POINT AND CROSSING ASSEMBLY: Switch, Switch assembly, Crossing assembly, Assembly drawings of turnout, Turnouts for High Speed and Main factors affecting design of turnout ~ MECHTECH GURU

POINT AND CROSSING ASSEMBLY: Switch, Switch assembly, Crossing assembly, Assembly drawings of turnout, Turnouts for High Speed and Main factors affecting design of turnout

POINT AND CROSSING ASSEMBLY

Important assemblies of turnouts 

In order to improve the quality of maintenance of points and crossings, it is necessary to understand its assembly and other important aspects related to design. Turnout consists of mainly 3 sub-assemblies.

1) Switch

2) Crossing

3) Lead

Important terms and definitions used in describing various parts of a turnout assembly are :-

Switch

The switch comprises of pair of tongue rails between two stock rails. Both the tongue rails are connected to each other with the help of stretcher bars, so that they are operated simultaneously. The pair of tongue rails along with attached stock rails and all other fittings is called point. Point provides facility to divert the wheel passing over it in facing direction from one track to the other track.(Fig.1).

Components of turnout
1. Components of turnout

Right hand switch and left hand switch

Depending on the side to which a train traveling in the facing direction of the switch is diverted, the point assembly is designated as right hand or left hand switch. In order to know whether turnout is left hand or right hand one has to stand at the SRJ and look towards the turnout. If the turnout side is towards right hand, it is called right hand switch, if it is going towards left, it is called left hand switch (Fig. 2).

LH and RH turnout
2. LH and RH turnout


Facing and trailing point

The turnouts on which trains are to be received from SRJ side are called facing point for such trains and the turnout on which trains are received from crossing side from any of the two tracks are called trailing turnout for such trains. Now a days, many loops are being converted as common loops, hence most of the turnouts pass trains in both directions.

Straight switch and curved switch

Straight switches have straight tongue rails. On such turnouts, vehicle will move on one straight track followed by another straight track. In case of curved switch, tongue rail is made curved. But this curve is not tangential to main line and there is no transition between straight and circular curve (Fig. 3).

Switch assembly: Important parts and terms involved in switch assembly are :-

(a) Stock rail joint (SRJ): 

(a) Stock rail joint (SRJ) is the joint at which the stock rail is joined to the rail at the approach. Both the stock rail joints of a point are kept opposite to each other (fig. 1).

(b) Theoretical toe of switch (TTS): 

 Theoretical toe of switch (TTS) is point of intersection of gauge line of a tongue rail at its ATS and its stock rail in closed position in case of straight switches. In case of curved switches, it is a point of intersection of the gauge line of stock rail to imaginary tangent drawn at the actual toe of the switch (Fig. 3).

Straight and curved switch
3. Straight and curved switch


(c) Actual toe of switch (ATS):

 It is a point at which the tongue rail starts at the front end. It is the first tip of tongue rail visible to the eyes. At ATS, tongue rail is machined very thin and lower than stock rail. It is further provided a fillet of radius 12 or 13 mm at the beginning. After fillet, top of tongue rail is provided upward slope. Tongue rail for different turnout and rail section have been given slightly different machining (Fig. 9 and Table 1).

(d) Switch angle:

Switch angle is the angle between gauge lines of the tongue rail and its stock rail in the closed position, in case of straight switches. In case of curved switches, it is the angle between imaginary tangent drawn to the gauge line of tongue rail at ATS and the gauge line of the stock rail. It is also called switch entry angle (SEA). (Fig. 3)

In case of a vehicle moving towards turnout side from main line, it encounters a curve immediately after straight with out any transition curve. In case of a normal curve which is provided on plain track, it always starts with a transition curve followed by circular curve with the provision that transition curve should meet tangentially to straight as well as circular curve for smooth riding. However, on turnouts, circular curve does not meets straight tangentially, it meets at an angle i.e. switch entry angle. Hence, a jerk is felt when a vehicle passes through actual toe of switch. The amount of jerk depends upon switch entry angle. Switch entry angles for various turnouts have been given in the following table 1:-

Table 1


It can be seen that SEA for turnouts on PSC sleepers have been reduced to a great extent as compared to earlier designs. This is one of the important factors for increase in speed on PSC turnouts.

