Transverse vibration properties of hydraulic cylinders.

摘要

The study and prediction of the transverse vibration properties of the hydraulic cylinder can play an important role in applications such as feedback control systems, condition monitoring and fault diagnostics. Being a structure with varying geometry, the natural frequencies and mode shapes of the hydraulic actuator are functions of the rod extension. The hydraulic actuator is modeled analytically as beams with end masses separated by seals which are modeled as springs. Two analytical models are derived for an air-filled cylinder using free-free boundary conditions. The first model assumes that the cylinder components are rigid. Using the Lagrange's equations, this "rigid" model is intended to obtain the natural frequencies and mode shapes of the first mode. The second model takes into account the flexibility of the barrel and the rod. Using the Hamilton's principle, the model was used to obtain the natural frequencies and mode shapes of the first three deflection modes. A variation of the second model is studied to consider the effect of the oil mass in the natural frequencies of the hydraulic cylinders. An experimental modal analysis was performed on four air-filled and oil-filled hydraulic cylinders at several rod extensions to validate the models. Although the experimental mode shape of the first deflection mode looked like a rigid mode, the rod beam is deflecting inside the cylinder making the rigid model unsuitable for predicting its transverse vibration properties. The elastic model, in addition of providing a better approximation of the natural frequencies, it gives an insight of the mode shape of the rod inside the cylinder barrel. For the hydraulic cylinders studied, the error between the analytical and experimental natural frequencies was smaller for air filled cylinders. Although the modified elastic model improved the prediction of the natural frequencies of oil-filled cylinders by a few Hertz, it seems that the effect of the oil mass is not the only factor affecting the modal properties. The new understanding of the transverse vibration properties of the hydraulic cylinder may help to facilitate future research in vibration control of hydraulic machinery, structural condition monitoring, design of hydraulic cylinders, and other applications.