Originally Posted by
KRviator
We utilise shuttle valves - known as "double check valves" to us - in the rail industry as a part of the locomotive pneumatic braking system. Their function is to pressurise the locomotive brake cylinders at the higher of two pressures, those sources being the train's automatic brake, or the locomotive's independent brake. With brakes released and no pressure in either system, the valve just sits there, even with identical pressures with brakes applied the small amount of inherent friction in the valve body prevents the slide from moving, it will just sit in the 'last moved to' position. I've not heard of 'valve chatter' ever being a problem and this system has been around for decades.
Here's a rough idea how they're set up for us...Ignore the 'relayair valve' bit, that's for another device.
Thanks for the answers all, being a mostly electronics type myself I find details of other types of systems interesting.
Roughly speaking this is equivalent to using 2 diodes to power a device from 2 (or more) sources,
There is no path from source to source and the higher voltage will supply the load.
The 'spring biased' effect can be achieved by using 2 diodes in series on the backup side, as long as the sources are within ~.7V of each other the primary will handle the load.
The only significant difference is that if the supplies are exactly equal both will contribute to the load.
This brings up a question on the pneumatic case , what happens when the pressures are equal, is there a chance that the shuttle will balance in the middle blocking the output?
Of course given the friction induced tendency to stay in last position greatly minimizes the chances but in theory it could happen i transient pressure were just right, an instance of Mechanical metastability?
I realize the drawings are simplified so there probably are details that prevent this.