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stirrup pump

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Unread postAuthor: Gippeto » Sat Feb 07, 2009 2:31 pm

john bunsenburner wrote:Well i weight 65kg and need about 50bar

50*14.5=725

725/25=29

143/29=4.9=>5


I would need a 5mm pump bore to get 50bar@65kg if i used all my body weight...



Hmmm, no.

As I prefer to work in psi,

You weigh 65kg, that's 143 pounds.

You want 50 bar or 725psi.

143pounds/725psi = .1972 in.sq.

pi*r*r = .1972 in.sq. so r=.2505"

Diameter = 2*r so D=.501"

You need a piston bore of ~ .5 inches, not 5 mm.
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Unread postAuthor: john bunsenburner » Sat Feb 07, 2009 2:33 pm

Sorry, i should REALLY think about my maths before doign it... and i can get the needed pressure with a 1cm bore pump easyly(yes i know its 1.2cm but i prefer 1 cm, easyer to work with)
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Unread postAuthor: roughboy » Sat Feb 07, 2009 2:38 pm

which is better to build a floating o ring stirrup pump or a 2 check valve on the bottom pump?

and which style of pump can give more higher pressure?
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Unread postAuthor: Gippeto » Sat Feb 07, 2009 2:50 pm

With the piston at the bottom of the cylinder, the less volume between the piston face, and the check valve, the better.

Think of it like this,

If your pump cylinder has a volume of 10 cu.in. , and you have 1 cu.in dead space in the "pump head", you have a compression ratio of 10:1 .

This means that in "theory" you could compress the air 10 times to 145psi max.

If you cut the dead space to .1cu.in. , then the compression ratio goes to 100:1 with an attendant increase in theoretical maximum pressure.

Now, bore size comes into play, as you have only so much mass to throw around.

See previous post.

To sum up,

Regardless of bore size and the weight you have to put onto the piston head, if you utilize a "largish" dead space design, you limit the compression ratio, and the ability of the pump to create pressure.

The floating o-ring design is better IMHO.


:)
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Unread postAuthor: Ragnarok » Sat Feb 07, 2009 3:06 pm

Actually, there is a third option. My pump has fixed O-rings, and only one check valve (I started with two, then swapped down to one).

The trick is a vent hole in the side of the pump shaft. Once the pump is pulled back past it, it lets the air in and does it's job.

Fair enough, you have to pull the pump back against a vacuum, but as it can be no more than -14.7 psi relative (which it won't reach anyway, as there is always some dead volume), it's not hard to do that.

You can see said hole approximately 8 inches down the cylinder in this picture. Dead centre (to use the bricks in the background as a scale) of the third complete brick.

Not easy to see if you're not looking, I admit, but it's there.
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Unread postAuthor: roughboy » Sat Feb 07, 2009 3:09 pm

what should i use for the body if i can't get my hands on copper pipes? PVC or galvanized steel pipe?
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Unread postAuthor: john bunsenburner » Sat Feb 07, 2009 3:12 pm

The steel will hold alot mor epressure and is safer, i woudl go for that!
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Unread postAuthor: Ragnarok » Sat Feb 07, 2009 3:20 pm

Galvanised pipe may not have an interior smooth enough to use. That is not to say that it won't, but there's no guarantee it will either.

Personally my recommendation is that if you can't get your hands on copper, you look harder. ;)
It is generally the best material for home-made pumps.
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Unread postAuthor: psycix » Sun Feb 08, 2009 8:06 am

The steel will hold alot mor epressure and is safer, i woudl go for that!

Copper can be used up to 500 psi. So if he wants a pump that goes up to 500, then copper is just fine.
Aluminum could also be used.
BTW You should learn to use the spell check. I'm using the firefox spell check plugin and I highly recommend it.


Ragnarok, interesting that you used a mechanism that simply pulled vacuum. It is however, lifting 3.2 kilograms every stroke so it would be something you feel in your arms at the end of the day. But on the other hand, its some extra exercise.

Besides pulling vacuum most people go with a floating O-ring (minimized dead space while not pulling vacuum.) But using 2 check valves at the bottom has some use too. It may create some extra dead volume, but it does allow you to use the pump as a vacuum pump.


A little hijack: I might make an homemade compressor by building a stirrup pump like this and attaching a crank + motor. It would have to go up to 60 bar as I already have a fridge compressor which goes up to 30. Now the question is: Would I use a full-length pump with one heck of a crank mechanism or use a shorter stroke pump, being much more compact though having more problems with dead space. Note that I have access to a machine shop for this.
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Unread postAuthor: Ragnarok » Sun Feb 08, 2009 8:41 am

psycix wrote:Ragnarok, interesting that you used a mechanism that simply pulled vacuum.

In theory, it has the highest pressure potential. In practise, it was an easy solution to one of the check valves buggering up.

It is however, lifting 3.2 kilograms every stroke so it would be something you feel in your arms at the end of the day.

You clearly have no idea what my working backpack weighs.
Heck, I think the laptop alone is 3.8 kilos. (Screw those 3 lb Macbooks.)
Add in the spare batteries, and my books... it's a lot more.

Also, those 3 and a bit kilos is fairly irrelevant compared to the compressive forces.
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Unread postAuthor: john bunsenburner » Sun Feb 08, 2009 9:48 am

psycix i woudl use a smaller more compact cranck mech and maybe hook up two small pumps, its just easyer than trying to find a place for a 1m monster. Also the common motor(i suspect you are goign for a washign machine motor) has a fairly high rpm and so even if you shorten the pump it will still be pretty swift, i recomend you hook either your fridge or you workshop compressor onto it to make things faster.

P.s. Heck even my school bag weighs around 15kg, and i definately prefer pumping up my gun than doing pull ups... :twisted:
And go knows maybe you could even get people to do it for you, just paint you pump, stick it into a fitness room and put some instructions next to it, then attach the pump to a very large tank from which you take air as needed. :D
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Unread postAuthor: sputnick » Sun Feb 08, 2009 11:06 am

I like johns idea, we shall call it, the pumpflex! all we have to do is put some carbon rods behind it to make it look legit, and then you have people lining up to try it... it cannot fail! except for when no one vents the pressure or you don't have time to go shooting one week, and the enormous tank explodes... but apart from that hell yea!
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Unread postAuthor: john bunsenburner » Sun Feb 08, 2009 11:16 am

safety pop of valve is you answer...hmm i made a deal with a bording school to breed fish in their pool, maybe this is another little project of mine!
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Unread postAuthor: psycix » Mon Feb 09, 2009 11:03 am

Hahah lets start a fitness center where every movement of the people is used to compress air.
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Unread postAuthor: Brian the brain » Mon Feb 09, 2009 5:39 pm

And keep our teeth...

Appearance is everything these days...

I suggest you get a GOOD fridgepump as that will put out 60 bar.

Perhaps you should borrow mine to try it out on your....most dangerous thing I've ever seen before my latest creation...

We need to go and shoot Mr. Porter some time anyway..
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