Materials Handling - Weighing machines and weighbridges
A weigh-bridge is a very large set of balance scales used for weighing large vehicles, including horse drawn carts, lorries and railway wagons. The first practical machines of this type were invented by John Wyatt of Birmingham in the 1740s and the external appearance then changed little until the 1960s. The typical installation for road vehicles consists of a large metal plate set into the road surface with a small hut beside it containing the scale. The works are all underneath the plate and need not concern the modeller however the basic arrangement is shown below, one point to note is that the timber or metal covering the link between the bridge and the hut should always be aligned with the centre of the bridge
Fig___ Weighbridge mechanism
The weighbridge was a feature of most railway goods yards, kit's are available from various sources some of which come with an etched brass bridge. The picture below shows a variation on the basic design, a large building is associated with a weighbridge but only the right hand third of the building houses the weighing mechanism. The remainder is a large office as this example was on a road leading to a small group of factories and a canal wharf and warehouse. Note that the plate covering the linkage between the bridge and the weighing room is located under the window, in the centre of the bridge not the centre of the building.
Fig___ Weighbridge with attached office
Road vehicle weigh bridges are associated with bulk materials handling, minerals such as coal and iron or and goods such as grain. Bulk liquids also require weigh bridge facilities but this depends to some extent on who is operating the depot and where the goods are going to and from.
Since the 1960s other types have come into use, one easily modelled modern type looks like a low bridge, built entirely of concrete. The sketch below is based on an example at the former rail-served Blue Circle cement depot at Northenden, south of Manchester. The photos were taken in the late 1980's from a passing train and show no office immediately beside the bridge, presumably there was some form of remote reading facility in a nearby office. The bridge was located on the approach road leading to the terminal and there was room for a lorry to pass clear of the bridge when required. The deck of the bridge was large enough to carry an articulated cement lorry and semi-trailer.
Fig ___ Modern Weigh-bridge
The bridge itself consists of a raised base approached by two ramps with mow concrete walls to either side. The photograph below shows the entire terminal, the bridge is located in the centre between the two diverging sidings. It has two oil stains on it and the sides of the base, where the lorry runs, are a slightly different colour.
Fig ___ Cement terminal and weigh-bridge
The railway wagon weigh-bridge uses an essentially similar mechanism with a platform and a small hut close by. The platform was usually covered with planking and can be easily modelled as shown below. The associated hut can be a normal brick or concrete type but in some locations this is set into the ground. This latter design is associated with hopper wagon loading points, it allows the weigh-bridge staff to check that hopper doors are closed as the loaded wagons are drawn over the bridge. Railway wagons had their tare (empty) weight marked on the side so it was only necessary to weigh the loaded vehicles, in the less regulated world of road haulage it was normal practice to weigh the wagon or cart as it went in and again when it came out to confirm the markings were correct.
The example sketched below was in a welsh goods yard, it was situated on a siding feeding the end loading dock and running past the heavier of two yard cranes. This meant not all wagons were weighed but those most likely to carry heavy loads would pass over it. I think the coal sidings branched off from this line, so it may have been used for the coal wagons, although I am not sure why the railway would wish to weigh loaded coal wagons arriving in the yard.
Fig___ Rail Weighbridge
On older rail weigh-bridges each wagon had to be positioned on the bridge in turn. Modern rail weigh-bridges, built since the later 1960s and where the traffic is constant, have been engineered to allow the wagons to be pulled over the bridge in a rake.