Showing papers on "Critical speed published in 1997"

Finite element analysis of frictionally excited thermoelastic instability

Abstract: The frictional heat generated during braking causes thermoelastic distortion that modifies the contact pressure distribution. If the sliding speed is sufficiently high, this can lead to frictionalfy excited thermoelastic instability, characterized by major nonuniformi-ties in pressure and temperature. In automotive applications, a particular area of concern is the relation between thermoelasticalfy induced hot spots in the brake disks and noise and vibration in the brake system. The critical sliding speed can be found by examining the conditions under which a perturbation in the temperature and stress fields can grow in time. The growth has exponential character, and subject to certain restrictions, the growth rate b is found to be real. The critical speed then corresponds to a condition at which b = 0 and hence at which there is a steady-state solution involuing nonuniform contact pressure. We first treat the heat sources Q at the contact nodes as given and use standard finite element analysis (FEA) to d...

Ambient-temperature passive magnetic bearings: Theory and design equations

Abstract: Research has been underway at the Lawrence Livermore National Laboratory to build a theoretical and experimental base for the design of ambient-temperature passive magnetic bearings for a variety of possible applications. in the approach taken the limitations imposed by Earnshaw`s theorem with respect to the stability of passive magnetic bearing systems employing axially symmetric permanent-magnet elements are overcome by employing special combinations of elements, as follows: Levitating and restoring forces are provided by combinations of permanent-magnet-excited elements chosen to provide positive stiffnesses (negative force derivatives) for selected displacements (i.e., those involving translations or angular displacement of the axis of rotation). As dictated by Eamshaw`s theorem, any bearing system thus constructed will be statically unstable for at least one of the remaining possible displacements. Stabilization against this displacement is accomplished by using periodic arrays (`Halbach arrays`) of permanent magnets to induce currents in close-packed inductively loaded circuits, thereby producing negative force derivatives stabilizing the system while in rotation. Disengaging mechanical elements stabilize the system when at rest and when below a low critical speed. The paper discusses theory and equations needed for the design of such systems.

On the transition from brittle to plastic failure in breaking a nanocrystal under tension (NUT)

Abstract: We are simulating the dynamical failure of three-dimensional notched solids under tension using molecular dynamics, simple interatomic force laws and system sizes from 10 to more than 100 millions atoms. The rare-gas solid will cleave brittlely at failure under mode I loading in the 110 direction. With sufficient loading, the crack velocity can reach a speed where a transition from brittle failure to ductile failure occurs. We show that this critical speed is independent of the loading history and is approximately 0.36 of the Rayleigh sound speed, consistent with our suggestion that this transition is initiated by an intrinsic instability in the crack dynamics.

Method of determining the traction of motor vehicle wheels

Abstract: In a method of determining the traction of motor vehicle wheels on wet pavements, a surge force generated by a water film on the pavement in front of at least one of the vehicle wheels is measured and from the measured surge force value and the instantaneous vehicle speed a critical speed at which the vehicle wheel will aqua-plane is determined and this critical speed is displayed to the driver or it is used to control a vehicle drive unit to keep the vehicle speed below the critical speed.

Engine idle rotation speed controller

Abstract: Engine rotation speed during idle running is feedback-controlled. A second target rotation speed which progressively decreases towards a first target rotation speed from a predetermined engine rotation speed, is set. The engine rotation speed is feedback-controlled based on a difference between a present rotation speed and this second target rotation speed. In this way, compared to the case where feedback control is performed using the first target rotation speed as a target value, the time period during which the engine rotation speed falls below the first target rotation speed is shortened and the engine rotation speed is made to stably converge to the first target rotation speed.

An Analytical Assessment of the Critical Speed Formula

Abstract: The Critical Speed Formula (CSF) is used in the field of accident reconstruction for the estimation of the speed of a vehicle that has been given a sudden unidirectional steer manoeuvre by the driver and when the tires develop a high enough sideslip to leave curved visible marks on the pavement. This and other uses of the formula are investigated in this paper. Reconstructions are done using computerized dynamic simulations of a turn manoeuvre for 3 different, driver forward control modes: braking, coasting and accelerating. The experimental results of T. Shelton (Accident Reconstruction Journal, 1995), are analyzed statistically, and are compared to the results of the simulations. Results show that the CSF can give reasonably accurate results but that the accuracy varies with several factors. One is where along the trajectory measurements are made to estimate the tire mark curvature. Another factor is the forward control mode; the accuracy is the highest when the vehicle accelerates through the turn and is the lowest for braking. The experimental data is also used to determine the statistical uncertainty of speed estimation. (A) For the covering abstract of the conference see IRRD 899758.

