train brake application in effect with pressure maintaining feature

[Deggesty]

Quite interesting, even to one whose locomotive running experience was limited to running a light engine around a wye going on forty and more years ago, though I did couple on to cars once or twice.

Anyone with a modicum of understanding of the physics of the matter should be able to profit from the article.

[tree68]

petitnj

Here is an excellent article on train brake operation: 

http://www.railway-technical.com/brake2.shtml

An excellent primer on railroad air brakes - should be required reading for anyone who will deal with them.

[petitnj]

Here is an excellent article on train brake operation: 

http://www.railway-technical.com/brake2.shtml

[oltmannd]

Big Jim-

I've been at my desk too long..... I need a few cab rides...

Thanks for setting me straight (again...)

[BigJim]

oltmannd

Moving the handle back toward release, increases the brake pipe pressures.  Any increase in pressure sensed at at each car's valve completely releases the brake.

Not quite the way it works.

In the "Frt" setting or on a freight only brake valve, once an application has been made, the handle will only cause the air pressure to increase if it is put in the 'Release" position, at which time it provides full equalizing reservoir pressure to the brake pipe. Once an application has been made, you can move the handle between release and the current application setting all you want with no effect.

If freight cars or non-graduated release passenger cars are operated with a brake valve in the "Pass" position and after an application is made, any movement of the brake handle back toward the release position will cause an increase in brake pipe pressure resulting in a release of the brakes on said cars.

[oltmannd]

When you move the brake handle toward release, the brakes release!

I suppose you want to know why....

In the old days, when you wanted the brakes to apply, you let some air out of the trainline, reducing it's pressure which applied the brakes on each car.

The brake valve on each car send air from the reservoir on each car to the brake cylinder until the pressure in the reservoir equals the brake pipe.

The way you used to do this on the locomotive was by moving the handle to "apply", then watching the brake pipe gauge.  When you had the reduction you wanted, you moved the handle BACK to lap.  (lap is closer to release than apply, so you are moving the handle back toward release)

Lap essentially bottled the train line. But all trainlines are leaky, so the leakage had the effect of applying more braking, as time went by - not an ideal situation....  The fix for this was pressure maintaining, where "lap" was essentially replace by a pressure regulator that would maintain the brake pipe pressure a the level you reduce it to.  

The brake valve "notches" were somewhat reconfigured so there was no more "apply" and "lap".  There was a variable "service" zone where the farther you moved the handle, the greater the brake pipe reduction - and the pressure would remain at that level until you released the brakes.

Moving the handle back toward release, increases the brake pipe pressures.  Any increase in pressure sensed at at each car's valve completely releases the brake.

That's a pretty big change from the way it was in the old days, hence the statement you quote.

*(all bets off if you are running passenger equipment with graduated release)

[GM1361]

Please elaborate on this sentence:
NOTE:when a train brake application is in effect with pressure maintaining equipment, do not move the automatic brake valve handle toward RELEASE unless a brake release is desired.