LWR - Long Welded Rail

LWR:Long Welded Rail (LWR)is a welded rail, the central part of which does not undergo any longitudinal movement due to temperature variations. A length of greater than 250 meters on Broad Gauge and 500 m on Meter Gauge will normally function as LWR. The maximum length of LWR under Indian Railway conditions shall normally be restricted to one block section.

LWR Sketch Diagram

As the central portion of LWR/CWR does not expand/contract i.e. it does not undergo any longitudinal movement, therefore, thermal forces build up in the central portion due to temperature variations. The thermal force (P), is to be resisted by a suitable track structure. 

                    P = A E α t 

Where,        A = Area of cross-section of the rail (sq.cm)

                         (A = 66.15 cm2 for 52 kg rail & A = 76.86 cm2 for 60 kg rail) 

                    E = Modulus of elasticity of rail steel, (2.15 x 106 Kg/sq.cm) 

                    α = Coefficient of linear expansion of steel, (1.152 x 10-5 /°C) 

                    t = Variation of rail temperature from td / to (°C) 

For a temperature change of 10C, the value of induced thermal force (P) works out 1.638 ton for 52 Kg & 1.903 ton for 60 Kg rail section. 

CWR: Continuous Welded Rail is an LWR, Distressing of which may be required to be carried out in parts. The maximum length of CWR under Indian Railways conditions shall normally be restricted to one block section.

SWR: Short Welded Rail is a welded rail, which contracts and expands throughout its length

  • Breathing Length is that length at each end of LWR/CWR, which is subjected to expansion/contraction on account of temperature variation.
    Rail Section Sleeper Density 1540 Sleeper/Km Sleeper Density 1660 Sleeper/Km
    52 Kg 66 meter 64 meter
    60 Kg 77 meter 74 meter
  • Buffer Rail is a set of rails provided in lieu of SEJ at the ends of LWR/CWR to allow expansion/contraction of adjoining breathing lengths due to temperature variation

SEJ: Switch Expansion Joint is an expansion joint installed at each end of LWR/CWR to permit expansion/contraction of the adjoining breathing lengths due to temperature variation.

  • Initial gaps at SEJ to be provided at td- 40 mm
  • Maintain gaps at SEJ- 40 to 120 mm

Improved SEJ

Different Type of SEJ

RDSO Drawing No. Rail Section Installation Gape Maximum Design Gape
RT-4160 52 Kg 40 mm 80 mm
RT-4165 60 Kg 40 mm 80 mm
RT-6902 60 Kg 40 mm 80 mm (Single Gape)
RT-6914 52 Kg 40 mm 80 mm (Single Gape)
RT-6922 60 Kg 40 mm 65 mm (Double Gape)
RT-6930 52 Kg 40 mm (Double Gape) 65 mm

Note:52 kg and 60 kg Rail Thrust on RDSO Drawing No. 80 mm gap combination SEJ should be laid as per RT-6782

Click Here-MCQ Test for LWR

Prohibited Locations for LWR

Due to the technical problems arising on account of the use of long welded rails, the Indian Railways have specified locations where the laying of LWRs is prohibited. These locations are listed below.

  • New constructions and doublings of lines, where the formation and the track are not fully stabilized. However, if mechanical compaction of the earthwork is done, LWRs may be laid on the new lines at the initial stage itself with the approval of the chief engineer.
  • Locations where rails are subjected to heavy wear, corrugation, or corrosion, or require frequent renewal.
  • Locations where the formation is weak and track deformations are extensive, which may lead to buckling.
  • Locations where the formation soil is susceptible to pumping and the ballast is likely to get heavily contaminated, thereby necessitating the frequent opening of the track and the screening of the ballast.
  • Locations where frequent breaches, flooding, and subsidence may occur.
  • On curves with a radius sharper than 500 m. In the case of reverse curves, the limiting value is 1500 m.
  • The steepest gradient permitted for LWRs is 1 in 100. At every change of gradient, vertical curves should have the minimum radii.

Track Structure for LWR

The minimum length required for a rail to function as an LWR depends upon the range of temperature variation, the section of the rail, the resistance offered by the ballast to the thermal expansion of the sleepers, and the resistance offered by the rail and sleeper assembly to any thermal expansion of the rails.

Formation

The formation should not pose any problems in the laying of LWRs. In stretches where the formation is bad, it should be stabilized before the LWRs are laid. A cross slope of 1 in 40 should be provided at the time of screening and laying of LWRs. In the case of concrete sleeper tracks, an extra cess width is provided to the extent of 90 cm for embankments and 60 cm for cuttings.

Ballast

A clean ballast cushion of a minimum depth of 300 mm for speeds above 130 kmph and 250 mm for speeds upto 130 kmph should be provided below the bottom of the sleeper for LWRs. In order to increase resistance, the shoulder width of the ballast should measure 350 mm in the case of straight tracks and the inside of curves and 500 mm in the case of the outer ends of curves. The ballast should also be humped to a height of 150 mm on both shoulders. The typical ballast profiles for LWRs for single-line and double-line BG concrete sleeper tracks with a 250 mm (10") ballast cushion.

Sleepers Prescribed for LWR

Sleeper density for BG On BG tracks the sleeper density on LWR/CWR shouldbe as follows:

GMT Sleeper Density
GMT > 20 1660 sleepers per kilometer for all routes except the E route, where 1540 sleepers per km may be permitted
GMT 10–20 1660 sleepers per kilometer for route A; 1540 per kilometer for routes B, C, D, and E
GMT less than 10 1660 sleepers per kilometer for route A; 1540 per kilometer routes for B, C, and D, 1310 per kilometer for route E (1540 per kilometer if LWRs are provided).

