Balancing Act

Cab readout devices for Vulcan scales, which take measurements from sophisticated load cells.

Ever spend an hour or more getting a concentrated or uneven load distributed right? You drive from the loading point to a truck scale, find that the load isn’t just where it needs to be, and then you return to the shipper’s facility to pester someone to shift it with their equipment. Furthermore, you may have to repeat the process.

The main purpose of an onboard scale system is to make it easy to get that load positioned before you leave the loading dock while the loading equipment is available. It will also alert you to an overload caused by underestimated cargo weight.

“Taking the shipper’s word for how much each piece of freight weighs is a sure way to find yourself overweight when you pick up that last piece of equipment,” says Air-Weigh’s Peter Powell, vice president of marketing. “Having an onboard scale means you can weigh each piece as you load and confirm its weight for the shipper.” Air-Weigh units even allow you to print a receipt so you and the shipper can sign and agree on the weight.

The concept of onboard weighing began years ago, using sophisticated load cells to measure the deflection of a piece of metal. These systems are now used more often in vocations such as log hauling, in which onboard weighing is absolutely critical, than in conventional trucking applications.

Widespread use of the onboard scaling concept really began with air suspension. After loading, a leveling valve in the suspension system adjusts the pressure in the air bags until the height returns to a constant setting. Many operators learn from experience that certain pressures indicate certain weights.

Sid Campbell, owner of Right Weigh Load Scales and the developer of that product, decided he could do more with this relationship. He patented a gauge design that put a scale in thousands of pounds at appropriate intervals around the gauge dial.

Tractors, trailers and air suspension designs differ in terms of the actual amount of air pressure at a given payload and axle weight because trailer and air bag designs differ. So, it’s necessary to further calibrate the gauge to the particular vehicle, Campbell says. After tapping into the air pressure system and mounting the gauge, the driver loads the vehicle to near maximum GCW and takes it to an accurate scale. He parks a certain axle or tandem on the scale pad and leaves the parking brake off. He then turns a screw in the face of the gauge till the indication precisely matches the weight reading.

Campbell explains that air suspension systems working with tandem or even tridem axles use a single load-leveling valve. This means the system applies the same pressure to all the bags in the tandem/tridem suspension. So, as long as you weigh the entire axle group together, once calibrated, the gauge will give the right weight. When using a lift axle in combination with a tandem setup, you just calibrate the gauge and then check the weight with the third axle lifted off the ground.

The Right Weigh system typically sells for about $105.

Air-Weigh scales also work mainly from the air pressure in an air suspension, but the system uses electronics to add some value and precision. Rather than connecting a gauge directly into the air system, Air-Weigh uses air pressure sensors, which are electronic transducers. They measure how much the suspension pressure compresses a spring and convert the tiny movement to an electronic signal. Separate tractor and trailer systems each have a small electronic processor module with a digital readout screen. The two communicate along a wire in the trailer electrical connector, using multiplexing, which allows the system to effectively share the wiring.

Installation demands little more than the amount of skill required to put a CB in a truck, the company says. Some 80 trailer manufacturers will install it as original equipment. In-dash weight readouts will be available as original equipment from all the truck manufacturers by the middle of this year.

The user calibrates each axle at empty and fully loaded, taking readings at a stationary scale and punching in the results on the control module. Working between two established points on the weight versus air pressure scales helps ensure accuracy, Powell says. Weights will normally be accurate within 300 pounds. If you should run different trailers, you can install a trailer system on each. When you connect the power connector, the tractor module will accurately read the weights from the new trailer’s module.

Air-Weigh’s systems have a list price of $1,085 for the tractor, $625 for a trailer.

The company also makes a slightly more exotic Trailer Payload Scale for those running end-dump trailers. It combines the standard air-based weighing of the trailer axles with hydraulic sensing of the lifting cylinder, so those who haul payload by the pound can get closer to legal maximum weight and make more money per trip. The systems can even be used to transmit load status to weigh stations and avoid a stop.

Powell says the owner may occasionally need to replace air and hydraulic sensors and transducers, which is a simple job. The system should be recalibrated in the process, however. If wiring is routed properly – where the original wiring runs – and properly tied in place, problems should be minimal. New wiring can typically be plugged in.

