Mechanization of track maintenance started in 1960s with purchase of UT machines, which used to tamp 1 sleeper at one time. With time, more & more activities are being mechanized. Mechanization has improved quality of track along with reduction in cost of laying & maintenance of track.
Following are the reasons, which go in favor of mechanization:
a) Total traffic & axle load has gone up, increasing requirement of attention to track.
b) Modem track structure is much heavier, which is difficult to be handled by manual means
c) In manual system of track maintenance, gang used to maintain track in between trains. During manual attention of track very awkward track parameters are formed, which are required to be smoothened before arrival of next train. With increase in number of trains, regular block margins are not available. So it is better to work in block using high output machines so that at the end of block safe track with better parameters can be achieved.
d) Average age of gangmen in gangs is very high; handling such a heavy track structure is difficult for them.
e) Quality of track achieved by mechanized means is much better than manual means.
(i) Plain Track Tamping Machines
06-16 Universal Tamping Machine (UT)- mostly phased out
09-32 Continuous Action Tamping Machine (CSM)
09-3x Tamping Express
(ii) Points and Crossing Tamping Machines
08-275-3 S Unimat
(iii) Multi-purpose Tamper (MPT)
At present most of Tamping Machines available on Indian Railways are of M/s Plasser make. The main functions of tamping machines are:
(i) Correction of Alignment
(ii) Correction of longitudinal level and cross level
(iii) Packing under the sleepers
For correction of alignment the machine can be worked in smoothening mode or design mode.
Two types of aligning systems are in use:
(a) Two Chord Lining System.
(b) Single Chord Lining System.
In 06-16 UT Universal tamping machine, two-chord system of lining is used while in other tamping machines, single chord lining system is used. Since now all the UT machines are phased out there is no need to describe two chord lining system.
(b) Single Chord Lining method This system is provided on all the tamping machines except UT machine and is controlled and executed with a combination of electronic, hydraulic and mechanical systems. Machines are provided with two systems of working i.e. 4 - point and 3 - point lining system.
In this case trolleys at A, B, C and D are pneumatically pressed against the rail selected for alignment measurement and correction, usually the high rail of a curve. The chord wire stretched between 'A' and 'D' represents the ‘Reference Line’ and the transmitting potentiometer (transducer) which are fixed to the measuring trolley 'B' and lining trolley 'C' are connected to this wire by means of forks and the rope drives. The trolley 'D' is called Front Trolley, 'A' is called Rear Trolley, 'B' is called Measuring Trolley and 'c' is called Lining Trolley.
The measured ordinate at B is multiplied through the electronic circuit by the specified ratio and compared with ordinate at C. Then electronic signal is emitted which activates the hydraulic control of the lining mechanism and necessary correction effected.
For circular curve, if we use a chord of fixed length and measure versines at two points which are at fixed distance from 'A' and 'D', the ratio of both the versines will be constant and independent of radius of curve. Typically the ratio ‘i’(called versine ratio) is approximately 1.21 for CSM. In this system, versines are measured at 'B' by transducer and the measured ordinate at 'B' is multiplied through the electronic circuitry by the specified ratio 'i' and compared with ordinate at 'C'.
As this system is true for circular curve, while working on transition this ratio may not be correct. Hence, while working on transition portion of curve, necessary correction can be applied by versine potentiometer on the front cabin. Such corrections are given in the catalogues of machine (every machine has diff catalogue). The pattern of application of versine correction can be understood by sketch given below
In 4 point lining principle it is presumed that all 4 points are on circular curve. But in field curve will be distorted because of which point D will be on displaced location, (although it is presumed that point A & B has been brought to proper circular alignment because of tamping done in this portion.). Because of shifted location of D base-line for measurement of H2 & H1 will be disturbed, so the track will not achieve its targeted alignment. But there will be a residual defect at C which will be equal to FD/n. here FD is the distance by which track is out of alignment at D. Value of n (error reduction ratio) is defined as
So track tamped by machine will not follow theoretical profile & error level at C will be limited to 1/6 of slew required at D.
