5 ms·

Are the cones mainly for keeping the train centered, or for allowing the outer wheel on a turn to be effectively larger? Not sure if the differential effect is

Are the cones mainly for keeping the train centered, or for allowing the outer wheel on a turn to be effectively larger? Not sure if the differential effect is an added benefit that isn't really necessary, or if it is the main goal of the design.

ben11kehoe 5y ago
They keep the train centered by the differential effect. The angle of the cone is very slight, nowhere near enough for gravity to overcome friction to cause the train to slip laterally into the center. And then keeping the train centered as the track turns results in the train turning with the track.
- GhostVII 5y ago
  Sure, but I guess I'm wondering if that is just a convenient effect of the centering, or if it is actually necessary to prevent the wheels from skipping.
lbotos 5y ago
my understanding is the later: https://youtu.be/agd8B-31bjE?t=106 https://youtu.be/agd8B-31bjE?t=106
aaaaaaaaaaab 5y ago
They do both.
AnimalMuppet 5y ago
It's the same thing. If the train becomes uncentered, then one wheel is the outer wheel, and will turn the axle back toward the center. Or, looked at the other way, when the track curves, then the axle becomes uncentered.
- smartscience 5y ago
  To demonstrate this more fully, consider the case of having the wheel flanges on the outside, with the conicity of the wheels pointing the other way. Gravity would still tend to centre this arrangement, but I'm told that if you build such a system in practice, then it won't run nearly as smoothly. (PhD was 'Residual stress in rails', for what that's worth. Judging from the profiles of the rails I saw, direct contact with the wheel flange plays a substantial role in keeping the train in place on curved track. But on roughly straight track, I'm satisfied that the argument about conicity applies).
  - lostlogin 5y ago
    > direct contact with the wheel flange plays a substantial role in keeping the train in place on curved track. The London Underground has some lines that are horrifically loud. The squealing must surely be at dangerous sound levels. I’d always assumed it was the flange against the rail, and you appear to be confirming that?
    - AnimalMuppet 5y ago
      That's called "flange squeal". Yes, it can be ear-shatteringly loud. But it mostly (totally?) happens on very tight curves. It shouldn't happen much or at all on gentler curves. (Of course, this is circular, because I'm kind of defining "gentler" and "tight" based on whether they cause flange squeal. Still, there's a point - there is something like a threshold of curve tightness where flange squeal becomes much more probable.)
      - idroveatrain 5y ago
        It can happen on straightrail on an incline, too. It's hard to assess precisely what's happening in the locomotive, but under traction I believe the running gear will toe out, and align based on the path of least resistance. I believe this leads to the flange pressing, with immense force, against the rail. You also get a lot of wheel slip in this condition. I surmised this running 2 motors up a 3%(?) grade with 20k ton gross at 10mph. It's about the only explanation I could come up with is that the running gear was twisting under the gravity and the energy being put down to work against it. It might also just be a stringline sort of effect dragging the motors to one side of the track and pressing the flange. Maybe one of the rail engineers will come holler at me for my poor trainhandling skills.
morpheos137 5y ago
The main reason other than turns is to avoid hunting oscillation on straight track. https://en.m.wikipedia.org/wiki/Hunting_oscillation https://en.m.wikipedia.org/wiki/Hunting_oscillation
jcrawfordor 5y ago
Both are factors in good centering, but mostly the change in diameter. In turns, there is a natural tendency for the train to shift towards the outside of the curve due to inertia. The wheel diameters become asymmetric which helps to re-center the train. It's usually not sufficient on its own, which is why superelevation is used as well - the outside rail is somewhat higher than the inside rail which shifts relative gravity to pull the train back towards the inside as well. The relationship between these two effects is a bit complex (depends on weights and speeds of trains) so it's usually all a bit approximate. The conical section of the wheels is mostly intended to prevent hunting on straight track, and the shape can't be made too aggressive without increasing the wear on wheels on rails. So on curves the superelevation is added to provide the extra force required. Because conical wheels do increase wear and can contribute to oscillation in their own way, there have been experiments with cylindrical wheels especially on higher-speed trains---BART is a well known example. It ultimately didn't work very well and so they have been re-trueing the wheels to a non-cylindrical profile, although still not quite a traditional conical one. Basically in higher-speed operation the re-centering effect is too significant and causes one wheel to "chatter," which over time creates a significant vibration in the rail. Trouble is cylindrical wheels tend to cause the same thing to happen on the other side. It was a very hard problem before computer modeling became available.
- iggldiggl 5y ago
  > It's usually not sufficient on its own, which is why superelevation is used as well - the outside rail is somewhat higher than the inside rail which shifts relative gravity to pull the train back towards the inside as well. [...] So on curves the superelevation is added to provide the extra force required. I've never heard about that theory as for why superelevation/cant is supposedly being used until now. Given that most of the time you'll end up with a remaining net force to the outside of the curve even after application of cant, it doesn't seem to make that much sense, either.
  - jcrawfordor 5y ago
    That's the conventional explanation of superelevation, although I worded it in sort of an odd way. But I'm describing the same thing that e.g. Wikipedia does. Superelevation directs the force of the car more "straight down" in relation to the rails which improves centering and balance of the load by the same token. The thing I said about "shifting gravity" is unnecessarily confusing because it depends on reference frame. I think for low-speed freight the balance needs to be pretty close on to ideal to meet regulations, e.g. FRA regulations give calculations for acceptable ranges. But since it's dependent on running speed it's hard to get correct for freight and passenger mixed operation which is the subject of this FRA report that has a lot of detail on the calculations: https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/19085/Superelevation.pdf https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/19...
tus89 5y ago
The goal is for the train to NOT be centered when cornering, the cones allow for this. If the train was centered during corning, or simply with non-coned wheels, the outer wheel would necessarily slip.

ben11kehoe 5y ago

They keep the train centered by the differential effect. The angle of the cone is very slight, nowhere near enough for gravity to overcome friction to cause the train to slip laterally into the center. And then keeping the train centered as the track turns results in the train turning with the track.

