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Where/how to make a plastic piston valve

  1. Oct 9, 2015 #1
    I have a project for school and need to make a hand water pump. The thing is, it needs to be a "mini" pump. I will be using 2 mini 1 way check valves and a t fitting, but the problem is i cannot find a way to make the actual piston pump. I also need a spring to draw the piston back up so i can manually push it down with my thumb. I need help with spring rate and how to make the pump in general. I was going to use the famous youtube "how to make a pvc pump" but i cannot find his stuff in a small enough size.

    Any insight on where i can buy the peices or how i could make them? The flow of water needs to be similair to a spray bottle
  2. jcsd
  3. Oct 9, 2015 #2


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  4. Oct 10, 2015 #3
    A water pistol pump.
  5. Oct 11, 2015 #4


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    You can make a piston from any hard plastic. The piston should be free to slide in the cylinder with minimum clearance.
    Use a lathe to cut a rectangular groove in the piston. Put a rubber O-ring in that groove.
    The O-ring must be big enough to gently touch the cylinder wall.
    The groove must be deep enough and wide enough so the O-ring is a loose fit and is not pinched.
    Water will move in behind the O-ring and seal the small gap between the piston and cylinder wall.
  6. Oct 11, 2015 #5

    Thanks for the reply. I actually found pvc small enough. But as you stated my problem is the o ring is too tight. Even with all the lubrication i have tried like vegetable oil wd40 and pb blaster its still a shove to get down. Its either too "loose" and the water leaks or its too tight. Any ideas on how i could get it to glide smoother?
  7. Oct 11, 2015 #6
    Theres no places these peices can even be bought in large quantaties! I have checked
  8. Oct 11, 2015 #7


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    Water is a natural lubricant with rubber and PVC. Lubricate the rubber only with water, anything else prevents the water doing the lubrication.

    The O-ring must not be tight in any direction. An O-ring seals because water pressure expands it slightly against the cylinder wall, while pressing it against one wall of the piston groove.

    You can make your own special size O-ring by cutting a small part out of a bigger O-ring. To find the ring size needed, rest an oversize cut ring in the cylinder without the piston. Carefully cut of the excess rubber that overlaps. Keep it dry, then carefully glue the joint with superglue if necessary.
  9. Oct 11, 2015 #8
    I have tried everything and it seems like i cannot get it right, i dont have the right tools for anything. Im cutting the grooves with a knife and i always dont cut enough or i cut to deep and it leaks air really bad

    Is theyre really no place i could buy this? Its seems impossible that i couldn't cause squirt bottles and water pistols have them.
  10. Oct 11, 2015 #9
    What is the capacity that you need to achieve. Is this just a demonstration or do you have an end transfer rate that you need to accomplish.

    Strictly for a demonstration my thoughts would be to visit a pharmacist or veterinarian. Either of these would likely have an irrigation syringe. These are typically in size up to 3 or 4 ounces. A quick trip to a store should come up with a compression spring that will overcome the seal of the syringe when lubricated. Adapt this to a tee between two check valves and bingo a hand pump. Clamp it in a holder and fabricate a cam to operate the plunger and there ya go mechanical pump.

    I was reviewing your original post. Seeing the you only need to approximate a spray bottle I would definitely go with a smaller syringe and maybe look for something that is close in outside diameter so that it could be glued into one of your PVC fittings.
  11. Oct 11, 2015 #10
    It would be helpful on the school type projects to know what the goal is for your learning experience. We could help with the calculation of seal friction for your project or we could also help with volumetric calculations. As an engineer often the first and sometimes most challenging goal is to identify your desired result. If you could help us with that we could assist further.
  12. Oct 12, 2015 #11


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  13. Oct 12, 2015 #12
    A hand pump normally uses a cup seal, as seen here,

    Same thing usually used for a water pistol, except the cup is a flexible plastic.
    Probably the same for sqeegie bottles.
    Binding and friction forces are minimized if done correctly.

    One could make that with a rod, a metal washer of less diameter than the bore of the cylinder, and a rubber washer (similar to that in your kitchen sink tap) slightly larger than the bore., and a screw.

    Or use a piece of leather cut to size so it forms a small cup without crinkling when in the bore, making sure their is enough clearance, plus a bit so it is not tight, between washer and bore to accomidate the leather thickness. Try substituting the metal washer with a rubber washer for flexibility if needed. Add a cup in both directions to obtain doubly acting pump on the up and down strokes.

    Into a wooden rod drill a hole in one end to accomidate the screw so that the wood does not split.
    Place the washer, then the rubber washer ( or leather ), and then screw to the rod.
    Put piston inside bore and enjoy.

    You have to experiment with your valves and entrance exit points, with regard to whether you make a single or doubly acting pump.

    Take apart a bycycle pump to see how that is made for extra insight.
  14. Oct 13, 2015 #13
    The syringe is a good idea. But even the syringe doesnt give "spray bottle like" ease with the trigger
  15. Oct 13, 2015 #14

    Intresting.. I took apart one spray bottle i had and saw that cup seal that you just mentioned.. Really interesting, the only gaskets and o ring i have ever used were parts for a car like pistons and misc parts. It seems to slide easy since its plastic against plastic, and the cup skirt is a tad flexible. I am still having a hard time understanding how the cup seal makes a perfect seal while the piston rises. Makes sense how the pressure inside would cause the skirt to open and seal while going down.

    I read online that to make the perfect seal you glue foam rubber sheets to the piston to make the perfect seal. Witch in theory should work, since it was saying the little air pockets in the foam rubber give room to be compressed when needed, but in my case it seems that the moment it "needed" to compress was on the up stoke thus not creating a vaccume to even hold water in my piston pipe
  16. Oct 13, 2015 #15
    Does anyone know where i can find a supplier for water pumps pistons like im looking for? How did they get it?
  17. Oct 13, 2015 #16
    The cup faces the higher pressure.

    Foam rubber - use a string wrap around the piston, coat with wax, or impregnate before hand . Do not have any knots touching the cylinder.

    You could also try a plunger type piston setup. The piston is longer than the cylinder length, and of lessor diameter than the cylinder so that there is no contact. The seal, or in this case, the packing, is at the top of the piston . Pushing the plunger into the water displaces the water pushing it out. Pulling the plunger out brings waterback in.

    You will need to experiment a bit. The first try would not necessarily work.
  18. Oct 14, 2015 #17
    I do not know about finding the cup seals that are as small as a pistol grip sprayer. I do know that these are common in hydraulics ( repair shops harder to find but wider range) and also in automobile brakes. ( Car parts stores pretty easy to find and often have sets of brake parts for rebuild of wheel cylinders or master cylinders). You can possibly find what you need there. As you search the seals in Master cylinders are typically smaller in diameter you may be able to find something that fits well in you PVC pipe.
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