Critical speed

About: Critical speed is a research topic. Over the lifetime, 2764 publications have been published within this topic receiving 31365 citations.

Optimum Design of Squeeze Film Dampers Supporting Multiple-Mode Rotors

Abstract: In this paper a study of the optimum design of squeeze film dampers for multimode rotors is presented. The optimum design program obtains the best possible damper parameters for a given rotor to satisfy the minimization requirements for the objective function. The objectives are to minimize the amplitude response of the rotor at the critical speed, minimize the force transmitted to the support at the operating speed, or maximize the power dissipated by the damper. A combination of these objectives can also be used, with weighting factors to weigh the importance of each of these objectives. These are the possible objectives for the design of squeeze film dampers for aircraft engine applications. The basis of the optimum design program is an extremely fast algorithm which is able to quickly calculate the unbalance response of a rotor, for circular centered orbits of the journal in the damper. A commercial routine is used for the optimization, and is based on a complex direct search technique. The variation of the optimum clearance, length, and retainer spring stiffness are plotted against various rotor parameters. Recommendations for the design of squeeze film dampers are made. Applications to an aircraft engine illustrate the power of the developed algorithm.

Modal parameter identification of general cutter based on milling stability theory

Abstract: In the field of CNC milling, chatter has been a hot research topic, which is related to machining quality, precision and cost. Stability lobe diagram (SLD) reflects the vibration of the machining system under different process parameters and cutter axis vectors that is significant for optimization. The accurate dynamic characteristics of the machining system is the prerequisite for stability analysis. Finite element simulation is mainly aimed at small diameter cutter system, and accuracy is poor. The most widely used method is hammer test, but the equipment is expensive, the operation is too professional and it cannot reflect the dynamic characteristics of the machining system in working status. This paper proposes an undetermined coefficient method for the general cutter system to identify the modal parameters, that are the natural frequency, stiffness and damping ratio, just based on the very simple experiment of three-axis half-immersion milling in horizontal plane. Firstly, considering the exact in-cut cutting edge and the instantaneous cutting force coefficient corresponding to the axial factor and the chip thickness, the dynamic model of three-axis milling machining for the general cutter is established. Secondly, two implicit conditions of stable critical speed and cutting depth are derived based on feedback control theory in the frequency domain. Thirdly, the two sets of critical cutting depth and the chatter frequency under arbitrary speeds are obtained by using the dichotomy. With the method proposed in this paper, one of the two is used to solve a series of modal parameter sets, and the other of the two is used to extract the optimal modal parameters in the modal parameter sets. Finally, taking the identified modal parameters as known conditions to search the points one by one in the two-dimensional space composed of the rotational speed and the cutting depth, and judge whether it meets the critical conditions. SLD can be obtained by connecting the points that satisfy critical conditions together. Based on the previous experiment of flat-end cutter, it verified the feasibility of the modal parameter identification method in the paper. In the designed three-axis milling experiment of the ball-end cutter, the in-cut cutting edge simulation and the cutting force coefficient identification were carried out, and the modal parameters of the cutter system were also obtained successfully. The plotted lobe diagram was verified by the spectrum analysis result of the vibration signal collected by the acceleration sensor.

Stability analyses for axially moving strings in nonlinear free and aerodynamically excited vibrations

Abstract: Nonlinear free vibration and stability problems are investigated for axially moving strings in transverse motions. The equation of nonlinear, free motion is derived and discretized using the Galerkin’s method. The method of multiple time scale is adopted to obtain the approximate response. It is pointed out that the motion stays stable for transportations with speed less than the linear critical speed. For taut strings that move with large transport speeds, the stability of the equilibrium configuration under steady aerodynamic excitation is explicitly analyzed. Based on the Routh–Hurwitz criterion, the condition for Hopf bifurcation is presented with multiple parameters for transverse motions perturbed in the vicinity of the equilibrium configurations.

Optimal Design of Squeeze Film Supports for Flexible Rotors

Abstract: Assuming central preloading operation below the second bending critical speed and full film lubrication, this paper presents a theoretical model which allows one, with minimum computation, to design squeeze film damped rotors under conditions of high unbalance loading Closed form expressions are derived for the maximum vibration amplitudes pertaining to optimally damped conditions The resulting vibration amplitude and transmissibility data of design interest are presented for a wide range of practical operating conditions on a single chart It can be seen that for a given rotor, the lighter the bearing the more easily one can satisfy design constraints relating to allowable rotor vibration levels and lubricant supply pressure requirements The data presented are shown to be applicable to a wide variety of rotors, and a recommended procedure for optimal design is outlinedCopyright © 1982 by ASME

Analysis of nonstationary processes in cylindrical shells interacting with a fluid flow

Abstract: The paper outlines a technique to analyze the nonstationary vibrations of cylindrical shells interacting with a fluid flow and subjected to external periodic pressure with slowly varying frequency. The dynamic processes occurring in the shell–fluid system as the resonance region is passed forth and back are analyzed