Schrader valve
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The Schrader Valve is a valve stem invented by August Schrader and is used on virtually all automobile tires and wider rimmed (lower pressure) bicycle tires. It is a poppet valve assisted by a spring. Schrader valves are also used on the fuel rail of some direct fuel injection engines and on some automotive air conditioning systems, in a diameter larger than those used on tires, to allow recharging with refrigerant. In addition, Schrader valves are sometimes used on irrigation systems to provide a place to temporarily attach a portable pressure gauge.
A Schrader valve consists of a hollow cylindrical metal tube with a threaded end. In the center of the end is a metal pin pointing along the axis of the tube; the pin's end is approximately flush with the end of the tube. Pushing this pin into the valve opens it. There is a spring inside the Schrader valve which pushes the pin outward to close the valve when the pin is released. The end of the valve is used to attach a hose fitting for pumping fluid (gas or liquid) in through the valve, to momentarily fit a pressure gauge to the valve for measuring the pressure behind it, and to secure a cap when nothing is attached. The installation tool pictured also attaches to the end of the valve, using the end threads. Generally, all Schrader valves used on tires have threads and bodies of a single standard size, so caps and tools generally are universal for the valves on all pneumatic tires on automobiles, bicycles, and even lawnmowers and wheelbarrows (at least in the United States). Most air hose fittings for inflating tires actually are not threaded; instead, they seal to the threads by compressing a rubber collar around the threads, or they simply seal to the end of the valve tube with a gasket (usually of rubber) which is sealed by manual pressure. For the former type, the threads are still important because they have essentially the same effect as concentric ridges around the valve tube, giving the rubber something to conform around for a firm grip against the force of pressure, which would tend to push the fitting off of the valve.
Depressing the pin is the method for manually relieving pressure retained by the valve (for example, when a tire is overinflated or needs to be deflated). A pressure gauge, when pressed onto the end of a valve, depresses the pin to open the valve so that the pressure inside can be measured.
A valve cap is essential on a Schrader valve because if one is not fitted, dirt and water can enter the outside of the valve, potentially jamming it or contaminating the sealing surfaces and causing a leak. Metal valve caps usually have a small rubber insert to permit a good seal against the valve body; a cap of this kind also helps to prevent air escaping from a slightly leaking valve. However, the vast majority of Schrader valves used for tires are fitted with plastic caps which effectively serve only to keep contaminants out of the valve stem.
As is true for a Presta valve, pressure inside a Schrader valve will hold the valve shut, but because the Schrader valve has a spring, it closes and stays closed even without pressure. Another difference is that the size of Schrader valve used for bicycle tires has a greater diameter than the Presta valve. The spring makes the Schrader valve better than the Presta valve for low pressure tires, but the larger diameter hole it requires can weaken the wheel rim. For this reason, Schrader valves are more common on mountain bikes--which have wider tires with lower pressure--than on other types of bicycles, particularly racing bicycles (which have narrow rims and tires and higher pressure).
The standard Schrader valve has the following thread:
External Thread.
METRIC: 7.7 mm OD, thread root diameter is 6.9mm x 0.794 mm pitch.
IMPERIAL: 0.305" OD, thread root diameter 0.302" x 32 tpi (threads per inch)
Internal Thread. (the valve itself)
METRIC: 5.30mm OD x 0.706mm pitch
IMPERIAL: 0.209" OD x 36 tpi.
