The scale was one of the first tools of civilization; ancient Egyptian tomb art from as early as 2500 BC depicts the use of balance scales, which continued to be used until the industrial age. The first incarnation of the modern strain gauge scale was introduced in 1938. This scale determined the weight of the applied load based upon the amount of force—or strain—that was placed on the gauge, which was attached to the most stressed section of the scale platform. In the past half-century, this technology joined the computer age, yielding higher-accuracy readings within a more convenient-to-use design. The electronic transducer, also known as a load cell, operates utilizing four or more strain gauges wired together, which establishes a “Wheatstone bridge” electrical circuit. The load cell became a popular selection for commercial weighing applications in the mid-20th century, but this new weighing method was not without its own shortcomings.
Electronic scales use load cells to convert mechanical movement, or applied force, into an electrical signal. The load cell deforms while bearing a load, and the compression or tension force that has been applied yields a corresponding electrical signal that is read by the weight indication system. While different designs are available, load cells based on the cantilever principle are most common; one end of the load cell is fixed and the other suspended to absorb the force of the applied load. The suspended end then reverts to its initial position once the load is removed.
As this mechanical movement must be precise in order to deliver highest accuracy readings, the early load cells were particularly susceptible to environmental forces that could influence this movement—such as shock and vibration. This made the load cell more challenging to apply in outdoor conditions, as well as in busy industrial environments. Additional mechanical devices such as expensive check rods and check plates were often required within these scale arrangements to help protect the load cell from damage. They also helped prevent extraneous loading from affecting the load cell’s operation. However, as with all mechanical scale parts, these devices are subject to wear, weather and other deteriorating factors that reduce their overall operational life. Continual—and often costly—maintenance programs were required to ensure proper scale operation, making the technology more expensive to apply.
THE FIRST WEIGH BAR
A better method was developed as a result of an innovative weighing solution designed for livestock farms in the 1960s. Art’s-Way Manufacturing, Inc. of Armstrong, Iowa enlisted the help of New Jersey engineer Dick Bradley to create an on-farm feed mixing solution that included an integrated scale, which would weigh each ingredient individually with far higher accuracy than the previous volume-based method. This scale would also need to withstand a broad range of environmental effects—including shock and vibration—and weather conditions, as Art’s-Way would market this system to farmers throughout the US. Existing systems employing mechanical components could wear easily in outdoor conditions, and undesirable loading effects influenced the load cell’s readings. Side loading, end loading and torsion effects would all impact the measurement of the applied load, reducing the accuracy and repeatability of the scale system.
Bradley’s solution was the Weigh Bar, an all-electronic strain gauge weight sensor designed to measure the applied load accurately without experiencing problematic loading effects. The Weigh Bar reliably delivered precise, repeatable readings, while its weatherproof and shockproof design allowed it to withstand environmental challenges. Its electronic design meant the Weigh Bar needed only minimal maintenance and provided a long usable life, allowing users to save significant expense both in the initial investment and in overall upkeep.
Unlike other load cells of this period, the Weigh Bar used an electronic signal to convey force measurements, rather than mechanical components. With this design, the Weigh Bar demonstrated exceptional accuracy, with no external mechanical apparatus required for dependable operation. The strain gauges, or electronic sensors, were positioned on the outside surfaces of the Weigh Bar, with two sensors on the top and two on the bottom—a design that ensured optimal performance.
The result was a new type of load cell that experienced reduced fatigue, with a low failure rate and exceptional overload protection. It also did not require complex vacuum-type hermetic seals that existing load cells used. Combining a simple potted seal with the device’s already-rugged design, the Weigh Bar provided the most robust and reliable weighing solution the industry had seen—all at a competitive cost.
As of 1969, Art’s-Way began selling industry’s first grinder/mixer with an incorporated Weigh Bar-based scale system. In 1971, Weigh-Tronix (now Avery Weigh-Tronix) incorporated as a subsidiary of Art’s-Way and sold the scale system itself to farm equipment manufacturers. Two years later, the company’s deck scale featuring Weigh Bar weight sensors became industry’s first all-electronic scale to earn legal-for-trade approval from the National Bureau of Standards (now the National Institute of Standards and Technology). Plus, the system’s reliability made it the only industrial scale to carry a two-year warranty; competitive model warranties rarely exceeded 12 months.
In the decades since the Weigh Bar’s invention, Avery Weigh-Tronix has continued to enhance its design. Today’s Weigh Bar features a multi-layer sealing process that further protects it from harsh environments. During manufacturing, the Weigh Bar also undergoes a three-step treatment process to further ensure strength, repeatability and low hysteresis. This combination empowers the Weigh Bar to withstand everyday jolts and moisture penetration—delivering long operational life.
Since the 1980s, the Weigh Bar’s construction has been strengthened through the use of aircraft-quality alloy steel, made up of a fine grain structure that further improves performance. Other Weigh Bar components such as the strain gauges and adhesive have undergone continuous improvement throughout the decades since its inception. With these enhancements, today’s Weigh Bar provides exceptional impact loading absorption and withstands in excess of one million cycles.
Several Weigh Bar models are available, including those designed with stainless steel construction featuring fully welded NEMA 4X (IP68) cans for sanitary applications, such as in food and beverage manufacturing. Models are also available specifically for use in hazardous areas. The same rugged Weigh Bar design can be implemented in weight sensors that vary in capacity from 100 to 250,000 pounds. The wide array of Weigh Bar configurations promotes its use in agricultural, petrochemical, pharmaceutical, food processing, material handling and transportation industry applications.