Experimental Evaluation of Dynamic Characteristics of Circular Arc Spring Dampers for Rotating Machinery

This paper presents an experimental evaluation of dynamic characteristics of Circular Arc Spring Damper (CASD™) which reduce the vibration of rotating machinery. CASD is a novel type of fluid film damper that has two or more arc-shaped beams and dual narrow slits (radial clearances) formed by Wire Electric Discharge Machining (WEDM). A small amount of viscous fluid (typically lubrication oil) is supplied into radial clearances through supply orifices, and outflow through clearances in end seals located at both ends of the damper. The fluid films in radial clearances and end-seal clearances provides viscous damping, while the arc springs provide constant support stiffness for the rotor and improve the linearity of the damping coefficients. A major advantage of CASDs over conventional Squeeze Film Dampers (SFDs) with squirrel-cage springs is their compactness and low weight. The concept of CASDs was originally proposed by Kanki et al. in 2005. However the basic characteristics of this type of damper, including the influence of the geometries, added mass coefficients, and cross-coupled terms of dynamic coefficients have not been investigated.

To clarify those characteristics, we conducted a series of excitation tests on three types of CASDs to identify their damping and added-mass coefficients. The feature of the newly developed test rig and the experimental results are presented in this paper. Tested dampers have same external dimension (160 mm) and damper width (44 mm) but have different arc patterns (2-arc or 4-arc) and radial clearances (about 0.3 or 1.2 mm). All dampers were tested both in open-end configuration and in end-sealed configuration. The influence of the end-seal clearances (about 0.1 to 0.4 mm) was also examined. In the tests, an electro-dynamic shaker provided unidirectional excitation force onto the outer-rim of the tested damper, which was in statically centered position. Lubrication oil was supplied to the test section at controlled pressure and temperature. Linearized dynamic force coefficients were determined using a frequency domain identification method based on a couple of sine-sweeps in orthogonal directions up to 300 Hz. The dynamic stiffness and the moving mass of structural parts were identified from the excitation in dry (no-oil) condition. The static load tests were also conducted to obtain the static stiffness of arc springs.

As a result, the following results were obtained: (1) 4-arc type CASDs have greater damping than that of 2-arc types; (2) CASDs have considerable amount of added mass coefficients, especially in end-sealed condition; (3) Cross-coupled terms are negligibly small; (4) Smaller end-seal clearances make the damping and added-mass coefficients greater significantly; (5) A practical level of damping can be produced with dash-pod configuration (extremely large radial clearances and small end-seal clearances), though the added-mass effect becomes more prominent. These new findings should be useful for the design and application of CASDs to real rotating machineries.