Forums

[nawalins]

Could someone please explain the difference between standard braking and dynamic braking, how they operate, and when is one braking system used over the other?

[chad thomas]

The Air brake or trainline brake works like most brakes by resistance of the friction between brake pads and the brakeing surface. In the case of railroads it's usualy the wheel surface itself although many cars in passenger service have disk brakes like your car. The brakes are activated by air pressure from the brake cylinder on each car that are linked to all the brake pads by the rods and levers of the brake linkage. All the cars are hooked up to a common air line that runs the length of the train.

Now the air brake system does not work directly on the air pressure. The air line is hooked to a "triple valve" that reads the air pressure to controll how much air goes to the brake cylinder. It is a complex system with it good points and bad. For a excellent explaination on how they work check out Al Krugs site:

http://www.alkrug.vcn.com/rrfacts/brakes.htm

One of the drawbacks of the air brake system used on almost all freight trains is that you can't make a partial brake reduction. It's all or nothing and to release takes time to recover the air. This recovery time can be measured many minuets on a long train and in real cold weather. If you are going downhill and make too big of an application and need to do a release, by the time you recover the air it could be WAY too late and the train will have accelerated to a point of no return and you have a runnaway. Now you can start a release and then make another application before the air recovers, but only once or twice or a few times, depending on how deep the applications. But on most steep grades you only have once or twice to get it right. This is a very simple explaination and you should refer to the above link for a good understandable explaination.

Dynamic brakes are kind of like downshifting your car on a steep downhill grade. It causes drag through the drivetrain. Only in this case it's an electrical drivetrain and the drag is cause electrically. A deisel-electric loco is an electric loco with an on board generator. The generator is the prime mover coupled through the crankshaft to a big electric generator and that supplys electricity to the traction (electric) motors that are geard to the axles of the locomotive. When going down hill these traction motors can generate electricity. If you hook up a load to these motors it will cause resistance and slow these motors down. In the case of dynamic brakes this load is a HUGE resistor grid. Basicaly like the heating elements in a toaster. Only instead of letting them get red hot, a portion of that energy is used to run a big cooling fan to keep the grid temprature from getting too hot. The fan is wat makes the whineing noise you hear.

Dynamics don't work in a linear fassion though. At a dead stop they don't work. As you increase speed they have more of an effect up to the point where there is maximum current and then they are modulated to not exceede 700 amps to the grid. This means they get more effective to a point then become less effective. At maximum dynamic brakeing that point is around 22 MPH (Extended range dynamic brakeing has an additionl range that would have another peek at 12 MPH). Above that peek there effectiveness tapers off and up around 60-70MPH they are a LOT less effective.



If practical an engineer will favor useing Dynamics over Air brakes because air brakes are quirky and the slack can be unprecictable on mixed freights. There is recovery time to consider and cars called dynamiters that do way more then there fair share of the brakeing sometimes show up. Plus with dynamic brakes there is virtually no where and tear, like there is with brake pads. Dynamics give you much more predictable results. when decending a hill the engineer will make a air brake application that will be a little less then what is needed and use the dynamics to controll the speed. With dynamic brakeing you have to plan ahead as when you are brakeing you are only doing it from the locomotives and if they are all at the head end you have a whole trains worth of slack action to controll. That is one of the reasons DPU is better for train controll as each DPU set can be controlled independantly. That allows you to keep the train streached or bunched reduceing the hazards caused by unpredictable slack action.

[nawalins]

Thanx for the response! The info you supplied really helpful in understanding how this system functions.

[chad thomas]

No problem, but be sure to check out Al Krugs site. He has a lot of good explainations of a whole bunch of things that are easy to understand. And his essays are interesting reading too, especialy his one on running steam engine SP&S 700.

[chad thomas]

One more thing I forgot to mention, Independant brakes. The independant brake is an air brake that works only on the locomotives. These brakes are more like the ones in your car. Yuo can set and release these almost instantly and in graduated amounts. Unlike the train brakes the independants can be partially released. These brakes are used primarily in switching when the cars air lines may not be connected. You can not use the independant brakes while useing the dynamics.

[ValorStorm]

QUOTE: Originally posted by chad thomas


One of the drawbacks of the air brake system used on almost all freight trains is that you can't make a partial brake reduction. It's all or nothing and to release takes time to recover the air.

There's probably a better way to say this. Although you continue with an explanation, a "newby" reading this thread could get the idea that every brake application is an emergency application.

[chad thomas]

I didn't mention emergency applications at all. I was trying to keep it simple. For a thourogh brake explaination refer to Al Krugs site

http://www.alkrug.vcn.com/rrfacts/brakes.htm

[ValorStorm]

QUOTE: Originally posted by chad thomas

I didn't mention emergency applications at all. I was trying to keep it simple. For a thourogh brake explaination refer to Al Krugs site

I realize you didn't mention emergency applications. That's why I made my point. The fact is that it is NOT all or nothing with air brakes. You CAN make partial applications. A "service application" is a partial application. The point that I think you were trying to make is a valid one: After a reduction it takes time to recover the air. But "all or nothing" in this context sounds like the only choice is to "big hole" the train every time. Al Krug's terrific site doesn't suggest anything like that.

HEY! 200 POSTS!

