Rotor Dynamics – Importance of Fundamental Understanding & Software tools

Rotor and bearings are the most critical components of any rotating machinery. Rotor lifetime and reliability depend, first of all, on the level of rotor vibrations. In order to meet highest requirements of reliability each step of the rotor design should be based on accurate Rotor Dynamics prediction.

Rotor dynamics is the branch of engineering that studies the lateral and torsional vibrations of rotating shafts, with the objective of predicting the rotor excessive vibrations. Rotor Dynamics is different from structural vibrations analysis because of gyroscopic moments, cross-coupled forces, critical speeds, whirling effect, etc. These difference makers are all due to the rotation of the rotor assembly.

Understanding of basic rotor dynamics phenomena and the various types of problems is absolutely mandatory when designing and developing rotor-bearing systems for various applications. Fundamental approach for Rotor Dynamics analysis generally is based on the following steps:

1.  Predict critical speeds.
2. Determine design modifications to change critical speeds.
3. Predict natural frequencies of torsional vibration.
4. Predict amplitudes of synchronous vibration caused by rotor unbalance.
5. Predict threshold speeds and vibration frequencies for dynamic instability.
6. Determine design modifications to avoid dynamic instabilities.
7. Calculate balance correction masses and locations from measured vibration data.

Another factor that determines accuracy of Rotor Dynamics calculation is rotor system simplification and the adequate modelling for rotor parts such as Impeller/disks, Sleeves, Balance pistons, Seals, Thrust collars, Couplings, Addition of Stiffening Due to Shrink Fits and Irregular Sections etc.

Influence of bearings on the rotor dynamics performance, life, and reliability of the machine cannot be ignored. Many of the problems faced with machinery today can be attributed to the design and application of the bearings. An understanding of how bearings work and some knowledge of the basic principles that underline their operation is therefore essential for making the proper choice for the particular design that best matches the service requirements of the machine in question.

SoftInWay Inc. helps to increase a level of rotor dynamics knowledge by providing classroom based and online training courses on Rotor Dynamics and Bearing Analysis. Basic knowledge mastered at the trainings allows to accomplish all rotor dynamics and bearing analyses steps without concerns in rotating machines reliability.

Dynamics of complex rotor systems is described by partial differential which can’t usually be solved analytically. Today with the availability of high-speed computers, a complete dynamic picture of the machine can be obtained using advanced codes for Rotor Dynamics and Bearing/Seals characteristics calculation based mainly on the finite element method (FEM). Some of the most popular and powerful specific software for Rotor Dynamics simulation are AxSTREAM® RotorDynamics and Bearing (SoftInWay Inc.), MADYN 2000 (DELTA JS Inc.), Dyrobes (Eigen Technologies Inc.) XLRotor (Rotating Machinery Analysis Inc.) ARMD (Rotor Bearing Technology & Software Inc.).

AxSTREAM® RotorDynamics modules allows to perform full scope of rotor dynamics analyses required by widely used standards:

• Static deflection analysis
• Critical speed and Critical speed Map analysis
• Stability analysis
• Unbalance response analysis
• Modal torsional analysis and Campbell Diagram analysis
• Time-transient torsional analysis

AxSTREAM® Bearing module calculates mechanical and hydrodynamic characteristics for various types of journal bearings (including oil, gas foil and ball bearing) and supplies RotorDynamics module with stiffness and damping characteristics.

AxSTREAM® Rotor Dynamics and Bearing software have some advantages in comparison with other codes:

1. Intuitive measuring software with user friendly interface. All functionality is well thought-out.
2. Advanced options for Structural supports modelling in AxSTREAM® Not only stiffness/damping characteristics and attached mass but also modal characteristics of supports can be considered to account the effect of supports vibration on rotor response with highest accuracy.
3. AxSTREAM® Bearing methodology takes into the account flow model (laminar or turbulent) and automatically correct calculating process to obtain more accurate results.
4. AxSTREAM® RotorDynamics and Bearing can be used as standalone software as well as integrated in the full AxSTREAM® Platform. Such an integration significantly accelerate the process of complete design and analysis of rotor-bearing systems.