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saurabhjain

Why there is no differential in a train. What happens when a train takes a turn?

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My guess would be the specifications for train tracks does not allow for a small enough radius turn to require a differential.  The small difference in distance traveled between the wheels on the inside vs outside of the turn is not significant enough and may be absorbed in wheel slippage, which may cause additional wear on the wheel.  I don't know the life a train wheels but I would expect it to be upwards of a million miles.

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 The small difference in distance traveled between the wheels on the inside vs outside of the turn is not significant enough and may be absorbed in wheel slippage,

David is correct; the differential distance in rounding a curve is taken up in wheel slippage. This is one reason why we hear train wheels screech when rounding a curve.

In any event, there would be no need for a differential; the wheels of a train car (other than the engine) are not driven. The difference could be taken into account by simply mounting the wheels with bearings between them and the axle. As far as i know, this is not done. I have seen large numbers of replacement wheel sets, two wheels rigidly mounted on a common axle, but never any indication that there was a bearing between the wheel and the axle.

DrD

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That is correct solid axle to wheel mounting. I learned that in some situations on very tight curves there are oiler units that lubricate the side of the track to reduce the friction. Train wheels are allowed significant wear on the diameter. I don't think you could replace just one wheel.

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The reason there is no differential on a train is the wheels are taperred with an anti-derailling stop (the larger diameter lip).

When the train goes around the corner, the larger diameter of the wheel moves to the outside of the outside rail, resulting in the greater distance of the outer rail being "absorbed" by the greater diameter of the wheel that is in contact....similarly the smaller diameter of the inner wheel covers less distance.

The angles are such that the appropriate diameters are self correcting, therefore there is no additional need for any form of differential.

For reference, the wheels are normally an interference fit on the axle facilitated by either "hot wheel fitting" or "cold axle fitting".

Any screaching is where the bend in the track is too tight for the particular wheel profile and therefore outwith this "self-correcting" zone of contact.

 

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Mr. Reid is correct in regard to the conical shape of the wheels, but there is no single cone that will be correct for all track radii. Thus this is simply one of the several things done to relieve the sliding resulting from both wheels being rigidly attached to the axle.

There are many ways to achieve a shrink fit, but none of them have any bearing on the question at hand.

Would you please explain the word "outwith" as used in your final sentence. I am not familiar with this word, and not quite sure I know what it means, although I think I understand the intent of your sentence.

I had not heard of the oilers mentioned by JAG, but this certainly makes sense if the sliding is a sufficiently great problem.

DrD

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I agree with Mr Reid

.Machining of wheel outside diameter with a taper profile eliminates need of differential in  rail wagon.This taper has got a direct relationship with curvature of the rail track.Also,the final machining of wheel outside diameter is accomplished with fine grinding operation.This helps to man over the axle assembly to slide left or right depending upon left or right turn.When the car takes a left turn,wheel on left track has to travel less distance than the right one hence the smaller diameter area of the left wheel will be in contact with the track.Similarly,right side larger diameter of wheel will be active so that right side wheel can travel more distance.

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Sajal, please explain the phrase, "This helps to man over the axle assembly ...." I don't know quite what this means.

As I stated before, there is no single cone that will be appropriate to all track radii. Try rolling a cone with a very wide apex angle; it will roll in a fairly small circle. Then roll a cone with a very small apex angle; it will roll in a very large circle. There is no single cone that will fit perfectly for all situation. Thus, while the cone helps, it is not a complete fix. It improves the situation, but there is still some sliding in most curves.

DrD

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What I meant was the wheel assembly will adjust itself when it comes on a curved track from a straight one..

Thanks for sharing your input Dr D.

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Apologies for delay in reply Dr D...have had Log-in issues (Saurabh...could you look into this please...?)

You are correct about the cone, however the situation is often exacerbated by inappropriate consideration being given to the wheelbase of the rolling stock with regard to the track layout....sometimes with disastrous consequences (The Golden Line in Mexic City). in this Metro build the track turns so tightly that the trains are at significant risk of derailing (in fact they have closed off a huge section of it as a result).,,had they ordered shorter wheel-base trains, this would be far less likely!

Basically the shorter the wheelbase between bogeys, the smaller diameter (obviously within reason) circle and hence tighter turn the train can safely make. It is to do with the lateral forces generated on the wheels as a result of the section still on the straight as the front is on the bend.

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