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So I have a new bicycle which has a Schrader valve on it. So I was wondering how does a Schrader valve actually work , the anatomy of it etc. The best I was able to find was something like this:

enter image description here

But I am still not sure about the mechanisms of the Schrader valve.For example:

  1. How does the Schrader valve allow air to flow one way but I can use the pin to let the air flow out the other way?
  2. Where is that spring attached to etc.

My old bike has a dunlop valve in it , which I do think I understand. Here is a picture were I understood it from:

enter image description here

Is there a similar explanation for the mechanicsm of Scharder valves?

Kripke Platek
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    https://en.wikipedia.org/wiki/Schrader_valve#/media/File:Schrader_valve_opening_and_closing_on_a_tire.gif The gif is animated, and shows you exactly how the valve is opened. This includes how the spring is stretched in the process. – cmaster - reinstate monica Jul 10 '21 at 12:37
  • @cmaster-reinstatemonica that is the exact gif I dont understand. – Kripke Platek Jul 17 '21 at 11:46
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    Well, there is that moving center piece. It is held up by the spring, closing the valve at the bottom. Part of the pressure difference also helps to close the valve when there is higher pressure below than above. Finally, the center piece protrudes to the top, so it may be pushed down mechanically, opening the valve. That's all there is to it. When you use the correct pump head, you have to close a lever at the pump head which simultaneously seals the pump head against the outer thread, and opens the valve by pushing down that centerpiece, allowing the pump to measure the pressure. – cmaster - reinstate monica Jul 17 '21 at 13:02
  • @cmaster-reinstatemonica Then how does air get through the valve when pumping? – Kripke Platek Jul 17 '21 at 14:05
  • Which part of "and opens the valve by pushing down that centerpiece" was unclear? – cmaster - reinstate monica Jul 17 '21 at 14:50
  • @cmaster-reinstatemonica I think you explained how the pump measures the pressure , not how the pump pressurizes the tire. – Kripke Platek Jul 17 '21 at 14:53
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    If air can flow out to reach the manometer, air can flow in to fill the tube, no? – cmaster - reinstate monica Jul 17 '21 at 17:53

2 Answers2

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Wikipedia pretty well sums up the history and functioning of the Schrader valve, as cmaster has noted in a comment. (As a bit of self defense, I independently chose Wikipedia as a good source after noting your question within the "newest" queue didn't yet have an answer). At any rate, the following information has been gleaned from that Wiki article and it's links:

Schrader valve components (from left to right) the valve core closed (top) and open (bottom), the order of assembly, end view of stem without core and with core (top) and stem with dust cap on (bottom). The core has a short outer thread which is screwed into the inner thread of the stem. The visible outer thread of the stem holds the dust cap.

The Schrader consists of a valve stem within which is the core. The core is a type of "Poppet valve" (AKA: mushroom valve) assisted by a spring. A poppet valve is a device used to control the opening, closing and the quantity of gas or vapor that flows into an internal combustion engine. They are used to control the admission and rejection of the intake and exhaust gases to the cylinders. The basic anatomy of it is a round opening or hole and a corresponding plug also oval or round that is connected by a valve stem. "The working end of this plug, the valve face, is typically ground at a 45° bevel to seal against a corresponding valve seat ground into the rim of the chamber being sealed. The shaft travels through a valve guide to maintain its alignment." (1)

With a Schrader valve, you have an externally threaded hollow cylindrical metal tube, typically of nickel plated brass. In the center of the exterior end is a metal pin pointing along the axis of the valve stem; the pin's end is approximately flush with the end of the valve body. A small rubber seal is located on the threads of the core to prevent gas or fluids from leaking through the threads. With this configuration, the flow of air is controlled through the valve by the spring holding the valve shut unless the pin is depressed, opening the tract to air flow. Air flows from the higher pressure gradient to the lower. Thus, when inflating a tire the pump creates a higher pressure than is within the tube or tire and air and additional air pressure is introduced. Depressing the pin to allow the opening of the valve without the seal or introducing air pressure of a pump causes the flow of air to escape the higher pressure of the tire into the lower pressure of the atmosphere.

The valve cap is important on a Schrader valve to prevent dirt or other contaminants from affecting the sealing surfaces and causing a leak.

(1) from the Wikipedia article on Poppet valve, https://en.m.wikipedia.org/wiki/Poppet_valve

MaplePanda
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Jeff
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  • I am still not sure how the two way mechanism of the schrader valve works. – Kripke Platek Jul 17 '21 at 11:59
  • @Prithu biswas. The mechanism is simply a plug that is held in place, blocking air flow. It is attached to a small rod and when the rod is pushed by a small tool or the center tip of an air chuck, it displaces the plug (aka valve body) which opens up the tract to air flow. The direction of that flow is determined by pressure differential and air flows from higher to lower pressure. The open valve is simply now an open tube in which air can flow either way. When mechanical pressure is released from the rod, a small spring exerts enough force to move the plug back into place, closing the system – Jeff Jul 20 '21 at 07:36
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The diagram makes it look a little more complicated than it really is.

A Schrader valve is one type of valve stem. It's unclear to me what particularly makes it a Schrader valve, though some of the other types of valve are thinner or longer etc.

The valve core is a "poppet valve", also called a mushroom valve because of the mushroom shaped plug inside it.
The following description uses directions like top, bottom, etc, in reference to the drawing below.

You have a tube (the patent calls it a "shell") that air can flow through, in either direction.

That tube is plugged by a mushroom shaped plug on the inside end (labeled #28, at the bottom of the diagram below) that holds a sealing washer (labeled #26)

The mushroom shaped plug (#28) is attached to a valve pin (labeled #20, the number is at the top of the diagram), that goes through the tube and sticks out the other end (the outside end, again that's at the top of the diagram).

The pin keeps the mushroom shaped plug lined up with the tube and provides a way to push the valve open from the outside.

Inside the tube, wrapped around the pin, is a helical spring (labeled #30, that's on the right side of the left-hand drawing, and most of the spring is not rendered in that drawing, just the top and bottom of the spring).

The helical spring pushes the pin upwards/outwards, making sure that the valve closes, i.e. the spring pushes the pin upwards, which pulls the mushroom/poppet at the inner end tight against the inner end of the tube. The compressed air inside the tire helps shove the valve closed also, but the spring makes it more reliable.

As you can see in the diagram, there are a lot more fiddly little details, but the above is the basic idea:

enter image description here

Here's a key bit of description:

It comprises a core body 12 including a machined head 14 and a drawn sheet metal shell 16. The lower end of the shell presses a seat 18. A valve pin 20 is supported at its upper end in a conventional bridge 22 and centered at its lower end by a conical centering flange 24. A sealing washer 26 is supported in a valve head 28 mounted on the bottom end of the pin 29.

A helical closing spring 30 surrounds the pin 20 and is disposed between an upwardly facing shoulder 32 formed in the shell and a stop shoulder 34 on the pin as is conventional.

For more details, check out the patent for the Schrader valve, patent #3712328, January 23, 1973. Read the whole patent here:

https://patents.google.com/patent/US3712328A/en?oq=3712328

Steven J Owens
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