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A Comprehensive Study of Self-Excited Vibration in Mechanical Systems

Self excited vibration is one of the main causes of vibration problems in mechanical systems. In our laboratory, the following research is done in order to find a systematic and uniform approach to self-excited vibration.

A Study of Generation Mechanisms and Countermeasures for Self-Excited Vibration in a Time Delay System

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Fig.1 Polygonal Deformation
Phenomenon of
Roll-Covering Rubber

In a time delay system, where the influence of a dynamic event generated at a certain time is fed back to the system itself after a certain period of time has elapsed, an unstable vibration occurs when the time delay meets certain specific conditions. Figure 1 provides the polygonal deformation phenomenon in roll-covering rubber as an example. This phenomenon is considered to be an unstable vibration caused by time delay due to a deformation generated at a contact point between rolls. A component of the viscoelastic deformation of roll-covering rubber is fed back to the contact point after the rotation period of one roll, and an unstable vibration near the natural frequency occurs at a rotating speed near the natural frequency divided by an integral number. An effective countermeasure has not yet been established, though the occurrence of similar vibration phenomena is observed in many rotating machines. In response to this issue, we have investigated the generation mechanism and possible countermeasures for this phenomenon in a time delay system from both theoretical and experimental viewpoints. Our current studies are developments of practical stability analysis and radical countermeasures, and in particular, an optimal design method for dynamic absorbers (Fig. 2 and Fig. 3).

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Fig.2 Experimental Apparatus for Polygonal Deformation
Phenomenon and Countermeasure by Dynamic Absorber

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Fig.3 Experimental Result of
Polygonal Deformation Phenomenon


A Study of Self-Synchronization Phenomena in a Mechanical System with Multiple Self-Excited Oscillators

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Fig.4 Prototype of Tamping
Rammer with a Generation
Mechanism for Synchronous Vibration


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Fig.5 Vibration Waveform at
Handgrip and Impact Plates
of Prototype of Tamping Rammer

The self-synchronization phenomenon, in which multiple nonlinear self-excited oscillators of difference frequencies are synchronized with a specific frequency, occurs as a result of the interaction between oscillators. Although general vibration phenomena are responsible for many problems in mechanical systems, the utilization of the self-synchronization phenomena will help us to develop new advanced technologies and mechanical systems. Today, however, there are few practical applications in the field of mechanical engineering, mainly because the occurrence mechanism of self-synchronization is not yet fully understood. Consequently, we have been investigating self-synchronization phenomena generated in various mechanical systems in order to elucidate the occurrence mechanism of the synchronization and its applications. In our current main project, we are focusing on developing a generation mechanism for synchronous vibration for hand-held vibrating tools. Figure 4 shows a photograph of a prototype tamping rammer used to compact cohesive soils and asphalt. In this apparatus, as shown in Fig. 5, it is possible to generate synchronous vibration, which enables the maintenance of a good balance between vibration control for the handgrip and excitation of the impact plates.

Mechanical Vibration Laboratory,
Department of Mechanical Engineering, Faculty of Engineering, Kyushu University
Professor Takahiro Kondou
Associate Professor Hiroki Mori
Assistant Professor Nobuyuki Sowa
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