April 1, 2009
Proper maintenance of truck scales ensures accurate tracking of material sales.
by Bill Murphy
Truck scales are a vital tool for aggregate operations, and the performance of the scale can have an enormous impact on profitability. The truck scale is the cash register through which materials are bought and sold and where profit and loss is measured.
A challenge facing many truck scale owners today is determining how long they should continue spending money on repairing an aging truck scale. When is it time to replace it? Today, the average cost of a truck scale service call can run anywhere from a few hundred dollars to several thousand dollars depending on the age, condition, type of problem, and design of the equipment being used. Chances are if maintenance costs are already significant, they will increase in the future.
Choosing the right truck scale for the application is the best long-term plan. Heavily concentrated and high-traffic volumes make rock and gravel applications some of the most demanding in the weighing industry. With that in mind, a well-designed truck scale and proper maintenance plan are the two most important elements in ensuring the sustained profitability of your aggregate facility.
Choosing the right truck scale
Truck scales are traditionally long-term investments that should provide decades of reliable and dependable service with minimum maintenance. Saving a few hundred dollars on the front end may cost thousands in unexpected maintenance costs later. In recent years, many truck scale owners have experienced higher maintenance costs and in some cases, even structural failures after just seven to nine years of reasonable usage.
Generally, the life expectancy of a truck scale is dependent on three primary factors, including the following:
Most truck scale products on the market have been designed for legal highway loading, which in most states means a gross vehicle weight in the range of 80,000 pounds. For a typical 18 wheeler, that means 12,000 pounds on the steering axle, 34,000 pounds on the drive axle, and 34,000 pounds on the trailer axle. The higher the axle load, the higher the stress concentrated on the weighbridge and load cell mounts.
It is important to ensure that trucks such as tri- or quad-axles are weighed with the tag or pup axles in the down position to ensure the load is spread over a wider area of the scale platform. Signs should be posted at the entrance of the scale to be sure drivers adhere to these regulations.
The design and type of steel used in the weighbridge is the most important factor in determining the eventual lifespan of the scale. The main support structure should be composed of structural I-beams, not channel, tubing, or bent-steel shapes. Because the steel weighbridge accounts for nearly 70 percent of a scale’s cost, some designers have taken shortcuts by using lighter and fewer steel components. These products are less expensive and may offer a lower capital price, but the result may be a shortened scale life and increased maintenance expenses.
When purchasing a new scale make it a point to understand how the weighbridge is designed. The quantity, type, size, and spacing of the I-beams are a good indication of a scale’s ability to handle demanding truck traffic.
Preventive maintenance priorities
Common sense maintenance initiatives for the following truck scale components can help avoid unexpected breakdowns and prolong equipment life.
1. Calibration. Every truck scale should be calibrated and tested by a state licensed servicing agency using no less than 25,000 pounds of certified test weight. The scale’s calibration interval is determined by its frequency of use. At a minimum, calibration should be conducted once every six months. For more heavily used scales, monthly or quarterly checks may be necessary. These calibrations will improve accuracy and expose potential problems before they become serious.
2. Weighbridge. Weighbridge maintenance is essential to prolonging the life of your equipment and keeping it in top operating condition. Removing the scale from the foundation, sandblasting all of its steel surfaces, and repainting it with a high solids urethane primer and top coat is something that should be done every eight to 10 years depending on environmental factors. Coating the underside of the weighbridge with an asphalt emulsion coating will also substantially reduce rust and corrosion of the weighbridge on non-visible surfaces.
3. Foundation. A scale’s performance is only as good as the integrity of its foundation. Keeping this foundation clean and free of mud, water, and debris will improve the performance of your scale. Material buildup around load cell stands, exposed cables, and wet junction boxes should be avoided. A quick visual check of sump pumps and drains should be done on a monthly basis. The concrete’s condition and the approach or pit coping should also be inspected regularly. Any cracks in the concrete or heaving due to frost can have an adverse effect on scale performance. Head walls and pit walls should be checked for alignment and structural defects. Approaches to and away from the scale should be level and ensure a smooth transition of truck traffic on and off the scale platform.
