Why are wheelsets designed so that the wheels are rigidly affixed to the axle? Why not have each wheel spin independently on the axle (like a car wheel). Going around curves the inner wheel would obviously "want" to turn slower than the outer wheel to the curve.. so the outside wheel ends up being dragged along while turning at a slower rate. I'm guessing that must contribute some to flat spots not to mention wear and tear to the track.
For openers, how would you make sure that the wheels stay in gauge on the axle if they are not rigidly attached?
CSSHEGEWISCH For openers, how would you make sure that the wheels stay in gauge on the axle if they are not rigidly attached?
I don't know.. maybe that's the reason. But the wheels on a car rotate freely on each axle yet somehow they maintain the same distance apart from each other.
UlrichWhy are wheelsets designed so that the wheels are rigidly affixed to the axle? Why not have each wheel spin independently on the axle (like a car wheel)?
We've had a couple of threads on this topic: your best bet is to find a copy of the Alan Wickens paper from 1965 ("The Dynamic Stability of Railway Vehicle Wheelsets and Bogies Having Profiled Wheels", linked in a couple of those old threads, not that it helps much now!) and read through it to see the kinematic steering effects.
Wickens also has a book on "Fundamentals of Rail Vehicle Dynamics" (of which you can get a free download if you are a member of scribd; I won't provide any link for copyright reasons) if you want a bigger firehose.
The early Talgo trains had to have separate wheels on stub axles, as the axle would pass through the depressed aisle way. As I recall at least some of the Train X designs on C&O featured individual wheels with special roller bearings, and I recall a design that split the axle of a conventional type of wheelset to allow the wheels to rotate at different speeds while being kept otherwise in gauge and alignment for maintenance. Later, Tom Blasingame went so far as to patent a replacement freight-car wheelset with this characteristic -- it did not sell, and the rights are back in the public domain now if you care to try selling the idea... as a design it has his typical well-thought-out attention to detail.
The problem (as you will read) is that kinematically, strange as it seems, the steering effect of a rigid wheelset is preferable to independent flanged wheels or coned-treads. Once that was determined, nobody ever went back to 'pastes or powders' again, at least not in high-speed design.
To the extent there is differential wear or flatting, it is far more easily handled as 'magic wear rate'-style periodic machining of treads to profile than in providing separate bearing and gauging arrangements (four are required, plus thrust in both directions) for independent wheels. As railroads increased their use of active reprofiling for 'other reasons' this one became more and more mainstreamed...
That's interesting, thank you. I must remember to check old threads..
Does the rigid axle apply to locomotive wheels as well?
Overmodplus thrust in both directions) for independent wheels.
Aye, There's the rub!
UlrichDoes the rigid axle apply to locomotive wheels as well?
Now you want a differential? How would you manage power to each wheel?
As has been mentioned, the wheels solidly fixed to a solid axle approach works well when combined with the self-centering effect of the wheel taper, or cone shaped treads. That tapered wheel concept is a stoke of genius that makes the connected wheel concept work so well.
The wheelset wants to run on track center just because the wheels seek the center to synchronize their RPM. And they must do that because they are fixed to a common axle. So departing from perfect center, presents each wheel with a different running diameter due to the cone shape of the wheel treads. This ingenuous idea forces the wheels to run at the same speed. If they run at the same speed, they have to track on perfect center of the railroad tack.
By giving the wheelset the natural tendency to center on the track, it becomes essentially self-steering without the solid guidance of the wheel flanges. This eliminates the need for the flanges to scrub against the sides of the rail head to prevent the wheelset from derailing. Eliminating the flange scrubbing reduces the rolling friction caused by flange scrubbing.
The same design does apply to locomotive trucks, although, I have heard that they have zero tread taper to maximize tractive effort adhesion. I am not sure if that is the case, or if possibly the locomotives have tapered wheels, but a different taper than rolling stock.
Another point to consider is that the tapered wheels and the railheads wear together to arrive at a perfect match. That match includes a rounding to the taper of the wheels to match a correspoinding rounding of the railhead.
There is more to this that comes into play when the cars round a track curve and the distance traveled by the outer wheels is fundamentally greater than the distance followed by the inner wheels.
It does, but there is an additional 'catch' to consider: the wheels are transmitting power and not just 'following' the easiest way.