(e) Throw of switch 

Throw of switch is the distance through which a tongue rail moves at its toe from its closed position to open position. This distance is measured from the gauge line of the stock rail to inside (non gauge face) of the open tongue rail. It is measured at actual toe of switch (Fig. 4)

Throw of switch
4. Throw of switch


Throw of switch is decided from consideration of minimum clearance required between the back of open tongue rail and gauge face of stock rail at JOH. As per provisions of schedule of dimensions (2004) for BG, minimum throw of switch allowed for existing works should be 95 mm, but for new works or alteration to existing works it is 115                                             mm.

Maintenance of proper throw of switch helps to:

1. Ensure proper bearing of closed tongue rail against stock rail.

2. Provide minimum clearance of open tongue rail required for passage of wheel flange at JOH.

Point machines provided by signaling department are designed for 143 mm throw and part of the stroke is made idle to achieve the desirable throw. Normally, signaling department provide stroke of 118 mm (3 mm extra for ensuring reasonable tightness). Now a days, new point machines have been designed which are useful for thick web switch. 

This is provided with a throw of 220 mm. Both the tongue rails are not moved simultaneously but the open tongue rail is moved first for 60 mm followed by simultaneous movement of 100 mm. Finally, initially closed tongue rail is moved for 60 mm. Which makes the total throw of 160 mm.

(f) Heel of switch

In case of loose heels, heel of switch is an imaginary point on the gauge line midway between the end of the lead rail and the tongue rail. In case of fixed heel switches, it is a point on the gauge line of tongue rail opposite the centre of the heel block. Heel block is the first block from toe of switch, fixed between the tongue and the stock rail with the help of bolts (fig. 5).

Loose and fixed heel switch
5. Loose and fixed heel switch


Loose heel

When the tongue rail and lead rail form a joint at the heel of switch, it is called loose heel switch. At such loose heel switch, fish plate is bent in front half to allow rotation of tongue rail. The 2 bolts provided towards ATS are kept loose to allow rotation of tongue rails where as the other two bolts towards lead are kept tight (Fig. 5).

Fixed heel 

In fixed heel switch, tongue rail extends beyond the heel and forms a joint with the lead rail. In case of fixed heel all the bolts are kept tight. All the modern turnouts are provided with fixed heel.

(g) Heel Divergence 

The heel divergence of the switch is the distance between the gauge lines of stock rail and that of tongue rail at the heel or in other words, it is the clearance between these two rails at the heel plus the width of the tongue rail head. It is measured right angle to gauge face of the stock rail (Fig. 6).

tabel 2
Tabel 2


Heel divergence of 1 in 8 ½ & 1 in 12 (tabel 2) curved switches on PSC sleepers is more because the heel is located at longer distance, at a place where the moveable length of tongue rail is flexible enough to be operated with a fixed heel.

Key dimensions of turnout
6. Key dimensions of turnout


Crossing assembly

It is a device introduced to permit movement of wheel flange at the inter-section of two running rails. For this purpose, it is necessary to provide gap for movement of flange of the wheels to travel across a running rail. Even after the wheel passes for some distance after throat of crossing, wheel load is still born by wing rail. Since the wing rail moves outward after throat, the outer portion of wheel tread remains in contact with wing rail. Since the wheel is constrained laterally because of presence of check rail, a gap is created between wheel flange and wing rail after throat of crossing. In this gap, nose of crossing is introduced. Top surface of crossing is machined lower by 6 mm at ANC. This machining starts from a distance 90 mm from ANC. Hence, load is transferred on nose only after some distance from ANC i.e. near 90 mm.

Important components and terms involved in crossing assembly are :-

(a) Wing rails

These are the two rails which start from toe of crossing. Wheel moves on wing rails up to ANC and further for some distance after ANC. Thereafter, wheel load is progressively transferred to nose of crossing. (fig. 7).


(b) Throat of crossing 

It is the point at which the converging wing rails of a crossing are closest to each other. (fig. 7).

