ROTATIONAL AND TORSIONAL VIBRATION ANALYSIS

WHAT ARE ROTATIONAL AND TORSIONAL VIBRATIONS, HOW DO THEY OCCUR AND WHAT DAMAGE DO THEY CAUSE?

Torsional and rotational vibrations are mechanical vibrations caused by time-alternating torques which are superimposed on the otherwise steady running speed of a rotating shaft. In automotive engineering torsional vibration is primarily caused by the fluctuations in engine power output. This results in crankshaft angular velocity fluctuations which cause twisting and untwisting of the shaft. The effects of torsional vibration are amplified by torsional resonance which occurs when a shaft‘s natural frequency coincides with its torsional frequency. Excessive torsional vibrations can result in unwanted noise, powertrain component wear and, in severe cases, broken shafts. To identify such effects in advance and adopt measures to avoid them before ex-cessive damage has occurred, the development engineer requires dedicated, state-of-the-art measuring equipment incorporating application-specific software to simplify measurement setup and provide quick analysis.

WHY DO TORSIONAL VIBRATIONS HAVE TO BE MEASURED?

Accurate measurement and analysis of torsional vibrations is a requirement in engineering and vehicle development departments. In recent years, torsional excitation sources have increased in power and complexity. In addition, the use of lighter materials in engines and drivelines makes them more susceptible to torsional excitations. Continuous optimization of the engine, drivetrain, and rotating components is required to mitigate the resulting comfort and durability issues in new vehicle and rotating component development. Drivetrain simulation models can help development engineers predict and identify torsional resonance scenarios, for example, and design out the problems during the development phase. Detailed and accurate data is essential for fine-tuning, checking and confirming all vehicle improvement measures. Without appropriate data, accurate and meaningful modeling is not possible, as dynamic test data is a prerequisite for parameterization and verification of modeling assumptions.

WHERE DO TORSIONAL VIBRATIONS OCCUR?

WHICH SENSORS ARE USED TO MEASURE SPEED?

The measurement of the rotational speed or angular velocity is usually carried out by means of

  • magnetic scanning of an existing gear or an easily mounted toothed disk
  • Using an optical or magnetic incremental encoder on the shaft
  • Scanning of black and white markings by means of laser optics.

The use of the respective sensor depends on the particular application, the environmental conditions as well as the required resolution.

ROTATIONAL AND TORSIONAL VIBRATIONS ANALYSIS PRINCIPLES AND MEASUREMENT

We provide specialist equipment for the measurement and analysis of torsional vibration. The company‘s core product is the Rotation Analysis System (ROTEC-RAS), a pc-based signal acquisition and analysis system [8]. Torsional vibration measurement re-quires detection of the times of occurrence of equally spaced angular positions around a rotating shaft (e.g. measurement of gear tooth or encoder pulse passage frequencies). Several types of transducers can be used to provide pulse signals which are proportional to a shaft‘s rotational frequency. The RAS speed channels use digital counters with a high-frequency clock (12 GHz, 40 bit) to record the time intervals between pulses. This angular sampling provides a fixed number of data points per revolution which is independent of the rotational speed. The momentary angular velocity of rotating shafts is thus measured, i.e. the mean velocity from pulse to pulse. The vibration angle and the angular acceleration are obtained by integration and differentiation of the measured angular velocity respectively. These two calculations are important when investigating torsional vibration problems. Another important calculation is the angle between two speed channels (angle of twist of a shaft, transmission error between two coupled shafts). The RAS analysis software, working primarily in the angle domain, provides comprehensive analyses in the time and spectral domain (FFT order and frequency analysis). The RAS‘s near real-time capability with display and analysis of all channels allows adjustment of test parameters during the measurement. Apart from digital, 10GHz speed channels, RAS systems are also fitted with additional measuring channels which facilitate conditioning and cap-ture of a variety of analogue signals with sampling rates up to 400kHz. The distinctive feature of ROTEC-RAS is the phase-matched acquisition of all signals: speed signal acquisition with variable discretization of time (angle-equidistant sampling) and acquisition of analogue signals – acceleration, force, pressure, torque, etc. – at constant time intervals (time-equidistant sampling).

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