By Josh Cosford, Contributing Editor
Most of the articles I write are assigned to me by super-editor Mary Gannon, and this article on strain gauges is no different. Oddly enough, I helped our Amtec Hydraclamp division technician assemble and test high-pressure spring clamps just yesterday, which we tested using a strain gauge. These clamps are 5, 10 and 15-tonne (yes, metric tonnes, not imperial tons) spring clamps and hydraulic release devices used to tension bolts for various applications. These clamps use disc springs so powerful that 450 bar (6,500 ps) is required to release clamping tension.
Without a strain gauge to help us adjust the preload of the retainer cap, we’d have no idea what the actual clamping output is, which would be unsafe for our customer’s application should the force be inadequate. As the term implies, these sensors measure the strain applied to a given material. Strain gauges use a conducting material such as thin wire or foil applied to a flexible membrane and arranged in a grid pattern.
Strain gauge
As the conducting material stretches or compresses, its electrical resistance changes and does so proportionally to the deformation of the material. The resulting change in resistance is measured, processed and interpreted into a force or pressure measurement, such as pounds or newtons. Strain gauge sensors must be calibrated because there is no direct and linear conversion between unrelated units such as Ohms and physical force.
There is also an intermediate circuit between the strain gauge and the electrical output, which is the interestingly named Wheatstone bridge. Named from — and I can’t make this up — Sir Charles Wheatstone, who improved and promoted an existing concept. It consists of a quadrant of resistors wired in a series-parallel bridge circuit that improves the sensitivity and accuracy of the strain gauge.
Finally, the bridge output is amplified and converted into a voltage, current, or digital signal that represents the force or hydraulic pressure in the sensor. Strain gauges are used in load cells, like my above example, but also in most hydraulic transducers, so an industry standard output signal is typically 0-5 V, 0-10 V, 4-20 mA or some form of digital signal transmitting within a network.
Strain gauges are also used to measure torque, where simply being placed upon a shaft is sensitive enough to measure how much the shaft deforms under torque. Even the strongest forged steel twists helically as torque is applied, so sensors added to the shafts of pumps or motors (or intermediate devices) can detect torque spikes or measure actual motor output.
Strain gauges play a crucial role in fluid power systems by providing accurate and real-time measurements of pressure and force. Their integration into pressure transducers, load cells, and torque measurement systems enhances operational safety, efficiency, and longevity. By leveraging strain gauge technology, engineers can optimize system performance while helping to diagnose any potential cause of failure. And before you ask, manufacturers produce load cells with outputs in both tonnes and tons.
Filed Under: Components Oil Coolers, Engineering Basics, Pressure Sensing, Sensors, Sensors & Gauges