Control apparatus for drive device for vehicle

Abstract: PROBLEM TO BE SOLVED: To suppress the generation of a speed-change shock by a method, wherein the speed of rotation of a motor generator is controlled in such a way that the input speed of rotation for an automatic transmission becomes a set target speed rotation during a speed change. SOLUTION: A control apparatus is provided with a target-speed-of-rotation setting means by which a target speed of rotation during the speed change of the input speed of the rotation of an automatic transmission 3 is set during a speed change and with a speed-of-rotation control means by which the speed of rotation of a motor generator 4 is controlled such that the input speed of rotation of the automatic transmission 3 becomes at set target speed of rotation during the speed change. Then, when a speed is controlled by the motor generator 4, the target speed-change time as a map value on a speed-change map which is set in advance at every speed change stage is decided, the target speed of rotation is decided, the speed of rotation of the motor generator 4 traces a desired target speed of rotation, especially only at the inertial phase during the speed change, and the input speed of rotation of the automatic transmission 3 is controlled with good accuracy. As a result, while the generation of a speed-change shock 7 due to a deviation is being suppressed, a speed change can be achieved in a desired speed-change time.

Nonstationary oscillation of a rotating shaft with nonlinear spring characteristics during acceleration through a major critical speed (a discussion by the asymptotic method and the complex-FFT method)

Abstract: Nonstationary oscillations during acceleration through a major critical speed of a rotating shaft with nonlinear spring characteristics are discussed. First, the first approximate solutions of steady-state and nonstationary oscillations are obtained by the asymptotic method. Second, the amplitude variation curves of each oscillation component are obtained by the complex-FFT method. It is clarified that the first approximation of the asymptotic method has comparatively large quantitative error in the case of nonstationary solutions. In addition, the influences of each nonlinear component in polar coordinate expression on nonstationary oscillations are investigated.

Critical speed for the avoidance of blow-up in a reactive-diffusive medium

Abstract: A concentrated heat source moves at constant speed through a diffusive medium which can support an explosive reaction. If the conditions are such that a blow-up would occur for a stationary source, it is shown that there is a critical speed above which the explosion can be avoided. The reactive-diffusive medium is assumed to be contained in an infinitely long tube of general cross-section. Both the cases of Neumann and Dirichlet boundary conditions imposed on the tube surface are considered.

Use of a Flexible Internal Support to Suppress Vibrations of a Rotating Shaft Passing Through a Critical Speed

Abstract: As is widely known, large vibrations may occur when a rotating shaft passes through one of its critical speeds. To suppress these large motions, it may be possible to modify the system as it is acc...

In-Plane Stress and Displacement Distributions in a Spinning Annular Disk Under Stationary Edge Loads

Abstract: It is well known that the in-plane stress and displacement distributions in a stationary annular disk under stationary edge tractions can be obtained through the use ofAiry stress function in the classical theory of linear elasticity. By using Lame's potentials, this paper extends these solutions to the case of spinning disk under stationary edge tractions. It is also demonstrated that the problem of stationary disk-spinning load differs from the problem ofspinning disk-stationary load not only by the centrifugal effect, but also by additional terms arising from the Coriolis effect. Numerical simulations show that the amplitudes of the stress and displacement fields grow unboundedly as the rotational speed of the disk approaches the critical speeds. As the rotational speed approaches zero, on the other hand, the in-plane stresses and displacements are shown, both numerically and analytically, to recover the classical solutions derived through the Airy stress function.

Multi-objective Optimization of a Flexible Rotor in Magnetic Bearings With Critical Speeds and Control Current Constraints

Abstract: This paper presents the single objective optimization and the multi-objective optimization for a flexible rotor system with magnetic bearings. The weight of rotor shaft and the transmitted forces at the magnetic bearings are minimized either individually or simultaneously under the constraints on the critical speeds and the control currents of magnetic bearings. The design variables are the cross-sectional area of the shaft, the bias currents of magnetic bearings, and the positions of the disk and the magnetic bearings. The dynamic characteristics are analyzed using the generalized polynomial expansion method and the sensitivity analysis is also studied. For single objective optimization, the method of feasible directions (MFD) is applied. For multi-objective optimization, the weighting method (WM), the goal programming method (GPM), and the fuzzy method (FM) are employed. It is found that the system design can be significantly affected by the choices of the bias currents of magnetic bearings, the position of the disk with unbalance, and the magnetic bearings. The results also show that a better compromised design can always be obtained for multi-objective optimization.

Composite electric motor shaft

Abstract: A composite electric motor shaft including axially distributed metallic and nonmetallic sections joined together exhibit increased mechanical damping, increased rotor critical speed, improved corrosion resistance and improved motor performance.