As value of n can’t be increased much because of machine construction; so we need to eliminate effect of D not being at ideal location. Hence front trolley is required to be moved to proper location. So a feed is to be given in the front cabin in slew potentiometer for moving location of front trolley. This movement does not take place physically but only electrical signals are emitted. This will ensure that track achieves ideal profile after packing.
Design mode for alignment can be actuated in three ways in machines:
a) For the machine not provided with on board computers, design lining can be done in the manner explained above. To do this first slews are calculated by taking versines of existing curve followed by calculation of slews either by manual calculation or by ROC software. Slews calculated for stations at 10 mts distance are to be interpolated for slews at every 1.2 mts by using linear interpolation between stations. The interpolated slews are also written on the top of sleepers along with direction.
b) Machines which are provided with on board computers there is no need to take versines by chord, but the machines are equipped with the system, which can be used to measure existing alignment of curve. To measure existing versines, machine is required to be run in measuring mode, in which machine will be run at 10 to 20 kmph. During measuring run no packing will be done only curves will be measured & versines recorded in computer. After curve measurement, with the help of ALC software, realignment solution will be found out keeping in view obligatory points and permissible slews and the file will be saved. While going for the tamping block the saved file will be executed while tamping. This will ensure application of slews automatically. For this operation, transition correction and interpolation of slews will be automatic. Operator is not required to read slews from the top of sleeper and feed in the machine. Quality of alignment expected from this treatment is supposed to be better than other system of alignment. However, measuring run will require additional traffic block.
c) In case of machines provided with onboard computer, where engineers are not willing to spare time for measuring run, versines can be measured by manual method and measured versines are fed into onboard computer of machine and realignment solution found by ALC software is saved in separate file. The saved file is executed when the machine goes for tamping block. In such case also transition correction and interpolation need not be written on the sleeper top. This methodology will save time for measuring run with reasonable quality of alignment.
The 3-point method is mainly used if:
- The track is to be lined according to specified radii or versines.
- The lining system is used in conjunction with a sighting device for straight.
The chord at measuring position 'B' is fixed by the fork and the transducer at 'B' is switched off. The ordinate at C only is measured on chord ‘BD’ and compared with preset ordinate value. For any circular curve, if the chord is extended and versine is measured at a fixed location, the measure versine should be, HI = 'BC' x 'CD' /2R. As for the system designed for the machine the length of 'BC' and 'CD' are fixed for a particular machine, the formula will change to, H1 = Constant/R. The value of Constant will vary from machine to machine, hence, while using 3 point lining method, constant for the machine which is working in this section will be asked from the machine in-charge. Radius of curve being tamped is known to the PWI so by using above formula, value of H1 can be calculated and fed in the versine potentiometer in front cabin. Machine will provide same at 'C' location. Transducer working at 'C' will check the versine at that location and any difference detected will activate the lining control to effect the necessary correction, so as to shift the track to specified versine.
As straight is a curve of infinite radius, the versine to be achieved on a straight is O. Hence, 3 point lining method can be used on a straight while keeping H1 value as Zero. The principle of three point lining method cannot be used directly on transition portion as value of H1 changes from place to place on transition. Hence, every machine catalogue provided with a methodology through which the value of H1 will be progressively increase from zero near the straight to full value of HI near the circular curve.
Treatment for transition length is limited to 6 different patches i.e. A, B, AB, C, D & CD. Length of every segment i.e. A, B, C & D is 14.75 mts. Versines for length A is calculated by dividing constant given in table (for A length) by a value R*L. Versines for length C can be calculated by subtracting value of versines calculated for A from value of H. Versines for length D is calculated by dividing constant given in table (for D length) by a value R*L. Versines for length B can be calculated by subtracting value of versines calculated for D from value of H. In AB length versines are increased at a rate of Hv=23617/R*L. Value of 23617 may change machine to machine.
