A strain gage telemetry system can be used to evaluate actual transmitted and dynamic torque in a shaft. A full bridge arrangement (4 gages) can measure torsional strain while negotiating strains due to to bending, axial and temperature effects. The voltage signal proportional to strain can be converted to torque or shear stress. The measured stress can then be compared to allowable levels. If possible, the gages should be located where maximum twist occurs, which requires knowledge of the torsional mode shapes of the system. Depending on the particular mode shape, this installation location may not be feasible. Installing the gages on the shaft near the coupling hub (but away from any keyways) is usually adequate.
For compressor systems, torsional measurements should be obtained at multiple conditions, including start-up, unloaded operation, all load steps (e.g. unloaders or pockets), and shutdowns. For variable speed machines, data should be acquired during a slow, smooth speed run. Cylinder pressure-time (P-T) cards are useful to correlate back to torsional excitations and can help identify acoustic resonances and/or valve problems. The EDI DAQ system simultaneously records all data channels to a digital file format.
EDI uses Binsfeld telemetry systems. Several real-world applications are given below: