The numbers I used was for a brake pipe charged to 90 psi, the standard for road freight trains. Some yard and transfer moves can be charged to less pressure.
Assume you released the brakes and immediately reapplied them. First, to get a release each control valve needs to see about a 2 psi rise in pressure. So let's say the brake pipe rises to 66 psi when you reapply them. Equalization will happen with about a 19 psi reduction. Brake pipe at 47, reservoir at 47, brake cylinder (2.5 x 19) at 47.5 psi. So you have braking force, but not the same force as from the fully charged brake pipe. I should note that our rules would require stopping when setting 18 psi or more. We wouldn't be able to give a full service application (from 90 psi) and then release while moving. On some larger trains in some places, I wouldn't do a running release even if I had only set 10 or 12 psi. Too much of a risk of tearing the train apart.
Speaking of pissing away the air. That is what's happening in this example. Usually it's from setting 10 to 15 psi, releasing it but not allowing a full recharge before again setting air. You can do that a few times before you have nothing left to give you any real braking effort.
Jeff
so if maximum brake force (non-emergency) is achieved when the brake pipe is reduced by 26 psi, can max braking can still be achieved after releasing the brakes by reducing the brake line pressure from 64 to 38 psi and once again from 38 to 12 psi?
greg - Philadelphia & Reading / Reading
It means when 10 pounds is released from the reservoir into the brake cylinder, it becomes 25 pounds of pressure in the cylinder. Maximum service braking is reached when equilization between the train line, auxilary reservoir, and brake cylinder is reached. From a 90 psi train line pressure that happens when about 26 psi is drawn off. The train line will be at 64 psi, the main reservoir will be at 64 psi and the brake cylinder will also be about 64 psi. (26 psi x 2.5 = 65 psi)
Any further draw down from the train line won't increase the pressure in the cylinder. The only way to do that is to place the brake valve in emergency, which draws off air from the emergency reservoir.
I know for emergency braking, if the train line falls below about 45 psi it may not transmit an emergency signal. Passenger car brake control valves will go to emergency when brake pipe pressure falls below about 20 psi as a safety feature. Freight control valves won't. During extreme winter conditions when cutting away from cars (and letting them go into emergency) may cause the control valve to stick, the procedure before cutting away is to draw down the brake pipe to 20 psi. That's to ensure the control valves don't go to the emergency position, but stay in the service position.
is there a limit to brake cylinder pressure?
AA Krug explained that the reservoir is 2.5 times the volume of the cylinders and therefore the cylinder pressure is 2.5 times the change in reservoir pressure. does this imply that if max reservoir pressure is 90, that max cylinder pressure is 225 psi?
is there a minimum brake line pressure below which the triple valve no longer works accurately?
will the same amout of air flow from the reservoir into the cylinders if the brake line pressure drops from 90 to 80 psi as from 20 to 10 psi?
any info on air flow rates thru the brake valve in the cab for different service rates?
i assume the cab brake releases air from the brake line at a fixed rate. this implies that brake line pressure will drop at a rate depending on the length of the line (i.e. #/length of cars)
Either the loaded car is underbraked, or they have one of these:
https://www.wabtec.com/products/1618/emptyload-equipment-offerings-elx-b
These were apprantly designed originally around 1970 and patent issued in 1972. Interesting training on them here: https://slideplayer.com/slide/1592622/ (warning robot voice - might want to turn off audio and study the diagrams)
Just like on pickup trucks you might find the hydraulic brake proportioning valve in the rear, attached to teh frame, with an arm to the axle, to adjust braking force on the rear axle relative to the load.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
i'm also a bit curious about brake cylinder pressure on MT vs loaded cars.
my understanding is that max brake force is ~10% of the car weight. would you really want as much braking on an MT as a loaded or would this only potentially be a problem if there was a long string of empties in the middle of the train?
on the other hand, extra braking on the MTs helps slows the entire train down and the engineer adjusts the brakes to whatever degree to slow the train down as much as desired. this may take longer with a fully loaded train.
mvlandswOn steam locomotives was the tender part of the independent brake system or did they have a separate brake system?
The "ET" in the #6 ET or #8 ET air brake stands for engine and tender. Yes the independent applied the tender brakes too. I don't recall ever seeing a retainer valve on a tender but, never say never. They would apply in automatic too and you would bail the engine/tender brakes off using the independent release.
Cheers, Ed
On steam locomotives was the tender part of the independent brake system or did they have a separate brake system? A story about helper operation on Cajon Pass mentioned turning up the retainer on the tender.
The idea of pressure maintaining came from the practice of bridging the old brake valves between the lap and the running positions. This allowed enough air into the train line to compensate for the leakage, but if you allowed too much air to flow the brakes would release. That could lead to runaways on steep grades which is where pressure maintaining is most useful.
