Rail Weigh-bridge

Schedule VII/ Part II/ Heading C of the Legal Metrology (General) Rules, 2011


A Rail Weigh-bridge is a weighing instrument used for weighing loaded rail wagons. It has a load receptor, inclusive of rails for conveying railway vehicles.
It is designed to record the weight of all wagons individually and collectively excluding the weights of engine and supportive wagons.
The weighing may be done in motion or in static position.

Working Principle

LC = Load Cell
TS = Track Switch
ADC = Analog Digital Converter
When a train comprising of several wagons passes over the load receptor, the load cells sends the load value to the Control Unit through the ADC.
At the same time, the track switches sends signals as to the type of vehicle passing over it. Accordingly, the control unit determines which values are to be accepted and which are not.
Gross weight of the appropriate wagons are displayed both on the indicator of the Control (Central Processing) Unit and on the Video Display Unit (Monitor). The key board is used to manually enter wagon number etc. The printer gives out the weighing sheet.

working principle

Major Components

  • Load Receptor (Load Cells)
  • Aprons
  • Vehicle Type Indicators (Track Switch)
  • Indicating Devices
  • Printer
  • Control Unit

Additional Devices:

  • Position sensors: Two sensors are used - one senses the arrival on and another the departure from the weighing platform of a train.
  • Additional sensors may be used to sense the gap between wagons.
  • Other sensors are used to record the total number of wagons passing through it, the wagon no and declared weight of individual wagons.

Wield-in or bolt-on Load Cells

Two or more load receptors, placed in series (on both the tracks), are used as a single load receptor for full draught weighing.

Apron

Apron is the part of the weigh zone that is not the load receptor but which is located on either end of the load receptor.

Track switch

Track switches are placed along the weigh rails to determine which weight is not to be recorded and which weight should be recorded.
They also determine the speed below or above which there will be no recording.

Control Unit (CPU) & Indicating Devices

  • There may be two indicating devices- an indicator on the Control Unit itself and on the display unit (Monitor).
  • Both the units should have same value of "d".
  • The reading on both the units should be same.
  • Check the reading with the Gross Weight shown on the printed receipt.

Typical Recording Instructions sent by Track Switches

Weight Type
Recording
Engine
No
Wagon
Yes
Guard Van
No
Speed above Max
No
Speed below min
No

How the recording is shown in print-outs

Serial No Wagon No Gross Weight t. Wagon weight t. Net Weight t.
1 Locomotive Engine      
2 SE 125749 84.70 22.50 62.30
3 SE 104912 83.80 22.50 61.30
4 CR 111102 84.90 22.50 62.40

Placing the Wheels

Wheels are to be placed on the Track Switches. However, in in-motion bridges, this is not a factor.

Reference (Test) Wagon

Wagons of known verified weight are used as Reference Wagons for testing in-motion weigh-bridges. When reference wagons are not available, full loaded wagons weighted on a static machine may be used as Test Wagons.

For Test Wagons, the MPE allowed will be 1/3rd of the value prescribed for Static Weighing of commercial wagons.



Weighing Types

  • Draught (in-motion)
  • Draught weighing may be full or partial
  • static



Full Draught Weighing

In this system, the mass of a vehicle that is entirely supported on the load receptor(s) is determined.

  • The wagon must be placed on the weigh zone consisting of a Load Receptor to receive the load.
  • The Receptor realizes a change in the balance of the instrument when a load is placed on it.

Partial weighing

In this system, Weighing of a vehicle is done in two or more parts successively on the same load receptor. The results are automatically added to indicate or print the vehicle weight.

  • There is only one set of load receptors.
  • In partial weighing weight on first pair of wheels is measured and then on the second pair.
  • The weights are added to give the result

Weighing Efficiency

Track requirement of the Rule

The LM (General) Rules, 2011 requires

  • A minimum of 500 metres of rail track length
  • The gradient of not more than 1:400 on either side of the automatic rail weigh-bridge.

