Torsional vibration

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Torsional vibration is the angular vibration of an object - commonly a shaft - along its axis of rotation. Torsional vibration is often a concern in power transmission systems using rotating shafts or couplings, where it can cause failures if not controlled. A second effect of torsional vibrations applies to passenger cars. Torsional vibrations can lead to seat vibrations or noise at certain speeds. Both reduce the comfort.

Contents

In ideal power generation (or transmission) systems using rotating parts, the torques applied or reacted are "smooth" leading to constant speeds, and the rotating plane where the power is generated (input) and the plane it is taken out (output) are the same. In reality this is not the case. The torques generated may not be smooth (e.g., internal combustion engines) or the component being driven may not react to the torque smoothly (e.g., reciprocating compressors), and the power generating plane is normally at some distance to the power takeoff plane. Also, the components transmitting the torque can generate non-smooth or alternating torques (e.g., elastic drive belts, worn gears, misaligned shafts). Because no material can be infinitely stiff, these alternating torques applied at some distance on a shaft cause twisting vibration about the axis of rotation.

Sources of torsional vibration

Torsional vibration can be introduced into a drive train by the power source. But even a drive train with a very smooth rotational input can develop torsional vibrations through internal components. Common sources are:

Crankshaft torsional vibration

Torsional vibration is a concern in the crankshafts of internal combustion engines because it could break the crankshaft itself; shear-off the flywheel; or cause driven belts, gears and attached components to fail, especially when the frequency of the vibration matches the torsional resonant frequency of the crankshaft. Causes of the torsional vibration are attributed to several factors.

If torsional vibration is not controlled in a crankshaft it can cause failure of the crankshaft or any accessories that are being driven by the crankshaft (typically at the front of the engine; the inertia of the flywheel normally reduces the motion at the rear of the engine). The couplings turn the vibration energy into heat. Therefore, and to ensure that the coupling is not damaged due to this (temperature could be very high, depending on the load), this is verified through torsional vibration calculation. [3]

This potentially damaging vibration is often controlled by a torsional damper that is located at the front nose of the crankshaft (in automobiles it is often integrated into the front pulley). There are two main types of torsional dampers.

Torsional vibrations in electromechanical drive systems

Torsional vibrations of drive systems usually result in a fluctuation of the rotational speed of the rotor of the driving electric motor. Such oscillations of the angular speed superimposed on the average rotor rotational speed cause perturbations of the electromagnetic flux, leading to additional oscillations of the electric currents in the motor windings. Then, the generated electromagnetic torque is also influenced by additional time-varying electromechanical interactions, which lead to further torsional vibrations of the drive system. According to the above, mechanical vibrations/oscillations of the drive system become coupled with the electrical oscillations of the motor windings' currents. Such coupling is typically nonlinear and presents a high computational burden.

Due to the highly nonlinear and coupled nature of electromechanical oscillations, approximations are often used, enabling such oscillations to be characterized analytically. To simplify the characterization of the oscillations between mechanical and electric systems, it is common to assume the mechanical and electrical components are uncoupled. Then, by holding either the mechanical or electrical aspect in steady-state, the characteristic of the other can be calculated. A common method is to apply electromagnetic torques generated by the electric motors as assumed excitation functions of time or of the rotor-to-stator slip, [4] [5] [6] which are usually based on numerous experimental measurements carried out for a given electric motor's dynamic behaviour. For this purpose, by means of measurement results, i.e., empirically, formulas have been developed that provide good approximations for the electromagnetic external excitations produced by the electric motor. [7] Although the electric currents flowing in the electric motor windings are accurate, the mechanical drive system is typically reduced to one or seldom to at most a few rotating rigid bodies. [8] In many cases, such simplifications yield sufficiently useful results for engineering applications, but they can lead to inaccuracies since many qualitative dynamic properties of the mechanical systems, e.g., their mass distribution, torsional flexibility, and damping effects, are neglected. Thus, an influence of the oscillatory behaviour of drive systems on the electric machine rotor angular speed fluctuations, and in this way on the electric current oscillations in the rotor and stator windings, cannot be investigated with a satisfactory precision, excepting numerical methods, which can provide arbitrarily high accuracy.

