Helical Coil Suspension Springs in Finite Element Models of Compressors

This paper discusses two approaches to modeling helical suspension springs for use in a dynamic finite element compressor model. The first approach represents the spring's stiffness characteristics with a simple spring element matrix. The appropriate spring constants are calculated using a separate finite element model of the spring. The spring modeling procedure is described and supplemented with classical solutions and experimental measurements. In addition, spring constants determined for a typical compressor suspension spnng are presented and compared. The second approach represents the spring with its own finite element mesh to include spring stiffness and mass effects in the compressor assembly model. This approach introduces spring surge which can significantly affect the compressor's modal properties. A major difficulty with the second approach is that incorporating a refined finite element mesh of each spring in the assembly model greatly expands the overall model size. To minimize this problem, we present procedures for making coarse-mesh spring models that maintain correct dynamic behavior over the frequency range of interest. Finally, the dynamic characteristics of a compressor assembly model calculated with both spring representations is compared. The number of system eigenvalues approximately doubles in a given frequency range as a result of spring surge frequencies included by the meshed-spring models. d D d.o.f. {d} E {f} g G 1aa J •• [kl Ka l N p y spring wire diameter spring mean coil diameter degree of freedom NOMENCLATURE column matrix of finite element nodal displacements modulus of elasticity column matrix of finite element loads gravitational constant modulus of rigidity area moment of inertia about the a-a axis mass moment of inertia about the a-a axis finite element stiffness matrix spring constant corresponding to the a-direction spring length number of active spring coils mass density specific weight INTRODUCTION Compressor suspension springs are key components in dynamic modeling and analysis of compressor assemblies. The springs support the compressor and are to provide vibration isolation between the compressor and the hermetic shell. This paper addresses two finite element method (FEM) approaches that may be taken to account for the springs and