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Forum Index : Other Stuff : OT: Air-brakes question...
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
Not exactly a controller problem, etc, hence posting it here, and there are very clever people here who will be able to clarify this for me: Just finished watching UNSTOPABLE - the movie about the runaway train. It was my understanding, that air-brakes on trucks or trains work on air-pressure. It's a simple enough concept: air pressure is used to release the brakes is my understanding, NOT the other way around, as is OFTEN shown in movies and TV. Loss of air-pressure would not result in no brakes, it should result in automatic brake application - correct? I might be wrong, but I always thought that was the way they worked, in order to be "Failsafe" in that(for example), if a truck had a leak in the trailer's air-brakes circuit, it would start breaking by itself, rather then the driver having to worry about it. Thus, if the trailer was to break away from the truck, the air-line would break, the pressure would be lost in the trailer section, and it would automatically brake the trailer to stop it. Taking that one step further, if that really is the case, then a runaway diesel train should not be possible as shown in the movie, where the initial driver of what becomes the runaway, does not connect the air-line. Would this not mean the brakes would be applied on the rest of the train? Also, I thought train engines had a form of "Dead-man's switch" - a foot-pedal or button that the driver has to keep pushing every minute or so, and if he/she fails to do that, the train automatically applies the brakes and stops itself without any additional help. I do understand that the term "Runaway train" really comes from the steam engine days, where if the driver was to pull open the regulator, then trip and fall off the footplate, then the train would puff away there with no-one to stop it until it ran out of steam. My understanding was that in modern diesel engines, this is not a possibility. Comments? Smoke makes things work. When the smoke gets out, it stops! |
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| MOBI Guru  Joined: 02/12/2012 Location: AustraliaPosts: 819 |
Your understanding is also my understanding, and I think, the correct one. However, never let the facts get in the way of a good story!!  David M. |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
Oh yes - I was not critising the story per-se' - I love trains, so a whole movie around them was fine with me, but it was just some of the specifics!  Smoke makes things work. When the smoke gets out, it stops! |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
It is still pretty rare to see spring brakes on a trailer. Most of the ones I see are just the plain ol single diaphragm brakes. If the trailer air tank is empty the brakes are off. If the red air supply hose is cut then a relay valve is triggered and the air pressure in the tank is used to apply the trailer brakes. It fools a lot of truckies because the trailer brakes are on as they start their truck in the morning and they have to wait for the pressure to get up before they will release. spring brakes or double chamber, double diaphragm or maxi's are usually only on the drive axles on the prime mover. 
The black chamber in this photo of a drive axle is the service brakes and the nickle plated chamber on the top is the chamber with the big spring in it. If you look closely there is a small bolt attached to the housing between the chambers, this is a manual release tool. If the diaphragm ruptures in the spring chamber then the bolt is fitted in the top and the spring is compressed by hand so the truck can be moved. I'm confused, no wait... maybe I'm not... |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Also very important to install the bolt when disassembling the Maxi for repairs or diaphragm replacement, or the spring will likely take your head off if not restrained when the retaining clamp is removed. Sometimes it just works |
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| M Del Senior Member  Joined: 09/04/2012 Location: AustraliaPosts: 155 |
I think spring brake chambers are more common than people think. I looked at the trailers at work today, 2 20-ton plant/8 4-ton pig/ 3 8-ton dog/ and 9 30000l bulkfuel tanker trailers. All had spring brake chambers. Mind you (in another life), we had a tilt bed trailer roll down a hill in the mid/late 80s, this was in the middle of the night. No spring brakes on it, just air operated. By memory these trailers were modified with spring/maxis by 1990, did some of them myself, lots of fun in the bush. All trailers purchased since then were fitted with maxi brakes on at least one axle. Would need to check the regs to see if all axles are now fitted with them, no longer on the tools (ahhh office hands). Mark Mark |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I used to be an engineer at Westinghouse Signals. You are correct that all trains have a dead man's throttle, which the driver has to constantly keep activating. Another safety feature is that there are motor driven arms bolted to the track that can swing upward. This arm deflects upwards a bar in front of the first wheel and cuts power, and applies the brakes if the driver attempts to go through a red light or approaches points that are set the wrong way, or an open level crossing. This can also be activated remotely, so that a stolen or run away train can be stopped even against the drivers wishes. There is also a system to detect the exact location of every train electrically on a long length of track, and to automatically bring up red lights if one train approaches too closely behind another train that is ahead. The braking systems themselves are also fail safe, and have to be released with compressed air pressure before the train can move. There is a huge amount of work goes into designing all these safety critical systems so that they will always fail in a safe condition. That is what I used to do there. Cheers, Tony. |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
Thanks for that info. I did not know about the arm that cuts the power - very interesting.
Despite it's blatant technical errors, I still like the movie. Anything with trains in it, has my yes. Smoke makes things work. When the smoke gets out, it stops! |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
http://en.wikipedia.org/wiki/Train_stop http://www.twincomfg.com/Automatic_Train_Stops/Twinco_Automa ticTrainStop.htm Cheers, Tony. |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
Thanks for the links!  Smoke makes things work. When the smoke gets out, it stops! |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
If you are interested in a real train-wreck there is a bit of info on Wikipedia on the Duffy Street crash in 1989. I saw a doco on the telly about it a few years ago. I'm confused, no wait... maybe I'm not... |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
Ta - checking link now... Smoke makes things work. When the smoke gets out, it stops! |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Any train crash is very bad. But in passenger miles travelled, rail is still the safest way go anywhere. It is a very strange industry to work in. The technology is mostly many decades old, positively ancient. The theory behind this is that if something very simple has worked reliably for sixty or more years, with a perfect safety record, why change it ? Cheers, Tony. |
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| Ron B Newbie  Joined: 26/04/2013 Location: AustraliaPosts: 5 |
All locomotives built in the west (USA,UK etc ) run Westinghouse air brakes. These use a train brake, which control the wagons behind the loco and there is a loco brake for the engine itself . There are several positions for the main brake controller, stop, lap, and full application and emergency. They work on pressure , around 75 psi. When the brake is partially applied , the pressure is reduced by 15psi. This applies the brakes and they can be released quickly by rebuilding the pressure . On full brake application all the air is released and the brakes are locked on . Each wagon or carriage has a reservoir to hold pressure , so the brakes can let off . The brakes cylinder which actuates the linkage for the brakes proper has a device called a triple valve . This is activated by the variation ion air pressure as regulated by the driver. So..if the air is lost ,the brakes come on,and stay on. Loco motives also have either dead man pedal and/or a vigilance button. The driver has to push the button every few minutes or a buzzer sounds. Another minute of so and the loco horn will sound or the brakes are applied. The buzzer is enough to wake the most tired driver. |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
Great information, thanks. Smoke makes things work. When the smoke gets out, it stops! |
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