Maintenance of Long Welded Rails
How to determine Stress-Free Temperature in the field
by Rajeev Shrivastava
Long welded rails are laid and fastened to the sleepers at rail temperature which is called de-stressing Temperature or Td. This temperature is kept 5° to 10° Celsius higher than the mean rail temperature (Which is the average of minimum and maximum rail temperature).
However, in service, rails have to be cut, replaced and welded at different point of time and the rail temperatures may not be equal to the Td at all times. Stress relaxation (plastic flow under stress), loss of toe load of the fittings etc., can cause movement of the rail even if tied to the sleepers and hence the effective rail temperature at which the rail would be stress free, can be different from the Td at any point of time and in fact, can also vary along the length of LWR at same time.
During maintenance, this stress-free rail temperature (say T0) has to be kept under watch and adjusted by de-stressing the long welded rail (LWR) if it varies by more than acceptable limits from Td. Attempt has been made below, to describe the procedure for judging the T0 for a long welded rail:
- A location is selected at least 200m away from the end of LWR (there is a switch expansion joint (SEJ) at the end of LWR). Two points are marked 6 m apart and these marks are transferred onto pegs erected on either side of the rails, independent of the track. These points can be called A and B. Two points (A1 and B1) are marked 100m in the rear of A and B and these are also transferred to pegs.
- A speed restriction of 30km/h is imposed and the LWR is cut at A and B and prevailing rail temperature (Tp) is measured. The rail closure A-B is removed from track in a traffic block (typically 1 hour) and track fittings are removed in A-A1 and B-B1 lengths.
- Rail in A-A1 and B-B1 is mounted on rollers to enable free movement. The movements at A, A1 and B, B1 are measured by stretching a chord over the pegs and measuring the displacement of the marks on the rails at these locations. Let's say, these movements are a, a1, b, b1 respectively. The movements should be taken with a positive sign if the movement is towards the gap AB and negative if it is away from AB.
- The net movement at A is a-a1 and at B is b-b1
- The T0 can be ascertained from the equation:
E = L*α*(Tp-T0) where
L = summation of lengths A-A1 and B-B1 which is 200m
α = coefficient of linear expansion = 1.152E-5
Tp = as measured by rail thermometer
e = Total net movement of lengths A-A1 and B-B1 = (a-a1) + (b-b1)
- The rollers are removed and rail is refastened from A1 and B1 towards A and B. Gap AB is closed with a suitable length rail closure and fastened with 940mm fishplates and clamps. Traffic block is removed and traffic is restored at 30 kmph SR.
- If the value of T0 is not within acceptable range, then the LWR is de-stressed. Otherwise, the closure is welded as below:
- a) The rail piece which was originally cut off from the track (and not the rail closure inserted to restore the traffic block) is shortened by 24*2-1 = 47mm by cutting on one side. This rail piece is inserted in track in a subsequent traffic block and one end is welded. Traffic can be restored by fish-plating the other end.
- b) In a subsequent traffic block, rail tensor (mechanical or hydraulic) is used to pull the rail ends at the fish-plated joints so as to get a gap of 24mm. While pulling, 100m length on either side (A1 to B1) is opened again (at a rail temperature lower than Td) and the rail is tapped with mallets before refastening. The other end is then welded, keeping the tensor in place.
- T0 is checked as above, every three years, after de-stressing, till need of de-stressing arises again.