The curves on a railroad are generally quite broad in comparison to what an automobile deals with. Even if there is a difference in relative speed, steel-on-steel will slip enough to make it a non-problem.
Thanks but i dont think that is spicific enough for my phyics teacher because he said it also had to do with the angle on the wheel and arch on the rail.
The wheels have a taper to them that steers the truck down the track. In theory when on the mainline with broad curves the flanges will not touch the rail. The wheel is bigger around at the flange then at the outside of the wheel. If the truck starts to head tward the outside of a curve, the outside wheel will be larger at the point of contact with the rail and the inside wheel will be a smaller diameter at the point of contact with the rail. With a solid axle this means that the inside wheel will cover slightly less distance then the outside thus the axle will steer itself back to the point which each wheel is the same diameter (and theoreticly in the dead center of the rails). The sharper the curve the further the wheels will need to move lateraly to steer into the curve. As the curve gets sharper you get to the point where the wheels cannot steer themselves sharply enough and the flanges come in contact with the rail and the rail guides the flange of the wheel. This greatly increases the resistance of moving the car because the wheel has to slide on the rail instead of just roll along.
In reality it sometimes does not work perfectly and a truck might not find its equaliberium and will “hunt” back and forth lateraly.
I’d say the simple answer is “they slip” - my experience is that when you get onto very tight curves (try a long-wheelbase 4 wheel brake van on a tight dockyard curve) you get a lot of squealing noises, presumably caused by the slipping and the flanges grinding along the rail. Having a differential of some sort might make a difference - if I shove two otherwise identical large scale models through a tight curve, one of them having solid axled metal wheels and the other having ball bearing wheelsets (on which the two wheels are free to rotate independently) there’s noticably less resistance with the latter. Not sure if this would scale up to 1:1 but I’d guess it might.
The taper is slight - 1:20 for frt, 1:40 for passenger. In sharp curves, the wheels just slip on the rails a bit. The solid axle actually can contribute to high lateral flange forces in such instances. There were problems with stack cars on sharp curves with freshly ground rail head profiles that required the TOP of the rail head to be lubricated. I don’t recall the specifics…
Having been in the freight carbuilding business I have been involved in tests on rolling cars and observed the following: On normal gradual curves, the wheel taper will handle the difference in rolling distance from one side to the other. On tight curves the taper is not sufficient and some wheel slip will occur. This is why you hear severe squeeling or snapping sounds with cars on tight curves. The more expensive steerable trucks eliminate much of this condition causing less wheel wear and easier rolling.
As noted the outside wheel travels farther than the inside wheel, so as the train corners one of the wheels will slip. If you ever hear of the flanges squealing, or the rails singing this is why.
The taper of the wheel tread is shown much better than we can explain it. The detail they call R-1 is the flange fillet that helps keep the flange from rubbing against the railhead (not 100% successful). Simply put, when running on straight level track, the railhead is contacting the wheel tread near the center of the tread on both wheels. When it hits a curve, the centrifical force pushes the wheelset toward the outside of the curve. The wheel on the inside of the curve is riding on the outer edge (away from the flange) of the tread. The wheel on the outside of the curve is riding on the inner edge (closest to the flange) of the tread, where the wheel diameter is slightly larger. Applying a bit of geometry, larger diameter is equal to larger circumference (diameter times pi equals circumference), yielding roughly the same effect as a differential on a car. This design goes WAY back in railroad history, and back in those days they had a tendency to do things the simplest way possible. This idea works so well, it’s still used today.
I seem to recall reading that the railroads/wheel manufacturers discovered a natural curve in the wheel tread caused by the wear of the wheel rolling down the track. They started casting the new wheels with that curve and improved handling and wear.
Current wheels are forged, not cast. The final wheel contour is cut in, and can be recut on a wheel lathe. I understand there’s even a device that will recountour wheels on a diesel without removing them from the locomotive.
Let’s not forget that top of the rail has a curved shoulder contour…bet it straight or curved track…the curved shoulder also assists in steering the wheel…the coutours of both the wheel and the rail are in theory designed to enhance the operation and life of both.
The “connection” between the wheel tread and the flange is not a sharp angle. There is a fillet in this angle, and the outside wheel on a curve will be forced to ride the fillet which, in effect, increases the diameter of the wheel at that point. If the curve is too sharp, the differential necessary will be too much for the fillet to handle and the result is slippage. Also if there’s not sufficient centrifugal force to make the outside wheel ride up on the fillet . . .
How long do you think that nice new wheel is going to keep it’s pretty profile? Not very long , a few months perhaps. The bottom line is the wheels will slip, this is the cause of rail corrugation on curves that any one who has ever ridden an engine can tell you about. This is the reason company’s like Loram exist.
Randy
I have seen these wheel cutting machines in action…pretty darn impressive. The older ones were lathes, the new ones mill the profile back into the wheel with an indexible carbide form cutter. Current wheels are cast. Pressure cast to be exact. It would be hard to begin with a blank of steel that large and forge it into the desired shape.
I agree with randy. If you look at a wheel that has been in service for awhile, there isn’t any taper left. They are actually worn in the center of the tread. This is why springfrogs have a false flangeway cut into them. It is relief for a false flange condition of a worn wheel so that the wheel does not ride on the outside of the tread over the frog.
Thanks Ken, pressure casting is another technique that I’ve only heard about. And the description I heard was REALLY vague. I couldn’t even visualize what they were talking about. I have seen a big wheel lathe in operation, but never the unit that recontours the wheel on the loco. That’s another one I’d like to see in action sometime.
Annother metal geek here too. I went to school for machining. Spent some time in metallurgy classes. Trains are cool, but, take me on a tour thru a steel mill and I’m grinnin…How screwy is that. Fellow metal geek…When I was in K.C. I saw that there was a wheel plant owned by griffen wheel co. They may have a website that goes into detail. BTW the wheel machine was made by Siemens and was CNC. Way trick. It had small measuring wheels that measured various faces of the wheel and diameter at the beginning of the cycle and then began the operation. I was impressed.
Ken
My father was a machinist (I have his WWII vintage Atlas lathe downstairs, and yes, it’s fully operational), and I’ve had training in it at the local Vo-Tech. I volunteer at Steamtown in Scranton at the Backshops. I’m currently working with the crew restoring the PRR K-4 that used to be displayed on Horseshoe Curve.