Rails

LWRs should be laid with 60-kg, 52-kg, or 90 R rails on BG and 90 R or 75 R rails on MG barring the portions already laid with 60 R rails. Generally, new rails with fish bolt holes should not be used for LWRs. Further, level crossings should not fall within the breathing lengths of LWRs, and switch expansion joints or buffer rails should be located at the end of the LWRs.

Rail Temperature is the temperature of the rail at the site as recorded by an approved type of rail thermometer as laid down in LWR manual Para 2.1. This is different from ambient temperature which is the temperature of the air in shade at the same place. The Indian Railways has been divided into 4 temp zones as under:

ZONE Range of Rail Temp.
I 40 to 50 Degree C
II 51 to 60 Degree C
III 61 to 70 Degree C
IV 71 to 76 Degree C

In Indian Railway maintenance of track is done according to temperature. Rail Temperature is measured at every section of Senior Section Engineer. The following three types of approved rail thermometers are used to measure the temperature of the rail.

  1. Embed Thermometer
  2. Dial Type Thermometer
  3. Continuous Rail Thermometer

Different Rail Temperature:-

Mean Rail Temperature (tm) for a section is the average of the maximum and minimum rail temperatures recorded for the section.

Installation Temperature (ti) is the average rail temperature during the process of fastening the rails to the sleepers at the time of installation of the LWR/CWR.

Destressing Temperature (td) is the average rail temperature during the period of fastening the rails to the sleepers after destressing LWR without the use of a rail tensor. If rail tensor is used, td for all practical purposes is equal to as defined in Para 1.13 of LWR manual(Stress-free Temperature). The Range of td or to shall be within the limits of rail temperature shown below

ZONE Rail Section Range of td
I, II, III All Rail tm to tm + 5 Degree C
IV Light Rail tm to tm + 5 Degree C
52 Kg or Above tm + 5 Degree C to tm + 10 Degree C

Destressing is the operation undertaken with or without rail tensor to secure stress-free condition in the LWR/CWR at the desired/specified rail temperature. Distressing is done in two ways.

  • Manually
  • Mechanically (with rail tensor)

Manually distressing: Manually distressing is done by the men when the temperature of the train is between 35 to 40 degrees. Distressing is always done in ascending order of rail temperature. The length of the destressing is decided on the basis of available manpower, blanks, and temperature.

Mechanical distressing: Mechanical distressing is done only at rail temperature less than T/T assumed by the rail tensioner. This distressing can be done up to one kilometer at a time.

Rail Tensor: Rail Tensor is a hydraulic or mechanical device used for stretching the rail physically.

Anchor Length (Ia)Anchor Length is the length of track required to resist the pull exerted on rails by the rail tensor at temperature tP, For practical purposes, this may be taken as equal to 2.5 meters per degree Celsius of (to – tP) for BG and 4.5 meters per degree Celsius of (to – tP) for MG track.

Maintenance of Long Welded Rails

Some important points which generally help in ensuring the safety and effective maintenance of LWR tracks are the following.

  • A well-compacted ballast bed should be available below the sleepers at all times to give adequate lateral and longitudinal resistance to the track in order to prevent buckling and excessive alterations in the sleeper lengths of the LWR track.
  • The track should be left undisturbed as far as possible and only essential track maintenance work should be carried out at temperatures close to the destressing temperature (td). Thermometer bulb Mercury bath Piece of rail head Mild steel clamp Seal Centigrade thermometer reading up to 100°C Handle
  • Regular maintenance should be completed well before the onset of summer and confined to the hours when the rail temperature is below td + 10°C. Track should not be lifted by more than 50 mm at a time in case of concrete sleeper s and 25 mm in other sleepers.
  • The track should preferably be maintained mechanically with on-track tie tamping machines.
  • At no time during manual maintenance should more than 30 sleeper spaces be opened in a continuous stretch between two stretches of 30 sleeper lengths that are fully boxed.
  • Packing or renewal of a single isolated sleeper may be done by a gang mate, provided that at least 50 sleeper spaces on either side are left untouched for a minimum of 24 hours after such work is done
  • Special attention should be paid to locations such as breating lengths, SEJ etc.
  • All fastening should be kept tight.

Buckling of Track

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.

Causes

A track can buckle due to the following reasons.
  1. Inadequate resistance to track due to deficiencies in the ballast
  2. Ineffective or missing fastenings
  3. Laying, destressing, maintaining, or raising the track outside the specified rail temperature range especially is hot weather
  4. Failure to lubricate the SEJs in time
  5. Excessive creep, jammed joints, sunken portions in a welded track
  6. Symptoms

    Buckling in a track becomes noticeable when the track displays the followingsymptoms.
    1. Presence of kinks in the track
    2. Absence of gaps in the SWR portion of the track in the morning hours of hot days.
    3. Expansion/contraction at SEJ is ±20 mm more than the theoretical range given in the LWR manual
    4. High percentage of hollow sleepers

    Precautions

    Buckling can be avoided by taking the following precautions.
    1. Proper expansion gaps as specified in the manual should be provided in the SWR portion of the track.
    2. 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.
    3. No work of track maintenance including packing, laying, aligning, major/minor realignment of tracks, screening of ballast should be done outside the specified temperature.
    4. 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 anti-creep fastenings, replacement and tightening of missing and loose fastenings, etc.

    Actions

    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.
    1. The temperature of the rail is brought down as far as possible by pouring water on the rails.
    2. Emergency or permanent repairs and destressing should be carried as specified in the LWR manual.
    3. 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.

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