DIY pneumatic retracts (built from GWS mechanical's)

[dogon1013]

It's been a while since I Built these, and so far I only have one working, but it works perfectly.

The piston and cylinder is made form a 30 cent 3ml Syringe you can get at your local drugstore. The retract part is a stock GWS blue retract. the only other parts needed are a return spring, strengthening wire, and a retainer clip (made from a paper clip or music wire).
The retainer clip goes thru the hole in the GWS retract actuater arm and it's sole purpose is to keep the spring pushing onto the retract arm. It is shaped in a C in my prototype and fits inside the bore of the syringe.

The spring is going to be the hardest thing to find, it has to be strong enough to push the rubber piston back into the syringe, which takes a good amount of force, and it needs to be just the right size to fit over the actuator arm of the retract, and into the syringe. I am using a piece of recoil spring from one of my Kel-Tec pistols (i have extras). You can probably order the springs from Kel-Tec, but I am sure some other spring can be found.

At first I tried to buy the syringes at the local Eckerds drugs (now CVS) but they said their policy was not to sell needles unless you have a prescription. I guess they think if you can easilly get needles, you will of course want to inject illegal drugs :rolleyes: . Luckilly Wallmart is always willing to take your money. I got the syringes at the neighberhood market across the street without any problems or strange looks.

BE SURE TO REMOVE THE NEEDLE ON THE SYRINGE. I just used pliers and pulled them out. The little plastic piece that the needle went into makes a nice quick disconnect for the syringe retract. It has threads and seals tight. I just trimmed the end and pushed the tube over it. A threaded retainer can be used to keep it tight or simply glue it on.


Enjoy


Now all we need is a cheap valve and air tank. :D

[DRETW]

That looks good. I thought I might offer... I'm a diabetic and I use an insulin pump. Every three or so days I have to change my infusion. These are 3cc syringes with some really small tubing attached. I don't believe I'm violating any law by offering you some used ones. There are no needles or componets that have come in contact with me and the only medicine in them might be a microdrop of humalog which is safe. They also come with a neat quick connect but its kinda in the wrong place on the hose.
You can PM me if your interested and I'll mail you some. I'll cut off the very oposite end from the syringe (the infusion). You can then pay me back the postage.

[dogon1013]

Thanks for the offer.

I don't think plastic pieces could be considered illegal, no mater where you live, unless it is considered to be biohazard, but if you clean it, it should be fine. There is no law prohibiting the sale of needles, it was just the policy of the store (Eckerds/CVS) not to sell the needles to anyone without a prescription.

Am I correct in assumeing that the only way you can buy these insulin pump pieces is if they have the drugs pre-loaded in them?

[DRETW]

No, they do not come pre-loaded, my concern was the biohazard part. Humalog requires a prescription but I would rinse that out. Also, looking closer, I THINK this tubing will screw right on to the ends of the "cylinders" your using now. If thats the case, you can probably go to a pharmarcy and purchase a 10 cc cyl. and with a spring or two," volia " instant reservoir. Pardon my french! I think when you get up to the larger sizes you can buy just the syringe with no needle. Then all you need is a valve.
New, infusion sets are 12 for 130.00. That dosen't include the reservoir (3.00 each). Used, they are worthless. I found them at minimed.com but I've never bought them there, I get mine from my insurance which requires a prescription anyway.

[RobinBennett]

Would fish tank valves work?

Otherwise you might be able to make a spool valve from a small syringe (basically there is a hole in the wall half way along, when the plunger goes past it the air can escape through the hole. You'd use a servo to move the plunger.

[dogon1013]

fish tank valves might work, as long is they are not too tight. I don't have a fish tank so I'm not sure what they look like.

I tried to convert a syringe into a spool valve, but it takes alot of force to push the syringe plunger, and i doubt a micro servo could do it. A full size might though.

[DRETW]

The valve is gonna be tricky, your system is air pres.-up, spring pres-down, right?

[DRETW]

Okay, I think I've got your super-easy valve figured out. Company right now, post later.

[dogon1013]

The valve is gonna be tricky, your system is air pres.-up, spring pres-down, right?

correct.

[DRETW]

It works! Not exactly super-easy but not that hard to do either. This one was just a proof of concept. The rod (control horn?) is 1.mm.dia wire and the weight is 2.9 grams. All you have ti do is add enough hoses on the right side for however many gear(cylinders) you need. You can also alter the slots on the sides for however much travel you need but if this valve was done carefully a micro servo shouldnt have any trouble moving it.

[dogon1013]

That does look good. Is that made from the insulin pump syringe? If so what volume is it?

That double o-ring plunger is perfect. how does it actuate? it sort of looks like the pin is solidly mounted to the piston and follows a curved path in the cylinder?

