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Short question topic

A place to ask general spud cannon related questions.
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Unread postAuthor: shardbearer » Sat May 19, 2012 3:11 pm

Lockednloaded wrote:Does anyone know if 1/2" cpvc fits snugly in 3/4" cpvc? I'd like to use it as a bolt, but I'm not sure if it'd be a tight enough fit


No they don't fit snugly, it's a pretty loose fit (maybe a 10th of an inch difference). If you sand out 1/2" PVC you can get CPVC to fit inside.
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Unread postAuthor: MrCrowley » Sun May 20, 2012 10:07 pm

Okay so I made some preliminary drawings just to see what everyone thinks of the design. I was originally planning on having the piston reduce to a smaller diameter in between o-rings to save weight but I figure the gap is so small (~25mm once you give the o-rings room on either side) that it might not be worth it.

About tolerances, how tolerant does the piston need to be? The actual diameter of the housing is ~51.3mm but I figure I don't want the piston itself to be tight so I made it 50.5mm in the drawing (perhaps 51.0mm is better? O-ring depth would change to 2.9mm if so), a tolerance of 0.1mm would surely be enough when it comes to machining it, right? The o-rings on the current piston are 52.1mm in diameter WHEN they are on the piston (so this is their stretched diameter) and as you know that seems too tight. I figure a stretched diameter of 51.8mm would be enough compression for the o-rings to seal but then again I have no idea about how compressed they should be.

The female threads at either end of the piston need to be M10 and ~20mm deep. Actually, I need to change the length from the front of the piston to the first o-ring to 5cm instead of 4cm so the overall piston length is 10cm. Not sure why I used both cm and mm in the diagram, it was just a quick mock up.

The o-ring dimensions are for the ones I currently have, I should be able to pick up some more as I wont be able to salvage the ones from the current piston. I figure a groove depth of 2.65mm will make the 50.5mm diameter o-rings stretch to 51.8mm.

Anyway, I don't intend for this drawing to be technical, accurate or to scale. It is merely so you guys get an idea of what I'm talking about and we can discuss tolerances and such.

Image

Current piston:
Image
http://i75.photobucket.com/albums/i290/ ... MG2515.jpg
http://i75.photobucket.com/albums/i290/ ... MG2514.jpg
http://i75.photobucket.com/albums/i290/ ... MG2513.jpg
http://i75.photobucket.com/albums/i290/ ... MG2512.jpg
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Unread postAuthor: jackssmirkingrevenge » Sun May 20, 2012 10:31 pm

#327 o-rings are 52mm diameter.

I would recommend a cheap set like this: http://www.ebay.com/itm/RUSSEL-AIRCRAFT ... 52&vxp=mtr

I would make the piston 0.2mm dia less than the tube it's traveling in, and the o-ring fitted diameter 0.2mm larger.

Also, you can get away with delrin/acetal for this component in my view.

Footnote, a plastic ruler MADE IN NEW ZEALAND?! :D
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Unread postAuthor: MrCrowley » Sun May 20, 2012 10:39 pm

Oooh, sexy o-ring kit... me likey :D
Thanks for the tip on dimensions, I'll adjust the shoddy diagram accordingly.

Footnote, a plastic ruler MADE IN NEW ZEALAND?!

Our plastic centimetres are better than Taiwan/Chinese plastic centimetres :D

Also, you can get away with delrin/acetal for this component in my view.
Yeah I could do but I'd rather not take the risk.
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Last edited by MrCrowley on Sun May 20, 2012 10:41 pm, edited 1 time in total.
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Unread postAuthor: Lockednloaded » Sun May 20, 2012 10:40 pm

Do toolies style valve generally perform as well as a standard piston valve? And do the same general rules apply for them with seat ratios and pressure differential apply, or should can the sealing face be just a hair larger then the seat and still open?
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Unread postAuthor: jackssmirkingrevenge » Sun May 20, 2012 11:14 pm

MrCrowley wrote:Yeah I could do but I'd rather not take the risk.


I don't think it would really be a risk, and for the same price you could probably get a spare made :D

http://www.engineersedge.com/oring_general.htm

some useful design rules. How even is the tube surface? you might actually want to put more compression, at least 0.5mm o-ring OD increase over tube ID.

Do toolies style valve generally perform as well as a standard piston valve? And do the same general rules apply for them with seat ratios and pressure differential apply, or should can the sealing face be just a hair larger then the seat and still open?


Same rules apply, but consider that the air has to flow past the sealing face, so if sealing face OD is close to chamber ID, you've created a flow restriction.
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Unread postAuthor: MrCrowley » Mon May 21, 2012 12:12 am

jackssmirkingrevenge wrote:
MrCrowley wrote:Yeah I could do but I'd rather not take the risk.


I don't think it would really be a risk, and for the same price you could probably get a spare made :D

http://www.engineersedge.com/oring_general.htm

some useful design rules. How even is the tube surface? you might actually want to put more compression, at least 0.5mm o-ring OD increase over tube ID.
Well the tube surface has had a seam ground down and smoothed with JB-Weld (for any pitting) so a higher compression might be good*. We have to remember though that the current o-rings are stretched to ~52.0mm and the piston requires a mini sledge to move it so I wouldn't want to exceed ~51.8mm in stretched o-ring diameter. There are definite signs of friction wear on the top of the o-rings so they would have been even larger in diameter when first installed.

