I am under the impression that a train tends to stay on the rails, as long as it is not going around a curve, because of its weight. This month’s Trains had a clip that, if I remember it correctly, stated that when a curve exceeds two degrees, the wheel flange engages the rail to keep the train on the tracks. Presumably banking the curve changes this dynamic. The clip raised a couple of questions that hopefully one or several of our technical participants can answer.
If I remember correctly ( I gave my issue of the magazine to a friend in El Paso), the author of the piece said that the wheel is concave, which helps keep it running straight on the track. Is this correct? If it is, how does this help the train stay on the tracks?
When a reference is made to wheel hunt, what does this mean?
The wheel tread is tapered from the smallest wheel diameter at the outside edge to the largest diameter next to the flange. Since the wheels are pressed onto the axle, they must turn at the same speed. When a wheelset is running centered on the track, each wheel is running on the same diameter. If another dynamic force such as rocking, for instance, developed, it might tend to force the wheelset off center to the track. This would cause each wheel to run on a different diameter from the opposite wheel.
Fundamentally, running on different diameters would require that each wheel turn at a different speed, which is not possible. So running off center to the track causes one or both wheels to slip slightly in its contact with the rail. The wheelset naturally seeks to resist the friction of this slipping by re-centering on the track, so that the slipping of the wheels is eliminated.
Lay two dowel rods, 2 to 3 feet long near each other (3 or 4 inches apart and parallel to each other) such that one end is a few inches higher than the other… sort of a RR track with a grade.
Take two paper cups that have sloped sides and tape them together wide end to wide end. Sort of a barrel shape.
Take two more cups and tape them together narrow end to narrow end. Sort of a bow tie shape.
Lay the 1st pair of cups (Barrel shape) on the dowel rods at the high end and let them roll down the “track”. You can probably lay them off-center and at some small angle to the track and they will still make it to the bottom of the “track” without going over the edge. Sort of self correcting for your not putting them centered and at right angles to the “track”.
Now try it again with the 2nd pair of cups (Bow-tie shape). Even if you are very careful to place them on the “rails” in the center and aimed to roll straight down the “Track” they will probably turn one way or the other and roll off the edge before reaching the bottom.
RR wheel sets (pairs on one axle) are like the cups connected at the wide end. The wheel treads are tapered like the cups are and that taper (like explained by Bucyrus) keeps the wheels centered on the track. On nice straight track the flanges will not touch the rails at all. Only when the track curves beyond some degree of curvature greater than the taper can compensate for will the flange on the wheels contact the rail sides.
There is even a taper to the flange (and the transition from tread to flange is a changing taper) that exagerates the change in diameters and performs the same action, but often cannot actually climb the rail for long enough to do much good.
I believe it is incorrect to say that the wheel treads are concave. They may wear into a concave shape, but their true configuration when new is a taper. This taper amounts to a cone shape, with is called conical, but not concave. Concave refers to a dished-out shape, which would be a curved or radiused profile. Did the article say concave or conical?
When a wheelset enters a curve, the distance traveled is fundamentally longer on the outside rail than on the inside rail. The centrifugal force induced by the curve forces the wheels against the outside rail, thus engaging the flange of the outside wheel against the outside rail. This shift causes the outside wheel to be running on its maximum diameter while the wheel on the inside rail of the curve runs on its smallest diameter. So, for every revolution, the outside whee
As the video explains, when a wheelset runs off center, it “steers” itself back to center. Wheel hunting occurs when a wheelset steers back to center so quickly that it runs past center, and approaches its opposite limit of off-center drift. Then it rebounds and repeats the cycle. This process can set up into a continuing harmonic cycle in which the wheel is continuously “hunting” to find its center.
Overall, it is accurate to say that the flange is only a backup device for keeping the wheels on the rails. But even at that, it is the fillet radius at the base of the flange that occasionally contacts the rail to keep the wheel on the track; it is not the flange itself. If the flange were flat-walled, and that flat wall actually contacted the side of the rail head, then that rotating contact would create lift, which would tend to lift the wheel off of the rail.
As the video explains, when a wheelset runs off center, it “steers” itself back to center. Wheel hunting occurs when a wheelset steers back to center so quickly that it runs past center, and approaches its opposite limit of off-center drift. Then it rebounds and repeats the cycle. This process can set up into a continuing harmonic cycle in which the wheel is continuously “hunting” to find its center.
