Calculating torsional natural frequencies is a common component of machine design and many specifications require a specific separation margin between calculated torsional natural frequencies and known exciting frequencies. However, a machine's actual torsional natural frequencies may not agree with what is predicted due to differences between what is modeled and what exists in the field. When the actual torsional natural frequencies coincide with a driving dynamic torque, resonance occurs and large torsional vibrations will be present.
Torsional vibration can damage shafting and drive components by producing high dynamic stresses in shafting, gears, couplings, etc. When torsional failures occur, there is generally little indication prior to failure since the dynamic motion is rotational rather than translational and cannot be measured or perceived without the correct equipment.
Types of machinery that are commonly tested for torsional vibration include:
The torsional natural frequencies can be determined by measuring either the torque or the twist of a shaft. We commonly measure the torque using strain gages and wireless telemetry. This type of torque measurement provides the static and dynamic torque in the shaft. Alternatively, the shaft twist can be measured using encoders. The dynamic torque and the twist give an indication of the torsional response of a system and with a proper test and analysis the torsional natural frequency can be determined.
Below is a comparison of the dynamic torque measured during two motor starts. For this project, a torsional analysis was conducted and recommended making a change to the system to reduce the torsional natural frequency. The two motor starts show the change in the torsional natural frequency.
Torsional natural frequency measurement before modification. The natural frequency was measured to be 2550 CPM.
Torsional natural frequency measurement after modification. The natural frequency was measuered to be 2330 CPM.