The typical weighbridge consists of a steel or steel-concrete composite deck structure supported on strain-gauge load-cells. There are two main structural designs – pit mounted and surface mounted. Surface-mount bridges usually have approach and exit ramps of steel or concrete. Pit weighbridges, as the name implies, sit in a preformed concrete pit with drainage.

Depending on the size and capacity of the weighbridge, the number of load-cells is usually six, eight or 10. To ensure optimum load input into the load-cells, they are usually contained within an integral mounting kit arrangement, which is designed to allow a small and controlled amount of lateral self-restoring movement in the weighbridge deck.

Reliable and accurate weighing is dependent on this deck movement and any build-up of debris in the gaps between the deck and the pit foundations or ramps can be problematic.

Such mounting kits also compensate for thermal changes in the weighbridge, which can be significant between winter and summer. Access to the load-cells is usually via hatches in the deck, although some drive-through designs have the load-cells out-board of the deck.

The key factors which can lead to problems are:
• Overloading and unnecessarily heavy braking
• Lack of cleaning of approach areas and the weighbridge deck itself
• Blocked drainage/flooding
• Build-up of debris around the load-cell assemblies
• Damage to cables caused by rodents or poor cable management
• Electric welding on the weighbridge deck or ramps.

Overloading and heavy braking
This can be a major problem, resulting in damage to loadcells, mounting hardware, the deck itself and its foundations. It can difficult to prevent this, but making drivers aware of the consequences, together with adequate signage, including speed-restriction signs, can help. A simple traffic light system can also be useful in reducing speed.

Lack of cleaning
This is the most fundamental, yet often most overlooked, factor in good weighbridge housekeeping. Ensuring the area around the weighbridge, together with the approach roads, is free of debris is very important for health and safety and operational reasons.

It is particularly important in winter months and after periods of heavy rain. Pit weighbridges usually have a ‘T’ section rubber seal around the edge of the deck to prevent ingress of debris. It is important to check this remains in situ.

Blocked drainage and flooding
This is probably the biggest cause of problems with weighbridges in quarries. Standing water around the weighbridge will exacerbate problems, especially with load-cells where the sealing may have been compromised through damage, fatigue or corrosion. If flooding does occur, extra checks should be made after the water has drained away.

Build-up of debris around the load-cells
This has a number of consequences which can result in premature load-cell failure. Any built-in lightning protection can easily be compromised and ongoing corrosion can result. Great care should be taken when cleaning around load-cells and no chemicals should be used. It is worth asking for advice on the best way to carry out the cleaning.

Cable problems
Rodents love to chew cables and can easily do thousands of pounds worth of damage to load-cells and electronics. Rodent-proof cable and ducting can prevent this.

Electric welding
The high currents involved with electric welding can easily damage load-cells and electronics.If it is necessary to carry out repairs, professional advice should be sought from the weighbridge supplier or service provider.

Checking the operation of weighbridges
A quick check can be done to regularly test the basic operation of a weighbridge – simply by driving a small van or car of approximate known weight on to the weighbridge deck.

Most weighbridges are calibrated in units of 20kg. In other words, the weighbridge weight indicator display moves up and down in steps of 20kg. The vehicle should be positioned at three different points – at the two ends and in the middle. Ideally, the displayed weight reading at the different positions should be in close agreement.

It should be noted that if critical components, including load-cells and instrumentation, are replaced on a legal-for-trade weighbridge, there is a legal requirement for the weighbridge to be re-verified by an approved company certified to carry out such verification. This will require the weighbridge to be loaded with calibrated weights. Failure to comply can result in fines for the operator and the risk of the weighbridge being taken out of service.

Predictive maintenance – the future
The frustration of not knowing ‘when’ or ‘if’ your weighbridge might fail and for what reason, could become a thing of the past. Remote predictive maintenance using Cloud technology is one of the latest tools becoming available for weighbridge operators. This technology is set to revolutionize weighbridge support by allowing service providers to view individual weighbridge performance, remotely in real time.

With such a system installed, engineers can log in to view the performance of the weighbridge. A key feature of this technology is that the system will immediately flag up any load-cell or instrument malfunction. The system can also show ‘when’ and ‘how often’ the weighbridge is being used, highlighting any spikes in load caused by overloading.

The technology is ideal for single- and multi-site installations. This real-time vision allows rapid response to site issues and ensures that engineers know what the problem is before they arrive. It also ensures the correct spares are available and allows the service provider to co-ordinate the calibration truck to ensure minimum downtime.

Depending on the particular site, it should also be relatively straightforward to upgrade existing weighbridges to take advantage of this technology. It is expected that such systems will also have the capability to allow operators themselves to view basic information relating to their weighbridges.

Summary
When it comes to looking after your weighbridge, basic good housekeeping will pay dividends and should minimize any unwanted breakdowns. Understanding fundamental weighbridge design and operation will help operators carry out effective preventative maintenance. Driver guidance and training, together with effective site signage will also help with long-term reliability.

Finally, new remote predictive maintenance technologies are set to improve ongoing weighbridge diagnostics and ensure any problems can be dealt with quickly and effectively.