Weight watching
Dialing in your axle weights can be simplified with the use of onboard scale systems

By Paul Abelson, senior technical editor

When equipping trucks, onboard scales may be the last things you might consider … unless you’ve had them before. Then you know the benefits they offer and how they contribute to your profitability while saving you time.

They save time by eliminating trips to a public scale, unless your revenue depends on accurate weights from a certified scale. Also, you can get a quick idea of how well your load is positioned and how much is on each axle set. No need to keep making trips to the scales after each adjustment. You can be sure you’re legal on each axle set.

Back when he started hauling steel, my son found out how important this could be. One shipper had a platform scale. If you’re not familiar with steel hauling, carriers drove their rigs onto the scale and the shipper determined how heavy a coil could be loaded without exceeding 80,000 pounds. The driver was responsible for determining where the coil was to be placed. My son eyeballed it and, after securing and tarping the coil, went on his way.

Sure enough, the scales were open. He found out he was several thousand pounds over on his drives and, of course, light on his trailer tandems. He could have checked on the platform scale, but being new he hadn’t yet learned how to do that. The lesson cost him almost a month’s rent.

Excessive fines and wasted time aren’t the only things you’ll save by using onboard scales. Tire life can fall victim to uneven weight distribution. When loaded too lightly, tires act as if they’re over-inflated. Center wear increases. Ride is harder, and unbalanced trailer tires tend to bounce, generating uneven wear patterns.

When a tandem is overloaded, conditions resemble under-inflation. Edge wear increases. Heat buildup inside the tire can shorten life and, if extreme, cause a catastrophic failure. In measurements taken at truck stops a few years ago as part of an FMCSA study, the vast majority of tires gauged were under-inflated by at least 10 percent. That under-inflation coupled with overloaded tandems will eventually lead to carcass failures and blowouts. After fuel and your pay, tires are the next highest cost of operating your truck.

Onboard scales are made for both air and spring suspensions, including walking beam models. As air suspensions are loaded, the leveling valve increases air pressure in the air bags to keep the ride height constant. As weight is removed, the valve bleeds off air pressure. Gauges read and display the air pressure.

Early scales displayed just air pressure. The driver had to consult tables to determine what the actual weights were. Today, virtually all displays are direct reading, showing weights in pounds of load rather than pounds per square inch. Most scales today are digital, displaying steer, drive and trailer weights and calculating gross vehicle or gross combination weights.

For other-than-air suspensions (spring, rubber block, walking beam and others), sensitive strain gauges are used. All steels deflect under load, either by bending or compressing. These movements are virtually microscopic, but they alter the metal’s electric properties. Those changes are measured, and computers translate the changes into the loads that caused them and display the results in pounds.

For onboard scales to function properly, they must be calibrated. The process is simple and straightforward. When your truck is empty, have it weighed on a certified scale. Enter that weight into the scale’s computer or control unit. Then load the truck to its fullest. Go to the same scale and enter those weights. Do it for each axle or axle grouping where you have an onboard scale. The computer will determine the strain or air pressures when loaded and empty and will adjust accordingly. That’s all there is to calibration. It should last for years, but it’s a good idea to recalibrate about once a year.

Once installed, onboard scales require little maintenance beyond periodic calibration. Because they are exposed to spray from corrosive chemicals, metal fittings can be attacked, but this can be minimized by locating the fittings in shielded areas. Wiring, especially at connectors, is also subject to corrosion. The same practices that protect lights and other wires work with scale wires. Use heat-sealing connectors when joining wires, and cover connections and splices with heat-shrink tubing. Inspect wiring and pneumatic fittings when winterizing and summerizing.

The benefits are real, and the purchase price for even the most feature-filled onboard scale is reasonable. That leads to a rapid return on investment and savings that will definitely improve your profits. LL