Frits Petit

On the Attachment Location of Dynamic Vibration Absorbers

In mechanical engineering a commonly used approach to attenuate vibration amplitudes in resonant conditions is the attachment of a dynamic vibration absorber. The optimal parameters for this damped spring-mass system are well known for single degree of freedom undamped main systems (Den Hartog). An important parameter when designing absorbers for multi degree of freedom systems is the location of the absorber, i.e. where to physically attach it. This parameter has a large influence on the possible vibration reduction. Often however, anti nodal locations of a single mode are a priori taken as best attachment locations. This single mode approach loses accuracy when dealing with a large absorber mass or systems with closely spaced eigenfrequencies. To analyze the influence of the neighboring modes, the effect the absorber has on the eigenfrequencies of the undamped main system is studied. Given the absorber mass we determine the absorber locations that provide eigenfrequencies shifted as far as possible from the resonance frequency as this improves the vibration attenuation. It is shown that for increasing absorber mass, the new eigenfrequencies cannot shift further than the neighboring anti resonances due to interlacing properties. Since these anti resonances depend on the attachment location, an optimal location can be found. A procedure is described that yields the optimalabsorber location. This procedure combines information about the eigenvector of the mode to be controlled with knowledge about the neighboring anti resonances. As the neighboring anti resonances are a representation of the activity of the neighboring modes, the proposed method extends the commonly used single mode approach to a multi mode approach. It seems that in resonance, a high activity of the neighboring modes has a negative effect on the vibration reduction.

Torsional Vibrations on a Hopper Dredger Due to Transient Conditions

Torsional vibrations are known to be a major threat to the drive line of every ship. While classification societies demand a thorough analysis of the regime behavior, the transient behavior is not studied. Transient loads involve among others starting/stopping the engine, engaging/disengaging clutches, altering the vessel’s speed and changing the pitch of the propeller. These high load changes are known to be a source of severe damage to gear boxes and flexible couplings. Further more, they tend to disturb the interaction between the mass-elastic system and the speed controller (governor instability). In this article, the different transient loads are described in detail for the specific case of a trailing suction hopper dredger. The focus lies on the engaging and disengaging behavior of the pump clutch. The different factors contributing to this transient load and their influence are explained without the use of a complex simulation model.Copyright

Multifunctional Pilot Plant for Mechanical Engineers

Cross-course projects are very important in mechanical engineering curricula. Based on the dynamic response of a structure to a ground motion, a pilot plant for hands-on training has been developed which incorporates structural analysis, experimental modelling, mechanical vibration, servo control, signal processing, kinematics and dynamics of machinery.

FEASIBILITY STUDY OF VIBRATION ABSORBERS WITH SIMPLIFIED SHIP PROPULSION MODELS

Abstract Torsional vibrations on the propulsion drive train of a ship due to harmonic loads are thoroughly analyzed by classification societies. During operation however the drive train is subjected to transient loads and the mass- elastic system interacts with the speed controller. Here we derive a smart reduced model of the ship propulsion system capable of revealing the low frequent dynamics which dominate the transient response. A possible approach to counter the damage caused by transient loads, is the attachment of a vibration absorber. The simplified model allows us to check the feasibility of this approach.