The Purposes and Objectives of Rotor Dynamics Analyses

Hello and welcome to the next edition in our introductory series to rotor dynamics analysis. In this installment, we’ll be looking at the specific purposes and objectives of performing rotor dynamics analyses; as well as the differences between lateral and torsional analysis. If you haven’t read the other entries in this series, you can find them here:

  1. Series Preface
  2. What is Rotor Dynamics? And Where is it Found?
  3. Why is Rotor Dynamics so Important?
  4. What API Standards Govern Rotor Dynamics Analysis?
  5. Basic Definitions and Fundamental Concepts of Rotating Equipment Vibrations

­

In earlier posts, we’ve established what the basic definitions and concepts are;  shown the consequences of improperly performing rotor dynamics analyses; as well as what standards are in place to ensure these breakdowns and catastrophes are avoided. That raises the question, what exactly are we trying to do by performing rotor dynamics analyses?

We know based on our previous articles we are trying to determine the critical speed and in turn determine if further damping and other measures need to be taken to ensure that there is a proper separation margin between the operating speed of the machine and its critical speeds. But what will this accomplish? The answer is actually 3 answers!

  1. We want to minimize and/or eliminate unplanned failures as much as possible, especially when these machines are counted on to keep planes in the air, the lights on, and in some cases, people alive! Have a look at this article from Halloween if you want an idea of what would happen if all our turbomachinery just stopped working.
  2. Secondly, we want a low vibration level. In addition to providing comfort in the case of an aero engine or a car’s turbocharger, lower vibration levels ensures less undue wear and tear on expensive rotor train components.
  3. Lastly, and this correlates with the others, ensure low maintenance requirements. Naturally the machine that can do its job the longest will be the one people desire, and rotor dynamics analyses play a large role when it comes to maximizing the service intervals for turbomachinery.­

­

Overall, the purpose of rotor dynamics analyses, is to ensure maximize machine reliability in the interest of time, money, and most importantly, safety. Now let’s get into the differences between lateral and torsional rotor dynamics.

RotorDynamics Block

Rotor dynamics is typically split into two distinct kinds of analyses which are constantly brought up; lateral and torsional rotor dynamics. So, what is the difference between the two?

Lateral Rotor Dynamics refers to the vibration and bending phenomena that can be seen in rotating equipment. In simplified terms, lateral=bending. Lateral analysis can be performed on a single machine like an overhung centrifugal compressor, or on the entire rotor train, e.g. the electric motor, the compressor, and anything else that is attached to the rotor train. (Figure 1)

Gas Turbine Rotor Train in AxSTREAM RotorDynamics
Figure 1. Gas Turbine Rotor Train in AxSTREAM RotorDynamics

Torsional Rotor Dynamics refers to the twisting phenomena that occurs between coupled strings of rotating equipment. In simplified terms, torsional=twisting. In torsional analyses, you are typically looking for torsional natural frequencies that occur as a result of multiple machines being coupled together in one single rotor train, such as a compressor and its motor, or a gas turbine being coupled to a fan and a compressor in the case of an aircraft engine, (Figure 2)

Diagram of a Shaft Undergoing Torsional Vibration
Figure 2. Diagram of a Shaft Undergoing Torsional Vibration

Key Differences: Let’s have a look at some more distinctions now that we have a general definition of lateral and torsional analysis.

  1. Lateral vibration is easily detected through standard instrumentation. Large lateral vibration amplitudes are often noticed because of rotor seals and process wheels rubbing.
  2. Instrumentation for torsional vibration is not usually installed. Large amplitudes can occur silently and without much effect on the housings and foundations. The first indicator of a torsional vibration problem is usually a broken coupling/shaft.
  3. Natural frequencies of lateral vibration are influenced by rotating speed.
    Natural frequencies of torsional vibration are independent of rotating speed
    and can be measured with the machine at rest if excitation can be provided.
  4. Lateral vibration in rotating machines can become unstable.
    This is very rare for torsional vibration in machines without speed control feedback.
  5. The most common excitation of lateral vibration is synchronous (1×RS) from rotor imbalance. Rotor imbalance has no effect on torsional vibration.
  6. Lateral analysis can usually be performed on each body in the train separately.
    Torsional analysis must include all the rotors in the train.

­

A Steam Turbine Rotor train and its rotor response in AxSTREAM RotorDynamics
Figure 3. A Steam Turbine Rotor train and its rotor response in AxSTREAM RotorDynamics

Having established the purposes for performing rotor dynamics analyses, and determined the differences between lateral and torsional analyses, we can get into the step by step objectives of a rotor dynamics analysis. (Figure 3)

  1. Predict lateral critical speeds – speeds at which vibration due to a rotor’s unbalance is maximum and should be avoided
  2. Determine the necessary design modifications to change the critical speeds.
  3. Predict the torsional natural frequencies.
  4. Calculate balance correction masses and locations from measured vibration data.
  5. Predict the amplitudes of synchronous vibration caused by rotor unbalance.
  6. Predict dynamic instability (including oil whip).
  7. Determine design modifications to suppress dynamic instabilities.

­

There are 4 general design considerations to keep in mind when adjusting the design of a machine or rotortrain to meet industry and company standards for rotor dynamics.

Rotor Dynamics primary design considerations:

  1. The positions of the Critical Speeds and the Separation Margins (SM).
  2. The peak response amplitudes going through the critical speeds, aka the Amplification Factors (AF).
  3. The steady-state response amplitudes within the operating speed range (Steady-State Vibration Limits).
  4. The rotor stability under various operating conditions (Stability Criteria).

­

Busted Bearing

Now to say this was a lot to digest might be the understatement of the year, but we’ll be breaking this down step by step in the coming entries. In the meantime, watch those amplification factors and keep your bearings lubricated! Or disaster will occur as shown in Figure 4.

Coming up in next month’s rotation…

Next month, we’re going to break down the importance of accurately modeling a rotor train, and then delving into the procedures of lateral and torsional analysis more specifically. We’ll see you then!

If you want to learn more about the importance of rotor dynamics, or about the tools our engineers and thousands of others around the world rely on for their turbomachinery designs, reach out to us at info@softinway.com