See the last step in this Instructable for an explanation of how the valves work and to see a short movie of the pump working.
This Instructable shows how to make one barrel of a treadle pump in order to show the valve system working.
Scissors or a sharp knife
A bicycle inner tube - doesn't have to be new or in serviceable condition
The rest of the pump cylinder
Less than 1m of 50mm PVC pipe
2 T joints
An end cap
8 large metal washers
An old flip flop
Bicycle inner tube = free from a local bike repairer... it was beyond repair.
The rest of the pump cylinder
PVC fittings = $3.80
Threaded bar, washers and nuts = $1.50
Old flip flop = free... worn and broken beyond repair
Grease = maybe around $0.10
Aziza's Place, Cambodia.
Step 1: Principle
This self-closing ability is the principle behind the valve system.
Step 2: Inlet Valve Assembly
Follow the notes on the photos...
Step 3: Pump Barrel and Plunger
To make the pump plunger, cut 2 x 50mm circles from an old flip flop with a sharp knife, then finish their edges with sandpaper for a good fit inside the pump barrel.
Cut a hole in the centre of each plunger to accommodate a length of threaded bar.
Put the plungers onto the bar 2 to 3 inches apart and secure them with a large washer and nut on each of their sides. The bottom plunger should be as level as possible with the end of the threaded bar.
Add a good amount of grease to the upper side and edges of the lower plunger, then run the whole assembly into the pipe.
Using anything handy, make a "stopper" for the threaded bar that will not allow the bottom plunger to pass down beyond the end of the 50mm PVC pipe and into the T - I used 2 more washers and nuts to hold a car exhaust support rubber in a good position... so stopping the bottom plunger 5mm short of the end of the PVC pipe.
Make a pump handle from another 2 washers and nuts at the top end of the threaded bar.
Using a short section of PVC pipe, fit the inlet valve assembly T to the pump barrel T.
Step 4: Outlet Valve
Don't leave a longer inner tube outlet as shown in the picture, which is only to show the pipe kinking. A longer outlet will have multiple kinks and may twist itself entirely closed.
Place the pump in a bucket of water, bend the inner tube against itself to lead up and out of the water, and start pumping.
The first 1 or 2 strokes of the pump will be "dry", so the pump may suck in air through the outlet hose - keep going until a little water/air mixture comes out of the outlet, after that the pipe closure will be good.
IMPORTANT - most of the outlet tube must be at least 2 to 3 inches below the surface of the water in the bucket for the valve to work - see the next step to find out why.
Step 5: So, How Does It Work?
At that point, the outlet valve is already pressed closed by water pressure from the water in the bucket. This pipe closure begins at the kink in the pipe and is magnified as the upstroke progresses by the suction created within the outlet inner tube, which is pressed entirely flat along its length at the end of the upstroke. At the end of the upstroke, the inlet valve naturally closes as the water flow through it stops.
2. At the beginning of the downstroke, the inlet tube is already closed at its kink. This closure is magnified by the pressure within the pump barrel as the inlet tube cannot straighten to allow water to pass.
At the outlet tube, the pressure from within the pump overcomes the water pressure on the outside of the tube from the water in the bucket, opening the kink in the outlet inner tube and providing an escape for the pressurised water within the pump barrel.
For a short video of the pump working, see here.
The next step is to enclose the outlet tube within a large diameter pipe so that pumped water can provide the back-pressure needed to close the inner tube valve, and to feed a pipe to the pump inlet . This would remove the need for the pump to be immersed to work.
*Update - the valves work equally well when the pump is not immersed. The pump outlet valve/pipe was fed into another T and set up in the same way as the inlet valve on the pump barrel. This allows the new T and new barrel to hold the head back-pressure without that pressure re-opening the valve - see picture 3.
Video of the pump working without immersion.
Cost of further PVC parts = $4.05
Original parts = $5.40
total = $9.45
Valves proven, pump next...