4. Load cells. Load cells are the heart of every electronic scale. They should be inspected for frayed cables, cracked or loose connectors, loose bolts, improper alignment, and potential buildup of mud and debris around the load cell. Canister type load cells should also be checked for rust or holes. Even stainless-steel load cells can rust. To reduce damage from lightning or surge voltage, a transient bypass cable should be installed at the load cell. This can dramatically reduce the effects of ground surges.
5. Junction box. Internal condensation is one of the most common problems associated with junction boxes. Moisture can occur not only from heavy rain and snow, but also from changes in barometric pressure and temperature. A stainless-steel junction box promotes internal condensation and is often more vulnerable to this type of problem. Newly developed junction boxes are made from fiberglass reinforced polyester and can reduce or eliminate the effects of internal condensation. These boxes also contain superior gasket design to help equalize pressure within the box.
6. Bumper bolts. Although there are truck scale designs that eliminate the need for bumper bolts, many older designs still require them. Frequent inspection is required on these models.
Bumper bolts minimize the momentum of the weighbridge as vehicles move on and off the scale. They are normally positioned at the ends of the weighbridge and are designed to bump the plates embedded in the end wall during scale movement. Bumper bolts need to be fastened in place and maintain clearance whether the scale is empty or loaded. Bolts should be adjusted with approximately 1/4-inch to 3/8-inch clearance from the end wall plates.
Seasonal weather changes can cause thermal expansion and contraction of steel and concrete in the bridge, resulting in potential binds. Bumper bolts that are too tight against the stops can result in errors that may go unnoticed for extended periods. Bumper bolts that are too loose can result in potentially damaging side loads to the load cells, or even, in some cases, a sudden collapse of the bridge structure. A bridge that does not swing freely or is noisy as vehicles enter and exit may require immediate attention from your scale service provider.
7. Grounding. To avoid the existence of multiple zero references that may create havoc with data lines and invite lightning damage, a single-point grounding system is recommended (see Grounding 101). Over time, this has proven to be a dependable means of lightning and transient protection for electronic scale systems.
Once the single-point grounding system is established, check the AC power supplied to other peripherals such as remote displays, printers, and computers. A remote device may not have the same AC power source as the digital instrument; therefore, each device may not be grounded to the same point. Again transient protection devices should be grounded to the same wire as the peripherals they are protecting. Measure the resistance between the AC-power ground points. The volt meter should read less than 1 ohm.
A copper transient bypass cable should be connected between modules and across each load cell mount to ensure transients pass through it instead of the load cells.
With proper maintenance and grounding, aggregate producers can extend the life of their truck scales and keep their operations’ “cash register” ka-ching-ing.
Checking a single-point ground involves two steps. First, verify the ground system of the AC power supply. Using a digital volt meter, check the resistance of the AC outlet ground to the actual ground rod of the AC power coming into the building. It should read less than 1 ohm. Then, measure the AC voltage across the ground and neutral of the AC outlet. The result should be 0 volts AC, not to exceed 0.5 volts AC. If you discover grounding issues with the AC power supply, notify the building owner or the power company.
Next, check the scale weighbridge grounding. Before this is done, make sure all peripherals are plugged into some type of transient protection device such as an uninterruptible power supply. Electronic scales are easily disturbed by any number of voltage distortions, so installation of power conditioning equipment is the best first line of defense.
Run a bare 10-gauge copper wire from the scale frame to the grounding lug on the j-box and then, lastly, to the ground rod from the power company. This wire can be buried in the soil from the scale to the AC ground. If a scale also uses an uninterruptible power supply, that device needs to be grounded to this wire as well.
Finally, measure the resistance between the scale sections and the AC ground. The reading should be less than 1 ohm. Higher ohm readings are typically caused by corroded connections.
Bill Murphy is the heavy capacity sales director for Rice Lake Weighing Systems and has been with the company since 2003. He has more than 30 years of experience in the weighing industry with the majority of that time revolving around truck and railroad scales.