In three point method point “D” is not on circular curve (as presumed in principle of lining), although we have presumed that point “B” is on tamped portion hence at correct location. Because of 'D' being at distorted track the base point for measurement of versine at 'C' gets shifted. So even after tamping, the track will not reach to the designed circular geometry and the residual slew at 'C' will be equal to FD/n. The value of 'n' = BD/BC which is close to 3. So the residual slew at 'C' will be almost 1/3rd of slew required at 'D'.
The value of 'n' cannot be increased much because of the machine design. Hence, we need to make value of FD = 0, by physically/electrically shifting point D to the designed circular geometry. This can be done by taking the versines of existing curve, with the help of ROC Software, slews required can be decided. Slews, as decided by software are the value of FD. Software will provide slews at every 10 meters interval which should be reduced to every 1.2 meter by linear interpolation. At the time of tamping such slews are fed into machine through slew potentiometer in front cabin. This will ensure theoretical alignment after tamping.
Tamping by three point lining method is very sensitive to position of starting point (i.e. TP). In case mistake is made in selecting TP full curve may go inside or outside, which may lead to sever misalignment at fixed structures like girder bridges or OHE masts. Hence while using this method in open line utmost care should be taken. Four point lining does not pose such problem.
To attend straight track alignment few machines are provided with laser based alignment system. Few CSM & Tamping Express machines were purchased with this facility. Laser emitting equipment & machine are placed at points which can be taken as good point. Now laser is directed to receiver provided at the front of machine standing on good point. By setting direction of laser in this way, good straight alignment is set; track will be slewed to this alignment after tamping.
While tamping straight track slew for the design mode should be measured in the track at chord length ranging minimum 80 to 100 meters. If possible it can be made longer to achieve better geometry. Measuring alignment at such a long chord length may not be possible except by optical instrument. People have tried different methodology with theodolite; any method will give good track alignment. However, measurement system with standard tripod may take very long time to level the theodolite at the track centre. One stool kind of stand was successfully tried in Kota Division, WCR by PWI Shri Malkhan Singh. Details of his method and the small stand which can be fabricated by carpenter easily can be seen in IRICEN Journal of Civil Engineering, June 2008 issue. With the help of this instrument long chord measurement can be done quickly and the results ate satisfactory.
====== (ii) Correction of Longitudinal
and Cross Level ======
By this process, longitudinal level and cross level of track are corrected. The principle of correction of longitudinal level is also called proportional leveling because the lift provided at 'M' is equal to lift provided at 'F' /Lifting Ratio. In this system, there are three points 'F', 'M', and 'R' are used.
R- Rear measuring point (Rear Tower)
M- Measuring and correction point (Feeler Rod Tower)
F- Front measuring tower (Front Tower)
In this system, chord is stretched between 'R' and 'F' as both are equally high. 'M', (i.e. Feeler Rod Trolley) is situated very close to lifting unit. In case the point 'M' goes down to the chord stretch between 'R' and 'M', the machine will actuate lifting of track by lifting cum lining unit to such an extent the top of 'M' again reaches to the chord level. This is how leveling system works. Separate 'F' 'R' and 'M' and ' chords are provided for both the rails.
In UT machines, as soon as the track is lifted to the required level, the contact board touches the chord wires cutting off the lifting process. In Unomatic, Unimat, Duonatic and 09-CSM tamping machine, instead of contact board, height transducers have been provided which cut-off lifting process as soon as required level of lifting is achieved.
On front tower, a pendulum is incorporated between the two chord wires through a PCB. If a general lift is given to datum rail chord wire, the other chord wire will automatically get lifted or lowered via the pendulum and PCB as to maintain correct cross-level.
On UT machines, the wires move physically up or down as per generated signal and total lift required at right or left rail at tamping zone is automatically achieved. But in case of other machines, the signal goes to microprocessor and wire does not move physically up or down. The microprocessor gives left/right rail required lift at tamping zone. The amount of lift is same as the wire would have moved up or down physically.
