Why do the train wheels not slip on the railway track despite of both are metal or metal drives on metal?
Remember the law of friction.
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.
Take your one palm on another palm. Now start rubbing. It may slide easily. Now give pressure by both hands and start rubbing. This time it will not be easy.
What is the difference in the second time? The pressure given by your hand increases the frictional force. And this frictional force opposed the sliding.
In Practical field of Railways.
Come to the railway. The heavy weight of wagons can be treated as a huge force. This force creates a frictional force between wheels and railway. So wheels don't slip on railway tracks when pulled by a locomotive.
How does frictional force actually act on wheels and track for the running of trains on truck?
As per Amonton’s first law of friction, “The force of friction is directly proportional to the applied load”. Here Force applied means the weight of wagon, coach, or loco itself.
This load (force) come through the wheel and ultimately on to the track. This weight force of the wagon, coach, or loco creates fictional force.
The weight of the wagon transfers to the railway track through ball bearing fitted on axle. Here two frictional forces are created.
One in ball bearing and other between wheel and track. In ball bearing the frictional force is lubricated frictional force.
And between wheel and track the frictional force is dry frictional force. Normally the dry frictional force is greater than lubricated frictional force.
When locomotive starts pulling, the train starts to move by rolling movement of wheels, as the ball-bearing frictional force is less than the frictional between wheel and track.
I upload a picture of wheels arrangements on track to better understand.
But in practice there is wheel slip on run in various situations.
Yes, wheels slips also occur on railway tracks. The loco pilot has to control wheel slips by some technical application and also by his own experience.
We the Loco Pilots use three kinds of activities to reduce wheel slipping in rainy season.
1st, we used to spray sand on the railway tracks through two sand boxes fitted in two sides of front wheel.
These sand boxes must filled with sand and it is the Loco Pilot’s duty to check whether the boxes are filled with sand and that the sanding system is in working condition before taking charge of Loco.
The loco pilot can spray sand on tracks with a paddle switch from the loco cab. The spraying of sand is very useful to create more frictional force between tracks and wheel and hence reduce wheel slip.
2nd, we reduce the current flow in traction motors through the master controller.
Loco Pilots press a special push-button switch fitted in the cab that reduces current difference between all traction motors by connecting a temporary resistance with each traction motor.
It reduces current difference between traction motors and hence reduces wheel slip.
3rd, is practical experience. When wheel slips start we use partial application of loco brake, that reduces wheel slip effectively.
If after all these techniques, wheel slip is not stopped, the loco pilot has to stop his train. After a while when situation will normal he should start his train.
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