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stirrup pump construction questions

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stirrup pump construction questions

Unread postAuthor: SilentCyan » Tue Jan 06, 2009 8:02 am

I am currently in the process of building a stirrup pump but have run into a few problems. my first problem is that i could not find any o-rings that are an exact fit in 3/4" type L copper. would it be better to use one that's slightly to small and stretch it over something or is cutting a piece out of a large one so that it's an exact fit and super gluing it back together acceptable?

my second problem is with my check valve. it's a washer i filed down to fit inside a 3/4" copper thread adapter(very similar to the one Gippeto made in his tutorial). the problem is that i had a friend solder the pipe and fitting together but he forgot to solder the washer as well so it leaks air like crazy. is there any good way to seal it without having to take it apart? i'll post some pictures later in the afternoon.
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Unread postAuthor: Gippeto » Tue Jan 06, 2009 2:42 pm

3/4" type M has an id of .811" and the #114 o-ring has an od of .813

The next smallest size is 113, which has an od of .75" and is too small.

3/4" type L has an id of .785"

You'll either have to swap out the type L for type M, try to find metric o-rings, or cut and glue an oring and hope it works.

I suppose I would try modding an o-ring first.


Remove the bolt and spring, and solder the washer from underneath.
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Unread postAuthor: Sticky_Tape » Tue Jan 06, 2009 3:01 pm

Actually he won't need a new piece of pipe I used type L with #114 orings on my shock pump. The oring can compress enough which will aid in sealing/the oring doesn't need to fit perfectly. At least that's my exsperience. BTW you don't need to file a washer to fit into the male threaded fitting you can use a 5/16'' washer.
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Unread postAuthor: SilentCyan » Tue Jan 06, 2009 6:30 pm

selection was kind of limited seeing as how i was shopping at my local lowes. they had a small selection of o-rings none of which fit nice enough as they were. i already cut and glued an o-ring(fairly tight fit but i bought some silicon lubricant) so i'll see how that goes. as for the washer i grabbed the closest fight that i could find and then ground it down but i'll be sure to keep an eye out for that size if i ever happen to need another.

i guess i'm going to try soldering it and hope i don't make a big mess of it. only problem is its going to have to wait until tomorrow as i don't currently own a soldering iron. does it really matter what type of solder i use?
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Unread postAuthor: psycix » Tue Jan 06, 2009 6:52 pm

From what I remember, I say that an o-ring needs to be somewhat to big in order to seal.
The force to seal is gained from squeezing the o-ring, Im not very sure, but I thought O-rings had to be squeezed 5%-10%.
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Unread postAuthor: Gippeto » Tue Jan 06, 2009 7:56 pm

Actually, the force to seal comes from compressing the air. The compressing air, forces the o-ring against the solid ledge of the piston, and also against the wall of the pipe.

But I'll agree, it does help if the o-ring is at least touching the pipe wall initially. :)

I suppose if Sticky's pump works with the 114, yours should too. :)

You should be using a torch when soldering copper pipe, but if you happen to have a big enough soldering iron, what the heck.

After you solder something together, is not the best time to ask if it's okay to use a different product.

Get the lap shear strength for it and calculate the failure pressure of the joint. You should find the lap shear strength on the manufacturers website. If not, try google.

Also, get the melting temp.
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Unread postAuthor: Sticky_Tape » Tue Jan 06, 2009 8:46 pm

Just for fun I made this diagram of a floating oring piston.
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Unread postAuthor: SilentCyan » Wed Jan 07, 2009 7:10 am

hmm, i haven't soldered with a torch before.... this should be interesting :D
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Unread postAuthor: kenbo0422 » Wed Oct 14, 2009 10:03 am

O-rings can be stretched. Industry recommendations for o-rings:

Up to 5% stretch, best at 2%

40% compression for non moving parts, 30% for moving parts.

This should give you an idea of the groove depth to the wall of the outer pipe for the compression for a given size.

To seal a 1/16" gap, you could use a 1/4" cross-section o-ring in a 1/8" deep groove. This will give you about 25% compression, or close to the 30% recommended.

Make your grooves deep enough to prevent the o-ring from rolling out. Alternatively, square profile o-rings can be used which are easier to keep in place, and also easier to glue together, should you buy a roll of it.
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