Differend Machines have different c/a ratio for lifting as under:
Ratio(c/a) Machine Type 4.3 UT 3.0 UNO,DUO,CSM,MPT 3.2 UNIMAT \\
In case of humps of small length, when point 'M' comes over hump, the top location of 'M' may go above the chord 'RF'. In such condition, no correction will take place at such humps or on approach of such humps and machine will leave this patch uncorrected in longitudinal level. Hence, there is a need that the front tower should be raised to such an extent that the chord 'RF' is always above 'M'. This will ensure correction at all the locations including small humps. Such raising of front tower is called general lift, which should be selected by PWI considering the longitudinal levels likely to be encountered during the days work. The general lift so decided is to be ensured throughout the patch to be tamped except the 'start' and 'end' position. At the start of work a ramp up at the rate of 1 in 1000 (1 mm in 1 meter) is provided in the general lift. Similar ramp down is provided at the end of work. Care is required to be taken that the start of work (ramp up) should be done at the point from where ramp down was started for the last block. This will ensure that full track is lifted to a proper level and no location of ramp down or ramp up left after completion of full stretch.
At all the three locations, (i.e. 'F', 'R' and 'M'), pendulums are provided which measure cross level difference at their locations. While starting work datum rail is required to be selected, this controls lifting of other rail. On a straight track, rail which is relatively higher is selected as datum rail. On double line, normally non-cess rail is higher but on single line the higher rail is required to be decided by PWI. On curves, always lower rail (i.e. inner rail) is selected as datum rail.
In case of long sags (length of sag is more than length of machine) front tower of machine will be on sag while the rear tower is still on level track. Because of which the chord stretched between 'F' and 'R' will not be horizontal. Hence at the location of lifting unit the lifted track will not go to the true horizontal. There will be a residual error at 'M' which will be equal to FD/Lifting ratio. Lifting ratio of a machine is equal to 'FR'/'RM'. For most of the machines, lifting ratio ranges from 3 to 3.2. Hence, 1/3rd of the longitudinal defect at 'F' will still remain at 'M' even after packing. Although, even with this residual defect, track still shows reasonably good running up-to 100 to 120 Kmph but for even better riding design lifting should be done.
In order to achieve perfect level the residual defect at 'M' should be zero which can be achieved by making FD = 0. This means that if front tower is raised by an amount equal to FD, track at 'M' will be brought to true level.
So we need to calculate FD value. This is done by taking levels of a track section by using leveling instrument. Levels should always be taken for the rail which is going to be used as datum rail for tamping. The existing vertical rail and formation profile should be plotted on a graph sheet with the length of track as abscissa and elevation of rail top and formation as ordinate. The scale adopted should be: Horizontal Scale- 1 :1000 i.e. 1 cm = 10 m and Vertical Scale- 1 :10 i.e. 1 mm = 10 mm. The redesigned profile should aim at easing the sags and humps with manageable lift and lowering. It is not necessarily the intention that the original longitudinal section of the line should be restored. Generally, the redesigned profile should be so arrived at as to have lifting only, as machines have lifting facility only, and lowering shall be resorted to in exceptional circumstance. After plotting existing track levels, proposed track levels should also be plotted on the same scale. & lift amount can be read from graph.
Final longitudinal level are decided keeping in view gradients, vertical curves and obligatory points such as FOB, ROB, Washable apron, Platform track, etc. The required lifts are finally recorded on every alternate sleeper. Leveling is carried out by entering the lift values at the front tower. By adopting design mode of leveling and lining desired track geometry can be achieved.
While working in transition curve, one part of the machine may be situated in the transition portion and the other part on the straight or fully canted track. This may lead to lifting of outer rail on straight track un-necessarily. To compensate for this, correction is applied at the front tower.
The amount of correction value 'K' = 36*SE/R. This K value is deducted from the amount of lift to be given in the front tower.
