A pneumatic piston, or air-powered hydraulic, is a cylindrically shaped bore in a piece of metal, plastic or other airtight medium, with a movable cylindrical insert called the piston, which is able to move in out and with response to the amount of pressure inside of the cylinder. Drilled into the top of the cylinder are holes, equipped with controllable values that can be used to add or remove pressure from inside the cylinder. Usually, one value is connected to the input pump and the other is connected to an output pump. In this way the fluid inside the system can maintain a singular directional flow while allowing the piston to move in either direction. The force with which the piston acts is proportional to the surface area of the piston exposed to the air pressure and multiplied by the air-pressure differential, which is the difference between internal and external absolute pressures. Low-pressure pneumatic pistons with large cross-sectional piston areas are cable of exerting large amounts of linear force. High-pressure versions are even stronger.
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Things you need
- 1 foot by 1 foot PVC pipe
- 1 foot PVC end cap
- Plastic rod
- 1/4 inch threaded valve
- 1/4 inch by 10 foot hosing
Insert the PVC end cap over the 1 foot length of PVC pipe. The fit will be snug and may require tapping to set it fully in place.
Drill a single hole into the centre of the PVC end cap with a drill bit sized appropriately to accommodate the threads of the screw-in valve. If no standard sized drill bit fits exactly right, use a drill bit one size too small and file the hole slightly larger with a cylindrical file.
Screw the threaded valve into the PVC end cap. Since the end cap is simply drilled and not threaded, the valve will not screw in easily. If it is too difficult to screw it in, use pliers to finish screwing the valve into place.
Insert the plastic rod into the open end of the PVC pipe. This rod will act as the piston, so a proper fit is vital. For this reason, it is necessary that the diameter of the plastic rod should match the inside diameter of pipe, such that the rod diameter is equivalent to the PVC pipe hole diameter, plus or minus, about one 5,000th of an inch. Custom machining might be required to find a fit that is this precise. If this is the case, start with a plastic rod larger than the hole diameter of the PVC pipe and use a lathe machine to spin the rod while cutting it to the new diameter.
Attach the hose to the valve connector simply by forcing it over the barbed ends of the connector. This will require some force, but you should be able to achieve this manually. Once the hose is slipped over the barbs, it will resist detaching and will make a nearly perfect seal.
Blow into the hose by mouth. The small air pressure generated by the lungs and diaphragm should be able to produce about two PSI relative to the ambient pressure. Once the compressed air reaches the inside of the 1-inch cylinder, a total of about 0.713kg. of linear force will drive the piston, extending it out away from the opening of the cylinder. The force is calculated by multiplying the area of the piston ("A" equals "PI" times "r" squared, which means "A" equals 3.14 times 0.5 square inches) by the pressure inside the cylinder (0.785 square inches times 0.907kg. equals 713kg.). If instead, the cylinder was 2 inches in diameter the formula would have worked out to yield 285kg. of force, which is more than two times the force that would have intuitively been expected by simply doubling the diameter of the pipe.
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