A measured approach

As with any piece of equipment, truck scales must be maintained to continue operating at their best. To prolong the life of a scale, it’s critical to adhere to a regular maintenance schedule.

A thorough checkup every six months, at the very minimum, is the general rule. Regarding timed maintenance intervals, owners also should consider their number of weighments, the operating climate and the value of the product being weighed.

Maintain accuracy

Every vehicle scale should be calibrated and tested by a state-licensed servicing agent with no fewer than 25,000 pounds worth of certified test weights.

Keep a solid foundation

Even the toughest scale on earth is vulnerable on a poor foundation. Cracked foundations can lead to movement or settling, which causes chronic calibration errors. Letting little cracks become big cracks could require owners to remove part or all of the foundation and pour a new one for the scale to accurately weigh.

Look at the deck

Rust or crumbling concrete can weaken a scale’s structure and cause accuracy problems. Clean and paint rusted steel decks to keep them smooth and shiny. 

Binding also is a common cause of inaccuracies. A rock as small as a big toe can become wedged between the foundation and deck. Installing T-strip molding between the deck and foundation can prevent debris from getting caught or falling into the scale. 

Some aboveground scales have end clean-out plates that can be removed, allowing technicians to clean out debris that builds up on the foundation’s surface closest to the end of the scale.

Check devices

Checking devices on some scales must be inspected to ensure they are working properly. This can be done by inspecting check rods to make sure the attachment points are solid and checking for binding. If there is a problem, weight readings will be inconsistent from section to section. After inspecting hardware to ensure jam nuts are tight, rods are level and free of foreign material and washers are not rusted or distorted, operators can replace corroded hardware components so they don’t fail in the future.

Bumper bolts also should be inspected. Unlike check rod binding, bumper bolt displacement is easy to spot. Bumper bolts need to be tight so they don’t move on their own, yet also must maintain clearance when the scale is empty and loaded.

Seasonal temperature and weather changes can cause concrete and steel to expand, and bumper bolts can bind tight against the contact area. If binding occurs, adjust bumper bolts to maintain the proper clearance for all conditions.

Examine the load cells

The load cell area could contain dirt and debris that has built up since the scale’s last check. Load cells must have clearance to deflect through their capacity range. The same debris that can bind the scale deck also can prevent the load cells and suspension components from moving freely. 

Additionally, operators should inspect load cell cables for damage. If damage is present, the seal is likely degraded, as well. To combat this problem, some manufacturers offer metal conduit to run the cable through, providing a layer of protection that’s nearly impervious to damage from rodents. Although steel over braided cable provides more protection from damage than cable sheathed with polyurethane or plastic, cables sheathed in steel also should be checked.

Wet home run cables or load cells should be replaced. When exposed to moisture, cables can function as wicks. Water can be sucked through the cable’s length by capillary action until it enters the load cell or junction box, where it can damage components and seep into the inner layer of the cable.

Wet cables might not cause signal distortion immediately, but they could slowly corrode the copper wires or shield beneath the insulation, causing trouble over time.

Corrosion of the shield limits the cable’s effectiveness in blocking electromagnetic interference and radio frequency interference, while corrosion of individual wires will degrade the signal.

To prevent moisture from entering the cell from the cable, bend the cable or flexible conduit downward to create a drip loop at the location just before it enters the cell.

Prevent moisture in the junction box

Check the junction box for internal condensation, which is a common moisture problem. This occurs from normal air exchanges from heating and cooling cycles and over-aggressive washing that damages sealing components that haven’t been properly maintained. If left unchecked, wiring terminals and other components in the enclosure will corrode.

Stainless steel junction boxes are more vulnerable to internal condensation than a fiberglass-reinforced polyester (FRP) one. If stainless steel is not required, install a junction box with an FRP enclosure. A desiccant bag in the enclosure will help control excess moisture. 

Follow grounding procedures

Scale manufacturers suggest a variety of grounding procedures. To avoid multiple zero references, which can affect data lines and attract lightning damage, single-point grounding is recommended. Checking a single-point ground involves several steps:

• Verify the ground system of the alternating current (AC) power supply. 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 and should not exceed 0.5 volts AC.
• Install power conditioning. Electronic weighing systems can be disturbed by voltage distortions. Installing power conditioning products should be the first line of defense against power problems. Use a braided transient bypass cable around load cells and between scale modules.
• Check the ground again. Connect a bare 10-gauge copper wire to the frame of the scale platform and the grounding lug on the junction box board. Then, run the copper wire back to the ground rod provided by the power company. This wire can be buried in the soil from the scale to the AC ground.
• Check the AC power supplied to peripherals, such as remote displays, printers and computers. A remote device might not have the same AC power source as the indicator; therefore, peripheral devices could require their own ground points. Transient protection devices should be grounded to the same wire as the peripherals they are protecting.

Quality counts

No matter what type of scale an operator owns, a portion of the preventive maintenance plan is dictated by the manufacturer. Many scale understructures are left untreated and vulnerable to corrosion. Steel deck scales that have a welded bottom plate harbor corrosion because water that seeps into the scale cannot run off and evaporate. In these cases, you can protect the scale components mentioned earlier. But the best protection of the understructure comes from the manufacturer.

Invest in a scale that does not include a bottom plate and does include a corrosion-resistant undercoating.

This article was submitted by Rice Lake Weighing Systems, based in Rice Lake, Wisconsin.