they don't give FB like a stepper / gear arrangement can.

Maybe something like below:

I did consider pumps, but i would probably have to make them custom to fit in the size/ performance i need. a concern i had was the 1 second minimum to maximum actuation requirement - would a small pump be fast enough? likely driven by nema8 stepper.

i considered that first (Using air) and did real world testing with that concept (3 way solenoid + stepper motor needle valve). getting the varying actuation amounts and speed in a small package (still haven't achieved size, but i can with a custom machined manifold) was very hard, and I still don't even know how i was (or am) going to get the air source to fit, thus a non-compressible liquid was worth a shot to try next i figured, since it would be entirely closed loop - thus no air tank, compressor, etc. also lower power draw.

as for @DTJ 's proposal, i don't believe using your method would give me any extra room, and would actually take up more space. if i could have rods sticking out the back, it would be nice, but due to size constraints i cannot.

since the ideal solution, sealing the axis of the actuator seems impossible as told to me by two people here so far, I have designed this non ideal solution, that only gives me two inches of fluid actuation at about 0.65" ID diameter. I believe this should be enough though. here are some images.

i am still very open to suggestions. thanks.

Andy Watson:
Could you possibly re-configure the lead-screw to run through the middle of the stepper motor - like these:

Maybe you can get a reasonable seal with O-rings if you use some rounded thread. But I do not have much hope for that.

A partial solution is to accept some leakage. Then close the end of the tube and combine it with some method  to return the leaked oil to a reservoir.

Do all pistons have to move at the same time?
I'm thinking more along the line of a single bigger pump and valves to distribute the fluid.

The power needed is a simple multiplication between flow rate and pressure.
Stepper motors are also not the most energy dense motors. BLDC motors are much better at this.

I don't think a rounded thread would help with an O-ring.  Any continuous ring would need to pass over a thread. 


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