By Peter Targett

During the Locomotion weekend at Palmerston North the inclines and slippery track were proving to be a problem for 1466 Dart and it was clear that I had to do something about the traction. The springs on the loco had always been a bit of a stop gap to get it running and were just some Century springs I’d got off the rack at Mitre 10 and chopped in half. There had always been an intention to investigate a weighbridge since Keith Wright’s article in ME on the unit installed at the Colchester Club track in the UK. Now was the time to do something about it.

Leading wheelset of Dart in position for weighing.

The Colchester weighbridge used bathroom scales as means of measuring axle load. However, I had just completed my Dynamometer in a box ‘DynoBox’ using two sensors from electronic luggage scales. The DynoBox had moved on from its original design now with real time clock and data logging to a 4Gb SD card so I had an early version of the circuit board available. Time and data logging isn’t required in a weighbridge.


Using two sensors, one on each rail, allows the weight on each wheel on the axle to be balanced as well as setting up the weight distribution between axles.

The sensors from the luggage scales are strain gauges, devices used to measure strain on an object. The most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to a beam by a suitable adhesive. As the beam is deformed, the foil is deformed, causing its electrical resistance to change.

This resistance change, is usually measured using a Wheatstone bridge. A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of a Wheatstone bridge is its ability to provide extremely accurate measurements (in contrast with something like a simple voltage divider) Its operation is similar to the original potentiometer.

“The Wheatstone bridge was invented by Samuel Hunter Christie in 1833 and was improved and popularized by Sir Charles Wheatstone in 1843” (Wikipedia). The full range output from the Wheatstone Bridge is very low, in the order of 10mv, so an accurate amplifier is required to get the voltage level from the sensors to a level suitable as input to the analogue to digital convertor in the micro controller. The gain of these instrumentation or operational Amplifiers are set by the value of the gain resistor, RG in the diagram, and it is selected to amplify the 10mv signal up to around 4v at full scale. As each sensor is capable of measuring a load of 50Kg each the weighbridge is capable of measuring the axle weight up to 100Kg per axle.

The micro-controller takes regular readings from both sensors converts the raw value into the weight in Kg, or Lb, and sends the result to the Bluetooth module for display. In addition, there are controls for setting the zero value and the ability to recalibrate each sensor with a known weight. I have some 9Kg cast iron ballast weights so have set up the software to be able to calibrate at 9Kg or 18Kg.

The micro controller

Like Dynobox the display is to an Android mobile phone using the free application BlueTooth Electronics available in the Android Play Store.

Above - Circuit diagram for the Wheatstone bridge etc. And below the  information output on the android mobile phone.


The mechanical components are fairly basic two rails to house the sensor beams and the connections and spacer bars to set the gauge. Currently set at 7.25”  but a simple job to make four more spacers for either 5” or 3.5” gauge. The designed intention is for the rail unit to fit onto the end of the steaming bay rails with an extension on the other end

suitably supported at the other end to take the weight of the loco.

Using the Weighbridge

Once I’d got the weighbridge working and had ensured that the track was level in all directions, it was time to weigh the loco at each end and find the centre of gravity, then weigh each axle in turn. With these initial  results it remained to work out how to  interpret the results and set up the axle loading. I’ll admit I hadn’t got that far when I found the article by Artisan in ME 4470 (Nov Dec 2013) “Locomotive Springing, Adhesion and Pulling Power”.

Using the method described in the article the optimum weight distribution is relatively easily determined. My initial data shows that my three axles were pretty evenly balanced, much like the prototype design, furthermore the Mitre10 springs were just about right for the main driver, but not the coupled axle. For models its recommended to get as much of the load onto the drivers and coupled wheels and as much as possible off the non-driven wheels, without introducing ‘porpoising’. To do this requires a bit of empirical fiddling and the availability of a range of springs, which I now have on order.

Spreader bar being used to weigh the front of the loco Dart.

This article first appeared in Blast Pipe, the magazine of the Hutt Valley MES, New Zealand.