If you leave a little extra length on the cylinder then you can put a screw in the side, to use as an overtravel stop and maybe as a place to mount it too.(on both sides of the piston)

[DRETW]

Yes, thats one of the pump reservoirs. Its about 5/8" in diameter and holds 3 ml (cc).
The double o-rings are nice but their still spaced kinda close together (about .160"). The angled slots along the sides cause the piston (spool) to move for and aft when rotated and the legnth of the slots control the travel. I think the more shallow the angle the smoother the spool works. I just made this one to see how/if it would work and then cut off the extra.
I like your idea of leaving a tab at each end for mounting.
The tubing is about 1.5 mm outside dia. and at first I wondered if it would work the cyl's fast enough, now I'm thinking it would be fine and could be adjusted (pinched) for a more scale operating speed.
When I finshed the other night I was supprised that mine held air good and with a cyl hooked to each end it even worked but I believe you'll have to do some "fine tuning" to get it exactly right. I've got plenty, mess up, just start over.

[dogon1013]

Yea I see that your tubes are very close together.
you could use 2 plungers in the same tube, just glue or screw them together with the correct space in between the O-rings. Then just pop off one or 2 of the orings that aren't needed.

[DRETW]

Yes you could but the minimum amount of spool movement keeps the resistance low. I think you would only want to use two plungers for a pressure up-pressure down system.
Also, I measured the internal diameter at .475" and the usuable stroke at 1." you could maybe get 1/8 or 3/16 more if you had to.

[dogon1013]

I started back on this project since GWS is coming out with a nice large P-38 like plane (called the GWS-38). I hope to use that plane as the flying testbed.

I ended up machining a very small spring retainer button. the picture is fuzzy but you get the idea. it has a slot in it for a tiny pin, and a slot going thru it for the arm of the retract. Once it's together it makes a nice neat package.

I am now planning on replaceing the wire retainer with some aluminum angle that attaches to the mounting base of the retract. this should stiffen up the retracts as well as make it easier to replace worn or broken parts. (no more glue).

[dogon1013]

Here's the latest update on this project.

I made it so it can be disasembled if necesarry. I removed the threads on the end of the syringe for the quickchange needle's so that there was a good sized shoulder to help hold the syringe in place. I drilled out the end of the syringe and pressed in a brass barb-fitting to hook up the air. I made a frame out of fiberglass sheet and wood that was gorilla glued together.

It does work, and I will try to upload a video of it.

One thing I noticed is that the syringe does have a lot of friction, so the return spring has to be pretty strong, and that means the air pressure has to be rather high too. I'll have to get a pressure gauge to see how high.

[dogon1013]

here's the video file (compressed in ZIP).

The pressure was a little too low and you can see ti doesn't retract all the way, but with the correct pressure it works well.

[lowdive]

great idea/design and thanks for sharing! i've got a set of these mech. retracts sitting around and now i think i know what to do with them. i'm thinking that one of them flavor injectors might also fit the bill for this. i've seen some that would be too big but i think there are smaller ones out there. i love it!

[dogon1013]

Here's a picture of the parts breakdown. I expect I will just glue the frame into the model and then install the retract and syringe.

A flavor injector may work, but I think they are usually too big. the ones I used are exactly the same width of the center part of the retract so it made making the frame very easy.


I did recently see syringes (with non-sharp needles) for sale at the hobby shop, in the glue area.

Toby

[lowdive]

need syringes... sans pricklies?
http://www.allegromedical.com/syringe-191879.html

(full selection: http://www.allegromedical.com/categories/shop-by-category/syringes/syringes/4056.html)


just remembered i had one of these things at home and looked it up. $15/100 pieces. $0.15/each!

[dave morris]

here's the video file (compressed in ZIP).

The pressure was a little too low and you can see ti doesn't retract all the way, but with the correct pressure it works well.


Nice work! :)

[crxmanpat]

Toby,

Are you still considering putting this in your GWS-38? I'm drooling over the possibilities for that and the GWS-262!

Pat

[dogon1013]

Yes, I think the GWS-38 might be the test-bed. I still want to simplify and hopefully lighten the design a little first. And I want to get to know the 38's flight characteristics a little more before I hack into it.

The 38 will be a cramped install (not a whole lot of room in the pod), but it should be able to easily handle the added weight.

[stacker]

dogon1013,
I just came accross this thread and wanted to let you know what a great idea you have had. Thanks for sharing it.

Cheers--Stacker

[osvaldoljunior]

you can try to use a pet bootle for gas, the pets can hold easy up to 100 PSI and are very litgh for a plane. I used to put up to 70 psi on coca pets (the max i can get from my bike pump) to presurize my water rockets!

[mech9]

Be very careful using plastic bottles, although they can take high pressure if they heat up (say from the sun) while under pressure they can explode.

Cheers Rob.