The recent rash of bad weather has crippled the Metra operation in Chicago because of frozen brake rigging and related equipment. This circumstance causes me to wonder if tread brakes aren't a better overall choice, especially since brakes shoes are now made of some type of composite material. I am interested in hearing thoughts on this subject.
Disc brakes have a wide variety of advantages on passenger equipment -- more swept area and greater 'friction' (pad) area, multiple brakes possible (as with Acela) on a single axle, faster acting (and easier to modulate in slips/wheelslide), little effect on suspension action or tread profile, for example. Yes, much of the 'working parts' are inboard in the truck structure, and this makes them somewhat more difficult to maintain, but remember that most of the time tread-brake systems suitable for high speed will also have some linkage inboard, even if the air cylinders and piping are external. (There is no reason disc brake air cylinders need to be inboard, btw.)
On the flip side, adding one or more brake discs contributes to both unsprung mass and rotational inertia, and also to gyroscopic effects related to the rotating mass. You may remember the excitement over cracked brake-disc spokes on Acela. More work is involved for wheel or axle servicing, and of course the cost is higher.
Now, having said all this, you may recall that many of the original Amcoaches were retrofitted with tread braking -- this may be a case of 'very little bit helps' (I well remember the stinking clouds of smoke under Clockers run with Heritage equipment in the early '70s, with a nice heavy GG1 on the point; much more fun with G's on Amfleet...) but certainly seemed to be welcomed by many in the operating community.
You will note that the 'composite' brakeshoes are themselves the cause of significant problems, one being the recent runaway(s) on Seventeen Mile Grade. The limited contact area possible between shoe and tread leads to very high heating in repetitive-high-speed stopping situations -- such as frequently encountered in Metra operation. If outgassing in the tread material requires more pressure to overcome than the brake-air pressure can safely provide, you may be in for something that the term 'brake fade' does not at all do justice to.
One place tread brakes can be significant is in keeping proper contact between the train and the signal system (or train-control system) via the track (you might think that something so heavy would be assured electrical contact, but that is definitely not assured!) At least one early design of disc-braked passenger car had a couple of "token" brakeshoes to keep enough of the tread shiny to assure conductivity. Wireless methods of connection may make this point less significant today.
You seem to have missed the point of heat disappation. Disk brakes have much less mass to absorb heat energy and depend on fan action to get rid of the heat. Tread brakes on the wheel take advantage of the wheel's mass to absorb and radiate the heat energy. The tread brake also continuously cleans and polishes the tread improving wheel rail adhesion. The San Francisco BART cars have disk brakes, a design mvoe to minimize unsprung weight. However, the consultants assumtion that dynamic brakes would do all the work 99% of the time has not been validated by experience. Dsik failures, necessitating total removal of the axel for replacement haave a bad effect on availability. Wheel rail adhesion also suffers, resulting in wheel flat spots and corregated rail. Ride the system and you will hear the noise.
Amtrak's 2 disk, 2 tread brake system on the Acela cars takes advantage of the best characteristics of both systems while providing high thermal capacity..
Run-away trains are the result of faulty train handling and were much more apt to occur with cast iron brakes shoes. Unfortunately there is no sure way to prevent stupidity.
OvermodOne place tread brakes can be significant is in keeping proper contact between the train and the signal system (or train-control system) via the track (you might think that something so heavy would be assured electrical contact, but that is definitely not assured!)
Boston and Maine had to equip its RDCs with "exciters" to be sure to trip automatic grade crossing protection for single car trains. The Budd disk brakes with "rolokron" antilock were indicted in at least one rear-end collision.
Rotor failure on a disk brake can create an amazing amount of noise and vibration.
As I recall, from a maintenance standpoint, disks require more - especially in the inspection department.
As a regular train crew member, I can inspect one of our trains and the brakes thereon (and do so every time I work) by doing a traditional "Class One" walkaround.
As I understand it, disks must be inspected by the mechanical department.
While that's probably not a problem for Amtrak or a transit agency, for us it would be a real problem as we don't have the staff to do it.
A major problem for us in the fall is leaves on the tracks, which can be like grease. As mentioned, tread brakes will clean that off (and you can see that on the shoes). Disks would not, which would greatly reduce braking ability and increase the probability of sliding a wheel.
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
sps The recent rash of bad weather has crippled the Metra operation in Chicago because of frozen brake rigging and related equipment. This circumstance causes me to wonder if tread brakes aren't a better overall choice, especially since brakes shoes are now made of some type of composite material. I am interested in hearing thoughts on this subject.
Carl
Railroader Emeritus (practiced railroading for 46 years--and in 2010 I finally got it right!)
CAACSCOCOM--I don't want to behave improperly, so I just won't behave at all. (SM)
CShaveRRCan disc brakes be applied to a degree that wheels will slide (on a train, I mean!)? Or can a failure to release after a service application result in slid wheels?