Mark Vinski
dehusman Emergency vents the train line to atmosphere, it is a one and done. In steam era systems when the brakes were in lap, the train line and reservoirs were not being recharged. The only time the reservoir was recharged was when the brakes were released. If you applied the brakes, you would draw down the reserviour pressure. If you made an additional application you would draw down the reservior pressure more. If you made numerous small applications without releasing the brakes, you could draw down the the reservior pressure so low, it could no longer effectively apply the brakes. The railroad name for that was "pissing away your air". The point of retainers was to allow the engineer to release the brakes, recharging the train line and reservoirs, yet still maintain pressure in the brake cylinders.
Emergency vents the train line to atmosphere, it is a one and done. In steam era systems when the brakes were in lap, the train line and reservoirs were not being recharged. The only time the reservoir was recharged was when the brakes were released. If you applied the brakes, you would draw down the reserviour pressure. If you made an additional application you would draw down the reservior pressure more. If you made numerous small applications without releasing the brakes, you could draw down the the reservior pressure so low, it could no longer effectively apply the brakes. The railroad name for that was "pissing away your air". The point of retainers was to allow the engineer to release the brakes, recharging the train line and reservoirs, yet still maintain pressure in the brake cylinders.
That's still how air brakes work. The only difference is the lapping is no longer manual, it's done by the engine brake valve equipment.
(Except on historical equipment that retains the older brake valves. I've had the chance to run some old switch engines that had the older 6 schedule brake valves.)
"Pissing the air away" is still possible. The wide spread use of dynamic brakes has made the use of retainers rare, but cars are still equipped.
that's my understanding
i wonder how accurately the ProtoThrottle models brakes
Dave H. Painted side goes up. My website : wnbranch.com
OvermodSee Al Krug's pages
thanks. very helpful
Those brake valves were a little before my time (heh), but normal position for running/charging the brake pipe would be "running".
Release provides higher pressure than normal -- this goes back to earlier control valves on the cars that could "stick", and the extra blast from "release" could free up the valve.
To make a brake pipe reduction you would move the handle from running to service, and "draw down" the amount you wanted, and then move the valve back to the "lap" position to hold the pressure there.
Emergency was a quick vent of the brake pipe -- of course, to throw the control valves to emergency and "dump" the air.
The independent brake valve (the "little brake") controls only the locomotive brakes. Looks like that one has two application positions "slow" and "quick". They do as advertised, rate of pressure increase corresponds to valve position. When you have the pressure you want, move the handle back to "lap". I believe the release position on those would release the engine brakes without affecting the train's application. Like I said, before my time.
No pressure maintaining on these -- you couldn't keep a brake application on the train for too long, the brake pipe pressure would "leak down" and the brake application would get stronger whether you wanted that or not. Hence the need for "retainer valves" on the cars so the engineer could release the brakes (but still keep a certain amount of pressure in the train brake cylinders, then recharge and reapply the application.
By the time I came along, 26L was the standard brake. Self-lapping (no more lap position) and pressure-maintaining. Much easier to use.
See Al Krug's pages on the finer points of one-pipe brake systems, then have Ed describe the difference between 6ET and 26RL in locomotives, which is a primary interest on late steam power.
You also need to know the difference between the Westinghouse brake on the train and the independent (which is a proportional straight air brake) on the locomotive. These systems are arranged to apply the independent along with the Westinghouse, and there will be some technical discussion necessary as to how that was proportioned and what the best approach to bailing off such an application would be done.
Note that pressure maintaining was not introduced until after the age of late steam.
I believe at least some of the excellent ICS course material on air brakes is now available for download on the Web. While I think much of this predates you're period of interest it is still meaningful, and accurate, background.
i'm interested in practical descriptions of how westinghouse braking available on late steam locomotives work. i'd like to avoid esoteric details, but would like details
my understanding is the variable service modes either simple reduce brake line pressire or allow it the to drop to a specific level, for example, 1st service may allow it to drop by 10 psi. The rate the brake line pressure drops depends on train length.
a reduction in brake line pressure opens a valve in each car allowing air to flow from the car reservoir into the brake cylinder until the car reservoir pressure equals the brake line pressure. it's not obvious what the brake cylinder pressure is except to say it is proportional to the reduction in brake line pressure.
i believe the "lap" brake setting isolates the brake line, but pressure may drop due to leaks
i believe the "release" brake setting increases the brake line pressure from a reservoir and/or pumps which recharges the car reservoirs and exhaust the brake cylinder pressure (release the brakes) because the brake line pressure is greater than the reservoir pressure
i would expect the brake would normally be set to release. set to service until adequate braking achieved and then moved to lap. service would be set if additional braking required and returned to lap. finally returned to release when brakes no longer required and not needed for the time it takes to recharge the car reservoir pressure.
setting the brake to "emergency" causes a rapid reduction in brake line pressure. my understanding is the maximum braking force is limited to ~10% of a cars weight. it's not obvious to me if brake line pressure is adjusted depending on average weight of car depending on # of empties in train.