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 negating strains due 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 during loaded and unloaded shutdowns. During start-up and shutdown, many transient events can happen quickly, so time-domain data obtained with a high sampling rate is the preferred way of plotting. Coast down time can be maximized by removing the compressor valves, thus obtaining a higher resolution waterfall plot. For variable speed machines, data should be acquired during a slow, smooth speed run so that waterfall plots and/or order tracks can be obtained. Cylinder pressure-time cards are also useful to correlate back to torsional excitation and can help identify acoustic resonances and /or valve problems. All data should be simultaneously stored using a multi-channel recorder.
