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Railway Engineering :Elements of Geometric Design
Geometric Design of Railway Track
Geometric design should be such as to provide maximum efficiency in the traffic operation with maximum safety at reasonable cost.
GradientAny departure of track from the level is known as grade or gradient.
Purpose of providing gradient:
Superelevation on Curves (Cant)
Cant is defined as the difference in height between the inner and outer rails on the curve. It is provided by gradually raising the outer rail above the inner rail level. The inner rail is considered as the reference rail and normally is maintained at its original level. The inner rail is known as the gradient rail.
Function of superelevation:
– Neutralizes the effect of lateral force.
– It provides better load distribution on the two rails.
– It reduces wear and tear of rails and rolling stock.
– It provides smooth running of trains and comforts to the passengers.
Equilibrium speed: It is the speed at which the effect of centrifugal force is exactly balanced by the superelevation provided. It can also be said that when the speed of a vehicle running on a curved track is such that the resultant weight of the vehicle and the effect of radical acceleration is perpendicular to the plane of rails and the vehicle is not subjected to an unbalanced radical acceleration, is in equilibrium then its particular speed is called equilibrium speed.
Cant deficiency is the difference between the equilibrium cant (theoritical) necessary for the maximum permissible speed on a curve and the actual cant provided there. As per Indian Railways, Cant deficiency is recommended as follow:
- BG Track: 75 mm
- MG track: 50 mm
- NG track: 40 mm
When a train travels on a curved rack at a speed lower than the equilibrium speed, then the cant excess occurs. It is the difference between the actual cant provided and the theoretical cant required for such lower speeds. Maximum value for cant excess is
- BG track: 75 mm
- MG Track: 65 mm
When a body moves on a circular curve, it has a tendency to move in a straight direction tangential to the curve. This tendency of the body is due to the fact that the body is subjected to a constant radial acceleration.