Mechanical vibrations and deformations are phenomena associated with the operation of the majority of railway vehicle drivetrain structures. The knowledge about torsional vibrations in transmission systems of railway vehicles is of a great importance in the field of mechanical system dynamics. [9] Torsional vibrations in railway vehicle drivetrains are generated by many coupled mechanisms, which are very complex and can be divided into two main parts:

  1. The electromechanical interactions within the railway drivetrain system, including the electric motor, gears, and the driven parts of the disc and gear clutches. [10]
  2. Torsional vibrations of the flexible wheels [11] [12] and wheelsets caused by variations of the adhesion forces in the wheel-rail contact zone. [13]

An interaction of the adhesion forces has nonlinear features which are related to the creep value and strongly depend on the wheel-rail zone conditions and the track geometry (especially when driving on a curve section of the track). In many modern mechanical systems, torsional structural deformability plays an important role. Often the study of railway vehicle dynamics using the rigid multibody methods without torsionally deformable elements are used [14] This approach does not enable analysis of the self-excited vibrations, which have an important influence on the wheel-rail longitudinal interaction. [15] A dynamic modelling of the electrical drive systems coupled with elements of a driven machine [16] [17] or vehicle is particularly important when the purpose of such modelling is to obtain an information about the transient phenomena of system operation, like run-up, run-down, and the loss of adhesion in the wheel-rail zone. The modelling of an electromechanical interaction between the electric driving motor and the machine also influence the self-excited torsional vibrations in the drive system. [18] [19]


Measuring torsional vibration on physical systems

The most common way to measure torsional vibration is the approach of using equidistant pulses over one shaft revolution. Dedicated shaft encoders as well as gear tooth pickup transducers (induction, hall-effect, variable reluctance, etc.) can generate these pulses. The resulting encoder pulse train is converted into either a digital rpm reading or a voltage proportional to the rpm.

The use of a dual-beam laser is another technique that is used to measure torsional vibrations. The operation of the dual-beam laser is based on the difference in reflection frequency of two perfectly aligned beams pointing at different points on a shaft. Despite its specific advantages, this method yields a limited frequency range, requires line-of-sight from the part to the laser, and represents multiple lasers in case several measurement points need to be measured in parallel.

Torsional vibration software

There are many software packages that are capable of solving the torsional vibration system of equations. Torsional vibration specific codes are more versatile for design and system validation purposes and can produce simulation data that can readily compared to published industry standards. These codes make it easy to add system branches, mass-elastic data, steady-state loads, transient disturbances and many other items only a rotordynamicist would need. Torsional vibration specific codes:

Bond Graphs can be used to analyse torsional vibrations in generator sets, such as those used aboard ships. [22]

See also

Bibliography

Related Research Articles

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<span class="mw-page-title-main">Synchronous motor</span> Type of AC motor

A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. Doubly fed synchronous motors use independently-excited multiphase AC electromagnets for both rotor and stator.

<span class="mw-page-title-main">Brushless DC electric motor</span> Synchronous electric motor powered by an electronic controller

A brushless DC electric motor (BLDC), also known as an electronically commutated motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields that effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the current pulses that control the speed and torque of the motor. It is an improvement on the mechanical commutator (brushes) used in many conventional electric motors.

<span class="mw-page-title-main">Synchro</span> Variable transformers used in control systems

A synchro is, in effect, a transformer whose primary-to-secondary coupling may be varied by physically changing the relative orientation of the two windings. Synchros are often used for measuring the angle of a rotating machine such as an antenna platform or transmitting rotation. In its general physical construction, it is much like an electric motor. The primary winding of the transformer, fixed to the rotor, is excited by an alternating current, which by electromagnetic induction causes voltages to appear between the Y-connected secondary windings fixed at 120 degrees to each other on the stator. The voltages are measured and used to determine the angle of the rotor relative to the stator.

<span class="mw-page-title-main">Dynamometer</span> Machine used to measure force or mechanical power

A dynamometer or "dyno" is a device for simultaneously measuring the torque and rotational speed (RPM) of an engine, motor or other rotating prime mover so that its instantaneous power may be calculated, and usually displayed by the dynamometer itself as kW or bhp.

<span class="mw-page-title-main">Drive shaft</span> Mechanical component for transmitting torque and rotation

A drive shaft, driveshaft, driving shaft, tailshaft, propeller shaft, or Cardan shaft is a component for transmitting mechanical power, torque, and rotation, usually used to connect other components of a drivetrain that cannot be connected directly because of distance or the need to allow for relative movement between them.

