What is Rotor Dynamics? And Where is it Found?

Welcome to this next edition of our “Introduction to Rotor Dynamics” series! In this edition we’ll be covering the definitions of rotor dynamics, and how it is an important factor in the lifetime of a rotating machine. So, for starters, what is rotor dynamics?

Well, if you read our preface which can be found here, you probably knew the answer already; or if you’ve been working in this field, you probably also have a good answer. For those of you new to rotor dynamics, however, it’s a branch of applied mechanics in mechanical engineering and is concerned with the behavior of all rotating equipment; considering phenomena like vibration, resonances, stability, and balancing. It accounts for many effects: from bearings, seals, supports, loads and other components that can act on the rotating system.

Is rotor dynamics vibration analysis?

Yes partially, but there is much more that needs to be considered as you can guess from the above definition. Vibration analysis simply isn’t enough, because the rotors in these machines spin at such high RPMs and are so heavily loaded. Something as simple as the bearing’s position and stiffness, or a slight asymmetry from blade creep can affect a rotor’s behavior.

Where can rotor dynamics be found and analyzed?

The short answer is, there are numerous machines where rotor dynamics can be considered.  In fact, it’s probably easier to list the numerous applications where rotor dynamics doesn’t exist.

A 3D model of a gas turbine rotor train
Figure 1: A 3D Model of a Gas Turbine Rotor Train.

Below is a very short list of some examples where rotor dynamics can be considered:

  • – Steam and Gas Turbines
  • – Jet Engines
  • – Wind Turbines
  • – Turbochargers
  • – Fans
  • – Pumps
  • – Compressors
  • – Turboexpanders
  • – Hard drives in Computers
  • – Electric Motors and Generators
  • – Reciprocating Crankshafts (Piston Engines and Compressors)
  • – Ship Propeller Shafts­

 

A 3D model of a gas turbine
Figure 2: A 3D Model of a Gas Turbine.

Not all these machines are considered turbomachinery, but they all rotate at high speeds. As a result, they will feel the effect of rotor dynamics based on components such as seals; the weight of the rotor; the bearings used; the attachments and loads on the rotor itself, and where those loads are positioned.  For example, on a ship’s propulsion shaft, consider the gearbox coupling, the seals and bearings found through the hull and on the outside of the ship, where on the end at least one propeller hangs over to propel the ship. Such a heavy, asymmetrical load on the equipment can influence the rotating behavior of the propeller shaft.

That example is a simple drivetrain on a ship needing rotor dynamics analysis. It’s especially important to consider rotor dynamics when you have a machine like a turbine or a turbopump that costs hundreds of thousands if not millions of dollars to develop and manufacture. Something as simple as a bearing’s stiffness and position can jeopardize the entire machine’s performance. We’ll discuss that in our next edition on rotor dynamics.

Interested in learning more about rotordynamics?

Check out our upcoming trainings here: http://www.softinway.com/en/education/classroom-training/rotordynamics-bearings-design/ 

Register for our latest webinar on “How to Apply API Standards to Turbomachinery Rotor Dynamics for Lateral Analysis