They sure can! and this was a concern very early on when disc brakes were applied to passenger cars. In the original lightweights, there was a significant difference between 'loaded' and 'empty' mass, for example. That is one important reason why the decelakron/decelostat devices were introduced (before the days of modern anti-lock systems). As noted in a post above, if there is any gunk on the rails or treads, you might get sliding if full release isn't made. So a modulating device separate from brake-pressure control (which the decelxxxx systems are) is desirable to avoid problems.
I saw a suggestion somewhere that braking energy be stored on cars via axle-mounted generators. Involved, yes...
Perhaps more likely in the days of Spicer drives to car-mounted generators for individual-car electrics. Nowadays we have HEP for the train power, and the 'economics' for individual axle-driven generators isn't really there. (You would need a generator for each axle to be braked, with full field control (which is not difficult) and ideally with the ability to sync phase with an external reference (which is not that difficult either) -- then you would need some control protocol for them all, ideally NOT carried through special trainline connectors. I think I'd do it with a wireless protocol as there would be fewer concerns with equipment not specifically provided with through-connections (analogous to the air pipe provided on non-air cars 'back in the day')
And then a buncha battery packs, or flywheels on commuter equipment, or maybe Karman-style reservoirs... I'm not sure the ROI is going to be there, although it's tempting to consider a 'poor man's MU' assisting part of the acceleration curve for something like push-pull trains.
... consider that trainlined dynamic braking would probably save a lot of energy and use of the air brakes.
Therein hangs a bit of a tale. With some care and electronics, the 'generators' can not only be modulated to produce braking on the usual curve, but actually reverse-motored at low speed to provide what is essentially dynamic braking up to adhesion limits, right to the equivalent of short-term rating at zero rpm (for DC machines). It is also possible, but somewhat more expensive, to produce an appropriate rotating polyphase field in AC motor windings to give positive "brake" torque at very low RPM. So yes, in theory, we have now arrived at a point where much of the train control can be handled without need for friction braking...
... the problem then being exactly the sort of problem faced on Seventeen Mile Grade: you won't or can't rely entirely on dynamic for all your moment-to-moment braking needs, and if at any moment the dynamic fails or is degraded, how can the crew comprehend and respond in time to assure a safe deceleration or stop with the 'normal' brakes and procedures?
Jerry Pier You seem to have missed the point of heat disappation. Disk brakes have much less mass to absorb heat energy and depend on fan action to get rid of the heat. Tread brakes on the wheel take advantage of the wheel's mass to absorb and radiate the heat energy.
You seem to have missed the point of heat disappation. Disk brakes have much less mass to absorb heat energy and depend on fan action to get rid of the heat. Tread brakes on the wheel take advantage of the wheel's mass to absorb and radiate the heat energy.
I am well aware of the characteristics of heat dissipation (note sp.) both due to high-speed braking and repeated braking. Much of the supposed heat capacity of the wheel itself is compromised when lightweight modern profiles (which put minimal metal in the tread and rim region to reduce rotational inertia) are used; this (and not the absolute thermal mass) is what I would use when starting to compare disk vs. wheel heat dissipation after x amount of braking. There are also significant issues with spot and differential heating when the wheel tread has been hardened (one consequence being the possible development of radial cracks in the tread surface similar to heat checking in journals). It's beneficial to actually look at the radiated IR from physical wheels to see where the braking heat goes ...
The tread brake also continuously cleans and polishes the tread improving wheel rail adhesion.
There is no doubt that this is significant. One difficulty is that the cleaning and polishing action usually also tends to take the tread profile away from optimal, meaning that the wheels may have to be turned more frequently for reasons other than the flat spots, etc. that Jerry subsequently describes. There were and are "patent" devices that can be attached to the brake shoe or beam to do the equivalent of Lidgerwooding and reestablish the true tread profile; I don't think these things are used as much as they could be.
<snipped >
Disk failures, necessitating total removal of the axle for replacement have a bad effect on availability.
Do they actually fail that often? One point I neglected to make is that some forms of brake-disc failure involve disc warp (this occurs very frequently in heavy automotive service!) and I am assuming, perhaps incorrectly, that the great majority of this 'disk failure' is in fact this kind of distortion and not actual failure of the disk structure.
THEORETICALLY you could machine the disk on the truck, with an attachment to the caliper, by raising the affected axle and providing a drive to rotate it. I do not know of anyone that chooses to do this vs. just dropping and replacing the affected wheelset and then working on the disc in a shop environment. The question Jerry raises is, I think, a bit different: how long the car will have to continue to operate after the disc warp occurs before it either reaches a terminus or some logical place it can be cut out of a consist or receive wheelset replacement. My suspicion is that on BART the amount of disc warp may have to become substantial before it 'qualifies' for attention... and if that is so, but the number of functionally distorted discs is substantial, you might easily see the overall effects described. (Given the momentary "microsliding" that will tend to occur each wheel revolution, where there is momentary loss of adhesion as the wheel rotates through the 'warped spots' but almost immediate recovery.
