Ed King's Air Brake Article, April 04 Trains

[Anonymous]

A rather egregious mistake slipped into Ed's article in the April 04 issue on air brakes -- our mistake, not his. The incorrect information is that leakage in a brake pipe leads to a brake release. That's so obviously wrong I can't believe I didn't see it. The correct information is this:

"Leakage, if allowed to be more than the allowable limit (5 lb/min) can greatly increase the time required to charge a train; with long trains it can take a lot longer to charge. Then, after you make a brake application, leakage affects the time required to get a train to release (some of the air you want to get back to the rear to make the brakes release has been lost to the atmosphere).

With the AB valves as we covered in the story, they're designed to put some of the air from the emergency reservoirs into the brake pipe to assist in releasing brakes farther back in the train, but that air must also be replaced. Leakage
makes that a more difficult task. Without engine brake valves with pressure maintaining, when the brake valve handle is placed in lap after making an
application, leakage will cause the application to increase; remember, in lap, no air is being put into the train line. If this happens, the brakes will eventually apply hard enough to cause a stop. Two things mitigate this condition - the pressure maintaining feature, which will hold the brake pipe pressure at the level of the application against reasonable leakage (most were effective for more than the legal leakage), and retainers. Retainers held the brakes on a train while the first application was made and then released so the brake pipe could be recharged.

An example in practice: I was running a long train one very cold night with a locomotive in the lead without pressure maintaining. All that was necessary was to lap the brake valve; leakage would make a brake application for me."

-- Ed King

[ironhorseman]

Interesting. I've read most of the article. There's more to braking than I could ever imagine. The article is almost 7 pages and I was struck by the statement: "A full discussion of air brakes is far beyond this article's purview." I'll have to make a trip to the university library.

[mvlandsw]

Under the right circumstances a leak can cause the brakes to release. If there is an intermittent leak, when it is leaking the pressure near the leak will be lowered. This causes the brakes to apply or apply harder than the amount called for by the engineer. When the leak stops the pressure rises causing the brakes to release on the nearby cars. Since some of the emergency reservoir air is put into the brake pipe to get a better release a chain reaction is started and all of the brakes can be released. Leaks of this type frequently develope in the collection of hoses, pipes, and fitting used to keep the air hoses lined up on cars with cushion under frames or end of car cushioning units. The joints flex as the slack moves in and out, opening and closing the leaks. Blowing the back up whistle on a caboose could have the same effect. I have also seen this happen when an EOT device is reconnected after a rear end helper cuts off.