A rail track is liable to get distorted, particularly in hot weather when the compressive forces in the track exceed the lateral or longitudinal resistance of the track. The buckling of the track is a matter of grave concern as it may lead to derailments and even serious accidents.
A track can buckle due to the following reasons.
(a) Inadequate resistance to track due to deficiencies in the ballast
(b) Ineffective or missing fastenings
(c) Laying, destressing, maintaining, or raising the track outside the specified rail temperature range, especially is hot weather
(d) Failure to lubricate the SEJs in time
(e) Excessive creep, jammed joints, sunken portions in a welded track
Buckling in a track becomes noticeable when the track displays the following symptoms.
(a) Presence of kinks in the track
(b) Absence of gaps in the SWR portion of the track in the morning hours of hot days.
(c) Expansion/contraction at SEJ is ±20 mm more than the theoretical range given in the LWR manual
(d) High percentage of hollow sleepers
Buckling can be avoided by taking the following precautions.
(a) Proper expansion gaps as specified in the manual should be provided in the SWR portion of the track.
(b) As buckling is likely to occur between the 11th and 17th hour of the day, rosters of key men should be so adjusted that there is proper patrolling of the LWR portions of the track when the temperature exceeds tm + 20°C.
(c) No work of track maintenance including packing, laying, aligning, major/ minor realignment of tracks, screening of ballast should be done outside the specified temperature.
(d) Wherever the track structure is weak and vulnerable to buckling, immediate action should be taken to strengthen by the provision of extra shoulder ballast, increase in sleeper density, provision of adequate anticreep fastenings, replacement and tightening of missing and loose fastenings, etc.
As soon as a tendency towards buckling is detected in the track, traffic should be suspended and the track should be fully protected. The track should be stabilized by heaping the ballast on the shoulders up to the top of the web of the rail. When buckling takes place, traffic on the affected track should be suspended and remedial work should be carried out in the following stages under the personal supervision of a PWI.
(a) The temperature of the rail is brought down as far as possible by pouring water on the rails.
(b) Emergency or permanent repairs and destressing should be carried as specified in the LWR manual.
(c) In the case of fish-plated or SWR tracks, a gentle reverse curve may be provided in the rear of the buckled track to ease out the stress. The buckled rail should then be cut at two places that are more than 4 m apart. The track should then be slewed to correct the alignment and rails of the required lengths should be cut and inserted to close the gaps.
Short welded rails (SWRs) are rails that are welded in panels of two, three, or five rails. In an SWR, the entire length of the rail is subjected to expansion or contraction due to changes in temperature (thermal expansion). In the case of an LWR (long welded rail), however, only the end portions of the rails are subject to thermal expansion. The central portion of an LWR remains clamped and does not undergo any change in length. Continuous welded rails (CWRs) are rails that are welded in lengths greater than 1 km. LWRs and CWRs are part of modern high-speed tracks and require very little maintenance as compared to fish-plated tracks.
1. Welded rails have played an important role in the modernization of the railway system. Explain the benefits of using welded rail tracks. Do they have any associated disadvantages?
2. Given the following track data, calculate the minimum theoretical length of a long welded rail beyond which the central portion of the rail would not be subjected to thermal expansion.
Rail section adopted-52 kg of cross-sectional area = 65.15 cm2 Coefficient of thermal expansion of rail steel = 11.5 x 10-4/°C Temperature variation in rail after laying of track = 30°C Modulus of elasticity of rail steel = 2 x 106 kg/cm2 Sleeper spacing = 65 cm
Average restraining force per sleeper per rail = 300 kg
3. What are the advantages of welding rail joints? What initial precautions should be taken to prevent the possibility of buckling in long welded rail tracks?
4. What are the different steps involved in the maintenance of LWRs? Explain the function of switch expansion joints.
5. Discuss the concept of long welded rails. How can an existing short welded rail track be converted into an LWR track?
6. What are the recommendations regarding track structure with respect to formation, ballast, sleepers, and rails when LWRs are to be used? What are the permitted and prohibited locations for long welded rails on Indian Railways?
7. Discuss the theory regarding the maintenance of LWR tracks and how the same is put into practice.
8. What is the concept behind LWR? What is the measurement of the breathing length and what is the expansion in this portion? How is this expansion accommodated?
9. What do you understand by short welded rails? Briefly describe the standard specifications for short welded rails.
10. What do you understand by the buckling of a track? What are its causes? Briefly describe the steps to be taken when buckling takes place.
11. Compare the salient features of short welded rails, long welded rails, and continuous welded rails.