This is why I would rather go to a place locally as I can bring along the tube and the machinist would have a pretty good idea how best to machine the grooves and piston. Though if I can get Crna to do it, 3-10 day mail would only be about ~$10 with materials under $50; which is probably cheaper than outsourcing the job locally. No worries if you can't do it Crna, I'm sure I'll work something else out :)


*I'm aware I should probably pick up a S/S barrel nipple or something that doesn't have a seam but it can be quite difficult to find a place open on the weekends that sells anything like that. I remember I enquired about it when I first bought the galv. barrel nipple and I was told they only knew of a few stores that sell in trade quantities. I'm still keeping an eye out.
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Unread postAuthor: Crna Legija » Mon May 21, 2012 1:37 am

ill call a plastics place i know but its rather expensive last time I went their.
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Unread postAuthor: MrCrowley » Mon May 21, 2012 2:35 am

Yeah I remember from shop class in high school a meter length of nylon or something was ~$50 and my teacher couldn't order the stuff in shorter lengths.

If we can get aluminium for under $50, I'm happy using that. Which reminds me, how important is the quality (alloy?) of the aluminium?
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Unread postAuthor: Crna Legija » Mon May 21, 2012 3:07 am

If i find something at work it will be 6061-T6 if i buy some it will be from off cuts from a metal store so it would be unknown doubt it will matter.
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Unread postAuthor: MrCrowley » Mon May 21, 2012 11:17 pm

Now I'm thinking something like this if it isn't too much trouble:
Image

Ignore the front sealing face bit. The only difference really being the reduced diameter front of the piston to save weight but it's not really crucial so it can be omitted from the design if it makes things easier.

I think I will keep the diameter of the piston at 50.5mm, o-ring grooves at a 2.60mm depth—as that should add 1.4mm to the diameter totalling 51.90mm with a ~20% o-ring compression—and 3.5mm width (not sure how much width I should allow for compression). From the rear of the piston to the first o-ring groove will be 20mm, the gap between the two o-rings will be 30mm and there will be 10mm in front of the first o-ring before the piston diameter is reduced down to 35mm. This reduced section would be 40mm long. The front and rear of the piston would have M10 threads that penetrate each end to a depth of 20mm.

The big space behind the piston is reserved for the bumper which takes up about 40mm. It's difficult to accommodate for because it's difficult to tell how compressed it gets when the piston slams back against it. For all I know, I could have the piston almost riding right up with the bumper, so there was no piston travel, if I knew that the bumper was compressed a significant amount but I think it's better to leave a bit of extra room to be sure.

edit: and yeah I f*cked up the drawing, I was drawing it while I had a lecture at uni :oops:
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Last edited by MrCrowley on Tue May 22, 2012 12:19 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Mon May 21, 2012 11:28 pm

MrC, just a though - why not simply get a bunch of disks machined to the right diameters you need and fit them on your threaded rod piston? Less work for everyone, lighter, cheaper...
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Unread postAuthor: MrCrowley » Tue May 22, 2012 12:16 am

Well it's not less work for me, I'll tell you that :D

I'd rather pay money for a proper piston than re-do a threaded rod design. They're great if you don't have machining facilities and are relatively cheap but they're a bugger to put together correctly. Some of the disks on mine aren't actually straight, it's noticeable when the spool rod is attached as it doesn't centre in the pilot port but it doesn't appear to affect performance or operation.

Hmm actually... to expand on that idea, you could make two hockey puck-sized things about 3cm wide, each accommodating an o-ring. Then all you need is an M10 thread running through both of them. The problem with this is with the bumper, on my current piston you can see that weird conical washer at the rear which helps stop the washers and stuff from being bent out of shape when the piston shoots back. In this case, it would be needed to stop the rear hockey puck from being deformed in one way or another (the epoxy wont take the repeated blows). Some of the washers on the front o-ring of my current piston are bent out of shape and I'm not even sure if it's from piston bounce or slamming in to the back of the pilot area and before its recent modification, about half the washers were actually loose. With the solid body design, I would only have to worry about leaks in the o-ring area, not where the rods are threaded in to the body.

But yeah basically it comes down to me wanting a solution that is relatively permanent. I'm done with half-fixes that save money, if I'm going to get some machining done I may as well get something solid that requires little maintenance and has little room for things to go wrong. There's nothing really wrong with the piston I have at the moment, it seems to be in a stage where I've sorted out each individual problem over several piston designs except now it's a bit too tight.
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Unread postAuthor: Crna Legija » Tue May 22, 2012 2:16 am

That's all right but make a drawing lol. It would be much easier to make a hole piston then have to part off a hole bunch of washers.
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Unread postAuthor: MrCrowley » Tue May 22, 2012 2:17 am

Yeah sorry I'll try get round to a proper drawing tomorrow when I have a 2 hour evening lecture :D

edit: or I'll figure out how to use some CAD software...probably easier, my drawings are rubbish.
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