When I watch a UP crew Tie their Locmotives into a Railcar. The Conductor connects the Air Hoses between the Car & the Unit I hear Air rushing from the Unit to the Railcar. How does this work out for the cars Brakes to be Relesed?
Is it the Locomotive pumping the air into the cars brake line? Or does the Locomotive have to suck the air out of the cars line for the brake to work?
Thanks,Allan.
The locomotive pressurizes the train line, it is not a vacuum system. Each locomotive has an air compressor.
Each car has a two part air reservoir, air tank, on it. The train line gets charged up, then the if the train line pressure is higher than the pressure in the reservior and the brakes release and the system charges the air reservoir.
When the air pressure in the train line drops a set amount less than the pressure in the reservior the brake valve on the car puts air from the reservoir into the brake cylinder to set the brakes. When the pressure in the train line gets higher than the pressure in the reservior, the brakes release and the reservoirs are charged back up.
That is a very simple outline of how it works.
Dave H.
Allan,
Dave sums it up nicely, but if you want a detailed description go here:
I thought Air brakes on trains worked the same way Truck and Bus Air brake systems work, namely, air pressure is built up to RELEASE the brakes, not apply them. The locomotive, once coupled to the train, has to build up the air pressure in the trainline before it can start moving the train. This way, if the part of the train ever becomes uncoupled, the lack of air pressure will automatically stop the disconnected portion of the train.
You’re half right - train and truck brakes both use a drop in train line pressure to cause the brakes to be applied. In both cases, however, the force to apply comes from air in a reservoir (on each car in train brakes). The drop in train line pressure causes a valve on each car to open, admitting air from the reservoir to the car’s cylinder and applying the brakes. When the train line pressure rises, the reservoirs are recharged to provide air for the next application.
The downside of this system is that since train brakes only have full release (which vents the air in the cylinders to the atmosphere for a speedy release), and since it takes time to recharge the reservoirs, repeated application and release of train brakes will leave you without brakes until the reservoirs recharge. This happened to a CP train on the Laggan sub in 1997 - the crew escaped unscathed, but both Spiral Tunnels were plugged for some time with derailed cars and spilled grain.
Air-brake equipped trucks are the same way - too many application/release cycles too close together and you’re riding a sled till the reservoir(s) are filled. Think about that the next time you pass a truck on the road and give him a bit of extra room when you pull in. [B)]
Al Krug’s site (linked in a post above) is the best explanation of how train brakes work that I’ve ever seen. There’s a great piece on signalling too.
Leon,
In a truck brake system there are two lines. One takes POSITIVE pressure to release a spring that applies the brakes when parked. The other one takes POSITIVE pressure to apply the brakes for service applications. They work on positive pressure to apply, but if the air is lost the spring will apply the brakes due to lack of pressure to release the spring.
Actually that is only half right.
18 wheelers use a air compressor to generate air which is filtered and sent to the dry tank. That tank is the supply for the braking system.
On each of the 10 wheels (Not 18 -remember we are dealing with duals on a wheel) the brake pads (Or discs) are applied using air pressure to remove force from a spring that is extremly powerful.
There is a spring inside the chamber that always “Wants” to apply the brakes. If you over come the pressure with sufficient air you will release the brakes.
If you lose all your “Primary” air and use up the secondary you will find yourself with 10 springs attempting to stop you wherever you are at that time. You dont have a choice in the matter.
Usually air loss is a result of driver error or a failure (Has happened) of a part somewhere in the system. If it was driver error in mountain driving then usually the brakes are over heated and unable to provide any braking at all. What you are actually generating is a ternimal high speed last ride with each wheel catching fire to boot.
I dont want to get into too much detail. But older trucks (Prior to 1975) have a different braking system. I dont see too many of these older vehicles these days.
I have had people attempt to jump between my cab and trailer and disconnect the emergency line side hose as that one is what carries the charge full-time to keep the springs released. A disconnect of
That is true, but it that drop in pressure only triggers a seperate sub system on each car to use a tank of air to force the brakes against the wheels.
Adrianspeeder
Highiron2003,
Thanks for that great explaination. Could you tell me something, when a tractor / trailer gets towed, how/where do they tie in the brakes? Is there built in provisions for this or does the tow truck driver carry something special for this? And do both air lines need to be tied into or can they get away with only the “release” line (on the level)?
Oops. Guess I know more about train brakes than truck brakes…[;)]
I dont know much about train brakes =) But I do know a “Big Hole” when I hear one.
You know those air line couplings that auto mechanics use for thier air tools? "**SNap wheiiirr! We have the same thing in specific areas on trucks. The tow driver simply brings a 100 foot coil of hose and connects the dead truck to his own rig’s air system. Presto both have brakes.
I understand that different forms of towing might not actually have functional brakes on the towee but I leave the fine points to the towers to tell.
I heard tell of a friend who suffered a break in his air line. he simply stripped a bic pen down to the plastic tube, put both ends of the severed rubber hose that supplies the air onto it, clamped it and off he went.
Upon review of the writing I used the term “Dry tank”
here is a twist on the dry air and wet air. Air is taken in and used by the compressor to generate a charge in the holding tank. Ideally you want this air to pass thru the wet tank, to the filter and finally to the dry tank.
The wet tank fills over time with oil, grease, sludge and well… water. Ideally you drain this out nightly while you sleep. Failure to drain (Or use a in-line filter) will cause your spring brake c
Hi group
air pressure on a freight train is 90psi 100psi on passenger. When the air hose is connected from the locomotive to the car or cars and the angle***is opened the chargeing process begins, after the rear car pressure is within 15psi of the regulating valve we the crew can do an class 1 air test by makeing a 20psi break pipe reduction.