Stress-Tek, maker of Vulcan Onboard Scales, produces sophisticated load cells that allow precise weighing directly from the deflection of springs and other suspension parts. The company also makes products that weigh from air suspension pressure. They manufacture an air-based system that includes not only all the wiring and air sensors, but ride-height control valves that help ensure predictable system pressure at a given weight.

Eric Elefson, director of sales and marketing, says the advantage of Vulcan scales is that if you are running leaf springs on your front axle, or even on trailer axles or under the tandems of an off-road dump truck, you can still accurately weigh every axle. Such a system can combine indirect measurement of weight via pressure on air-suspended axles and direct measurement via load cells measuring deflection on the sprung axles to provide a maximum of information at a minimal cost. All the readings will show up on a single screen. System sophistication and price can be geared to the precision needed.

Vulcan systems can, for example, use a replacement fifth wheel incorporating a load cell to weigh the front end of the trailer; measure hydraulic pressure to check the weight of the forward end of a dump body; and can even measure the pressure at the hangers of a spring suspension system. All systems have digital readouts. They can even transmit axle weights or payloads to your office.

As with Air-Weigh, separate tractor and trailer systems allow you to switch trailers and still read the trailer axle weights by merely plugging in the tractor-to-trailer electricals. Elefson says prices range from about $1,800 to as much as $5,000 for the most advanced systems that use only load cells. Self-diagnostics help find a problem in the event a component or wire would fail.

Onboard scales aren’t for everyone. Elefson, for example, recalls a basket hauler who never carries more than half the maximum allowable weight. “There’s no way he would ever have an overweight axle or need to adjust his load,” he says. “The question is to make it justifiable – something you can get a payback on.”

Count costs before dismissing scale system.

There’s more than one factor to consider when figuring if an on-board weighing system will pay for itself. Make your own estimates to see the true costs of not using an onboard scale:

  • Unpaid miles driving back and forth to scales and time waiting for a fork lift or crane to reposition a load. “Especially with the new hours rules, you must list time spent running back and forth to a public scale as on-duty time,” says Right Weigh Load Scales owner Sid Campbell. Since fewer hours taken to get legal means more hours earning money, he has seen even company drivers install his system on company-owned vehicles.
  • Lost income, when you’re paid by the pound or hauling less-than-truckload, because you often “load light to avoid fines and check-weighing,” says Air-Weigh’s Peter Powell.
  • Overweight fines. These can run from $85 to $150 or more.
  • Scale fees of $8 or more each time the truck is weighed.
  • Increased maintenance expense because of overloaded tires, brakes, springs and other components.


    A five-axle tractor-trailer with standard tandem axles on the trailer must run with 34,000 pounds on each set of tandems and 12,000 pounds on the steer axle. And, if an officer checks your weight distribution with a portable scale and finds that it’s off by more than a whisker, you’re likely to pay a big fine.

    Just why does weight distribution matter so much to authorities? It’s about protecting roads and bridges.

    “Axle spacing is as important as axle weight in bridge design,” says Peter Powell of Air-Weigh. “A bridge is analogous to thin ice on a pond. Walking on the ice concentrates a person’s weight on the small area covered by the individual’s feet, and the ice may break. Lying down, however, spreads the same weight over a much larger area, and the ice is less likely to break.”

    Proper axle loading benefits more than the nation’s infrastructure – it also helps you. With either air or spring suspension, overloading will defeat the suspension’s ability to reduce the frequency of vibration, thereby putting more stresses on both the trailer components and the load itself. Overloaded suspensions are also more likely to create vibrations that can shift the load, which could aggravate the situation.

    “Lightly loaded vehicles last longer, including the braking system,” says Jim Feddern, manager of motor carrier enforcement of the Ohio State Patrol and chair of the Commercial Vehicle Safety Alliance vehicle committee. A Vulcan On-board Scales publication notes the advantage of “increased safety by keeping weight within legal limits, allowing braking distance to remain constant and tracking around corners to be more predictable.”


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