Research in Mechanical Engineering

VIBRATION AND NOISE

Staff Involved: I.M. Howard, R.D. Entwistle, L. Morgan, K.K. Teh

Project Description(s):

Dynamic modelling of gearbox vibration

To improve the current generation of diagnostic techniques, many researchers are actively developing advanced dynamic models of gear-case vibration to ascertain the effect of differing types of gear-train damage. This research investigates various simplified gear dynamic models aimed at exploring the effect of localised gear tooth damage on the resultant gear-case vibration. Dynamic models have been developed incorporating the effects of variations in gear tooth torsional mesh stiffness, friction, geometrical errors, and multiple shafts. Frictional forces have been included between the teeth by modelling the frictional effects as coulomb friction whereby the forces and moments are dependent on the geometry between the contact point and the gear rotation. Comparison of the results with and without the effects of friction, geometrical errors and tooth damage have been completed using Matlab and Simulink models developed from the differential equations. The effects that the various model factors have on the frequency spectrum, and on the common diagnostic functions of the resulting gearbox component vibrations, have also been investigated. Experimental work is underway to validate and improve the gear dynamic models. Extensive FEA modelling has been completed on the effects of localised tooth damage and geometrical tooth pitch and profile errors on the resulting static transmission errors and the mesh stiffness.

Smart Sensors

Traditional condition monitoring for rotating machines requires a maintenance engineer with expensive equipment to visit every machine in an industrial plant and record data for trending and diagnostic analysis. This equipment may include an accelerometer or velocity probe, charge amplifier, spectrum analyser, data collector and a computer. Each reading may take several minutes to position the accelerometer, set up the equipment, take readings, determine the diagnostics and if required do further computer analysis. Condition monitoring of this type usually occurs on a periodic basis, typically once a month, depending on the cost and critical nature of the machine.

Recent technological developments now permit the monitoring task to be accomplished with cheaper and smaller equipment; much if it being able to be permanently mounted. This research is aimed at eliminating the need for a professional maintenance engineer to perform condition monitoring. Rather, each smart sensor will have the capacity for machine-health monitoring including trending and diagnostics. This will allow an engineer sitting in an office environment, remote from the machine, to ascertain machine condition via the Internet.

The main objective of the current research is to design and prototype a Smart Sensor that is capable of performing detection and diagnostic functions and communicating the condition information to maintenance management systems via Ethernet, Bluetooth or the Internet.

Performance and Condition Monitoring of Large Scale Diesel Engines

In conjunction with DSTO, a research project has commenced that investigates performance and condition-monitoring issues associated with large-scale diesel engines. The research will investigate the use of various transducers, including accelerometers and pressure transducers, to detect changes to the operational performance and condition of the combustion cycle. Signal analysis techniques such as inverse filtering will also be investigated.

Reduction of ship noise and vibration

Aluminium hulled high-speed ferries are a relatively new class of vessel, which have tended to suffer more severely from noise and vibration than their predecessors. The current demands for high speed from more powerful engines, low weight, and economy of construction and operation all tend to exacerbate the inherent noise and vibration problems of any passenger-carrying vehicle.

The hull of the ship can be considered to respond to external forces in two ways, one is the rigid body motion in response to a seaway and the other is the elastic or flexural response of the hull or other structure to internal and external forces. Rigid body motion is not traditionally referred to as a vibration but is considered under the general subject of sea keeping. Flexural vibration can be excited in the form of overall horizontal and vertical bending, torsion and axial modes of the hull main structure, as well as local vibration of substructures and components. The primary sources of vibration excitation are wave motion, main and generator engine firing and propeller induced vibration.

The engine room is inherently noisy. This noise is transmitted through the passenger space floor. It is also carried out of the vessel through the engine room air vents and is emitted by the exhaust pipes. In cases where the exhaust is discharged underwater, close to the engine room, the exhaust noise emerges from below the water when the ship rolls. Any noise on the outside of the ship may re-enter the passenger space through glazing and side panels. In some cases it is regenerated by panel vibration. The application of constrained layer damping to these panels may reduce the regeneration of noise, and may prove effective in reducing overall noise levels. However, the additional weight (around 20% for each panel) means there may be a significant reduction in vessel speed.

Isolation of the passenger space by the use of a 'floating' floor has been attempted but with limited success. Potentially, this could be the most effective means of increasing passenger comfort but its successful application depends on an understanding of the interaction of two non-rigid bodies with many modal responses, separated but constrained by an elastic medium. Numerical modelling, finite-element modelling and testing of an experimental structure is being used to investigate the potential for reductions in noise and vibration from the use of flexible separation of the hull and superstructure.

Axisymmetric vibration modes of thin circular discs

The various vibration modes of circular disk-like components are of significance in many engineering applications. Examples include disk-brake squeal, circular saw vibration leading to noise, increased kerf width and heat generation, grinding wheel vibrations and turbine rotor disk motions. All of these are examples where the transverse vibrations are detrimental.

Little research has been undertaken into the effects of the axisymmetric in-plane modes of vibration that may be present and excited in all of the above examples. In the cases where transverse stationary loads are present such as in grinding, sawing and disk brakes, the interaction between the transverse and torsional modes may be significant.

This research aims to investigate the implications of the axisymmetric modes of vibration in the important areas cited above.

Torsional vibration in machine tool chatter

Machine tool chatter theories have generally assumed that the cutting forces are essentially independent of the cutting speed and that the rotating components revolve at constant speed. These assumptions have also been applied to grinding chatter models. However, unlike most other machining processes, the grinding force models in contemporary use show a dependence on both the workpiece surface speed and the grinding wheel surface speed. It follows that if the workpiece and its drive system, or the grinding wheel and its drive system, are torsionally flexible then the cutting forces would be modified by any torsional vibrations present. Furthermore, the chatter response of the grinding machine may be affected by the presence of these torsional flexibilities.

This research tests the hypothesis that torsionally compliant workpiece and grinding wheel drive systems will modify the chatter response of cylindrical grinding processes.

A review of the literature, which includes the historical development of chatter theories in general and grinding in particular, shows that all previous grinding chatter models have taken the drive systems to be torsionally stiff and assumed constant rotational speeds.

A mathematical model that includes drive system torsional compliances has been formulated and a solution technique developed which enables the stability boundary to be located. The model has the capability to estimate the growth or decay rate of chatter vibrations. An alternative mathematical model has been developed in order to provide confirmation. Numerical experiments have been conducted using the model and these reveal that the presence of torsional compliances has a significant effect on the system dynamics, particularly if the torsional system resonances are in the vicinity of the chatter frequency. Three new chatter modes have been discovered which have not been previously associated with grinding. They are (a) a coupled mode chatter involving the transverse and workpiece torsional degrees of freedom, (b) regeneration that has a chatter frequency less than the natural frequency of the machine tool structure and (c) a grinding wheel torsional chatter.

The broad conclusions of the research to date are that torsionally compliant workpiece and grinding wheel drive systems can have a significant influence on the stability and chatter growth rates in cylindrical grinding.

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