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Long Welded Rails

Introduction

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. lwr_1.jpg

                             Fig.1 Outline of LWR
                             
                             

Dream of every railway engineer- “jointless track”

Various innovative techniques have been used to obtain this dream:

  • Increasing the length of rails by rolling longer rails.
  • Welding the rail joints.

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.

Historical Background

  • Although welding of rails was started in 1905 itself, commercial welding on any considerable scale became common only after 1932.
  • The weights and lengths of standard rail sections varied from 22kg/m to 65 kg/m and from 5.5 m to 27m respectively (reference1). The length of welded panels varied from 18m to 380m.
  • Welding process most commonly used : ‘ thermit’
  • Although longer length rails were being used, track engineers were unsure of the expansions which these longer length rails would undergo under temperature variations.
  • The ability of the welded panel to withstand compressive stress buildup due to constrained expansion
  • British transport commission conducted a number of studies in the 1950’s to go into the aspect of lateral stability of the LWR under temperature induced compressive stresses.
  • They found: welded panels derive resistance to buckling not only from the stiffness of the rail itself but also from the rail sleeper fastenings and ballast resistance.

==== 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:

  • rail section and range
    1. 52 kg & heavier - tm +5°C to tm+10°C

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:

  • stress-free temperature is applicable for any destressing operation wherein rail tensors are used.
  • stress-free temperature is a finite value.

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

  • for other than concrete sleepers

when ballast compaction done with hand operated compactors

  • BG 3,00,000 gross tonnes
  • MG 1,00,000 gross tonnes

mechanized shoulder and crib compactor

  • BG 50,000 gross tonnes
  • MG 20,000 gross tonnes or minimum of two days whichever is later
  • concrete sleepers
  • BG 50,000 gross tonnes
  • MG 20,000 gross tonnes or minimum of two days whichever is later
  • at least one round of stabilisation by DTS
  • for newly laid LWR/CWR, at least 3 rounds of packing ( last two with on track tamper)

Rail Temperature:Rail temperature is different from ambient temperature which is the temperature of air in shade. It affects behavior and stability of LWR.

Temperature zones:

^Zone^Range of rail temperature (°C)^

|(i)|40-50|

(ii)51-60
(iii)61-70
(iv)71-76

       Fig.2 Showing Variation of Temperature

Theoretical Background

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

  1. Creep resistance on account of friction between the rail and the sleeper at the rail seat
  2. Creep resistance further offered by the rail-sleeper fastening

This results into less expansion/ contraction in LWR rail sleeper frame as a whole tends to moves (due to toe load ).

  • rail/sleeper frame is restrained by ballast resistance
  • assumed constant
  • build up progressively from end of LWR
  • thermal forces are developed
  • temperature variation
  • rail section
  • importance of measurement of rail temperature
  • forces in rail to be kept in limit
  • maintenance operations

Permitted Locations for LWR

Alignment: shall not be laid on curves sharper than 440m radius both for BG & MG.

In temperature zone-I up to 5° on BG

  • 52 kg rail on PSC, m + 7 with following precautions
  • increase shoulder ballast to 600 mm on outside and provide for 100 m beyond the tangent point
  • reference marks every 50 m for creep
  • SEJ 100 m away from tangent point
  • each curve greater than 250 m length – provide SEJ on either side.
  • may be continued through reverse curves not sharper than 875m radius.
  • for reverse curves sharper than 1500m radius, shoulder ballast of 600mm over a length of 100m on either side of the common point should be provided

gradients

  • steepest 1 in 100
  • vertical curve if algebraic difference is equal to or more than 4 mm/m or 0.4 %,as per para 419 of IRPWM

^ ROUTE^ minimum radius^^

BGMG
A4000 mall routes2500 m
B3000 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 ====

  • A detailed plan shall be made showing the exact locations of SEJs and of various other features mentioned in sub
  • A sample of the detailed plan may be seen at fig.3 . the plans may be prepared to a horizontal scale of 1:5000

              Fig.4 Typical Layout of LWR

Temperature records

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:

  • Four numbers of 6.5 metre or longer rail pieces of the same rail section as LWR two sets of SEJs with sleepers and fastenings adequate numbers of 1 metre long fish plates with special screw clamps/ joggled fish plates with slotted grooves & bolted clamps.
  • Rail closures of suitable sizes 1 metre
  • 10 cm straight edges callipers and feeler gauges (2mm to 0.1mm)
  • Rail cutting equipment
  • Destressing equipment i.e. rollers, mechanical/hydraulic rail tensor, mallet and side rollers for curves. Alumino-thermic / mobile gas pressure welding, equipment and consumable materials, equipment for protection of track, equipment for night working.

