Page 446 - IRSEM_Main Book

Section 4: Thick Web Switches (TWS)

19.4.1 Conventional over-riding type of switches suffer from geometrical deficiencies
like:

(a) Weakness of tongue rail due to severe reduction of section by heavy
machining. Tongue rails are further weakened by heavy wear and tear by
traffic. So these are to be replaced frequently.

(b) Obstruction to the wheel flange at the actual toe of the switch due to
thickness of the tongue rail as well as due to switch entry angle. Hence a
jerk is experienced by vehicles on account of these obstructions.

(c) Twist in the track over a short distance on tongue rails machined from rail
of same cross section, due to overriding nature of the tongue rail.

(d) Structural weakness of the entire switch assembly to withstand lateral
forces at higher speeds.

19.4.2 Thick Web Switches (TWS) overcome deficiencies of conventional over-riding
type of switches. These are non-over riding type:

(a) Thick web switch has wider switch opening of 160 mm (as against of 115
mm of conventional over riding type of switches).

(b) Ground connection along with clamp lock used for operation of TWS and
direct locking of tongue rail with stock rail shall be of approved type.

(c) For operation and clamp locking of TWS, electric point machine of approved
type with requisite throw shall be used.

19.4.3 For ground connection and clamp lock as per RDSO Drg. No. RDSO/S-3454
for 60 Kg rail and RDSO/S-3455 for 52 Kg rail, IRS point machine of 220 mm
throw is used.

(a) Out of 220 mm throw, during first 60 mm throw closed switch rail gets
unlocked and open switch rail moves towards stock rail. During subsequent
100 mm throw both switch rails move simultaneously thereby open switch
gets closed and closed switch gets opened by 100 mm. During final 60 mm
throw, closed switch gets locked and open switch rail moves 60 mm further
resulting in total opening of open switch by 160 mm.

(b) Provides direct locking between tongue rail and stock rail in closed and
open position.

(c) No stretcher bar is used and both the tongue rails move independent of
each other.

Chapter 19: Installation, Testing & Maintenance of Signalling Equipment Page 394 of 535