(c) Toe of crossing 

It is the joint where wing rail of crossing meets the lead rail. Fish plated joint (6 bolts) is provided at this location. The joint should be machined joint to reduce the excessive hammering. (fig.7).

(d) Heel of crossing

It is the last fish plated joint (6 bolts) at the end of crossing (fig. 7). This joint should also be machined joint to reduce the hammering effect of the wheel. (In case of turnout on concrete sleepers, the track going towards the turnout side is required to be made straight up to the centre of last long sleeper).

(e) Crossing angle 

It is the angle contained between the gauge lines of the of the crossing measured at the theoretical nose of crossing.

Components of crossing
7. Components of crossing


(f) Number of crossing 

The number of crossing is the cotangent of angle of crossing. If the angle between legs of crossing is “F”, the number of crossing “N” will be equal to “cot F”. The number of crossing can be found out in field by measuring the spread between two gauge lines of crossing at an approximate distance of 1 m from ANC on both sides. If the spread is approximately 8.5 cm, crossing is 1 in 12, if it is approximately 12 cm; it is 1 in 8.5 crossing.

(g) Point rail

In case of built up crossing, it is the machined rail, which extends up to the actual nose of crossing (fig.7). Front end of point rail is machined but kept thick enough to take the impact (if any) coming on it. Normally width of point rail is kept equal to the web thickness of the corresponding rail.

(h) Splice rail 

It is the rail which forms a part of nose of crossing but does not extend up to ANC. It is connected to the point rail with the help of bolts. Point rail and splice rail together form “V” of crossing (fig. 7).

In case of CMS crossing, there is no concept of point or splice rail since it is monolithic.

(i) Theoretical nose of crossing & actual nose of crossing

Theoretical nose of crossing is the theoretical point of intersection of the gauge lines of a crossing, which is used as a reference point for all layout calculations specially for the turnouts laid on curve (fig.7). The actual nose of crossing is the point at which the spread between the gauge lines of a crossing is sufficient to allow for adequate thickness, from consideration of manufacture and strength. Normally, ANC is provided with a width equal to thickness of web for the corresponding rail section.

Other important aspects of turnout

a) Switch Length 

Switch length is the effective length of a tongue rail which moves laterally during setting of the points; or in other words, it is distance from the heel of the switch to the actual toe of switch. Normally, length of switch should be more than the longest wheel base or the maximum distance between any two wheels of the adjacent wagons on safety consideration (fig.6).

b) Lead of turnout

It is the track portion between heel of switch to the beginning of crossing assembly. Lead of turnout is measured from the theoretical nose of the crossing to the heel of the switch measured along the straight track (Fig.6).

c) Overall length of turnout

It is the distance from the stock rail joint to the heel of the crossing measured along the straight track.

(d) Turn in curve 

Turnouts are always provided to connect 2 tracks, hence on divergent side after heel of crossing (or last long sleeper in case of PSC sleepers) track is laid to connect it to adjoining track. This part of track may be straight or curving in any direction. If it is curved in same direction, it is called connecting curve. However, if this curve is in the direction opposite to the direction of lead curve, it is called turn in curve. (i.e.) track portion between the heel of crossing to the fouling mark (Fig.8).

(e) Machining of tongue rail 

Tongue rails are machined heavily so as to make tip of tongue rail such a thin and low that when pressed against stock rail, wheel can move from stock rail to tongue rail without hitting to the tip of the tongue rail. To ensure it, tongue rail is machined in various stages.

Turn in curve
8. Turn in curve


Following are the stages of machining of tongue rails (fig.9):

Stage 1- 

Machining starts from top level of tongue rail at JOH and tongue rail is machined lowered by 22 mm at ATS for 1:12 and 1:8.5, 60 kg turnout. In case of 1:8.5, 52 kg turnout this figure is 12 mm for PSC sleepers.

Stage 2- 

Starting from point where tongue rail head width is 13 mm to ATS, it is lowered by another round of machining. Causing the front end be further lowered by 6 mm for 1:12 and 1:8.5, 60 kg turnout. In case of 1:8.5, 52 kg turnout this figure is 13 mm.

