There is more to rotating machinery than meets the eye, explains Consultant Greg Nelson.
What connects turbines, pumps, turbochargers, and ship tail shafts? They all rotate, and they all vibrate. Rotating machinery is everywhere – it comes in all sizes and speeds, and it all vibrates. Rotor dynamics is the study of the vibration of rotating machinery, and it’s a niche skill but one that enables much of modern life.
It’s impossible to perfectly balance rotating shafts – the centre of mass will always be minutely out from the axis of rotation. As the rotor spins, that puts a rotating centrifugal load into the shaft, causing vibration of the system. In addition, the shaft will have a stiffness and mass distribution, and the bearings holding the shaft will have a stiffness, meaning that the system will have a number of natural frequencies, involving both radial movement or torsional modes.
In some senses, rotor dynamics is much like other types of vibration analysis – we want to find out the natural frequencies to avoid excessive vibration at running speeds, we want to make sure the system is stable and not liable to shake itself to bits, and we want to figure out how to solve problems with existing kit or design away problems before they occur. But there are a number of additional difficulties unique to rotor dynamics – gyroscopic effects significantly affect the natural frequencies, and there are fluid/structural coupling effects in the bearings, seals and other places along the shaft. These can cause catastrophic instabilities if not configured correctly, and rotor dynamics is the tool we use to make sure this doesn’t happen.
We are now pushing the boundaries of traditional rotor dynamics assessments, using probabilistic techniques to look at changes in risk, and leveraging Bayesian Inference to use rotor dynamics models in condition monitoring. There is more to rotating machinery than meets the eye!