On the cars their is a reservoir which has (2) sections they are the auxiliary and emergency. When air pressure is taken away from the brake pipe i.e. a minimum reduction this is most often a 6-8psi drop of the brake pipe air is admitted to the brake cylinder through the contorl valve this is at a 2-1/2 to 1 ratio so with a 6psi reduction I will have 15psi in the brake cylinder.
The cars brakes will begin to set up in a serial setting from the front to the back and releaseing in the same mannor.
The emergency acts the same way except at a much faster rate at the same time I must place the etd in emergency to dump air from the rear also.
Rodney
Thanks Highiron.[8D]
High iron I do not think electric brakes will be used in the near future same problem as on the RR need to have a 2nd brake system overlay the regular brakes. Airbrakes have a simple advantage oveer electric brakes even if the computer goes out on the ABS you still have brakes. One other thing you missed is what happens when only one brake loses pressure that is a GAURENTEED JACKNIFE. I had one so I know what happens. It is not fun at all when you see the nose of the trailer start to come through the back of the sleeper. Worse case scenrio you lay it ons its side best case you need a CLEAN PAIR OF UNDERWEAR[:P][:D]. Had 2 of those and that is the reason I con not drive for a living anymore old brain bucket took to many impacts.
I havent had too many problems in the way of jack knife.
The milk tanker truck on a R model mack I drove suffered the off-ramp scenario (On a downgrade to boot) and I believe if that one chamber locked first it would have created a very bad jack knife. I basically juggled the air the rest of the day in a race between stopping power versus losing air.
My wife was driving in 2000 winter crossing knoxville on light ice we hit a curve and the tractor lost it’s footing and I reached over to grab the wheel and break the angle. Number two hit that spot and made it thru but numbers 3 thru 7 did not. The last I saw of any of them was a scene of chaos as they struggled to stay up right while gravity dragged the whole group into the median. I believe one of them was actually attempting to “Power out” and save his flat bed load with the horsepower.
I had one incident on filthy rain in the cross bronx expressway (Aka I-95 NYC) where a car cut me off trying to grab the off ramp for one of the bridges. He or she then stopped dead in front of my 70 mph rig (Speed my fault but you dont go slow in the bronx) ABS saved the day.
The rest of my recollections are related to winter driving when you cannot get enough traction and things get exciting and stressful.
My probelms were related to moisture in teh air system I had one were the hose blew off the parking side of the chamber. That one led to a fatal wreck with a drunl driver. The other one was in SD in the winter had a nice slug of ice get in the air lines the pressure literally blew the lines off the valve no one hurt in that one. What lead me to getting off the road in sept of 2000 was a rear end collison in my car and 3 weeks later I had a medical card disqualfiing incedent that puled my CDL permently I still can’t drive a normal car. Liek they say I have the T-shirt.
Ok I now understand now. But here is another Question? Like some Railyards across America… In most cases, When there is no Power Coupled to the waiting Train on the departure Track,there is a Hose running up to the first car of the Train that has no Power. The Hose that is connected to the cars Brake Hose,Is it call,Pre-tested? Like down in the BNSF Gibson Yard BNSF Pre-tests their Brake pipe for their Intermodel Trains. Allan.
They probably do. YArds want the trains gone asap. It would benefit them to have a pre-charged train line.
Cars left out in sidings probably needed to be tied down.
A few months ago TRAINS had an article explaining how conventional air brakes and the new electronics work. Interesting reading. Unfortunately I do not remember which issue.
Mention was made of wet air earlier. A little physics here. A compressor increases the air pressure from atmospheric to the 90-110 psi of the train. One of the bad effects of compressing air is that you go through a dew point and will have water condense out. As mentioned earlier, this water is no good. Besides causing corrosion it can be detrimental to the operation of the brake system. If you fill a cylinder with an essentially non compressible fluid, such as water, then try to compress it when the machine thinks it is air sometning has to give. I saw this in the oil filelds many years ago. We were purging a gas gsthering line that went to a gas storage compressor. The water in the line jumped the slug catcher and was sucked into the compressor cylinder. The valves closed and the 18 foot diameter flywheel (It was a very old compressor and I am dating myself) tried to compress the water. Something had to give. The 12 head bolts on the 18 inch cylinder sheared and the cylinder head went through the corrigated building, through a chain link fence, across the highway, and planted itself in the road cut. Needless to say the compressor never ran again.
Moral: Get rid of the water.
Allan,
Most yards that originate trains have a big, and I mean big, set of compressors, often housed in a building by themselves, with pipes to each end of the departure tracks, complete with valves and pressure guages.
We do the inital terminal air test without a locomotive, because it allows the car men a chance to change brake shoes that might have been missed…and do a good look over one last time.
And, as Highiron pointed out, it also allows us to have trains ready to go before the crews show up, without having to tie up locomotives to pump up the air in the cars.
Most yards require that at least two working hand brakes must be applied to the leading end, to hold the cars in place with or without air brakes applied.
In my yard, it is forbidden to leave any car not coupled to a locomotive standing without a hand brake, unless it is coupled to a cut of cars that have at least two handbrakes applied.
Any car standing alone must have a hand brake applied.
Any cut of cars must have two working hand brakes applied,
The term you are looking for is ground air, as opposed to locomotive air, and we call the hose and valve in the track the air spot…so next time you hear someone say they are spotting up a yard track, or putting a track “on the spot” you will know what they are doing…
Ed