Torsional Vibration

Rotating components, such as shafts and couplings, rotate when torque is applied. If a shaft or coupling is rigid and the torque applied is constant, the rotation of the shaft or coupling is stable and does not generate torsional vibration. In real-world applications, however, shafts and couplings are not rigid and the torque applied might not be constant. Moreover, the components that transmit the torque to the shaft or coupling also can generate inconstant torque because of the mechanical defects. Thus, when torque is applied, a shaft or coupling twists slightly and generates torsional vibration.

Every rotating system has one or several critical torsional frequencies. When torque is applied to a rotating system, the system generates an excitation torsional frequency. The closer the excitation torsional frequency is to one of the critical torsional frequencies, the higher the torsional vibration of the system becomes. When the excitation torsional frequency equals one of the critical torsional frequencies, the rotating system generates the strongest torsional vibration. Strong torsional vibration in a rotating system can cause the failure of rotating components or damage to the system, thus monitoring and controlling torsional vibration is critical.

You can measure the torsional vibration of the rotating components from the change in angular position, angular velocity, or angular acceleration. The NI Sound and Vibration Measurement Suite measures torsional vibration from the change in angular velocity, or the rotational speed, of the rotating components. You can use either analog or digital tachometer signals as the input signals. When you use analog tachometer signals as the input signals, set the sampling rate of the analog tachometer higher than the sampling rate calculated in the following equation:

sampling rate_tacho = 10 * samples per revolution * max speed (RPM)/60

The tachometer system you use to obtain tachometer signals might have geometric imperfections. For example, suppose you use an optical transducer to observe pieces of reflective tape attached to a shaft. The intervals between two pieces of reflective tape might vary. The variation of the intervals can affect the accuracy of the rotational speed profile that the tachometer measures. You must compensate for these geometric imperfections. If you use an encoder that can generate several hundred or more pulses per revolution, you do not need to compensate for geometric imperfections. An encoder can generate accurate speed profile results, so you can ignore the geometric imperfections in the tachometer system.

Rotating systems like compressors, turbines, and electric motor drives usually generate torsional vibration at fixed frequencies. Rotating systems like combustion engines usually generate torsional vibration that changes with rotational speed or generate torsional vibration at fixed orders.

By measuring the torsional vibration of a rotating system, you can monitor the condition of the system to identify faulty components and avoid system failure. You also can perform noise, vibration, and harshness testing on the rotating system to identify unwanted vibrations and improve system performance.

Because the Sound and Vibration Measurement Suite measures torsional vibration from the change in angular velocity, the measured signals typically contain some unwanted DC components. You can ignore the DC components when you perform further frequency or order analysis. You also can apply a highpass filter to remove the DC components before you perform further frequency or order analysis. If you apply a highpass filter to remove DC components, you usually can achieve better results for constant-speed torsional vibration signals than for variable-speed torsional vibration signals.