Why trains have no differentials ?

Question

Apart from a two wheeler any three or more wheeled vehicle have differentials then why not Trains ?

Answer 1

The shape of the wheels and the fact that there are no sharp turns completely disregards their use.

As the track begins to turn, the train is still travelling in a straight line. this means the track is then in contact with a different part of the right hand side wheel to the left hand side wheel.

By makeing the wheels with conical tendancies, the wheel on the outside of the corner will have a slightly larger radius at the point it is in contact with the track then the wheel on the inside of the corner.

This makes the wheels follow the corner round. You can prove this by making cardboard wheels (one bigger than the other) and using a pencil for a axel. It will not roll in a straight line.

It is a very simple principle, and it works. To introduce a differential on every driven axel would just be introducing un-needed complication.

Answer 2

The wheels are not even across the tread.  

The tread is tapered.  The amount of taper increases, smaller on the outside of the wheel, growing larger as distance to the back of the wheel increases,  so that when traversing a curve, the outside wheel in the curves, with flange hard against the rail, moves its position on the rail inward toward the taper, and the slight difference created in the diameter of the wheel because of the taper, wherein the taper increases toward the back of the wheel, it is in effect,  larger in diameter than the inside wheel, which allows it to run a little "faster" (poor choice of word but demonstrates well) thereby off-setting the greater distance the outside wheel must travel through a curve.

The newer EMD AC engines also have additional compensation that allows for the wheels as mounted to "cant" slightly to help the wheels stay more centered through the curve, (similar to the old "zero tracking error" high end turntables in home stereo gear) reducing "wheel creep" and wheel "slip."  This provides for more traction and less wear on the rail.

The older, latter day steam locomotives, with their long driving wheel base, had additional "play" in the drivers in the middle to assist in negotiating curves, and some had these same center wheels with no flanges.  These were only found on the larger engines of the day.

Even so, in tight curves, the shrieking of steel om steel is quite noticeable.  There are many industrial spurs and tracks in industrial parks that have very sharp curves.  Larger, six axel "SD" type engines cannot operate in these areas due to their truck wheel base.  This is why the shorter, general purpose "GP" four axel type of engines are used in these areas.

Answer 3

Modern trains do not directly use engine power to drive the wheels.  They use diesel generators to create electrical current which is in turn used to power electric motors attached to each wheel of the engine.  The losses of using mechanical systems to route power to each wheel are much higher than converting the energy of the motor into electricity and then back to work using an electric motor.  Because of the very small contact patch between each wheel and the track all the wheels of the locomotive must be driven (and also use sand injection to aid in traction) to move the train.  At higher speeds the small contact patch makes the rolling resistance lower increasing efficiency overall.

Answer 4

A differential allows each wheel in the axle to be able to rotate at different speed while having the equal amount of torque. This allows the vehicle to make smooth turns and greatly reduces the turn radius.

On a train, its not much of a concern. For one thing, trains don't make small turns. given their length.  In addition, the trains run on a steel track on steel wheels. And lastly on modern trains, each individual train wheel is powered by its own motor, thus negating the need for a differential.  Furthermore, and most importantly, having a differential reduces the amount of torque force applied to the wheel.  On a train torque is most important than hp because a lot of torque is needed for pulling the other carriages.

Answer 5

Purpose of differential is to transmit power to rear wheels ; this thing is achieved in trains by coupling one compartment to another, engine drags all wagons attached by coupling- a kind of attachable and detachable joint.

Answer 6

A train's wheels are turned by electrical current not by mechanical movement. All the motor does is build up a electrical charge like a generator and transfers it to the traction motors on each wheel. The sensors on the train tell it when to take power away from and put power back on each wheel.

Answer 7

No no, differentials would be BAD on a train!  

They would defeat the self-centering mechanism built into railroad wheels.

I've ridden on small rail vehicles that have differentials.  They "hunt" terribly, bouncing and slamming left and right.  That wouldn't work for real trains, it would shred flanges and track.

No, trains have beveled wheel treads. Look very closely

http://www.railway-technology.com/contra...

Even clearer (but PDF file):

http://www.narcoa.org/info/how_to/Wheelp...

The flange doesn't normally hit the side of a rail, not even in a gentle curve.  Imagine the wheels are left of center... the left wheel is riding on a slightly larger diameter than the right wheel.  The axle wants to turn "right" back to center.

You've seen the "make the cone roll uphill" toy....

http://plus.maths.org/issue40/features/u...

Nevermind the uphill trick, why does that cone always stay centered on the rails?  Why doesn't it simply turn sideways and fall between the rails!?    Same exact principle.  Railroad wheels are in fact cones.

Here is a more exaggerated picture of the goings-on.  

http://members.aol.com/stevef88/puzzles/...

Note that these wheels have a very exaggerated wheel tread, but no flange at all!

The flange isn't used at all 99% of the time.  This effect is sufficient.   But in sharp curves, the flange does indeed earn its pay.  The cone effect is "maxed out", and the wheel has walked sideways until the flange hits the side of the rail.  After that, you are in the situation for which you would want a differential... the wheels "need" to turn at different rates, and there is much scuffing of wheel tread and scraping of flanges.

Oh well. That's what makes the scraping and grinding noise in sharp yard curves, and the singing noise in mountainous mainline curves.

Answer 8

Because they are on tracks and will not be making any sharp turns.  It is steel on steel.