Stage 3- 

A corner fillet of radius 12/13mm is made at ATS. From machining pattern of tongue rail tip (fig.10) it can be understood that a there is a projection of 6mm from gauge face of tongue rail in 1:8.5 turnout, but no projection is there in case of 1:12 turnout on PSC layout. This has got relation with the gauge which is required to be maintained between two stock rails at ATS as per para 237(1)(g) for different design of turnouts.

Assembly drawings 

Assembly drawings numbers of various turnouts being utilized on Indian Railways are given in the (Table 4)

Sketch of tongue rail machining
9. Sketch of tongue rail machining


Table. 3


Machining at ATS for 1 in 8.5, 60 Kg tongue rail
10 a. Machining at ATS for 1 in 8.5, 60 Kg tongue rail

Machining at ATS for 1 in 12, 60 Kg tongue rail
10 b. Machining at ATS for 1 in 12, 60 Kg tongue rail


Important dimensions of the most popular turnout assemblies, which include sleeper spacing, clearances, offsets for turnouts, rail closures etc. are given in following chapters. These dimensions are very important and have to be scrupulously adhered to at the time of assembly of turnout in order to achieve trouble free maintenance during service.

Turnouts for High Speed 

When the speed on straight track is above 250 kmph, High speed turnouts with speed on curved track from 80 to 100 kmph are warranted.

Assembly drawings numbers of various turnouts
Table. 4


Main factors affecting design of turnout are:-

(i) Kink in the turnout route at the toe of switch rail

(ii) Entry from straight to curve without transition

(iii) Lead curve without super-elevation

(iv) Entry from curve to straight without transition

(v) Gap at the V of crossing

As the wheel negotiates the toe of switch, there is abrupt change in direction resulting in lateral jerk on bogie and corresponding heavy lateral force on tongue rail. The magnitude of force primarily depends on switch entry angle. By reducing the switch angle, entry gets smoothened and flange force gets reduced. The small switch angle is obtained by providing curved/ tangential switches. In tangential type, very small switch angle is possible. Tangential types of switches are used over foreign railways for HSR. As per D72 ORE report and trials over SNCF railway, higher speed can be permitted over T/out by reducing SEA.

Absence of super elevation over Turnout causes unbalanced lateral acceleration and affects safety and comfort. In high speed turnouts, Switch Entry Angles are small and the permissible cant deficiency on the TO curves becomes main criteria for evaluating the permissible speed.

Up-gradation in turnout technology in the railway system has been guided by the following considerations:

(i) Higher speeds on straight and curved tracks with reasonable level of passenger comfort. Designs have been evolved for a speed up to 230 Kmph on turn out track.

(ii) Least life cycle cost with minimum traffic interruption for repairing.

(iii) Track geometry maintainability comparable with the normal track

(iv) Safety and comfort

(v) Planned maintenance without emergencies

The result of the trial made on the SNCF Railway have given very favourable results by:-

(i) Adoption of tangential layouts for higher speeds and Thick web switches.

(ii) Flatter Switch entry angle by tangential layouts thereby reducing the angle of attack and reduced lateral forces resulting in increased passenger comfort.

(iii) Use of spring operated switch setting device to ensure proper flange way clearance.

(iv) Use of movable nose crossings housed in a specially designed cradle, thereby avoiding gap at crossing.

(v) Introduction of transition curves thereby improving the running characteristics of the curved tracks.

(vi) Use of asymmetrical profile section ZU- 1in 60 forged to standard rail profile (UIC 60) at the end.

(vii) Continuation of canting of rails through turnout resulting in smoother ride over turnouts.

(viii) Use of higher UTS steel, further hardened to reduced wear.

(ix) Effective holding of stock rail.

(x) Use of non-greasing eco-friendly base plates.

(xi) Use of specially designed synthetic rail pads for reduced vibration of switch assembly.

(xii) Use of flatter angle of crossing i.e. 1 in 20 or 1 in 24.

(xiii) Sophisticated pulling techniques including introduction of hydraulic systems.

(xiv) Surface hardening of load bearing areas.

By these modifications, the forces, accelerations and rolling movements, are found less than the normally allowed limits. Further, the actual sensation felt by the passenger was very good.



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