PI control of HSFDs for active control of rotor-bearing systems

Abstract: This paper describes the proportional integral (PI) control of hybrid squeeze film dampers (HSFDs) for active control of rotor vibrations. Recently it was shown that the automatically controlled HSFD based on feedback of rotor speed can be a very efficient device for active control of rotor vibration when passing through critical speeds. Although considerable effort has been put into the study of steady-state vibration control, there are few methods in the literature applicable to transient vibration control of rotor-bearing systems. Rotating machinery may experience dangerously high dynamic loading due to the sudden mass unbalance that could be associated with blade loss. Transient run-up and coast down through critical speeds when starting up or shutting down rotating machinery induces excessive bearing loads at criticals. In this paper, PI control is proposed as a regulator for the HSFD system to attenuate transient vibration for both sudden unbalance and transient run-up through critical speeds. A complete mathematical model of this closed-loop system is simulated on a digital computer. Results show an overall enhanced behavior for the closed-loop rotor system. Gain scheduling of both the integral gain and the reference input is incorporated into the closed-loop system with the PI regulator and results inmore » an enhanced behavior of the controlled system.« less

Speed controller for vehicle

Abstract: PROBLEM TO BE SOLVED: To provide a speed controller for vehicle with which comfortable/safe travel can be provided by decreasing the driving operation load of a driver by traveling a vehicle basically without requiring any accelerator operation to the driver SOLUTION: Speed control is executed by adjusting the driving power of an engine 24 so that real car speed can be coincident with target car speed but this speed control is executed by utilizing a navigation function Namely, a CD-ROM 5a stores map data expressing a road map and reference speed determined based on critical speed on that road at least and based on the current position of a present vehicle, the reference speed is read out of the map data corresponding to the road where the present vehicle is traveled at present Then, the target speed is determined based on that read reference speed and the speed control is executed

Receptance Method for the Sensitivity Analysis of Critical Speeds to Rotor Support Stiffness

Abstract: A rotor system can be studied via various approaches, e.g., FEM, the transfer matrix method, etc. The receptance method has been one of the methods used for frequency analysis of rotors. The authors, herein, have shown an application of the receptance matrix for sensitivity analysis. Examples of critical speed sensitivity to support stiffness were illustrated and a computing algorithm was developed. Numerical examples proved the approach to be valuable for rotor engineers in quick evaluation and understanding of the support effects.

Electro-hydraulic pump unit and small diameter pipe propulsion device

Abstract: PROBLEM TO BE SOLVED: To use a hydraulic source which is small in energy loss, is not affected by the contamination, and is small in the capacity and the outer diameter SOLUTION: In an electro-hydraulic pump unit in which the drive source of a hydraulic pump 3 to drive a cylinder 6 is a three-phase induction motor 1, an inverter 8 is interposed in the power supply line of the three-phase induction motor 1 The critical discharge Q0 in the range in which the three-phase induction motor 1 is not overloaded, and the critical speed v0 of the three-phase induction motor 1 are obtained by a computing element 11 based on the discharge pressure signal S2 detected by a hydraulic pressure detector 10, and the inverter 8 is controlled so that the speed of the three-phase induction motor 1 is not more than the critical speed v0 , ie, so that the discharge flow rate of the hydraulic pump 3 is not more than the critical discharge Q0

Method for locating critical speed paths in integrated circuits

Abstract: A method for locating a critical speed path within a integrated digital circuit. First, the area containing the critical speed path is isolated by selectively enabling a delay of clock driver circuits in the integrated circuit. Isolating the location of the speed path consists of determining a source clock driver that clocks the source of the speed path and a destination clock driver that clocks the destination of the speed path. The possible data path that may be the speed path are then further narrowed down by examining a connection database that lists all the data paths between various circuit areas. Specifically, all the data paths that do not originate at a flip-flop clocked by the source clock driver and end at a flip-flop clocked by the destination clock driver are eliminated. Next, information from a logic simulation trace is examined. The exact time at which the error occurs is identified on the logic simulation trace. At this point, only the source flip-flops that change state during this time period can be related to the speed path. Thus all paths that do not include a changing source flip-flop are excluded from possibility. At this point, the possible suspect data paths that may be the speed path have been reduced to a very small number, possibly one.

System identification using vibration monitoring for fault diagnostics in manufacturing machinery

Abstract: This paper outlines a different technique in vibration analysis, namely modal testing/analysis for preventive/predictive maintenance in productive machinery Instead of just using a signal analysis approach as has been used widely, a system analysis approach is proposed in this paper The system analysis has been widely used for passive structures (structures with no moving parts), however its application for machines with high speed rotating components has yet to be explored An experimental set-up is used to demonstrate the feasibility and practicality of this technique with all the problems associated with this type of structure Several excitation techniques are used, such as transient, random, sinusoidal, pseudo-random, to perturb the system Several experiments are carried out with different combinations of excitation and response measurement locations (rotating and nonrotating) and the nature of the forcing function on the system It is shown that the predicted gyroscopic effect may be detected by exciting the foundation and carrying out measurement on the rotor and vice-versa The gyroscopic effect results in bifurcation of the critical speed of the rotor and this is reflected in the frequency response function measured on the system The various results are compared and the damping effect is also measured

On the Transient Response of a Spinning Disk Under a Space-Fixed Load

Abstract: The titled problem is studied analytically by an eigenfunction expansion method. In the early stage following the excitation, symmetric deformation is observed when the rotation speed is around the first critical speed. At a rotation speed ten times higher than the first critical speed, at which the convective velocity of the media is comparable to the wave propagation speed, asymmetric deformation becomes apparent.