It is certainly 'faulty train handling' in an absolute sense, but the characteristics of outgassing of the composite shoes make the situation much worse, in my opinion. When the critical speed difference for irrecoverable loss of braking is in the range of 3 mph, between 22 and 25 mph, and the 'faulty train handling' is excessive reliance on dynamic braking to hold the train in that speed range, I submit that there are implicit difficulties that are far more effectively dangerous than just reliance on cast-iron shoes would induce. I would be interested to see whether there was actually a high percentage of trains run at speeds technically close to the V-never-exceed for composite shoes, relying implicitly on functioning dynamic brake operation... ISTR in the threads on the CSX failures, this was discussed in some detail.
Overmod I saw a suggestion somewhere that braking energy be stored on cars via axle-mounted generators. Involved, yes... Perhaps more likely in the days of Spicer drives to car-mounted generators for individual-car electrics. Nowadays we have HEP for the train power, and the 'economics' for individual axle-driven generators isn't really there.
Perhaps more likely in the days of Spicer drives to car-mounted generators for individual-car electrics. Nowadays we have HEP for the train power, and the 'economics' for individual axle-driven generators isn't really there.
Not to mention that I've been told by folks who deal regularly with vintage cars that the traditional axle mounted generator set-up recently became illegal (or against the regs, or whatever).
Overmod Now, having said all this, you may recall that many of the original Amcoaches were retrofitted with tread braking -- this may be a case of 'very little bit helps' (I well remember the stinking clouds of smoke under Clockers run with Heritage equipment in the early '70s, with a nice heavy GG1 on the point; much more fun with G's on Amfleet...) but certainly seemed to be welcomed by many in the operating community.
There's one Amfleet equipped trainset for the Pacific Surfliner service (known by the riders as the "old train") and the brake shoe smell when stopping is pretty potent. One of my co-workers says it is strong enough to make her nauseous.
Are you sure that outgassing in the brake shoe is the cause of reduction in braking effort? Note that I am asking the question out of curiosity as opposed to disagreement. The NTSB report on one of the runaways on the Seventeen Mile Grade stated that the coefficient of friction for composite brake shoes drops when the wheel tread temperature reaches ~600F, but did not explain the mechanism for the fade. The report stated that the maximum safe speed for descending that grade with 100 ton cars was 17(?) mph as a higher speed would lead to wheel tread temperatures in excess of 600F.
What I took away from reading the report was that the wheel tread temperature was the critical issue for braking ability. It would seem that using a larger diameter wheel would allow for higher downhill speeds due to the larger radiating area - at 600F, heat removal will be largely by thermal radiation. It also raised a couple of questions. The first being how long does the downgrade need to be for the wheel tread to reach 600F at the maximum safe speed? The second being what is the safe descent speed as a function of car weight and gradient? I seem to recall that somewhere below 1% that the safe descent speed is not limited by wheel tread temperature - perhaps due to a combination of increased convective heat transfer and reduction of braking effort needed due to air resistance.
- Erik
Jerry Pier The San Francisco BART cars have disk brakes, a design mvoe to minimize unsprung weight. However, the consultants assumtion that dynamic brakes would do all the work 99% of the time has not been validated by experience. Dsik failures, necessitating total removal of the axel for replacement haave a bad effect on availability. Wheel rail adhesion also suffers, resulting in wheel flat spots and corregated rail. Ride the system and you will hear the noise.
The San Francisco BART cars have disk brakes, a design mvoe to minimize unsprung weight. However, the consultants assumtion that dynamic brakes would do all the work 99% of the time has not been validated by experience. Dsik failures, necessitating total removal of the axel for replacement haave a bad effect on availability. Wheel rail adhesion also suffers, resulting in wheel flat spots and corregated rail. Ride the system and you will hear the noise.
I was attending UC Berkeley during BART's first 6 years of operation and remember the issue with the brakes. Based on a presentation given by a BART employee at the EECS departments Power Systems seminar ca 1975, the issue with the "dynamic" braking was due to problems with switching the braking resistors. The cars were actually set up to use regenerative braking with the resistors being switched in across the line from the third rail shoe when the voltage rose above the nominal 1000V. When the line voltage hit 1200V, the control system would shut off the regenerative braking and rely on friction braking.
The BART substations were designed for single quadrant operation (don't know if they were plain rectifier or single quadrant SCR) and thus could not accept regenerated power. The presenter did think that dual quadrant substations (i.e. ones that could accept regenerated power as the M-G sets used on the Milwaukee) would have reduced the wear on the disk brakes.
Our community is FREE to join. To participate you must either login or register for an account.