Thanks everyone for your replies. I think I understand why such a combination wouldn't be worth it for high-speed railways.
Just for some background: The reason I asked was because I'm thinking of designing a miniature H-Bahn/SAFEGE system that will support a Knuffingen-style Kai Tak airport. It will have both gentle curves and tight corners, with lots of switches. Based on what I can see in photos, I suspect the H-Bahn bogies use rubber tyres, with lateral wheels to keep the bogie centred on the track. I also saw a picture of their "tongue" switch, and it looks like there has to be a vertical wall for the lateral wheels to push against to guide the whole bogie in the correct direction. However, I'm having difficulty convincing myself that such a switch would work if the turnout is very tight.
http://www.monorails.org/movies/ShonanSwitch.gif
I'm guessing H-Bahn/SAFGE would have to use differential gears with the rubber-tyred system. But I wonder if they could have used coned steel-wheels with solid axles instead, using the edges of the slot as railheads? That way, there wouldn't be a need for vertical walls within the switch tongue against which the sidewheels would push, but the steel wheels would be guided by the slot edge itself just like a traditional rail-borne vehicle in a yard, or a tram rounding a corner.
The other reason I asked was due to sheer curiosity.
Something I learned here some time back was that the squealing one hears on sharp curves is not the flanges, but the results of one wheel or the other sliding on the rail.
We have some 5.5 degree curves - we get a fair amount of squealing there.
Differentials would, indeed, introduce another point of failure. A good reason to avoid them.
If you want to hear some squealing wheels, check out the Deshler railcam. Loaded coil cars on the southwest transfer (the sharpest of the three) make sleeping difficult at oh-dark-thirty...
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...
See the original rail/wheel dynamics paper by Wickens (1965). This was done for high-speed guiding of four-wheel "waggon"-style vehicles in Britain, and the rather surprising result is that coned wheels on a rigid axle have a self-centering and self-steering effect.
Tom Blasingame went to the trouble of taking out a patent for a three-piece truck wheelset with a 'center bearing' allowing differential rotation of the two wheels while preserving the structural advantages of a rigid axle in that type of construction. It expired recently without anyone formally expressing interest in manufacturing it, even though it could use the same kind of grease-loaded package roller bearing and seals that give a life far in excess of the wheel wear limits.
For driving axles, the situation is a little different. A locking differential that would work with a 'typical' bull-gear-driven nose-suspended traction motor would require a considerable expansion of case size and weight around the center axle area, with what might be a substantial increase in unsprung mass, and it might be difficult to design this for correct inspection of what would be a dangerous potential point of failure (causing the effect of a broken axle) where current alternatives have none.
Current practice in HSR is to use very little, if any, cone in the wheelsets. That requires extremely good external methods of controlling truck or frame hunting or other motion, but it should also be remembered that this involves very gentle and progressive minimum curves, on the order of a 12-mile radius or better, and long radii of vertical curvature as well.
The wheel tread profile is slightly tapered, I suppose one could call it conical. That's how rigid wheelsets roll through curves, they shift sideways a bit until they find the right spot.
Sharp curves are a operational headache as they restrict both speed and the type of locomotive that can be operated there.
Differentials require lubrication, and represent a collection of moving parts whose failure could easily result in a derailment.
The current rigid wheelset design with roller bearings and tread brakes is a perfect example of the KISS principle. It is low maintenance, low cost and easy to repair.
Why change from something that works?
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-an Articulate Malcontent
Hi there, question: why aren't there trains that have both coned steel wheels on rails, plus differential gears? My understanding is that coned wheels are great for gentle curves, but don't do well through very tight curves, whereas differential gears allows bogies to navigate tight corners. And for coned wheels to navigate sharper curves, the cone would have to be more exaggerated. But exaggerating the cone, according to my understanding, will make the wheelset hunt more on straight track.
Why don't rail-borne vehicles combine the two "solutions"? Why not have a slightly-coned wheel that won't hunt at high speeds, can navigate gentle curves, but also rely on the differential gear at very sharp corners too?
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