Wherever a vertical curve is provided, some correction is also required in the front tower while front tower enters from graded track to vertical curve. The value of correction is X = 83586/R. Value of Constant 83586 will change from machine to machine. The correction is applied positive when we enter from upgrade to vertical curve (i.e. ridge portion), it is negative when we enter curve from downgrade (i.e. valley portion); the pattern of increase and decrease can be obtained from machine catalogue.
While tamping by machines on PSC sleeper track, if the lift is limited to 30mm single insertion is sufficient. If Lift is between 30 to 50mm, double insertion is resorted to. In case double insertion is not given for lift more than 30mm, retentivity of tamping is badly affected. In case of lift requirement more than 50mm, in first round, lift is limited to 50mm with double insertion and rest of the lift is executed in separate round of packing after consolidation period is over. On metal sleeper, double insertion is required irrespective of amount of lift. For other than PSC sleepers lift is limited to 25 mm.
—-Working of Design Lift—-
The reduced level of track ie, the longitudinal level of rail top shall be taken at regular interval of 10m.The levels can be reduced in Excel sheet station wise like KM.444.40, 444.41, 444.42, 444.43, 444.44, 444.45, 444.46, 444.47, 444.48, 444.49, 444.50, 444.51, 444.52, 444.53 and reduced level 100.00, 100.10, 100.16, 100.25, 100.3, 100.415, 100.575, 100.68, 100.775, 100.88, 101.00, 101.14, 101.275, 101.395 The reduced levels can be changed to versine by V2=L2-(L1+L3)/2.ie,versine at 444.41=100.10-(100.00+100.16)/2 =0.02m and multiplying by *1000 it becomes 20mm.This cell shall be selected and dragged down to convert all. Realignment of this versines are done by any method,and the maximum value of negative slew is added to all slews Now all slews have become positive and this gives the value of Design lift.
Alternately,with the Initial Rail longitudinal level,reduced with respect to stations with equal intervals in Excel sheet,shall be plotted in chart and Proposed/Final longitudinal level with value of lift shall be obtained as under; Select all cells,including corresponding station number,Right click and in Insert menu select`Chart' and choose Custom and then click Smooth line.(In XP,select Scatter with smooth line option)You will get a Chart showing stations in X axis,and RL at Y axis..You have to edit the range of RL by clicking on the Y axis. For Proposed RL,click on the Initial level graph,then Right click ,and select `Add Trendline'.Choose Polynomial option and select order of 6,and Display Equation on chart.You will get a proposed Railevel.This shall be directly used for New Track,or new road work. But to get the Design lift,Follow these steps. Select the formula,right click,in the Format Trend line,select Number and choose Decimal places 28.You will get the exact Equation of the Proposed level.Substitute values of x ie,1,2,3,…. and get the final level.Final-Initial will give the +ve & -ve values. ADD maximum -ve value,ALL values,to get only Lift for Tamping.This will give the Best smoothness with little Design Lift.
Another method to work out the same is to do the reverse: Do not take levels but take the versines on top of the rail using the normal wooden blocks and 20 m chord and then use the realignment solution as explained above. http://iricen.gov.in/IRICEN1/documents/Shared/G2098848/VERT.VERS.bmp This solution might work in some situations but not for stretches where the track surfacing is bad or there are obligatory points.
For bad stretches, use of track surfacing programs such as TS.exe and Track surfacing available in IRICEN website for downloads may be used.
It becomes more complicated,while tamping the turnout in symmetrical split. 1 in 8.5 turn out in symmetrical split is as good as it is laid laid on 4D curve.In Smoothening mode also,the correction factor got to be applied.In Design mode it is difficult to feed the high values of Versines HA,HB within a short distance of 25m(ie.ATS TO ANC). SO THE UNIMAT OPERATOR PREFERS TO DISPENSE THE LINING. Only option left is to slew in the manual mode without lining.For this ANC-ANC distance shall be measured,and corresponding track centre D shall be calculated from layout software.The existing track centre at ATS D' shall be corrected as per sketch in the following link. http://iricen.gov.in/IRICEN1/documents/Shared/G2098848/symmetrical.bmp