Torsional Vibrations

Torsional Vibrations in Rotating Machinery, Cars + Trucks, Ships

Basics

The engineering jobs contain calcu­lations and simula­tions of TOR­SIONAL VIBRA­TIONS of straight, branched, and inter­mesh­ed drive sys­tems (drive­lines, power­trains) also con­sider­ing non­linear cha­rac­teristics (e.g. rubber coup­lings, gear stages with back­lash, shift and en­gage effects in clutches, etc.) in the time or fre­quen­cy do­main. We can follow steady-­state condi­tions or time-­tran­sient effects in the si­mu­lation en­viron­ment. Further­more, con­trol effects may be part of an ex­panded tor­sional vibra­tion mo­del.

The CAE results show the natural fre­quencies (critical speeds) usual­ly pre­sented in so-called „Campbell“ dia­grams (inter­ference dia­grams, water­fall dia­grams), as well as the gra­phi­cal inter­pre­tation of the vibra­tion modes (modular forms, mode shapes). We also get simu­la­tion re­sults showing the ex­pected ampli­tudes and magni­tudes (usually taken from torque signals, velo­cities and speeds, accelera­tions) depend­ing on the exist­ing excita­tions (i.e. excita­tion torques of the motor, the engine, the work­ing process, and the operat­ing con­ditions).

Procedure

A typical torsional vibration analysis (TVA) is structured as follows:

• MODEL GENERATION
  creating the computer model according to the task
• ANALYSIS OF EXCITABILITY
  calcu­la­tion and inter­pretation of the natu­ral be­havior in­cluding reso­nance ana­ly­sis depend­ing on the sys­tem exci­ta­tion (studying inter­ference dia­grams), first mo­del vali­da­tion
• SIMULATION
  determination of the system response like torques in the shafts, couplings, gear stages, etc. – either time dependent or speed dependent
• POSTPROCESSING + INTERPRETATION
  analysis of the results compared to measure­ments and expe­riences, final mo­del vali­da­tion and model refine­ment (if necessary)
• OPTION: FURTHER CASE STUDIES, SYSTEM OPTIMIZATION
  specification of a technically and economically reasonable „best“ drive system configuration

Simulation Methods

We distin­guish between two different simu­la­tion methods for our CAE analysis:

a) the „STEADY-STATE SIMULATION“ (i.e. the simu­lation of the steady-­state operat­ing con­dition with con­stant operat­ing speed also known as „simulation in the frequency domain“)

b) the „TIME-TRANSIENT SIMULATION“ (i.e. the „simu­la­tion in the time domain“ to deter­mine the time-­transient response of the drive system depend­ing on time-variable operat­ing con­ditions also consider­ing non­linear effects like gear back­lash)

Possible resonances can be pre­destinat­ed („simu­lat­ed“) – simi­lar to a „wea­ther fore­cast“ of the dyna­mic beha­vior of the com­plete drive sys­tem (drive­line, power­train); here: example of a reci­pro­cating com­pressor drive­line.

Source: RBTS, Inc. / USA (Software: ARMD)

Special Analysis, NVH Studies

Even dynamic-­acoustic effects can be pre­dict­ed as part of so-called NVH studies (NVH = „Noise, Vibra­tion, Harsh­ness“) which are im­por­tant when opti­miz­ing auto­motive drive­lines and com­plete power­trains (cars, trucks, any other vehicles). Spe­cial re­ferences and ex­tend­ed in­for­ma­tion par­ticular­ly on this sub­ject are available on re­quest.

Source: ESI-ITI, Dresden / Germany (Software: SimulationX)

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