GhostVII 5y ago
Sure, but I guess I'm wondering if that is just a convenient effect of the centering, or if it is actually necessary to prevent the wheels from skipping.

lbotos 5y ago

my understanding is the later: https://youtu.be/agd8B-31bjE?t=106 https://youtu.be/agd8B-31bjE?t=106

aaaaaaaaaaab 5y ago

They do both.

AnimalMuppet 5y ago

It's the same thing. If the train becomes uncentered, then one wheel is the outer wheel, and will turn the axle back toward the center. Or, looked at the other way, when the track curves, then the axle becomes uncentered.

smartscience 5y ago
To demonstrate this more fully, consider the case of having the wheel flanges on the outside, with the conicity of the wheels pointing the other way. Gravity would still tend to centre this arrangement, but I'm told that if you build such a system in practice, then it won't run nearly as smoothly. (PhD was 'Residual stress in rails', for what that's worth. Judging from the profiles of the rails I saw, direct contact with the wheel flange plays a substantial role in keeping the train in place on curved track. But on roughly straight track, I'm satisfied that the argument about conicity applies).
- lostlogin 5y ago
  > direct contact with the wheel flange plays a substantial role in keeping the train in place on curved track. The London Underground has some lines that are horrifically loud. The squealing must surely be at dangerous sound levels. I’d always assumed it was the flange against the rail, and you appear to be confirming that?
  - AnimalMuppet 5y ago
    That's called "flange squeal". Yes, it can be ear-shatteringly loud. But it mostly (totally?) happens on very tight curves. It shouldn't happen much or at all on gentler curves. (Of course, this is circular, because I'm kind of defining "gentler" and "tight" based on whether they cause flange squeal. Still, there's a point - there is something like a threshold of curve tightness where flange squeal becomes much more probable.)
    - idroveatrain 5y ago
      It can happen on straightrail on an incline, too. It's hard to assess precisely what's happening in the locomotive, but under traction I believe the running gear will toe out, and align based on the path of least resistance. I believe this leads to the flange pressing, with immense force, against the rail. You also get a lot of wheel slip in this condition. I surmised this running 2 motors up a 3%(?) grade with 20k ton gross at 10mph. It's about the only explanation I could come up with is that the running gear was twisting under the gravity and the energy being put down to work against it. It might also just be a stringline sort of effect dragging the motors to one side of the track and pressing the flange. Maybe one of the rail engineers will come holler at me for my poor trainhandling skills.

morpheos137 5y ago

The main reason other than turns is to avoid hunting oscillation on straight track. https://en.m.wikipedia.org/wiki/Hunting_oscillation https://en.m.wikipedia.org/wiki/Hunting_oscillation

jcrawfordor 5y ago

Both are factors in good centering, but mostly the change in diameter. In turns, there is a natural tendency for the train to shift towards the outside of the curve due to inertia. The wheel diameters become asymmetric which helps to re-center the train. It's usually not sufficient on its own, which is why superelevation is used as well - the outside rail is somewhat higher than the inside rail which shifts relative gravity to pull the train back towards the inside as well. The relationship between these two effects is a bit complex (depends on weights and speeds of trains) so it's usually all a bit approximate. The conical section of the wheels is mostly intended to prevent hunting on straight track, and the shape can't be made too aggressive without increasing the wear on wheels on rails. So on curves the superelevation is added to provide the extra force required. Because conical wheels do increase wear and can contribute to oscillation in their own way, there have been experiments with cylindrical wheels especially on higher-speed trains---BART is a well known example. It ultimately didn't work very well and so they have been re-trueing the wheels to a non-cylindrical profile, although still not quite a traditional conical one. Basically in higher-speed operation the re-centering effect is too significant and causes one wheel to "chatter," which over time creates a significant vibration in the rail. Trouble is cylindrical wheels tend to cause the same thing to happen on the other side. It was a very hard problem before computer modeling became available.

iggldiggl 5y ago
> It's usually not sufficient on its own, which is why superelevation is used as well - the outside rail is somewhat higher than the inside rail which shifts relative gravity to pull the train back towards the inside as well. [...] So on curves the superelevation is added to provide the extra force required. I've never heard about that theory as for why superelevation/cant is supposedly being used until now. Given that most of the time you'll end up with a remaining net force to the outside of the curve even after application of cant, it doesn't seem to make that much sense, either.
- jcrawfordor 5y ago
  That's the conventional explanation of superelevation, although I worded it in sort of an odd way. But I'm describing the same thing that e.g. Wikipedia does. Superelevation directs the force of the car more "straight down" in relation to the rails which improves centering and balance of the load by the same token. The thing I said about "shifting gravity" is unnecessarily confusing because it depends on reference frame. I think for low-speed freight the balance needs to be pretty close on to ideal to meet regulations, e.g. FRA regulations give calculations for acceptable ranges. But since it's dependent on running speed it's hard to get correct for freight and passenger mixed operation which is the subject of this FRA report that has a lot of detail on the calculations: https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/19085/Superelevation.pdf https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/19...

tus89 5y ago

The goal is for the train to NOT be centered when cornering, the cones allow for this. If the train was centered during corning, or simply with non-coned wheels, the outer wheel would necessarily slip.