Switch Expansion Joint
IMPROVED DESIGN OF SWITCH EXPANSION JOINT
In the typical design of Switch Expansion Joints (SEJs) presently used in welded track on most of the railway systems the Stock and Tongue rails forming part of the SEJ have two sharp bends in close proximity.
At the time of design of the initial types of SEJs, the rails in common use on many railway systems were of about 72 kg/sq. mm UTS with Percentage Elongation (PE) of about 14.0% (min). Since 1980s, most railway systems have switched over to use of 90 UTS or even higher UTS rails.
The Percentage Elongation of 90 UTS rails is, about 10% (min). The new types of rails are, therefore, highly sensitive to bends, nick-marks etc. which are normally caused during bending/ machining jobs involved in the production of SEJs from the rolled rails. As a consequence, there have been several cases of fracture of stock/ tongue rails of SEJs fabricated from high UTS rails as compared to almost nil fractures reported when the SEJs were made from 72 UTS rails.
Since SEJs are used in welded tracks on which trains travel at high speeds, the possibility of such fractures at bends introduces a serious element of safety hazard. M/S. Bina Metal Way Limited (BMW) have, therefore, invented a new type of Improved SEJ which does not have any bends in stock and tongue rails thus eliminating the safety risk.
One more advantageous feature of BMW's Improved SEJ is the provision of two gaps of 40 mm (nominal) each between the tongue rails and the stock rail as compared to the use of one gap of 40 mm (nominal) in the old designs. Under temperature changes in welded track, the gap in Improved SEJ of BMW make would vary from 33 mm to 57 mm. In the old version of SEJ, the single gap of 40 mm (nominal) could range from 28 to 74 mm under temperature changes. The smaller maximum gap of 57 mm in the improved design of SEJ as compared to 74 mm maximum gap in the earlier design reduces impact caused at the joint when the wheel negotiates the gap in the SEJ.
Another feature of BWS's design of SEJ is that the minimum head width of tongue rail in 60 kg design is over 61% (of the full rail-head width) whereas in the earlier design of SEJ it was only 35%. Therefore, there is the advantage of lesser wear and longer life associated with larger head width in the new design. Further, the wear-prone areas of heads of stock and tongue rails are pearlitized by a special process to achieve a hardness of 320 ±15BHN.
After examining the advantageous features of BMW's design of SEJ, the Indian Railways procured more than 850 sets of Improved SEJs for use on Central, Eastern, Northern, South Central, South Eastern and Western Railways. Out of these, over 700 sets have been laid in tracks as per information available with BMW. They are performing to the satisfaction of P-way field staff. Their riding characteristics as monitored by Track Recording Car has also been found to be satisfactory.
BMW closely interact with the field staff on all the zonal railways with a view to assisting them in the laying of the Improved SEJs to eliminate field problems, if any, and for further improvements in the design.