Piston pumps are found in almost every area of human endeavor. They are used to inflate bicycle tires and air mattresses. In their liquid embodiment, they pump water from wells or bail water out of boats. Small piston pumps used in the medical field are known as syringes--used to inject medications through a hypodermic needle, or to extract blood for samples. Industrial uses are myriad. Constructing a useful homemade piston pump is the perfect rainy day project with housebound youngsters. Here's how to do it.
Principle of Operation
Piston pump strategy is simple: a piston withdrawing from a cylinder first draws in a predetermined amount of air or water by increasing the sealed volume. A one-way check valve allows the medium only to be pulled into the cylinder. A second check valve--the exit valve stays shut for the moment until the vacuum pressure ceases when the outward stroke stops. When the piston is pushed back into the active side of the cylinder, the first check valve shuts and the other check valve opens to allow pumped medium to be pushed out of the cylinder.
Design Your Pump
A homemade piston pump can be easily produced with a 2-foot section of 3-inch white schedule 40 PVC pipe. It will pump up air mattresses and pool floats quite easily. The piston is comprised of a 3-foot piece of 2-inch schedule 40 PVC pipe along with a matching end cap and coupling union. The check valves will require two, half-inch tube barbs with ½ NPT male threaded ends, 2¼ x 20 flat-head Phillips drive stainless steel screws with a matching locknut each, two 3/8x5/8-inch neoprene O rings, and two light coil springs 7/16-inch outer diameter by ½-inch long. The sealing piston ring will be a neoprene-beveled toilet flush seal 3 inches outer diameter and 2 inches inner diameter. A 5-foot length of half-inch inner diameter vinyl tubing plus a small tube or can of PVC cement is all that's needed to get started.
For the inlet check valve, push one of the small O rings onto one of the screws and insert into one of the tube barbs from the NPT side of the fitting. Put one of the springs on the other side of the screw, and thread on the locknut until the spring is just slightly depressed. For the outlet check valve, repeat the above, except start from the tube barb side of the fitting. Assemble the 3-inch end cap to the 3-inch pipe without cement by tapping. Drill two 11/16 holes into the side of the end cap on opposite sides and thread in the two check valve fittings. Cut a 3-inch piece of the 2-inch pipe, and cement it into the end cap, put the rubber seal over the pipe stub, and cement the union onto the end of the pipe stub. Cement the rest of the 2-inch pipe into the coupling. Cement the 2-inch T onto the other end.
Using the Pump
Spread petroleum jelly liberally on the end of the cylinder and insert the piston, slide several times, and then attempt to inflate a mattress adapting with supplied fittings. In an emergency, this unit also will pump water like a bilge pump.
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