Hybrid Synergy Drive (HSD), also known as Toyota Hybrid System II, is the brand name of Toyota Motor Corporation for the hybrid car drive train technology used in vehicles with the Toyota and Lexus marques. First introduced on the Prius, the technology is an option on several other Toyota and Lexus vehicles and has been adapted for the electric drive system of the hydrogen-powered Mirai, and for a plug-in hybrid version of the Prius. Previously, Toyota also licensed its HSD technology to Nissan for use in its Nissan Altima Hybrid. Its parts supplier Aisin offers similar hybrid transmissions to other car companies.

<span class="mw-page-title-main">Motor drive</span> Piece of machine equipment

A motor drive is a physical system that includes a motor. An adjustable speed motor drive is a system that includes a motor that has multiple operating speeds. A variable speed motor drive is a system that includes a motor that is continuously variable in speed. If the motor is generating electrical energy rather than using it, the motor drive could be called a generator drive but is often still referred to as a motor drive.

Engine balance refers to how the inertial forces produced by moving parts in an internal combustion engine or steam engine are neutralised with counterweights and balance shafts, to prevent unpleasant and potentially damaging vibration. The strongest inertial forces occur at crankshaft speed and balance is mandatory, while forces at twice crankshaft speed can become significant in some cases.

<span class="mw-page-title-main">Swashplate</span> Mechanism to convert between reciprocating and rotary motion

A swashplate, also known as slant disk, is a mechanical engineering device used to translate the motion of a rotating shaft into reciprocating motion, or vice versa. The working principle is similar to crankshaft, Scotch yoke, or wobble, nutator, and Z-crank drives in engine designs. It was originally invented to replace a crankshaft, and is one of the most popular concepts used in crankless engines. It was invented by Anthony Michell in 1917.

<span class="mw-page-title-main">Rotor (electric)</span> Non-stationary part of a rotary electric motor

The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor's axis.

In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity to mechanical power while an electric generator converts mechanical power to electricity. The moving parts in a machine can be rotating or linear. While transformers are occasionally called "static electric machines", since they do not have moving parts, generally they are not considered "machines", but as electrical devices "closely related" to the electrical machines.

<span class="mw-page-title-main">Harmonic damper</span> Vibration damping system in an engine

A harmonic damper is a device fitted to the free end of the crankshaft of an internal combustion engine to counter torsional and resonance vibrations from the crankshaft. This device must be an interference fit to the crankshaft in order to operate in an effective manner. An interference fit ensures the device moves in perfect step with the crankshaft. It is essential on engines with long crankshafts and V8 engines with cross plane cranks, or V6 and straight-three engines with uneven firing order. Harmonics and torsional vibrations can greatly reduce crankshaft life, or cause instantaneous failure if the crankshaft runs at or through an amplified resonance. Dampers are designed with a specific weight (mass) and diameter, which are dependent on the damping material/method used, to reduce mechanical Q factor, or damp, crankshaft resonances.

On maritime vessels, noise and vibration are not the same but they have the same origin and come in many forms. The methods to handle the related problems are similar, to a certain level, where most shipboard noise problems are reduced by controlling vibration.

<span class="mw-page-title-main">Geislinger coupling</span>

The Geislinger coupling is an all-metal coupling for rotating shafts. It is elastic in torsion, allowing it to absorb torsional vibration.

<span class="mw-page-title-main">Centrifugal pendulum absorber</span>

A centrifugal pendulum absorber is a type of tuned mass damper. It reduces the amplitude of a torsional vibration in drive trains that use a combustion engine.

<span class="mw-page-title-main">Drivetrain</span> Group of components that deliver power to the driving wheels

A drivetrain or transmission system, is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the drivetrain is the components of a motor vehicle that deliver power to the drive wheels. This excludes the engine or motor that generates the power. In marine applications, the drive shaft will drive a propeller, thruster, or waterjet rather than a drive axle, while the actual engine might be similar to an automotive engine. Other machinery, equipment and vehicles may also use a drivetrain to deliver power from the engine(s) to the driven components.

Electromagnetically induced acoustic noise, electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials vibrating under the excitation of electromagnetic forces. Some examples of this noise include the mains hum, hum of transformers, the whine of some rotating electric machines, or the buzz of fluorescent lamps. The hissing of high voltage transmission lines is due to corona discharge, not magnetism.

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References

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