• Locomotive Brakes

  • General discussion about locomotives, rolling stock, and equipment
General discussion about locomotives, rolling stock, and equipment

Moderator: John_Perkowski

  by GonzoBright
Hello All,

Just as most large Trucks/Lorries have air brakes, trains do too. If one keeps pounding on their brakes, they incur a condition called brake-fade. Does anyone know how engineers combat brake-fade on their locomotives? (Especially while heading down hills)
  by litz
It certainly can happen ... there have been runaways because brakes applications happened faster than the pressure recovery ...

If you start at 90psi, make a 20psi application, your brake line pressure is now 70psi.

If you release and reapply 20psi when the pressure has only rebounded to 80psi, now you're at 60psi ... and you can see the spiral downwards from there ...

Engineers are taught during training how to avoid this ... and, of course, locomotive independent brakes are basically immune (it takes practically zero time to recover pressure ON the locomotive) ... and dynamic brakes are immune.

You'd be surprised how effective 4-5 locomotives worth of independent brake effort can be ...
  by NorthWest
Locomotive engineers are qualified on their territory, and operate trains on the same section of railroad day after day. They learn where to use braking power, and where to not. Usually, dynamic brakes are used to their maximum effect before using air brakes, because it reduces wheel and brake shoe wear and saves air.
  by Desertdweller
Dynamic brakes are often effective in controlling train speed, but they depend on the loco's wheels turning to develop braking force, and the slower the speed, the less force is generated. Using independent brake creates the possibility of sliding the wheels.

Controlling train speed with dynamic brakes alone (or a heavy-handed independent application) will result in slack being run in against the power. This is unavoidable, and is not necessarily a bad thing if done in a controlled manner. The automatic (train) brake will have the effect of stretching the slack out. You can combine both methods (blended braking) in a very gentle manner (watch your head-end device to see when the rear end is setting up). The situation you have to avoid is having slack running out in one part of the train while running in at another part. This usually results is a very unhappy situation (somebody is going to have to change a knuckle).

Dynamic braking works best on short to medium-sized trains, where speed can be controlled by that method alone. It is quite possible for a long, heavy train, or a train on a steep grade, to overwhelm the dynamic brake. If that happens, air will have to be taken quickly to control speed, and the slack will be in the wrong location to do that gently. Anytime air is set under the train, the brakes under the locomotive (independent system) will need to be released, because they will set up much faster than the brakes under the cars, and the speed differential will result in a violent whack against the locomotives.

If you are going through a sag (between two steep grades), you will need to set a little air under the cars before you enter the sag to keep the slack stretched, then open the throttle as you come out of the sag to keep ahead of the slack. Once you get enough cars on the uphill side of the sag, you can knock the air off and keep on the throttle as you head upgrade.

  by Pneudyne
I wonder perhaps if we are dealing with two somewhat different phenomena here. With road vehicle brakes, the term “fade” usually, although perhaps not universally refers to the loss of friction in the foundation brake mechanisms (drums or discs) that occurs because of heat build-up from continuous or repeated operation, and not to any degradation of the actuating mechanism realizable pressure, such as occurs with direct-release type automatic air train brakes if they are recycled too frequently.

Heavy truck-trailer air brake actuating systems are essentially of the straight air type, neglecting for the moment the spring-type secondary/emergency and parking brake systems. To put it in locomotive terminology, the primary braking system is operated from the brake pedal by what might be called an “application and release” pipe (although truck terminology is different). At the trailer end, the brakes are not operated directly, but by a relay valve that uses air from a local reservoir that is in turn fed from the truck air system by what might be called a “main reservoir” pipe. Thus truck-trailer primary brakes are somewhat analogous to locomotive independent brakes, and as such unlikely to suffer loss of air pressure through excessive use given typical air compressor and main reservoir capacities. The foot operated brake valves are self-lapping and thus pressure-maintaining. Truck/trailer and railroad air braking systems make an interesting comparative study, and there do appear to be overlaps. Way back in the 1950s, Westinghouse UK developed an electro-pneumatic braking system for truck-trailer combinations, I suspect based upon its extensive experience with railroad EP braking systems for EMUs, etc. It was not taken up in practice, though.

  by Train Detainer
I'm not that familiar with truck brakes, but if brake fade does in fact represent a loss of friction, then it's not something that locomotive engineers deal with much. Modern rail equipment, both cars and locomotives (for the most part) uses HF Comp (HIgh Friction Composition) brake shoes that are designed for high heat applications. HF Comps actually have better friction/adhesion quality at higher temperatures (above ambient) and engineers have to accommodate the extra time needed for the shoes to warm up when they make an initial brake application. If you wait until your 12,000 ton train is already cresting the top of a heavy grade at speed before applying the brakes, you will at best be speeding before the brakes warm up. At the worst, your speed will have become excessive and you have a runaway. There are many other variations on what can happen in between to recover or fail, but like another poster said above, engineers must be qualified on their territory and plan their actions accordingly.

The only thing similar to the described fading would be on a very long grade where already thin shoes are worn thinner and their ability to take the heat diminishes to the point that they basically self-destruct and are no longer functional or in a true emergency situation where control of the train has been lost and the wheels/shoes have become so hot that they have actually begun to melt. This has happened to varying degrees in runaways over the years. See Cajon Pass runaway reports for details.

The issue of fading in dynamics isn't related to friction, but is a concern depending on equipment and speed. Older flat type dynamics had a limited effective range, quickly becoming worthless below 20MPH. Newer DC locomotives had Extended Range dynamics that considerably lowered the lower end of the effective speed range(6-7 MPH), but it's still a curve that flattens to 'almost useless' at lower speed ranges, requiring the engineer to transition to independent brakes to actually stop the train. Newer AC locomotives are capable of dynamic braking virtually to a stop. The dynamic brake effort curve also tapers off at higher speeds with DC (generally above 25 MPH), but less so than at lower speeds. The newest DC locomotives have even higher capacity dynamics than the older ER types, too. It all depends on what you're operating.