Vibra Screw has been designing batch and continuous gravimetric control systems for decades. We were quick to adopt useful technology by incorporating digital and graphics displays, as well as an ever-growing number of I/O interfaces into our products. As expected, customer response to these improvements was overwhelmingly favorable.
However, all gravimetric control systems, no matter how fancy the user interface, measure weight using a transducer called a load cell. (See Figure 1) The load cell converts mechanical weight into an electrical signal. The most common type of load cell employs a strain gauge transducer (See Figure 2) in a Wheatstone bridge and can be found in devices ranging from deli scales to systems designed to process solid rocket fuel.
While computer technology has enabled high tech gravimetric control systems to evolve significantly over the last 30 years, a load cell is still the basic workhorse at their core. And, though it may be hard to believe, the strain gauge load cell technology got its start in the late 1800’s. In 1843 Sir Charles Wheatstone perfected and popularized his Wheatstone bridge. (See Figure 3) A resistance change in the bridge produces a proportional change in voltage across the bridge. Then almost 100 years later, in 1930’s, Edward E. Simmons and Arthur C. Ruge invented a device that could measure strain in an object.
It wasn’t long before the strain gauge and Wheatstone bridge technologies were combined and the “load cell” was born. This transducer changes mechanical energy into a proportional electrical signal. Even with the invention of the transistor following World War II, the load cell still found it’s primary application in laboratory test instrumentation. The small signal created by the load cell wasn’t ready for commercial/process gravimetric control systems. Lever scale and balance remained the preferred method of measuring weight.
Finally, in the computer age of the late 20th century, electronic technology had advanced enough to accurately measure small signal changes and the load cell started to replace the balance and mechanical lever scales used in processing industry. Even in today’s cutting edge technology, over 170 years later, the strain gauge load cell continues to be the preferred transducer used for process weighing and gravimetric control and the one used by Vibra Screw.
Weigh Feeders & Batchers
For over a half-century, Vibra Screw has been a major supplier of hoppering and feeding equipment used to make much of the United States solid rocket fuel and ordnance. Vibra Screw vibrating systems, without mechanical agitation or conveying, are ideal for safely discharging and moving solid rocket propellants and explosives. Vibra Screw’s Bin Activator’s and Feeders and over 60 years of bulk material handling experience provide proven solutions.
Hercules Aerospace, now Alliant Techsystems [ATK], constructed a solid rocket fuel manufacturing facility in Utah to process solid fuel for the Space Shuttle and nearly all the nation’s other launch vehicles and strategic missiles. It required precise weighing of the ingredients that make up the fuel. Design criteria included batch accuracies of 0.05%, safety factors that permit remote operation and do not require periodic recalibration and self-cleaning characteristics that permit total elimination of residue between cycles. Vibra Screw was awarded the contract to provide the turnkey system. Based entirely on scale mounted Bin Activated Bins and Vibrating Tube conveyors to deliver precise weighed batches of propellant ingredients, the design was the first time screw-less conveyors had been used for these materials, creating a much simpler and safer operation.
Thiokol Corporation, also now part of ATK, has used Vibra Screw equipment continuously since the 1970’s for similar propellant production. The unusually sticky nature of some of their materials led to Vibra Screw developing a cleaner profile Bin Activator, without internal cross bracing, that greatly enhanced flow from storage while reducing welds in the material contact areas.
The nation’s major ammunition depots have also used Vibra Screw equipment for decades. Again, the simplicity of vibration-only movement of dangerous material provided many safety benefits when handling explosive powders.