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 metre on broad gauge (BG) and 500 meter on metre gauge (MG) will normally function as LWR(fig. 1). The maximum length of LWR under indian conditions shall normally be restricted to one block section.
Fig.1 Outline of LWR
Dream of every railway engineer- “jointless track”
Various innovative techniques have been used to obtain this dream:
Apart from the logistic considerations of rail transportation and its loading and unloading, the length of the rail was governed by the length of cooling boxes in the rail manufacturing steel plant, as controlled cooling after the rolling process was necessary.
==== Definitions ====
LWR: is a welded rail whose central portion does not exhibit any longitudinal movement on account of temperature variations.
SWR: is a rail which expands and contracts throughout its length: 3-rail panels, 39m in BG and 36m in MG.
CWR: An LWR which continues through station yards including points and crossings is described as a continuous welded rail.
Breathing length :is that length at each end of LWR/CWR which is subjected to expansion/ contraction on account of temperature variations.
Switch expansion joint (SEJ): is an expansion joint installed at each end of LWR/CWR to permit expansion/ contraction of the adjoining breathing lengths due to temperature variations.
Buffer rails: are a set of rails provided in lieu of SEJ at the ends of LWR/CWR to allow expansion/contraction of the adjoining breathing lengths due to temperature variations
Destressing: is the operation undertaken with or without rail tensor to secure stress-free conditions in the LWR/CWR at the desired/specified rail temperature.
Rail temperature: is the temperature of the rail at site as recorded by an approved type of rail thermometer.
Mean rail temperature ™:for a section, is the avg. of the max. & min. rail temps. 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 avg. rail temperature during the period of fastening the rails to the sleepers after destressing LWR without the use of rail tensor. If rail tensor is used, td for all practical purposes is equal to to as defined below. Range of td or to shall be:
others - tm to tm + 5°C
Prevailing rail temp (tp): is the rail temperature prevailing at the time when any operation connected with destressing is carried out.
Stress-free temp (to): is the rail temperature at which the rail is free of thermal stress. For all practical purposes, this is equivalent to destressing temperature, but there are two essential differences:
Rail tensor: is a hydraulic or mechanical device used for stretching the rail physically.
Anchor length (la): 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 2.5m per degree celsius of (to-tp) for BG.
Hot weather patrol: is the patrol carried out when the rail temperature exceeds td+20oc/ td+25oc.
Cold weather patrol: is the patrol carried out during cold months of the year in specified sections as per instructions of chief engineer.
Consolidation of track
when ballast compaction done with hand operated compactors
mechanized shoulder and crib compactor
Rail Temperature:Rail temperature is different from ambient temperature which is the temperature of air in shade. It affects behavior and stability of LWR.
^Zone^Range of rail temperature (°C)^
Fig.2 Showing Variation of Temperature
A metal rod supported on frictionless rollers can theoretically expand and contract freely with variations in temperature = l α t
where, l = length of the metal rod α = coeff. of linear expansion (1.152×10-5/°C) t = change in temperature e.g. if l = 13m, t=20°C; change in length = 3mm
Rail is restrained due to
This results into less expansion/ contraction in LWR rail sleeper frame as a whole tends to moves (due to toe load ).
Alignment: shall not be laid on curves sharper than 440m radius both for BG & MG.
In temperature zone-I up to 5° on BG
^ ROUTE^ minimum radius^^
|A||4000 m||all routes||2500 m|
|C, D & E||2500 m|
* LWR/CWR plans requires the approval of THOD. however any deviation from the provision of this manual approval of PCE shall be obtained ==== Laying of LWR/CWR ====
Fig.4 Typical Layout of LWR
Ascertain tm, and maximum daily variation from records available with the pwi-incharge if not available adopt the tm and range as shown in rail temperature map of india.
Materials Required Following materials are required for laying one LWR:
Preliminary Works Deep screening of ballast along with lifting or lowering of track, if required, provide standard ballast section. All other preliminary works identified in above shall also be completed before laying of LWR/CWR.If any of the preliminary works can not be completed before installation of LWR/CWR such stretches should be isolated by providing SEJs.