Overall, it is accurate to say that the flange is only a backup device for keeping the wheels on the rails. But even at that, it is the fillet radius at the base of the flange that occasionally contacts the rail to keep the wheel on the track; it is not the flange itself. If the flange were flat-walled, and that flat wall actually contacted the side of the rail head, then that rotating contact would create lift, which would tend to lift the wheel off of the rail.
I recall reading here some years ago that the cause of the “singing” in sharper curves (and some of ours are up to 5.5 degrees) is one or the other wheel sliding over the rail as the other turns at rail speed, all due to the fixed axle cited earlier in the thread.
The point being that it’s not the flange that’s “singing,” it’s the tread.
But Larry…in the article in the current Trains…the article then points out, we see the outside rail does see wear on sharper turns, and wouldn’t that have to be from the flange…?
The turnout from the CN Holly Sub to the westbound Flint sub in Durand is a #20 turnout. The speed limit for a #20 is 45 MPH, but trains are restricted to 25. Even around that mild curve at low speed there is plenty of squealing between wheels.
If I remember correctly ( I gave my issue of the magazine to a friend in El Paso), the author of the piece said that the wheel is concave, …
I believe it is incorrect to say that the wheel treads are concave. They may wear into a concave shape, but their true configuration when new is a taper. This taper amounts to a cone shape, with is called conical, but not concave. Concave refers to a dished-out shape, which would be a curved or radiused profile. Did the article say concave or conical?
When a wheelset enters a curve, the distance traveled is fundamentally longer on the outside rail than on the inside rail. The centrifugal force induced by the curve forces the wheels against the outside rail, thus engaging the flange of the outside wheel against the outside rail. This shift causes the outside wheel to be running on its maximum diameter while the wheel on the inside rai
Depends on where the wear is. If it’s on the side of the rail, it’s the flange, if it’s on top, it’s the tread.
Considering weight distribution on the two wheels of each axle, it would make sense that the “inner” wheel (especially if the curve has any superelevation) might well have more weight, which would thus make the wheel adhere more to the inner rail. That would cause the wheel to slide on the outer rail.
Conversely, it’s possible that if the rails are more level, centrifugal force would cause the “outer” wheel to adhere better, causing the inner wheel to spin.
Lots of dynamics there that depend on lots of factors.
You can see the taper in the wheels on this truck from a locomotive…the wheel set is brand new so the taper is/appears pronounced.
Inside rail at the beginning of a crossover, note the “groove” worn in the rail head or ball of the rail, caused by the flanges rubbing on the rail as the rail and flange steer the truck through the crossover.
The light gray dust you see is metal dust from the flange, rail and tread of the wheel…when you are riding on a car through a curve, and the sunlight is just right, you can see the meatal dust motes floating in the air.
The ball or head of the rail in curves will misshape faster than wear away, the forces will push the metal to the outside of the rail, to the point it can have a 1" overhang, thin as a razorblade and just as sharp.
You will see this often on switching leads, we wear out the curved part of our lead once a year, then transpose the rail, wear them out the other way, then either grind them “reshape” or replace them.
You will not see this on any class 1 mains, as it is a FRA failure anywhere but in a yard.
Depending on the traffic, both type and frenquency, it can take as little as six months to flatten the ball of the rail.
If I get the chance, I will get a photo saturday of this.
On a curve where the speed is high enough to create enough centrifugal force to push the wheel running on the outside rail of the curve against that rail, that repeated contact will tend to wear that rail until the much of the flange face actually does contact the side of the rail. I have heard a prolonged high-pitched squeal at times, and also a high-pitched “zinging” sound that often happens in short bursts. I assume that at least some of those sounds originate from flange contact with the rail.
I have also heard a low-pitched sound that reminds me of a prolonged frog croak. That comes from the rail being sprung sideways and suddenly breaking free and snapping back to proper alignment. It is a cycle that happens many times per second, and you can actually see the rail move in a side-to-side vibration as the sound occurs. It occurs at slow speed
This rail was re-ground last year, transposed the year before.
Note the edge has mushroomed out to the side from the forces.
The rail closest to you is the inside rail of a curve…when we shove large cuts around and into the holding tracks, the speed and centrifugal force plus the weight/mass forces the wheels to the outside of the curve, hence the groove(3rd photo) from the flanges.
But on slow drags out with switch cuts, gravity pulls the car and the wheels towards the inside of the curve, which is the low side, hence the mushrooming to the outside.
As noted in an earlier post, messing with that edge can cause you to need stiches and an tetanus booster, it is as sharp as a razor blade.
