Identification of Blocked Forces and Moments of an Oil Pump Fixed on a Test Bench by Using an Inverse Vibration Technique

The major difficulty of the structure borne noise source characterization is the strong dependence of the coupling forces with respect to the input mobility of the reception structures and/or the mechanical link between the source and the receiver. The placement of a force sensor can then be too intrusive and the use of inverse techniques on the real reception structure is difficult to apply, because the accessibility can be limited and tests must then be applied for all structures likely to be linked with the studied source. The aim of this paper is to propose a general technique to identify the intrinsic characteristics that are the blocked forces and moments on a simple test bench hosting the source. The test bench consists in mounting the source on a vibrating beam where the mechanical link corresponds exactly to the link between the source and the final receiver. The vibration displacement field of the beam is then measured in two configurations: when the source is switched on and when the source is switched off while the beam is excited by a shaker. The application of the RIFF technique (also known as Force Analysis Technique) allows one to identify the force distribution applied to the beam. The second configuration (beam excited by the shaker) enable the identification of the force distribution due to the attachment of the source only. This force distribution can then be used to correct the beam equation of motion such that the effect of the link can be avoided. The RIFF technique can then be used to identify the blocked force distribution from the displacement measured in the first configuration (source switched on). The interest to identify the force distribution is that is it is possible to extract forces and moments applied to attached points by the source thanks to local spatial integrations around each attachment points. In this paper, the considered source is a oil pump that can be mounted on different engines. The study focuses on the blocked forces and moments applied by two close attachment points. Two post-processing are then proposed, the first consists in extracting forces and moments for each point, the second consists in calculating the equivalent force and equivalent moment applied by both points. Results are shown for all of these proposal, demonstrating the interest to consider the equivalent forces and moments in the low frequency domain.