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prevent oil/liquid from getting through threads on actuator

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super7800:
in the most simplest of terms, i need to design a device to create 4 variable pressure points (in this case small pouches) on a arm band, times 2 arm bands for a total of 8 actuators. they should be able to fully inflate or deflate within 1 second. to accomplish this, I plan to use oil and a stepper motor piston system (pictured below). I believe the device is for training animals, but my professors are under an NDA and thus cant actually tell us what it does (EENG 1st semester senior design project). The device must also be as small as possible. Knowing this, i have designed what is pictured.

the shaft will be ACME 5mm (please excuse my horrible representation of them in the cad lol), connected on one end to a nema 8 stepper motor, running the whole length of the tube, only other connection point is through the tapped hole in the center of the slider (the one with 3 o-rings). the two o-ring is stationary plug that has threads to connect to the fittings (which connect to the pouches). shown here as quick connect, but will probably be barbs or other type later on. It does not support the ACME rod. for scale, the tubes are 3/4" OD, 5" long.

my question is that since its pushing out a liquid, some might find its way into the back of the tube, where its not supposed to be. What could i use to seal this, to prevent any oil from going through the "3 o-ring slider nut part"? What kind of seal should i look into using? i'm an EENG student, not a MENG student so i have no idea what seals or other devices are out there.

i hope i have been clear and concise. feel free to ask for clarification.

Ian.M:
I don't think there is any product on the market that can effectively seal an actuator screw on its thread.   Also, what stops the 'slider' nut spinning?

super7800:
Hm, that could be problematic. perhaps a threaded seal if those exist?

friction. easier for screw to turn and push then for the 3 (or 4) o rings to turn in tube (hopefully). I've seen something similar done a few times on homemade solder paste applicators, so i think its worth a shot. i'm open to other ideas for the actuation of fluid, but it has to be very small. thanks.


EDIT:
shown is another method i could use, keeping oil sealed with 3 O-Rings. however, with this i am left with about 2.3" of actuation out of a 5" tube (due to end cap). the shown coupler is only for the prototype- if this works nema8 steppers with integrated lead screws (5mm) will be used instead to maximize actuation distance. Perhaps this is an option?

jpanhalt:
1) I echo the problem of sealing a thread.  Why not use a conventional seal at the motor/coupler end and let everything else be wet?  In fact, you could then get rid of the O-rings.  And since a really tight seal is not needed, you could use a splined cylinder and piston to prevent the piston from rotating in the cylinder.

2) If I read the drawing correctly, you are using a "1/4-18 NC."  Do you mean a 1/4-18 NPT?

super7800:
sorry for the confusion, it is 1/4-18 NPT threads, and I plan to use 1/4" tube. the central shaft is ACME 5mm.

I don't know if that would work for this application, would that allow for pressure to be maintained within the "arm band oil bags"? i would think that the oil would move the path of least resistance - to the back of the plunger and limited pressure would be achieved?

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