On completion of these works, such stretches may be welded, destressed and joined with LWR/CWR,welding of rails to form LWR.weld into 10/20 rail; panel,while unloading 880 grade (90 uts) or higher grade rails.Insert SEJs at the predetermined locations with proper gaps, provide closure rails of 6.5m or longer at LWR side/s of SEJ to facilitate adjustment of gaps during destressing operation.
welding of rails to form LWR
laying of LWR can be done at any time of the year, but
The gaps between the reference mark and tongue rail tip/stock rail corner at various rail temperatures shall not differ by more than ± 10 mm from the theoretical range, where fish plated or swp track is joined on one side of SEJ, the gap between the reference mark and tongue rail tip/stock rail corner on LWR/LWR side shall not differ by more than ± 10 mm from the theoretical range.
Maintenance Of LWR/CWR
Regular track maintenance in LWR/CWR includes following operations:- i) tamping/packing ii) lifting iii)shallow screening/shoulder cleaning iv) aligning including minor realignment of curves v) renewal of fastenings requiring lifting vii)maintenance of buffer rails
Special Maintenance for LWR/CWR
General (a) Track structure consisting of other than concrete sleepers:- in LWR/CWR the regular track maintenance in LWR/CWR shall be confined to hours when rail temperature is between td + 10°C and td – 30°C and shall be completed well before onset of summer. (b) Track structure consisting of concrete sleepers:- the regular track maintenance in LWR/CWR shall be confirmed to hours when the rail temperature is between td + 10°C and td – 30°C and shall be completed well before onset of summer.If rail temperature after maintenance operation exceeds td + 20°C during the period of consolidation the speed restriction of 50 km/h on BG and 40 km/h on MG shall be imposed.
ii) Ballast section shall be properly maintained, specially on pedestrian & cattle crossings, curves and approaches to level crossings and bridges. Cess level should be correctly maintained. Dwarf walls may be provided on pedestrian and cattle crossings to prevent loss of ballast. Replenishment of ballast shall be completed before onset of summer.
iii) Sufficient quantity of ballast shall be collected to provide full ballast section before commencing any maintenance operation, specially lifting.
iv) When crow bars are used for slewing, care shall be taken to apply these in a manner so as to avoid lifting of track. The crow bars should be planted into the ballast at an angle not more than 30 degrees from vertical; otherwise lifting of track may result.
v) Special attention shall be paid to maintenance of fastenings in LWR/CWR specially on concrete sleepers according to para 1411 of irpwm. checking correct driving of erc. clip applicator/ extractor. Checking for corrosion, rubber pads, checking of toe load (excess movement of SEJ’s.
vi)Check insulating liners, replace the cracked ones. renewal of fastenings measures to prevent corrosion and seizure of ercs with mci inserts
vii) All fastenings shall be complete and well secured. For correction of alignment, the shoulder ballast shall be opened out to the minimum extent necessary and that too, just opposite the sleeper end. The ballast in shoulders shall then be put back before opening out crib ballast for packing.
viii) Deep screening/mechanized cleaning of ballast provisions laid down in para 238 of irpwm will also apply mutatis- mutandis to LWR/ CWR with further provisions as mentioned hereafter in this para wherever mechanized cleaning of ballast is resorted,
Inspection and Records
Knowledge of staff in regard to prescribed maintenance practices shall be periodically checked and it shall be ensured that the work is done accordingly.Ultrasonic examination of rails should not be in arrears defective rails/welds should be replaced expeditiously.
Inspections of gaps at SEJ and creep/movement at centre of LWR/CWR by permanent way officials would be done as per following schedule:- i) pwi-incharge/pwi sub-section:-
maximum temperatures alternately by pwi incharge and pwi sub-section.
ii) Assistant engineer:-
Destressing Of LWR
i) the work of destressing shall be done during a traffic block under the personal supervision of a pwi. ii) it is preferable to impose a speed restriction of 30 km/h before actually obtaining the traffic block and to loosen/remove fastenings on alternate sleepers to reduce total duration of the traffic block.
When To Destress :-
Abnormal behavior of LWR/CWR whenever gets manifested in one or more of the following, destressing shall be undertaken.
Sequence Of Operations
The procedure to be adopted for destressing of LWR without rail tensor,it consists of the following steps:-
Destressing operation of LWR with the use of rail tensors
For destressing of LWR with the use of rail tensor, the following procedure shall be adopted:-