Railway Engineering :Elements Of Geometric Design
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Railway Engineering :Elements of Geometric Design

 
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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.

Gradient 

Any departure of track from the level is known as grade or gradient.

Purpose of providing gradient:

•To provide uniform rate of rise or fall,
•To reduce cost of earth work,
•To reach different stations at different level.
Types of gradient

1) Ruling gradient: The steepest gradient allowed on the track section. It determines the max load that the locomotive can haul that section. The steep gradient needs more powerful locomotives, smaller train loads, lower speed, resulting in costly hauling.
–In plains:   1 in 150 to 1 in 200
–In hilly regions:   1 in 100 to 1 in 150
2) Momentum Gradient: The gradient on a section which are steeper than the ruling gradient acquire sufficient momentum to negotiate them are known as momentum gradient.
3) Pusher gradient: As stated above a ruling gradient limits the maximum weight of a train which can be hauled over the section by a locomotive. If the ruling gradient is so severe on a section that it needs the help of extra engine to pull the same load than this gradient is known as pusher of helper gradient. In Darjeeling Railways 1 in 37 pusher gradient is used on Western Ghat BG Track.  
4) Gradient at stations: at stations gradient are provided sufficient low due to following reason:
–To prevent movement of standing vehicle
–To prevent additional resistance due to grade.
On Indian railways, maximum gradient permitted is 1 in 400 in station yards.
Grade compensation on curves

If a curve is provided on a track with ruling gradient, the resistance of the track will be increased this curve. In order to avoid resistance beyond the allowable limits, the gradients are reduced on curves. The reduction in gradient is known as grade compensation for curves.
•BG track:  0.04% per degree of curve
•MG track:   0.03 % per degree of curve
 •NG track:   0.02 % per degree of curve

Degree of curve
A curve is defined by its degree or radius. The degree of a curve is the angle subtended at the center by a chord of 100 feet or 30.48m.
If R is the radius of curve,
•Circumference of the curve= 2 ∏ R
•Angle subtended at the center by the circle = 360 degree
•Angle subtended by the arc of 30.48m = 1750/R
Thus, a 1 degree curve has a radius of 1750 m.

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.

Speeds

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.

Maximum permissible speed: This is the highest speed which may be allowed or permitted on a curved track taking into consideration of the radius of curvature, actual cant, cant deficiency, cant excess and the length of the transition curve. When, the maximum permissible speed on the curve is less than the maximum sanctioned speed of the section of a line, permanent speed restriction become necessary on such curves.

Cant deficiency

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

Cant Excess

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

Centrifugal Force

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.

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