Measuring Service Loads Applied to a Part

Knowing service loads is critical for improving existing designs — reducing weight and costs and increasing performance — and for proving that new designs meet specifications. The use of force transducers is a common practice when determining applied loads. But what if the structure or part doesn’t lend itself to the necessary transducer attachment, or the required transducer is too bulky or expensive?

A simple method for measuring service loads applied to a part is by installing strain gages on that part, essentially converting the article under test into a force transducer. Strain gages are small, rugged, and lightweight, and are simple to install and highly accurate. Installation involves bonding the gage to the test part using any one of several convenient adhesives (including cyanoacrylates), so practically any part shape or configuration is amenable to gage installation. The installation is completed by soldering necessary wiring to the gage solder tabs, which can be copper-plated to facilitate easy soldering.

Installing strain gages on a simple rod end bearing, or heim joint, is an excellent example of creating a force-sensing test part. A full-bridge strain gage arrangement is used, whereby four strain gages are wired into a Wheatstone bridge circuit, thus providing maximum force sensitivity. For the rod end bearing it is convenient to use two 90° tee-rosette gages, creating a Poisson bridge. One grid from each gage is aligned with the long axis of the part, and the remaining two are aligned perpendicular to the long axis. After wiring into the bridge circuit, the instrument leads are connected to the appropriate instrumentation — a Micro-Measurements P3 works well. The heim joint is then calibrated using an applied force, or load, of known magnitude. Output from the full-bridge is recorded at the applied load, and that relationship is subsequently used to determine unknown loads.