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TRACK TOLERANCES

NEETHU NARAYANAN

B060128CE

MURSHID

B060471CE

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Introduction

The safety and comfort of rail travel depend primarily on the track geometry and the standard to which it has been maintained.

Parameters governing the track geometry have always certain variations due to various reasons.

Track tolerance:

- may be defined as the limit of variability of various parameters pertaining to track geometry.

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TRACK PARAMETERS

The tolerances are generally laid down for following track parameters,

1) Gauge variation:this is measured as deviation from nominal gauge which is 1676 mm for

B.G. and 1000 mm for M.G. Gauge is measured as the minimum distance between running faces of two rails.

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2) Unevenness:this is measured in terms of difference in the longitudinal

levels over a fixed base. Unevenness is generally measured over a base of 3.5 meters of length and for left and right rails separately.

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3) Cross level difference:this is measured in terms of relative difference in the level of two rail tops

measured at the same point. Cross level difference also includes the variations in super elevation.

If a1 and a2 are the absolute height of the 2 rails at the same point, then

Cross level difference = a1- a2

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4) Twist:this is measured in terms of change in cross levels per unit

length of measurement. Twist is calculated by knowing the cross level difference between two points over a fixed base and dividing the cross level difference by the base length. This is normally denoted as mm/meter.

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5) Alignment variation:

this is measured in terms of variation in versines taken on successive chords of fixed length with overlaps of half chords. This is measured over a base of 7.5 meter length on left and right rail separately.

*** versine = versed sine Versin (θ) = 1 – cos (θ)

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TOLERANCES UNDER LOADED OR FLOATING CONDITIONThe various parameters of track geometry can be measured under loaded or under floating( non load) condition.

Normally there is not likely to be much difference in the measurement of gauge and alignment under loaded or floating condition.

cross level, unevenness, and twist differ considerably when track is under loaded condition from the floating condition so the measurements of these parameters are taken under loaded condition.

VALUE UNDER VALUE UNDER PARAMATERES FLOATING CONDITION LOADED CONDITION

Cross levels 6 mm 10 mm

Longitudinal levels 6 mm 12 mm

Gauge G mm (G+1-1.5)mm

Alignment A mm (A+1-1.5)mm

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TYPE OF TRACK TOLERANCES-- different type of track tolerances are stipulated depending upon the purpose for

which these are made.

1) SAFETY TOLERANCES:

These are the limits of variations at which dangerous or unsafe conditions arise for the movement of the trains.

Depends upon condition of track + type of vehicle, its riding characteristics and its maintenance standard.

BUT……….The factors governing the unsafe conditions are so variable and indeterminate that almost none of the railway systems in the world has laid down the safety tolerances.

It is felt that the track has to deteriorate too much for reaching these unsafe limits.

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2) SLOW DOWN TOLERANCES

These are the tolerances which when reached call for slowing down of trains.

It was considered that it is not necessary to fix these slow down tolerances for Indian railways in view of the past experience.

but now slow down tolerances are fixed

Investigations done by RDSO reveal that 4- wheeled wagon is the most critical rolling stock in empty condition.

Depending upon the values of unevenness and twist the speed of 4 wheeler wagons should be reduced as per the table…….

Speed( Kmph) Peak value of unevenness on 3.6 m (mm)

Peak value of twist on 3.6 m (mm)

75 14 13

60 16 15

45 22 22

30 24 25

15 33 30

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3) SERVICE TOLERANCES OR GOOD RIDING TOLERANCESthese are the tolerances to which the track parameters may vary any time during service.

These tolerances are based on the acceptable limits of riding comfort in relation to the vehicles in use.

SERVICE TOLERANCES FOR 120 KMPH ON B.G.

1. TRACK GAUGE: tolerance generally in isolated locations

a) On straight track +6 mm ------

- 3 mm ------

b) On curves up to 4o + 13 mm ------

- 3 mm ------

c) On curves > 4o for introduction + 19 mm ------

of high speeds.

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SERVICE TOLERANCES Contd….

2. Unevenness measured under loaded condition on a 3.5 m base.

tolerance generally in isolated locations

a) On long term basis 6 mm 10 mm

b) As an immediate measure 10 mm 15 mm

3. Twist measured under loaded condition on a 3.5 m base

tolerance generally in isolated locations

a) On straight and on curved 2 mm/m 3.5 mm/m

b) track other than on transitions. 1 mm/m 2.1 mm/m

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SERVICE TOLERANCES Contd….

4. Alignment measured as versine on 7.5 m chord

tolerance generally in isolated locations

a) On straight 5 mm 10 mm

b) On curves 5 mm 7 mm

5. Cross level not specified on consideration that small but constant cross level difference do not matter much.

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4) MAINTANANCE TOLERANCEThese are the tolerances which are achieved immediately after the track

is attended to.

these tolerances depend upon the track structure, tools and equipment used, method of maintenance adopted, supervision available and such other factors.

These tolerances are prescribed arbitrarily so that the results of maintenance can be assessed.

The Tolerances laid down by Northern Railway

Unevenness + or -2 mm over a 3.5 m base

Alignment + 2 mm over a 7.5 m base

Cross level + 2 mm

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5) INDEX TOLERANCESthese are the tolerances which when exceeded, would call for attention of maintenance units to undertake maintenance work.

These are also called maintenance schedule tolerances.

Index tolerances are somewhat lesser than the service tolerances.

Eastern railway proposed that index tolerances should be kept at 60-70 % of service tolerances.

Parameter index tolerance

a) Cross level 3 mm

b) Alignment on 7.5 m base 3 mm

c) Sleeper to sleeper variation in gauge 2 mm

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TOLERANCES FOR TRACK LAID WITH NEW TRACK MATERIALS

SL NO parameter location value

1 Gauge StraightBG curves<4o, MG<6oNG<14o

+-2mm-3 mm,1 mm

2 Cross level straight +- 3 mm

3 Packing -------- 20% sleepers loose

4 Alignment Straight(10 m chord)Curves of radius 600 m & 20 m chordCurves of radius 600 m & 20 m chord

+-2 mm+-5mm

+-10 mm

5 Sleeper spacing ------- +-20 mm

6 Joints (not square) --------On straightsOn curves

+- 10 mm+- 10 mm

.5*pitch of fish bolt holes

7 Low joints ---------- Not permitted

8 High joints ---------- 2 mm

9 Expansion gaps ---------- +-2 mm

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6) NEW TRACK TOLERANCES

These are tolerances prescribed for laying the new track

These are meant for testing the quality of the work done

Northern Railway has laid down desirable standards on an experimental basis for track laid with new track

Parameter length of chord standard deviation1. Unevenness 3.6 m 1.20

2. Unevenness 9.6 m 2.50

3. Twist 3.6 m 1.75

4. Alignment 7.2 m 1.50

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RIDE INDEXRide index is a value which will represent the riding comfort experienced by a passenger in a coach

R.I. is calculated as,

R.I.= 0.896 x (b3 x (f)/f)^0.1

R.I.= ride index

b = acceleration in cm/s^2

f = frequency in cycles/s

(f)= correction factor

R.I. 1 2 3 4 5

Appreciation very good good satisfactory tolerable dangerous

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Reference:

* Indian Railway Track – M.M. Agarwal

6th Edition

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