Weighbridge Protected Impact System
A lorry going onto a weighbridge has a similar effect to a battering ram. As the lorry charges onto the deck of the weighbridge and brakes, the combined mass of the lorry and weighbridge hits the end wall of the foundations with tremendous kinetic energy. This energy is multiplied considerably the further the weighbridge moves, even by only a few millimetres. Like a battering ram, repeated impacts will result in the foundations being damaged and they will eventually disintegrate. While most of the damage will be to the end foundation, especially with pit-mounted weighbridges a lorry may drive on at an angle and so move the weighbridge against the side foundations, with resultant damage.
Another important consideration is the effect on the speed and accuracy of the weighing mechanism, which is achieved through the load cells on which the weighbridge sits. A load cell will not measure accurately unless it is absolutely vertical. Any lateral movement will distort its measurement as well as taking longer to settle down, not to mention possible damage to the load cell.
It is critical that adequate protection is built into the weighbridge to prevent this happening. By its nature a weighbridge has to be "free floating" on its load cells in order to work, so any protection must not interfere with that. It also has to have adequate clearance between it and the surrounds to allow for deck expansion at higher air temperatures.
In the 1980s Shering patented the IDE Restraint Post system. It is essentially a solid stainless steel bar which is anchored in special high strength grout into the foundations. It engages with the weighbridge via a special Restraint Bush, which has a lateral clearance of only 1mm, large enough to ensure the weighbridge still "floats" unrestricted, but only allowing 1mm movement in any direction.
It is placed in the middle of the weighbridge where the effects of metal expansion are negligible.
Two additional IDE Restraint Posts are fitted at either end of the weighbridge to restrict slewing forces from lorries driving onto the weighbridge at an angle. One critical difference in these Posts is that their Restraint Bush has an oval rather than a round hole to allow for metal expansion lengthwise in the weighbridge, while still restricting slewing movement to 1mm in any direction.
The use of these IDE Restraint Posts solves the problems of potential damage, inaccurate measurement and, because the movement is so small, the bridge settles very quickly and so increases the vehicle throughput.