Critical speed

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

Analysis of Dynamic Characteristics of Gas Turbine Rotor Considering Contact Effects and Pre-Tightening Force

Abstract: Modern gas turbine rotors are usually designed as combined rotors in which disks are tied together by one central tie rod or several tie rods distributed along the circumference. This structure has the advantages in cooling design, light weight and processing assembly, however it brings some problems and challenges in predicting the dynamic characteristics of rotor. No matter how many tie rods are used to fasten the disks, the rotor is not an integral or continuous structure any more. The contact effects between contact faces and the pre-tightening forces of tie rods have great influence on the rotor’s dynamic behaviors. Traditional methods to calculate the critical speed in rotor dynamics such as Transfer Matrix Method and 2-D Finite Element Method (FEM), based on the integral and continuous rotor, fail to consider factors of the contact effects and the pre-tightening forces of tie rods. Although the 3-D FEM can exactly calculate the critical speed, it is still time and resource consuming to establish and calculate such complex three-dimensional structures, even on the most advanced computers at present. In this paper, the traditional 2-D FEM is improved by considering the contact effects and pre-tightening forces of tie rods. Contact faces in the rotor are dealt as elements with equivalent stiffness but without mass, thus the rotor-bearing system of gas turbine are composed of contact elements, elastic elements, rigid disk elements and bearing elements. According to the improved 2-D FEM, a program is developed to calculate the critical speed and unbalance response of gas turbine rotors. The equivalent stiffness, serving as an important input parameter in the program and elements in the stiffness matrix of the system, is mainly determined by the contact stiffness between contact faces and the pre-tightening force. To find out relationships between them, GW (Greenwood and Williamson) statistical model is used and the equivalent stiffness of complex contact faces is obtained. According to the results, certain curves showing the relationship between equivalent stiffness of contact surface and pre-tightening force are obtained. By these curves and the program, we can easily calculate the dynamic characteristics of gas turbine rotors with satisfying accuracy and less time. To validate this method, the critical speed of a real rotor of a certain gas turbine was calculated with the program and curves, and the results agree well with the measured data.© 2008 ASME

Experiments on the Stability and Response of a Flexible Rotor in Three Types of Journal Bearings

Abstract: Three sets of bearings were tested on a three-mass flexible rotor: axial-groove, pressure-dam and preloaded three-lobe, and the instability thresholds and unbalance responses were determined. The pressure-dam bearings were designed for maximum rigid rotor stability; the three-lobe bearings had preload factors of approximately 0.75. Rotor first and second critical speeds were observed at 2550 and 9800 rpm, respectively. The instability thresholds and unbalance responses at the first critical speed for the axial-groove and three-lobe bearings were found to be strongly dependent on the load orientation. Maximum stability limits of 6550, 7400 and 10 400 rpm were found for the axial-groove, pressure-dam, and three-lobe bearings, respectively. Thus, the preloaded three-lobe bearings were the only set that allowed this particular rotor to operate above the second critical speed. The experimental data were also compared to theoretical predictions. Differences in instability thresholds of up to 37 percent were rea...

Arrangement for balancing of a body rotatable about an axis

Abstract: In an arrangement for balancing of a main mass (14) having an imbalance and rotating about an axis (18) balancing masses (23; 24) are used which are relatively freely moveable with respect to the main mass (14) and disposed in at least one closed path (22) symmetrically positioned around said axis (18). The main mass (14) is adapted to be rotated at speeds both below and above the critical speed and the path (22) is connected with the main mass (14) so as to rotate in synchronism therewith. Locking means (34, 36; 42, 43) are provided for locking the balancing masses (23; 24) relative to the path (22) at rotating speeds below the critical speed and for releasing the balancing masses (23; 24) for movement relative to the path (22) at rotating speeds above the critical speed.

Imbalance Vibration Suppression of a Supercritical Shaft via an Automatic Balancing Device

Abstract: This research explores the use of automatic balancing (AB) devices or "autobalancers" for imbalance vibration suppression of flexible shafts operating at supercritical speeds. Essentially an autobalancer is a passive device consisting of several freely moving eccentric masses or balancer balls free to roll within a circular track mounted on a rotor that is to be balanced. At certain speeds, the stable equilibrium positions of the balls are such that they reduce or cancel the rotor imbalance. This "automatic balancing" phenomenon occurs as a result of the nonlinear dynamic interactions between the balancer balls and the rotor transverse vibration. Thus, autobalancer devices can passively compensate for unknown imbalance without the need for a control system and are able to naturally adjust for changing imbalance conditions. Autobalancers are currently utilized for imbalance correction in some single plane rotor applications such as computer hard-disk drives, CD-ROM drives, machine tools and energy storage flywheels. While autobalancers can effectively compensate for imbalance of planar, disk-type, rigid rotors, the use of autobalancing devices for nonplanar and flexible shafts with multiple modes of vibration has not been fully considered. This study explores the dynamics and stability of an imbalanced flexible shaft-disk system equipped with a dual-ball automatic balancing device. The system is analyzed by solving a coupled set of nonlinear equations to determine the fixed-point equilibrium conditions in rotating coordinates, and stability is assessed via eigenvalue analysis of the perturbed system about each equilibrium configuration. It is determined that regions of stable automatic balancing occur at supercritical shaft speeds between each flexible mode. Additionally the effects of bearing support stiffness, axial mounting offset between the imbalance and autobalancer planes, and ball/track viscous damping are explored. This investigation develops a new, efficient, analysis method for calculating the fixed-point equilibrium configurations of the flexible shaft-AB system. Finally, a new effective force ratio parameter is identified, which governs the equilibrium behavior of flexible shaft/AB systems with noncollocated autobalancer and imbalance planes. This analysis yields valuable insights for balancing of flexible rotor systems operating at supercritical speeds.