[chad thomas]

QUOTE: Originally posted by ValorStorm

QUOTE: Originally posted by chad thomas

I didn't mention emergency applications at all. I was trying to keep it simple. For a thourogh brake explaination refer to Al Krugs site

I realize you didn't mention emergency applications. That's why I made my point. The fact is that it is NOT all or nothing with air brakes. You CAN make partial applications. A "service application" is a partial application. The point that I think you were trying to make is a valid one: After a reduction it takes time to recover the air. But "all or nothing" in this context sounds like the only choice is to "big hole" the train every time. Al Krug's terrific site doesn't suggest anything like that.

HEY! 200 POSTS!

I didn't say the application was all or nothing. I said the release is all or nothing. Please don't put words in my mouth that I did not say.

This is what I said:

One of the drawbacks of the air brake system used on almost all freight trains is that you can't make a partial brake reduction. It's all or nothing and to release takes time to recover the air.

[ValorStorm]

QUOTE: Originally posted by chad thomas

QUOTE: Originally posted by ValorStorm

QUOTE: Originally posted by chad thomas

I didn't mention emergency applications at all. I was trying to keep it simple. For a thourogh brake explaination refer to Al Krugs site

I realize you didn't mention emergency applications. That's why I made my point. The fact is that it is NOT all or nothing with air brakes. You CAN make partial applications. A "service application" is a partial application. The point that I think you were trying to make is a valid one: After a reduction it takes time to recover the air. But "all or nothing" in this context sounds like the only choice is to "big hole" the train every time. Al Krug's terrific site doesn't suggest anything like that.

HEY! 200 POSTS!

I didn't say the application was all or nothing. I said the release is all or nothing. Please don't put words in my mouth that I did not say.

This is what I said:

One of the drawbacks of the air brake system used on almost all freight trains is that you can't make a partial brake reduction. It's all or nothing and to release takes time to recover the air.

Sorry. I didn't meant to come across as jumping to conclusions. But I did not put words in your mouth. The words that still stand out in your quote are these: "you can't make a partial brake reduction. It's all or nothing..." Your point about releasing the air is, as I mentioned, a good one. And I assumed that it was what you must've meant by "all or nothing."
I was never trying to belittle you. Whenever my mistakes are pointed out to me (and I make plenty on these threads) I simply edit my post. When that opportunity wasn't taken I made the point that "You CAN make partial applications. A 'service application' is a partial application." And it is.
Chad, you're among the top five most knowledgeable resources in the forum. I'm simply not. You have no reason to be so defensive.

[chad thomas]

QUOTE: Originally posted by ValorStorm

QUOTE: Originally posted by chad thomas

QUOTE: Originally posted by ValorStorm

QUOTE: Originally posted by chad thomas

I didn't mention emergency applications at all. I was trying to keep it simple. For a thourogh brake explaination refer to Al Krugs site

I realize you didn't mention emergency applications. That's why I made my point. The fact is that it is NOT all or nothing with air brakes. You CAN make partial applications. A "service application" is a partial application. The point that I think you were trying to make is a valid one: After a reduction it takes time to recover the air. But "all or nothing" in this context sounds like the only choice is to "big hole" the train every time. Al Krug's terrific site doesn't suggest anything like that.

HEY! 200 POSTS!

I didn't say the application was all or nothing. I said the release is all or nothing. Please don't put words in my mouth that I did not say.

This is what I said:

One of the drawbacks of the air brake system used on almost all freight trains is that you can't make a partial brake reduction. It's all or nothing and to release takes time to recover the air.

Sorry. I didn't meant to come across as jumping to conclusions. But I did not put words in your mouth. The words that still stand out in your quote are these: "you can't make a partial brake reduction. It's all or nothing..." Your point about releasing the air is, as I mentioned, a good one. And I assumed that it was what you must've meant by "all or nothing."
I was never trying to belittle you. Whenever my mistakes are pointed out to me (and I make plenty on these threads) I simply edit my post. When that opportunity wasn't taken I made the point that "You CAN make partial applications. A 'service application' is a partial application." And it is.
Chad, you're among the top five most knowledgeable resources in the forum. I'm simply not. You have no reason to be so defensive.

Well, There is no reason to argue about it. I guess I could have worded it better, my bad. I hope there are no hard feelings. And while I do have my moments, I don't know about top five, but thanks for the compliment.[;)][:D][8D]

[Anonymous]

The first post on this thread was very interesting. I was wondering about Dynamic brakes. but why would they be removed on a loco? IC removed them on some of the SD40-2's. Seems like air brakes have the drawback because of the recover time. I figured DB was preferred.

[chad thomas]

Well, Dynamic brakes are an added cost at purchase and add to the maintainance costs of locomotives so equiped. If you have a relativly flat railroad like the IC there is really no need for dynamics, although they would save wear and tear on brake pads.

[Anonymous]

I guess it does make sense than. Because many of IC's SD40-2's were purchased from BN and BN ordered them w/ DB but IC removed the DB on some of them.

[Anonymous]

QUOTE: Originally posted by chad thomas

One more thing I forgot to mention, Independant brakes. The independant brake is an air brake that works only on the locomotives. These brakes are more like the ones in your car. Yuo can set and release these almost instantly and in graduated amounts. Unlike the train brakes the independants can be partially released. These brakes are used primarily in switching when the cars air lines may not be connected. You can not use the independant brakes while useing the dynamics.

A little correction here. You CAN, with limitations, use the independent while using dynamics.
Development of approximately 15-18 psi of independent brake cylinder pressure, dynamic braking will be reduced to that of a standard range dynamic brake or, depending on locomotive brake cylinder pressure switch design, will eliminate all dynamic braking completely.
I really abhor the use of words such as: CAN'T, NEVER, ALWAYS, etc. on these rail forums