As noted, there are serious advantages to using two wheels on a stout axle: it would be difficult to build anything with nose-suspended motors otherwise (differential or no differential!) and relatively little more advantage to most systems of quill drive.
Where the advantage comes in is with radial steering, as in the GM HTCR trucks. Via a linkage these trucks turn their outer axles slightly to follow a curve, with the truck frame itself requiring only incidental pivoting so the center axles remain normal to the rails under the long underframe. We've discussed the clear benefits in traction and relative binding that occur when this is accomplished.
Meanwhile, on reciprocating steam locomotives rigid driver axles are necessary to keep the engine in precise quarter. There have been some amusing 'fantasy' engines that are drawn to show the drivers canted on the visible side, like the wheels on racing wheelchairs, but an engine built that way would not run reliably from side-rod drive even with low-lash bevel gearing conjugating the axles on both sides...
EuclidThe wheelset wants to run on track center just because the wheels seek the center to synchronize their RPM. And they must do that because they are fixed to a common axle. So departing from perfect center, presents each wheel with a different running diameter due to the cone shape of the wheel treads
Sounds very similar to the mechanics that keep flat belts centered on crowned pulleys.
Ulrich That's interesting, thank you. I must remember to check old threads.. Does the rigid axle apply to locomotive wheels as well?
There's a reason why limited slip and locking differential are made, keeping both wheels turning at the same rate usually improves traction. OTOH, with AC motors and an intelligent variable frequency drive, traction could be improved with a motor per wheel arrangement.
Erik_MagThere's a reason why limited slip and locking differential are made, keeping both wheels turning at the same rate usually improves traction.
The better approach, however, involves 'torque vectoring', which is closer to what you describe as being possible with proper VFD drive control.
The problem for a modern AC-drive diesel-electric, then, is that you'd effectively need a split AC motor and two bull gears per 'axle' to get the torque to the railhead doing that, in addition to providing a proper coupling in the center of the axle shaft (very likely not at the same OD of a regular axle...) I am not sure that the gain from this arrangement would justify the costs, including the need for twice as many inverters and control gear although admittedly of smaller peak capacity each.
Overmod I am not sure that the gain from this arrangement would justify the costs, including the need for twice as many inverters and control gear although admittedly of smaller peak capacity each.
I am not sure that the gain from this arrangement would justify the costs, including the need for twice as many inverters and control gear although admittedly of smaller peak capacity each.
Neither am I.
"intelligent VFD" implies sufficient # of sensors and processing power to properly do torque vectoring. Think the gains would come from the "intelligent VFD" in finding the optimum amount of slipping to maximize torque, in the same vein the MPP controllers get the maximum power out of a solar panel (first heard about this 1976 - 1977). The two rails might vary in the optimal slipping.
The idea of powering each wheel separately in the automotive world dates back before WWI. The Couple Gear built both battery powered and hybrid vehicles. The fire service was a big customer, replacing horses with various tractors, including Couple Gear.
In today's computer and solid state world, it's not a reach to think that a locomotive could be built with each wheel being a separate power unit. As noted, each wheel's performance could be tailored to it's own coefficient of friction.
Perhaps someone will try it. I would opine that building such a locomotive would likely benefit from getting away from the idea of essentially cutting the axle in half and putting a motor on each half. Couple Gear used a beveled gear drive system.
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...
Anyone remember LeTourneau Earthmovers. They had diesel-electric drive trains with one motor per wheel.
Electroliner 1935 Anyone remember LeTourneau Earthmovers. They had diesel-electric drive trains with one motor per wheel.
Johnny
Deggesty Electroliner 1935 Anyone remember LeTourneau Earthmovers. They had diesel-electric drive trains with one motor per wheel. Yes, I remember them; they were certainly odd-looking with only two wheels. I wonder just what became of the company.
Electroliner 1935 Anyone remember LeTourneau Earthmovers. They had diesel-electric drive trains with one motor per wheel. Yes, I remember them; they were certainly odd-looking with only two wheels. I wonder just what became of the company.
https://en.wikipedia.org/wiki/LeTourneau_Technologies
Never too old to have a happy childhood!
DeggestyYes, I remember them; they were certainly odd-looking with only two wheels. I wonder just what became of the company.
Euclid and Cat "pans" also had two wheel tractors. Euclid also made one with a motor in the back as well as the one in the tractor.
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