TORTRAN2

Program TORTRAN2 Overview

With the increased demand for high speed and high efficiency rotor applications, rotor systems are often constructed with operating designs that approach failure conditions. As a result, failure analysis has become a critical step in the design of industrial rotor systems. Program TORTRAN2 is a computational tool designed to aid in torsional failure prediction.

TORTRAN2 Version 1.0 computes the following information about the rotor system:

• Rigid body angular velocity & acceleration.
• Rotor startup time.
• Transient angular velocity, torque & stress.
• Estimated rotor lifetime.

TORTRAN2 can be run on any IBM or IBM compatible PC with at least 512 KB of RAM. A math co-processor is recommended to speed computation. GEOGRAF(TM) graphics libraries are required to view graphics output.

Torsional Forcing Function Options

TORTRAN2 has four different torsional forcing function options:

• Synchronous motor input. The synchronous motor option allows for easy analysis of the startup characteristics of synchronous motor driven systems.
• Arbitrary pulsating torque input. Any oscillating torque may be input that has average and pulsating components which are functions of running speed.
• Average torque input. Torque that is a function of running speed but does not oscillate may be entered with this option.
• Time dependent torque input. By entering torque values at discrete points in time, any arbitrary torque may be input as a function of time. The program will interpolate between values.

With these four options, or combinations of these options, nearly any torsional input function can be modeled.

TORTRAN2 Computation

TORTRAN2 first calculates the rigid body angular velocity and acceleration of the motor- rotor system. From this information the program determines the startup time of the machine. TORTRAN2 then calculates the transient vibrational characteristics by integrating the linear modal equations of motion that represent the system.

The integration results are used to compute the transient velocity, torque, and shear stress values found within the individual elements of the rotor system. The stress values are compared to known material stress limits to determine the cumulative fatigue buildup and to predict rotor life.

Transient Velocity Plot

Velocity, Torque & Stress Plots

Transient Torque Plot

TORTRAN2 provides plots of angular velocity, torque, and stress versus time. These plots may be used for comparisons, reports, and visual confirmation of the transient vibrational behavior. The stress plots compare actual stress values to the critical stress limits of the material.

Transient Stress Plot

TORTRAN2 Finite Life Estimation

Anytime a variation in torsional load is applied to an industrial rotor system, shear stress due to transient torsional vibrations may exceed the endurance limit (maximum applied stress for infinite life) of the rotor material. If this occurs, TORTRAN2 implements Miner's rule to assess the cumulative fatigue buildup within the rotor. Miner's rule is a linear damage theory that can predict material life based on applied cyclic stress.

In addition to Miner's rule, TORTRAN2 uses several other theories in the estimation of rotor life. These include the Rainflow counting method, the modified Goodman relationship, the nominal mean stress theory, and the residual mean stress theory.