INTRODUCTION
Train wheels and rails are made of high strength steel. A steel wheel requires very little energy to roll on a steel rail. Due to the low rolling resistance of the train wheels, trains in general are one of the most energy efficient means of transportfor freight and passengers.
Optimum cross-sectional profiles of both rail and wheel not only play a crucial role in maintaining low rolling resistance thereby reducing energy (fuel) consumption, they also assure that:
Contact stress (surface fatigue) is minimized
Trains can steer round curves
Trains run stable (passenger comfort)
Trains run safe and do not derail
Noise from trains is minimal (residents comfort)
Wear is minimal and thus life of components such as rails and wheels are extended
Impact on environment is minimized (due to early replacement of wheels and rails)
Impact on track and train components (and associated maintenance and replacement costs) is reduced
How Trains Curve
On a railway curve the outer rail is longer than the inner rail.
If a wheelset negotiates a curve, the outer wheel thus has to travel a longer distance than the inner wheel.
However both wheels on a wheelset or typically fixed on an axle without differentail thus one wheel cannot turn faster than the other wheel.
The wheels of a train are not flat. Typically wheels have a conical shape
When negotiating a curve, the centrifugal force of the train moving along the curve results in the outer wheel rotating with a greater diameter while the inner wheel rotates with a smaller one. Conical wheels thus allow trains to “steer” round curves
Note:
The tighter the curve, the more difficult it will be for a wheelset to “steer” round curves.
There is a limit to the degree of conicity which can be applied as trains still need to be able to run stable on straight tracks.
Creep Forces (Slip & Friction Forces)
Creep forces in the longitudinal direction occur when traction is applied or when wheelsets need to negotiate relatively tight curves
Creep forces are generated in wheel rail contact patch and they are critical for development of wear damage and surface defects on rails (and wheels)
Creep forces can be longitudinal, lateral or a combination of both
Creep forces in the lateral direction occur when wheelsets move/slip laterally from side to side in tangent tracks or in mild curves
Types of Rail Wear and Defects
Types of damage fundamental mechanisms:
Corrugation
Wear and plastic flow
Rolling contact fatigue damage