What is practical

  • 100 m straight track either side.
  • The track should be level tangent with gradient not exceeding "1 : 300"
  • Points or crossings up to 100 m of both the ends should be avoided.
  • Concrete sleepers up to 100 m & RCC base up to 20 m on either side.

Random errors of the weighing system

  • Error caused by improper alignment of axles.
  • Error caused by the influence of proceeding and succeeding wagons transmitted through the coupling system.
  • Dynamic error caused by the velocity of a train.

These errors may be taken care of by suitable structural designing of the weigh-bridge platform and through the proper alignment of the load cell circuitry. Necessary correction factors may also be introduced in the CPU configuration to eliminate the result of the errors.

Verification of Rail Weigh-bridge

Descriptive Markings

The Descriptive Plate should bear the following information:

  • Weighing Method
  • Max and Min Wagon Weight
  • Scale Interval "d" in kg or t.
  • Accuracy Class (0.2 / 0.5 / 2 /5)
  • Max and Min Operating Speed - (V max km/h and V min km/h
  • Max and Min number of Wagons per train

Testing In-motion Rail weigh-bridges

The Wagon no and weight is manually fed to the computer. The Gross weight is obtained from the control unit and the computer generates serial no and net weight.

Testing

  • Arrange appropriate number of test wagons as per Table IV and distribute uniformly throughout the train.
  • They should be weighed repeatedly and in each direction to yield at least 60 wagon weights.
  • Test will be done at various speeds from min to max operating speed.

 
Total No of Wagons in a Train Minimum No of Test wagons
Less than 10
All
Between 10 and 30
10
Above 30
15

Distribution of Test Wagons

Legal Metrology, Australia suggests that the test wagons are placed in the following sequences:

Table I: MPE Table (for in-motion weighing)

Accuracy Class During Verification During Inspection
0.2 ± 0.10% ± 0.2%
0.5 ± 0.25% ± 0.5%
1 ± 0.50% ± 0.1.0%
2 ± 1.00% ± 2.00%

Error Calculation

1. The value calculated as per table, rounded to the nearest scale interval.
2. The value calculated according to the table, rounded to the nearest scale interval for the weight of a single wagon equal to 35% of the maximum wagon weight as shown in the descriptive plate.
3. Value of 1d.
4. Find out the highest value of 1, 2 and 3.

Example:

An Automatic Rail weigh-bridge of class 2 has d = 0.5t with maximum capacity of 100t. It is used to weigh from 20t (empty) to 90t loaded) wagon.
MPE from 20 to 90t will vary continuously from 0.2 to 0.9t (at 1% as per table).
There will be two situations;

Situation A:

Weighing empty wagon of around 20t.
(a) 1% of 20 t is 0.2 t rounded to 0.5 t.
(b) 35% of 100 t is 0.35 t rounded to 0.5 t.
(c) d is 0.5 t
MPE at this point is 0.5 t (highest value)

Situation B:

Weighing loaded wagon of around 90t
(a) 1% of 90 t is 0.9 t rounded to 1 t.
(b) 0.5 t as above.
(c) 0.5 t as above
MPE at this point is 1 t (highest value)

Static Weighing

Table II: MPE Table for Static weighing

Load MPE
Between 0 and 500d ± 0.5d
Between 500d and 2000d ± 1.0d
Between 2000d and 10000d ± 1.5d

Table III: Checking relationship between Class, Value and ratio of Maximum wagon weight and "d"

(Applicable to both in-motion and static weighing testing)

Accuracy Class d in kg Maximum wagon weight / d
    maximum minimum
0.2 ≤ 50 1000 5000
0.5 ≤ 100 500 2500
1 ≤ 200 250 1250
2 ≤ 500 100 600

Testing empty wagon weight

Actual weight of empty wagons need to be checked with the declared weight as the recorded weights are supplied to the CPU either manually or read through a sensor from the declaration given on the body of a wagon. This serves as a tare weight to be subtracted from the net weight of a loaded wagon to derive the weight of the consignment.
This is to be done by static weighing

Sealing and Stamping


1. The Control Unit is sealed.
2. The descriptive marking plate is stamped.