Figure 1 Bent Rod End
Think about that for a minute. Would you want a surgeon to operate on you with kitchen utensils such as a serrated knife? Then why do some people (management cough) expect or ask the force calibration technician to calibrate load cells, truck & aircraft scales, tension links, dynamometers, and other force measuring devices with whatever they have in their laboratory. Adapters that could range from unsafe, to an improperly machined adapter that allows them to fixture the force measuring device in the frame to apply forces and record output at those forces. You must ask why is this happening? Is it just that people are oblivious to the importance of adapters in mechanical measurements? The purpose of this particular blog is to examine safety concerns with using older adapters and discuss some of the error sources associated with using the wrong adapters. We will leave the rest up to the reader on whether it makes sense to seek out the appropriate adapters and have skilled calibration technicians or simply hope the technician can maintain the “miracle worker” title. Though some calibration technicians may still wear the miracle worker hat when the manufacturer writes specifications that were achieved once and never repeated, but I regress as this is an educational post and we will not be discussing unrealistic specifications. Instead, we will be discussing using the adapters that will give a calibration technician the highest probability of meeting those specifications.
Figure 2 Grade 8 Bolt that Failed at 120,000 lbf and close to 350,000 load cycles
Let’s start with those old adapters that have been in use for decades. The service life for force calibration adapters depends on several factors including material, design, manufacturing, number of load cycles, and magnitude of each load. There may come a time where the material begins to lose strength as the result of fatigue and eventually breaks. Today, there are better material and manufacturing control processes in place that provides more reliable strength values for design engineers than decades ago. There are also computer programs that greatly help in modeling and conducting all kinds of stress analysis. We often get asked what we should do with older adapters? Our guidance is to visually inspect all adapters for any signs of wear or fatigue and replace if they show any signs of potential failure. We recommend replacing adapters that have been in use for more than 20 years or 100,000 load cycles (10,000 calibrations). Adapters today are designed for a life cycle of at least 500,000 load cycles (50,000 calibrations) and failure at close to 1,000,000 load cycles. Now that we have explained some safety recommendations, let’s start to discuss some examples where the proper adapters are going to yield better results.
Common Adapters to Reduce Force Measurement Error
Keeping the line of force pure (free from eccentric forces) is key to the calibration of load cells. ASTM E74-18 in note 5 states “Force-measuring instruments have sensitivity in varying degrees depending on design to mounting conditions and parasitic forces and moments due to misalignment. A measure of this sensitivity may be made by imposing conditions to simulate these factors such as using fixtures with contact surfaces that are slightly convex or concave, or of varying stiffness or hardness, or with angular or eccentric misalignment, and so forth. Such factors can sometimes be significant contributors to measurement uncertainty and should be reflected in comprehensive measurement uncertainty analyses.” ISO 376 has an entire Annex devoted to adapters.
Morehouse Tension Member Assemblies
Figure 3 Morehouse Tension Members
A good start to what makes a good tension or compression adapter is the ISO 376 standard. Annex A.4.1 of the ISO 376 standard says “Loading fittings should be designed in such a way that the line of force application is not distorted. As a rule, tensile force transducers should be fitted with two ball nuts, two ball cups and, if necessary, with two intermediate rings, while compressive force transducers should be fitted with one or two compression pads”. Figure 3 above shows Morehouse tension adapters that have two ball nuts and two ball cups built into them to align the load cell with loading line and eliminate misalignment errors in tension calibration. Website Link to Quick Change Tension Members
Morehouse ISO 376 Compliant Compression Adapters
Figure 4 Morehouse ISO 376 Compliant Compression Adapters