Several mechanical models with lumped mass elements were employed in the literature to represent strongly frequency-dependent characteristics of hydraulic mounts which result from so called inertia track dynamics. Although complex stiffness by the mechanical model showed good agreements with the measured values, there exists a critical pitfall. The fact is that model of the complex stiffness was derived at the driving point while measurements were obtained across the mounts. That is, the point stiffness model was compared mistakenly with the transfer stiffness.
The point stiffness of a vibration isolation element is the same as the transfer stiffness as long as the inertia effects in the element are negligible. The fundamental mistake of the mechanical model with lumped mass in previous studies was that one point-stiffness is not sufficient to describe its dynamic behavior as long as inertia effects are to be represented by a lumped mass. Another point-stiffness and one transfer-stiffness had also to be discussed.
In this paper, conflicts of the mechanical model with lumped mass are discussed by illustrating how different two point-stiffnesses and one transfer-stiffness defined by the lumped mass model are from each other. Also, actual measurement results of the stiffness matrix ; two point-stiffnesses and one transfer-stiffness are shown for a hydraulic mount, where the three measurements of stiffness are nearly identical. It is conclusively discussed that the mechanical models with lumped mass are inappropriate to represent the hydraulic mounts. Instead, a hydraulic model is shown to match well with the measurements.
