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davidh
07-11-2009, 12:09 PM
im trying to repair this hand powered air pump that is used to put pressure into a water tank. inside the brass pump the piston assembly is three peices bolted to the bottom of the pump rod. two are pressed sheet metal and the third is a good ol o-ring.

its so rusty that it bends and ceases to function in use so, being of sound mind and with a couple lathes and some aluminum i figured it was a no-brainer to just make a new one that would last forever. . . . WRONG !

as can be seen in the pictures the new one is one peice with matching diameters to the sheet metal one, and the 4 holes are in the same place and the same size as the original valve but, it will pump like crazy when depressed but will not allow air to go into the pressure side of the seal when its retracted. there is a check valve in the bottom end of the cylinder that works great so that is not the problem, the problem is with my new valve.

what the heck am i missing here ? my brain won;t function so i am asking for some collective help. please ?
http://i260.photobucket.com/albums/ii12/tooldoc/temp090.jpg
http://i260.photobucket.com/albums/ii12/tooldoc/temp091.jpg
http://i260.photobucket.com/albums/ii12/tooldoc/temp098.jpg
http://i260.photobucket.com/albums/ii12/tooldoc/temp093.jpg

Peter.
07-11-2009, 12:24 PM
I have one of them, I think we must have 60 or 70 on the company. If you need the end parts I could get you new spares.

P.S. generally, you replace the o-ring to get it working again. The discs in your picture look to be ok, in that it should still pump with them and a new o-ring. I would look at the profile of the curve - it's probably not quite the same on your machined part. Either that or the o-ring is too tight a fit in the groove. When you press the plunger the o-ring is forced against the holes, when you lift it the o-ring un-covers the holes and air is drawn through them, around the o-ring and past the solid disc.

Barrington
07-11-2009, 01:09 PM
When you press the plunger the o-ring is forced against the holes, when you lift it the o-ring un-covers the holes and air is drawn through them, around the o-ring and past the solid disc.Are you sure about that :confused: - I would have thought the o-ring presses against the solid disc on compression, sealing the disc to the bore. On the pull stroke the o-ring would press against the edge of the holed disc allowing air to pass through the holes and around the side of the solid disc ???

However it works it, as you say, it would be important that the o-ring can move easily back and forth between the discs, and with clearance on the inner diameter on the pull stroke...

Cheers

.

Circlip
07-11-2009, 01:14 PM
Peter has it right!

Regards Ian.

larry_g
07-11-2009, 01:15 PM
I have one of them, I think we must have 60 or 70 on the company. If you need the end parts I could get you new spares.

P.S. generally, you replace the o-ring to get it working again. The discs in your picture look to be ok, in that it should still pump with them and a new o-ring. I would look at the profile of the curve - it's probably not quite the same on your machined part. Either that or the o-ring is too tight a fit in the groove. When you press the plunger the o-ring is forced against the holes, when you lift it the o-ring un-covers the holes and air is drawn through them, around the o-ring and past the solid disc.
I think that Peter nailed it. If you look at the last picture I suspect that the plate with 4 holes in it is upside down. When assembling the piston make sure that the o-ring floats in the recess 'tween the two plates. If it is tight it will seal the 4 holes at all times and not act like the check valve it doubles as.

lg
no neat sig line

Barrington
07-11-2009, 01:54 PM
Hmmm, not convinced yet...

A bit of cut and paste shows there an awful lot of hole to seal - even if the o-ring could be flattened sufficiently.

http://i564.photobucket.com/albums/ss82/MrBarrington/Image1-1.jpg

:confused:

Cheers

.

Peter.
07-11-2009, 01:55 PM
Yes actually it could be that the replacement part was fitted with the holes towards the end instead of towards the handle. That would cause it to pull a vacuum in the tube rather than draw air into the space for pumping.

EDIT: My apologies I have it completely wrong. The holes do in fact go towards the end and the solid towards the handle. I just checked my own one. I guess the holes must be to allow pressure behind the o-ring to force it against the sides of the tube.

Peter.
07-11-2009, 02:09 PM
As you can see mine is in just as rusty a condition yet works perfectly. The O-rings tend to square-off and drag on the tube.

http://peterrimmer.myby.co.uk/images/pump1.jpg

http://peterrimmer.myby.co.uk/images/pump2.jpg

Circlip
07-11-2009, 04:24 PM
Sooooooooory Barrington, apex before tip, YOU got the order correct, I must get some better specs, but the holes are NOT sealed in any part of the operation, it won't work otherwise.

Regards Ian.

Barrington
07-11-2009, 05:02 PM
Apology not necessary, but why do you add:-
..., but the holes are NOT sealed in any part of the operation... I think I'm pretty sure that's what I thought I said ! :p

Cheers
.

davidh
07-11-2009, 05:48 PM
you guys saved my butt again. i opened up the width and depth of the groove and now it pumps better than new. the old sheetmetal one was so flimsey around the center hole that when an impatient employee started pumping too hard, it just kinda bent all out of shape.

again. thanks for the advice, and the offer and the picture.

davidh (the old guy)