# NW-Mailing-List Digest, Vol 62, Issue 3

NW Mailing List nw-mailing-list at nwhs.org
Wed Nov 3 17:42:38 EDT 2010

Hey, I actually learned something from the air brake portion of
mechanical in my engineer training! :)

You're right, to an extent, but there's more to it. If you have a
reservoir as big as Texas (at 90 psi) and a normal sized brake cylinder
(at 0 psi), the two will equalize pretty darn close to 90 psi. If you
have a reservoir and a brake cylinder of exactly the same volume, the
two will equalize at 45 psi. The larger the reservoir, the higher the
final pressure in the brake cylinder.

Thus, both reservoirs actually DO supply air at the same time, because
both are trying to equalize with the lower pressure of the brake
cylinder, and of course with each other when all three are connected by
an emergency application.

To take it a step further (Jimmy, you probably already know this, but
others might not).. If I make a service reduction, the aux. res. and
brake cylinder will equalize at some pressure, say around 50 psi. If I
then make an emergency application, the emergency reservoir pressure is
higher than either the aux. res. or brake cylinder, and it will then
cause the system to equalize at a higher pressure, closer to 70 psi.

Physics is never basic. :)
Ken Rickman

NW Mailing List wrote:

> Like I said, being as both cylinders have the same pressure, they

> cannot supply air to the brake cylinder at the same time. You can have a

> reservoir as big as Texas , but it still only has the same amount of

> pressure in it that the aux. res. has. Basic physics.

--
Kenneth Rickman - krickman1 at carolina.rr.com
Salisbury, NC