Q Bank for Civil Engineer LDCE Exam- Write Short Notes on (22)

Q No. 14. Define super elevation, cant deficiency, cant excess in curve & writes brief notes on

• Methods for setting out simple curve:

• Transition length

• Virtual transition.

• Criteria for Realignment of curve.

• Various steps to improve Running on curves.

SUPER ELEVATION:- Cant or superelevation is the amount by which one rail is raised above the other rail. It is positive when the outer rail on a curved track is raised above inner rail and is negative when the inner rail on a curved track is raised above the outer rail.

Cant deficiency:- Cant deficiency occurs when a train travels around a curve at a speed higer than the equilibrium speed. It is the difference between the theoretical cant required for such higer speed and actual cant provided.

Cant excess:- Cant excess occurs when a train travels around a curve at a speed lower than the equilibrium speed. It is the difference between the actual cant and the theoretical cant required for such a lower speed.

• METHODS FOR SETTING OUT SIMPLE CURVE:

TANGENTIAL OFFSET METHOD: This is the method employed for settin g out a short curve of about 100 metre (300 ft) length. It is generally used for laying turn-out curve. LONG CHORD OFFSET METHOD: This method is employed for laying out curves of short lengths. In this case it is necessary that both the tangent points are located in such a way that the distance between them can be measured and the offsets taken from the long chord. 

QUARTERING OF VERSINE METHOD:

This method is also used for laying out curves of short length of about 100 metres (300ft.) In this method, the location of two tangent points (T1 and T2) is first found out and the distance between them measured. The versine (V) is then calculated, using formula:- 

CHORD DEFLECTION METHOD: The chord deflection method is one of the most suited to confined situations as most of the work is done in the immediate proximity of curve. 

 THEODOLITE METHOD: The theodolite method is a very popular method for setting out a curve on the Indian Railway, particularly when accuracy is required. This method isalso known as Rankine’s method of tangential angles. In this method the curve is set out by tangential angles with the help of a theodolite and a chain on the curve having lengths.

Transition Length:- Transition curve is an easement curve, in which the change of radius is progressive throughout its length and is usally provided in a shape of a cubic parabola at each end of the circular curve. It affords a gradual increase of curvature from zero at the tangent point to the specified radius of circular arc and permits a gradual increase of super elevation, so that the full superelevation is attained simultaneously with the curvature of the circular arc. 

1. The desirable length of transition ‘L’ shall be maximum of the following three values –

 (a) L = 0.008 Ca X Vm 

(b) L = 0.008 Cd X Vm 

(c) L = 0.72 Ca 

Where: L = the length of transition in metres. 

 Vm = maximum permissibe speed in Km.p.h. 

 Cd = cant deficiency in millimeters. 

 Ca = actual superelevation on curve in millimeters.

• TO IMPROVE Running ON Curve

Running on curves:- (1) For smooth and satisfactory running on curves –

(a) There should be no abrupt alteration of curvature and/or superelevation (cant) 

(b) The superelevation should be appropriate to the curvature, at each point.

(c) Versine variotation should not be more than +5 to -5mm from theoretical values, station to stn. Varion should not be more than 10 mm 

(d) Cant gradient should not be steeper than 1in 360 

(e) Proper ballast section, cess width to be ensured 

(2) On Group ‘A’ and ‘B’ routes, gauge, versines and superelevation on each curve must be checked once in every four months and on other routes every six months. 

Criteria for realignment of a curve:

(1) When as a result of inspection by trolley or from the foot plate of locomotive or by carriage or as a result of Track Recording carried out, the running on a curve is found to be unsatisfactory the curve should be realigned. 

(2) The running over a curve depends not only on the different between the actual versine and the designed versine but also on the station-to-station variation of the actual versine values. This is because it is the station to station variation of versine which determines the rate of change of lateral acceleration, on which depends the riding comfort. Service limit for station to station versine variation for 3 speed group viz. 120 Km/h and above, below 120 Km/h and upto 80 Km/h and below 80 Km/h and upto 50 Km/h, should be considered as tabulated below: 

SERVICE TOLERANCES FOR 120 KMPH ON B.G.