NEED FOR POINTS & CROSSING
For transfer of railway vehicles from one track to other, a special assembly viz (Points & crossing) is required.
Points and crossings are necessary when the trains are diverted from one track to other where special arrangements are made for the movement of wheels.
The point facilities the vehicle to divert and crossing provides the gap in the rail to be crossed by flange wheels.
The points & crossing can be provided on parallel or diverging or converging track.
The points & crossing are also called turnouts. A turn-out is designed as right hand and left hand turnout depending upon whether the traffic is diverted to right or left.
Fan shaped switches
To facilitate the transfer of trains from one track to another switch and crossing assemblies are provided. All switch and crossing however complicated, is built up from three basic units joined together with necessary plain rails called closure or lead rails. These units are:
(b) Acute or Vee crossing
(c) Obtuse angle crossing.
Track B takes off from track A and the arrangement is called a Turn- out. This consists of a switch assembly and a Vee crossing assembly joined with a few closure rails.
A set of switch consists of four rails, the outer two are known as stock rails and the inner two as switch or tongue rails. A stock rail and a tongue rail match together to from a switch. A pair of tongue rails with their stock rail are commonly known as points. The switch rail are firmly held by stretcher bars and can be set to give a passage for traffic to either one track or the other. The switch rails pivot about a point known as the heel.
The switch rail is machined out of plain rail sections. In some modern design switch rails are formed out of special thick web rail sections. These have been provided in the new design of high speed passenger turnouts.
For switches, a number of design exist, but broadly they fall into the following two categories.
1. Fixed Heel or Spring Switches
In this design, the tongue or switch rails are rigidly connected to stock rails at the heel by means of distance blocks, bolts and nuts. Movement of the switches is effected by springing from the first block. Fixed heel switches are of two designs
(a) straight switches and
(b) curved switches.
Straight Switches :
In this type, the tongue rail is straight from the tip of the switch to the heel.
In this type the switch rail is straight from the tip of the heel to the same radius as that of the turnout.
In both the cases of straight and curved switches (interchangeability not possible) the tongue rail for left and right hand turn outs are interchangeable. In the case of curved switches, the switch rails are left straight by the manufacture and are spring to the appropriate radius when lying in track.
A few designs of partly curved switches have also been developed by RDSO for the MG system.
Loose Heel Switches
The switch rail pivot about heel joints held by blocks and fishplates. The fishplate holding the switch rails given an appropriate bend to permit free movement of switch rail. In view of comparatively sharper change of curvature, and consequent knocking caused to the rolling stock, loose heel switches are not favoured in new layouts.
|Loose Heel Switches
Design of Switch
They are generally of the following two types :
1. The ordinary or undercut switches in which the foot of the stock rail is planned to accommodate the tongue rail.
2. Over riding switch in which the foot of the tongue rail is so planned that it over rides the foot of the stock rail, which is maintained to full section in the over riding switch the head of the tongue rail for part of its length is kept higher than that of the stock rail by 6mm which reduces the amount of undercut in the foot of the tongue rail. This difference in height is adjusted by putting special machined bearing plates behind the heel. The main fault with over riding switches is their relatively thin blade tip which is liable to damage owing to lateral forces imposed by the moving wheel.
Over riding type of switches have been standardized in the Indian Railways.
Advantages with the adoption of over riding type of switches are as follows:
(a) The stock rail being uncut is not be a source of weakness.
(b) As all the planning is confined to the tongue rail, its cost is less than planning both tongue and stock rails.
|Over riding switches
(c) Although the tongue rail in the over riding switch may appear to be weaker than in the ordinary switch, it is supported by the stock rail for the whole of the weakened portion of its length and the combined strength of the two rails between sleeper is greater than that of the tongue in the undercut switch.