Prediction of Forces Applied on a Receiver by Two Vibrating Structures Installed in Series; Application to an Engine Cooling Fan Mounted on a Car Chassis

Abstract

Prediction of vibration and/or acoustic radiation of machines is an old problem which has in part been solved via a linear vibrations approach. Equipment mounted on a host structure, or receiver, is often responsible for vibration and radiation of the latter. The equipment is called source (electrical motor, ventilation system, cylinder and vibrating laminae in a music box . . . ). The host structure is passive (receiver structure of an engine, building supporting the ventilation system, music box itself. . . ). Beyond the case where one source structure is coupled with a host structure, there are a great number of configurations where the source structure excites an intermediary structure, itself mounted on the receiver. Our contribution pertains to such cases with an application to a car engine cooling fan system: the fan (source) is attached via elastic connections to a plate (intermediary structure), which excites the chassis (receiver) through rigid and elastic links. The vibratory behaviour of the assembled structure depends on those of the structures, which are known either by their technical specifications or because they are physically obtainable. According to the situation, there may or may not be a choice between different methods to predict the vibration and/or acoustic radiation of the host structure. In cases where choice exists, the predictions obtained by different methods vary and this paper attempts to show how to obtain the best prediction of forces imposed on the receiver structure. The originality is to be found in a metrological procedure more apt than another to predict the vibratory behaviour of an assembly in series of three structures in an industrial context. Indeed, when inertance matrices necessary for the prediction are measured on isolated structures, the prediction is of lesser quality than when a certain number of them are measured on coupled structures. This echoes a similar conclusion drawn recently from academic configurations.