Tire pumps are useful tools for cyclists--ask anyone who's ever had a flat tire. The pump is used to move atmospheric air into the tire, providing sufficient pressure to inflate it properly. There are many different designs for tire pumps, from small pocket-sized pumps to large stand-on, two-handed types, but the principles behind most tire pumps and their inner workings are basically the same.
Principle
The basic principle behind a bicycle pump is that of the compressor. When air pressure is greater in one space than another, air will be forced through any opening between these two spaces to equalize the pressure. A bike pump does just that, acting as a piston. It forces air, under pressure, from one space to another, in this case from the cylinder of the pump into the tire.
Construction
Bicycle pumps consist of a cylinder with a cap on both ends. One cap has a hole in it with a rubber gasket through which the rod passes. Another small hole in the top cap with a valve allows air into the cylinder, but prevents it from escaping. The outside end of the rod has a handle. Attached to the other end is a downward-facing leather or rubber cup. The cup fits closely to the inside of the cylinder. At the bottom of the cylinder is a port with a valve that allows air to pass from the cylinder to a hose. At the end of the hose is a valve stem clamp that fits tightly over the valve stem of a tire.
Charging
To charge the pump with air, the handle is raised. This draws air into the cylinder through the hole in the top cap. Because the cup at the bottom of the rod is flexible and faces down, it allows air to pass around it into the lower part of the cylinder. The air in the pump is now ready to be compressed.
Compression
When the cup reaches the top of the cylinder, the handle is pressed down. As the cup moves down in the cylinder, pressure is exerted on the air inside. This forces the edges of the cup against the wall of the cylinder, forming an airtight seal. The air within the cylinder is compressed by the downward motion of the cup. It is then forced, under pressure, through the port in the side of the pump, past the stop valve, down through the hose past the valve stem clamp and into the tire. The handle is then drawn back up to start the process again. During that time, the stop valve in the port prevents any air from escaping the tire.
CO2 Pumps
A recent innovation in tire pumps is the use of carbon dioxide contained in small cartridges. The carbon dioxide is first pumped into the cartridge, which is then sealed. The cartridge is later inserted into the grip of the pump, which holds it securely. A trigger valve, hose and valve stem clamp are attached to the top of the grip. The user turns a knob to puncture the cylinder, releasing the carbon dioxide and charging the pump. When attached to a tire valve stem, the trigger is then depressed, releasing the carbon dioxide, which passes through the hose, into the tire.
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