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The train’s wheels are not perfectly cylindrical, but slightly conical. In our opinion, this conical shape is a marvel of engineering that accomplished two major things, one - correcting the course of the train towards the center, and two - helping the train to achieve the differential action. To understand the first accomplishment, let’s consider a simple experiment with glued paper cups.
When I roll this set of glued paper cups on this tracks, you can see that they are moving perfectly straight. Even if I try to give a small tilt for the cups at the beginning, they are still managing. Now, what about this set? This is glued in the opposite way. When I roll the cups on the same track, it is failing. Railway wheels use this kind of conical shape. This angle makes sure that the wheels never leave the track, but the question is why.
This conical arrangement produces a self-centering force. To understand how, we need to check out the forces acting on the wheels. During a straight track movement the main forces acting on the wheels are shown here. The reaction forces will always be perpendicular to the surface of the cone. When the wheels are centered, the horizontal components of these forces cancel each other out.
Now, assume that, due to some reason, the wheels have moved to the right. One interesting thing happens to the train’s wheels when it moves along the axis. Did you notice? It is clear from the visual that the whole train wheel set tilts as shown. Along with this tilt, the normal forces also get tilted. If you do a force analysis in this condition, you can see that there will be net force towards the left direction.
This force will bring the wheels automatically to its center. As the wheels approach the center, the self-centering force will disappear. What a simple but brilliant technique to self-center the wheels, right? Flanges are fitted on both the sides of the wheels as an extra safety feature.
When I roll this set of glued paper cups on this tracks, you can see that they are moving perfectly straight. Even if I try to give a small tilt for the cups at the beginning, they are still managing. Now, what about this set? This is glued in the opposite way. When I roll the cups on the same track, it is failing. Railway wheels use this kind of conical shape. This angle makes sure that the wheels never leave the track, but the question is why.
This conical arrangement produces a self-centering force. To understand how, we need to check out the forces acting on the wheels. During a straight track movement the main forces acting on the wheels are shown here. The reaction forces will always be perpendicular to the surface of the cone. When the wheels are centered, the horizontal components of these forces cancel each other out.
Now, assume that, due to some reason, the wheels have moved to the right. One interesting thing happens to the train’s wheels when it moves along the axis. Did you notice? It is clear from the visual that the whole train wheel set tilts as shown. Along with this tilt, the normal forces also get tilted. If you do a force analysis in this condition, you can see that there will be net force towards the left direction.
This force will bring the wheels automatically to its center. As the wheels approach the center, the self-centering force will disappear. What a simple but brilliant technique to self-center the wheels, right? Flanges are fitted on both the sides of the wheels as an extra safety feature.
For fun, let’s assume that the train’s wheels are in the opposite angle. Here if you do the same force analysis during a right displacement, you can see the net force developed is again towards the right. This is why for this wheel geometry the train wheels always get thrown out of the track.
Now, let’s explore the second reason for giving a conical shape to the wheels. With this conical shape, the engineers were able to achieve differential action. Suppose the train has to take a turn as shown. Here, the left wheel has to travel more distance than the right wheel. However, when the wheels are connected using a common shaft, how is one wheel able to travel more distance than the other wheel? Here’s where the conical shape comes into play. To accomplish this, turning the wheels will cause it to slightly slide towards the left. If you consider the contact point of the wheels, the left wheel has a higher radius than the right wheels. In short, for the same angle rotation, the left wheel will travel more distance and achieve differential action.
Remember, to achieve differential action in cars, the engineers had to separate out the wheels and turn them under different speeds. Here, they achieved the differential action just by giving the wheels a conical shape. Interesting, right?
Of course, when the wheels slide towards left, it will produce a force automatically towards the right as we saw earlier. During a cornering situation, this force is provided to supply the centripetal force needed for the turn. Due to this, the wheels will not slide back to the center during the cornering.
At Lesics, we salute the brilliant engineers who accomplished two main engineering goals just by proving a taper to the wheels. Thank you for watching the video.
See you next time.
Now, let’s explore the second reason for giving a conical shape to the wheels. With this conical shape, the engineers were able to achieve differential action. Suppose the train has to take a turn as shown. Here, the left wheel has to travel more distance than the right wheel. However, when the wheels are connected using a common shaft, how is one wheel able to travel more distance than the other wheel? Here’s where the conical shape comes into play. To accomplish this, turning the wheels will cause it to slightly slide towards the left. If you consider the contact point of the wheels, the left wheel has a higher radius than the right wheels. In short, for the same angle rotation, the left wheel will travel more distance and achieve differential action.
Remember, to achieve differential action in cars, the engineers had to separate out the wheels and turn them under different speeds. Here, they achieved the differential action just by giving the wheels a conical shape. Interesting, right?
Of course, when the wheels slide towards left, it will produce a force automatically towards the right as we saw earlier. During a cornering situation, this force is provided to supply the centripetal force needed for the turn. Due to this, the wheels will not slide back to the center during the cornering.
At Lesics, we salute the brilliant engineers who accomplished two main engineering goals just by proving a taper to the wheels. Thank you for watching the video.
See you next time.