prevent oil/liquid from getting through threads on actuator - Page 1
Forum section: electronics / general

author: super7800
author_rank: Regular Contributor
timestamp: October 13, 2021, 06:19:06 am
content: 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 isthat 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.

author: Ian.M
author_rank: Super Contributor
timestamp: October 13, 2021, 07:07:59 am
content: 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?

author: super7800
author_rank: Regular Contributor
timestamp: October 13, 2021, 02:36:54 pm
content: 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?

author: jpanhalt
author_rank: Super Contributor
timestamp: October 13, 2021, 03:20:18 pm
content: 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?

author: super7800
author_rank: Regular Contributor
timestamp: October 13, 2021, 03:31:58 pm
content: 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?

author: DTJ
author_rank: Super Contributor
timestamp: October 15, 2021, 02:57:00 am
content: 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:

author: super7800
author_rank: Regular Contributor
timestamp: October 18, 2021, 04:14:05 pm
content: 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.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 18, 2021, 04:39:43 pm
content: Could you possibly re-configure the lead-screw to run through the middle of the stepper motor - like these:https://en.nanotec.com/products/2968-nema8-linear-actuator-20-mm

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 30, 2021, 01:24:10 pm
content: 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.

author: jpanhalt
author_rank: Super Contributor
timestamp: October 30, 2021, 01:50:07 pm
content: I don't think a rounded thread would help with an O-ring. Any continuous ring would need to pass over a thread.