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

  • after welding about 1km or longer, destressing to be done
  • if not possible within a reasonable period
    1. observe tp , and if tp rises above t + 20°C, (t = welding/laying temperature) then
  1. temporary destress at likely tmax - 10 °C until final destressing.
  2. if the rail temperature comes down appreciably, cold weather patrolling should be introduced.
  3. final destressing after consolidation of track
  • temporary speed restriction shall be imposed on the length of track where welded panels are joined by 1m long fishplates with special screw clamps or joggled fishplates with slotted grooves & bolted clamps as in fig. 4, 5, 6.

  Fig. 4

  Fig. 5

  Fig. 6
  
  

Gaps at SEJ

gaps at SEJ shall be adjusted at the time of laying/subsequent destressing of LWR/CWR, as illustrated in fig.7 and shall be as under:-

   Fig. 7

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:-

  • a) Every fortnight during the two coldest and two hottest months of the year at about minimum and

maximum temperatures alternately by pwi incharge and pwi sub-section.

  • b) Alternately once in two months during other months of the year.

ii) Assistant engineer:-

  • at least once in six months, preferably during oldest and hottest months.

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.

  • i) when the gap observed at SEJ differs beyond limits specified and exceeds the maximum designed gap of SEJ
  • ii) when tip of tongue rail/corner of stock rail crosses the reference line (c.s. no.10)
  • iii) after special maintenance
  • iv) after restoration of track following an unusual occurrence
  • v) if number of locations where temporary repairs have been done exceed three per km.

Sequence Of Operations

The procedure to be adopted for destressing of LWR without rail tensor,it consists of the following steps:-

  • i) Arrange the block at such a time when temperature will be tm+5°C to tm+10°C during the fastening down operation.
  • ii)Impose 30Kmph and remove fitting of alternate sleepers.
  • iii)During block,remove closer rail and adjust the gap at SEJ.
  • iv)Remove remaining fitting from SEJ to center of LWR.
  • v)Place roller at every 15th sleeper to permit the rail to move freely
  • vi)Struck the rail by wooden mallet horizontally to assist in their longitudinal movement.
  • vii)Remove rollers and start fastening from middle of LWR and Proceeding towards SEJs.
  • viii)Insert the closer of required length and weld the joints.
  • ix)Fastening for 20meter shaller be removed before welding.

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:-

  • i)Remove impediments to free movement of rail such as rail anchors, guard rails, check rails, etc.
  • ii) Create gap of about 1 metre at the centre of LWR during a traffic block and insert a closure rail there at a restricted speed.
  • iii) During the first traffic block, create a gap of 1 metre at location ‘b’ i.e. centre of LWR . introduce rail closure as required and fasten with special fishplates and clamps. allow traffic at restricted speed.
  • iv) Mark the anchor length a1 a2 and c1 c2 each equal to la at either end of the length a2 c2 to be destresed
  • v) Erect marker pillars w0 w1 etc., on each of the length a2b and c2b. transfer the marks w0 onto the rail foot
  • vi) During the second traffic block, when tp is less than the desired to (fig. 9), destressing operation shall be carried out for the lengths a2 b and c2 b as described below:-
  1. a) Remove the closure rail from location ‘b.’ unfasten and mount on rollers the portion from a2 c2.
  2. b) Fix the rail tensor across the gap at ‘b’ and apply tension so as to obtain some movement at w0 to remove any kinks or misalignment and to minimize the friction in the rollers etc. release the tension and note the movement y0 at w0.
  3. c) Transfer marks w1, w2,……. onto the rail foot and note temperature tp.
  4. d) Calculate the required movement at w1 as under:- movement at w1 =yo + elongation of length w0 w1 (l) due to temperature difference (to - tp) = y0+ lα(to -tp). Calculate the required movement at w2 as under movement w2 = movement at w1 + elongation of length w1w2 (l) due to temperature difference (to - tp).
  5. (e) After the fastening down of the last length a2b and c2b is completed, make a paint mark near free end of one rail at a distance of (6.5 metre + 2 x 25 mm – 1mm), measured from the end of the other rail across the gap spanned by the rail tensor.
  6. (f) Remove the tensor, close the 1 metre gap temporarily and allow traffic at restricted speed.
  • vii) During another traffic block, cut the rail at the paint mark, insert a rail closure of length exactly equal to 6.5 metre and weld one end thereof. if the gap at the other end is also 25 mm, it can be welded in the same block.
  • viii) During a subsequent traffic block, when tp is less than td, equalise the forces in the rail by releasing the fastenings over a length of 100 metre on either side of location ‘b’ and tapping with wooden mallets etc. fasten down the rail and allow traffic.
  • ix) During another traffic block, when tp is within the range of temperature specified for td in, destress the end 100 metre from SEJ. Thereafter, weld the closure rail next to SEJ duly ensuring setting of the SEJ
 
C:/wamp/www/doku/data/pages/long_welded_rail.txt · Last modified: 2010/11/01 09:34 by osdme