There are two types of weighbridges (often called truck scales in America ):-

Road weighbridges for truck weighing and other road vehicle weighing, and
Rail weighbridges for rail wagons and trains

Both road and rail weighbridges work on the same principle of either an electronic (compression electronic load cells ) or mechanical (lever) systems to measure load.

The mechanical lever system works by a series of "breakdown" levers that reduce the load on a lever system that "pivots" the load to reduce the load at the other end of the lever. This is the same principle as moving a tree trunk on the ground by using a rock and a pole near the tree to lever it.

Electronics has been introduced to weighbridges for a number of reasons, but chiefly to enable electronic displays and computerization of weighing scales. Electronic scales are easier to install. Electrical "spikes" and lightning strikes can cause havoc with the electronic equipment, so power conditioners and other surge protection devices are used.

Weighbridges have been used for trade since the late 1800’s, and as road vehicles and trains have become larger, the weighbridges have become larger, with weighing available for Semi-trailer, B-Double (25 metres) and even Road Trains (36 metres). Weighbridges are found in Industrial areas, Truck stops and near highways.

Trucks weigh for two main reasons. To weigh products being sold or to check whether they are overloaded.

Why weigh trucks? Overloaded trucks can damage road surfaces and bridges. Trucks are much more difficult to stop when overloaded and accidents are more likely to occur. For safety reasons it is illegal for a truck to be overloaded.

The Motor Transport Department (in Sydney the Roads and Traffic Authority) fine drivers from $300 to $3,000 for an overloaded truck. In some states in America a driver may be jailed immediately if unable to pay an on the spot fine.

Why weigh bulk goods? Goods such as vegetable oil for margarine vary from Tallow being $300 or $400 per tonne, to Palm Olein being over $1000 per tonne. The load on a semi-trailer with 25 tonnes nett is worth more than $20,000. The cost of weighing is small compared to being short delivered on a load being purchased! It is false economy for companies not to weigh product being purchased.

Certain vehicles may also require a weight certificate for registration purposes.

Weighbridges in Australia vary in length from smaller than 8 metres to larger than 30 metre sites. Scale capacity can vary between 30 tonnes to around 300 tonnes. Most road weighbridges are 3 metres wide, but dump truck weighbridges are 5 metres wide and approximately twice the cost to build, due to the massive steel required. Dump truck weighbridges allow the truck to be positioned anywhere on the scale surface as they are not easy to maneuver.

In Australia weighbridges for trade use must be pattern approved by the National Measurement Institute in Lindfield, Sydney and overseas by O.I.M.L., a standards body.

             
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