# Strain Gage for Shear Strain and Torque

When you twist a door knob to open the door, your hand is applying a torque to the knob.  Torque-force twists things, whether it be the massive propeller shaft on a cruise ship, or the humble door knob.

Torque loads cause shear strains in the structure to which the torsional loads are being applied.  This is a good thing, because strain gages can measure these shear strains.  That’s actually not a precisely true statement.  Bonded electrical resistance strain gages do not respond to pure shear strains.  Fortunately, associated with all shear strains are two normal components of strain, tension and compression, which strain gages easily sense.  This is why it takes two strain gage sensing grids to resolve one shear strain value.  For mathematical convenience, these grids are normally oriented at ±45° to the measurement direction (90° between the two grids), allowing the gage, when properly connected to a strain indicator instrument, to directly display strain magnitude.  But, the grids could be oriented at any angle relative to one another and the shear strain can be calculated.

Torque transducers provide a convenient way of measuring torque-loads being applied to a structure; shaft, bolt, door knob, and so on.  The sensor used in many high quality torque transducers is a Micro-Measurements Transducer Class shear-strain gage.  These precision measuring devices allow designers to optimize structures for anticipated loads.

Micro-Measurements produces two distinct categories of strain gages.  One is called Stress Analysis (SA) gages and the other is called Transducer Class (TC) gages.  Stress Analysis gages are intended for applications where measuring an unknown strain is required.  This includes, but is not limited to, such things as aircraft structures, bridges, automobile frames, drive shafts and other components, office chairs and anything which experiences service loads that could result in catastrophic failures.  Stress Analysis strain gages are carefully calibrated (gage factor and transverse sensitivity) for strain sensitivity and temperature response, allowing strain measurement accuracies of better than 0.5%.

Transducer Class strain gages are intended for use in devices, like torque transducers, that will be calibrated prior to putting into service.  These gages take advantage of the superb repeatability available from metal-foil strain gages, so that batch to batch transducer performance is consistent; which also provides simpler transducer manufacturing without ongoing, difficult calibration.  To reduce manufacturing costs, Transducer Class strain gages are ‘type’ calibrated for strain sensitivity (gage factor).  For example, most Transducer Class gages have multiple creep compensations available to support various transducer materials and capacities.  Only the first compensation from that gage type will be calibrated for strain sensitivity and the tolerance is assigned as nominal, because all other creep compensations of that type will assume the same gage factor value.

Torque measurement is a very common application, used to measure engine power output on propshafts and driveshafts, driver input through steering wheels, tightening bolts, or anywhere a component is subjected to a twisting force.

When the shaft is loaded in torsion, it creates a state of pure shear and the applied torque can be found by orienting rosettes where the gridlines are positioned at 45 degrees to the axis of the shaft.  Some common applications can be found in pulp and paper mills for measuring efficiency, output shafts for large ships for optimum engine setup, and even power monitoring for cyclists so they can determine how much energy was expended during training, and, of course, there are many others.   For most of the these applications, the strain gages are used to construct a full Wheatstone bridge circuit to double the sensitivity to torque and cancel unwanted effects due to axial or bending load components.

In the foreseeable future, at least, utilitarian robots are apt to be decidedly nonandroaidal in appearance, consisting largely of a disembodied arm terminating in some griping force/touch mechanism which can loosely be called an “organ” or a “hand”. Furthermore, the principle areas of application for robots, today, are not only in the home, but in those manufacturing, processing plants, hospitals and transportation services where their economic viability can be assured by the high cost and low productivity of unskilled and semiskilled labor.

Even under these circumstances, however, not much prescience is required to recognize that humanity and Advanced Sensors Technology strain gages stand at the threshold of a new and unsettling era - one that can be referred to with little danger of overstatement as a “robotic revolution”. Alongside the growth of the torque sensors , the Advanced Sensors strain gage has emerged as the next technology phenomenon. The new design of shear and torque strain gages are consider to be a game changer for the stress analysts society.

Domestic healthcare, domestic plants,  cobots , robotics , new automation, supply chain 2.0,  autonomous vehicles, autonomous mobile robots, drones , IOT and Technology 4.0  are examples of sectors that could benefit significantly from using a shear and torque strain gages.

Features:

·        Gage patterns designed for measuring shear strain and torque

·        Individual and multiple grid patterns including combinations which include torque and thrust

·        Gage lengths from 0.062" (1.57 mm) to 0.250" (6.35 mm)

Micro-Measurements announces successful integration of Advanced Sensors Technology into Shear Strain and Torque patterns strain gage sensors for Industry best-in-class design and performance characteristics. Advanced Sensors Technology applies tangible specification and manufacturing process improvements, along with industry-exclusive strain gage sensor design techniques, for direct customer benefit. The innovation behind Advanced Sensors Technology reflects many decades of Micro-Measurements R&D experience, gained across a global portfolio of thousands of successful applications, with other refinements achieved via ongoing customer feedback. Today, we already can see how Advanced Sensors Technology strain gages are making their way into the torque sensors design right along with popular concepts like artificial intelligence (AI), machine learning (ML), and the Internet of Things (IoT).

Filter Stress Analysis Shear and Torque Patterns

Filter Transducer Class Shear and Torque Patterns

Ron Zukerman