Overhung Rotor Research Articles

Part I—Theoretical Model for a Synchronous Thermal Instability Operating in Overhung Rotors

Atheoretical model has been developed for a synchronous thermal instability that is caused by differential viscous shearing in bearings of overhung rotors. This model employs an unbalance threshold criterion for instability instead of utilizing a traditional frequency‐domain stability analysis. The current model will be used to investigate several case studies for both plain and tilting pad journal bearing rotors in the second part of this article.

508 能動弾性軸受台で支持されたオーバーハング回転軸系の最適振動制御(磁気軸受・制御,OS-4 ロータダイナミクス)

This paper treats the vibration control of an overhang rotor system. The optimal regulator theory is applied to decide the control gain suppressing the vibrations of the overhung rotor. Influence of weighting coefficients in the cost function is investigated, and the optimal feedback gains to give the effective vibration suppression performance are provided. Frequency response curves are simulated numerically and the effective vibration suppression performance is shown. For practical use, the simplification of the rotor model for controller design and the application of the Kalman filter are also examined. The numerical results are confirmed by the experiments for an overhung rotor with the active vibration control system.

オーバーハング回転軸系の振動制御に対する最適レギュレータ理論の適用(G10-4 制御,G10 機械力学・計測制御部門)

オーバーハング回転軸系の振動制御に対する最適レギュレータ理論の適用(G10-4 制御,G10 機械力学・計測制御部門)

220 弾性軸受台で支持されたオーバーハング回転軸系の危険速度通過に関する研究

220 弾性軸受台で支持されたオーバーハング回転軸系の危険速度通過に関する研究

221 オーバーハング回転軸系の制振に対する最適レギュレータの適用

221 オーバーハング回転軸系の制振に対する最適レギュレータの適用

WAVELET ANALYSIS OF VIBRATION SIGNALS OF AN OVERHANG ROTOR WITH A PROPAGATING TRANSVERSE CRACK

This paper presents an experimental study of the dynamic response of an overhang rotor with a propagating transverse crack using the discrete wavelet transform (DWT)—a joint time frequency analysis technique. Start-up and steady state vibration signatures are analyzed using Daubechies (Db6) mother wavelet and the results are presented in the form of scalograms and space-scale energy distribution graphs. The start-up results showed that crack reduces the critical speed of the rotor system. The steady state results showed that propagating crack produces changes in vibration amplitudes of frequency scale levels corresponding to 1 X, 2 X and 4 X harmonics. The vibration amplitude of frequency scale level corresponding to 1 X may increase or decrease depending on the location of the crack and side load. However, the amplitude of frequency scale level corresponding to 2 X increases continuously as the crack propagates.

102 オーバーハング回転軸系の最適振動制御に関する研究(OS10 振動ならびに衝撃解析・最適設計・モニタリング)

102 オーバーハング回転軸系の最適振動制御に関する研究(OS10 振動ならびに衝撃解析・最適設計・モニタリング)

Active Vibration Control of an Overhung Rotor System Considering Stand Stiffness.

This paper describes a method to effectively reduce the unbalanced vibration of an overhung rotor system. We propose a hybrid type damping device in which a bearing pedestal is supported elastically by rubber springs and controlled actively by electromagnetic actuators. The damping device is mounted in a motor casing which is set on an elastic stand. When the damping device is supported elastically, the suppression effect often deteriorates. In order to prevent this deterioration, we apply a stiffness control which is achieved by conducting positive feedback of the displacement signal. In the analysis, we calculate the controller gains that minimized the compliance of the overhung portion in consideration of the stand stiffness. In the experiment, the results correspond to the analytical results, and we confirm that the application of the stiffness controller is a highly effective method for vibration suppression.

Active Vibration Control of an Overhung Rotor System Using Electromagnets.

This paper describes the suppression of vibration which is generated on rotating machinery with an overhung rotor. We propose a rotor-shaft system supported by rubber springs which are installed on the pole surfaces of the electromagnets. This is a hybrid type equipment in which a bearing housing is supported elastically by rubber springs and controlled actively by electromagnetic actuators. These actuators are controlled by displacement sensors at the pedestal and/or the roller and can apply an electromagnetic force which suppresses any vibration of the roller. In the analysis, we calculate the controller gains which minimized the amplitude of vibration at the roller. In the experiment, the results correspond to the analytical results, and we confirm that the hybrid type equipment is a highly effective method for vibration suppression.

Electromechanical Simulation and Testing of Actively Controlled Rotordynamic Systems With Piezoelectric Actuators

Past research on this subject has treated piezoelectric actuators as ideal devices that have tip displacements proportional to their input voltages, at any instant in time. This assumption neglected phase lag and amplitude change at high frequencies. This characteristic of any actuator may lead to coupled control system-structural system instability that limits the amount of active stiffness or active damping that can be obtained. The paper presents a method for simulating the coupled “electromechanical” system to predict rotordynamic stability and unbalance response along with control system stability. The piezoelectric actuators and their amplifiers are represented as equivalent linear electrical circuits. The electromechanical system modeling approach is utilized to correlate test results from a double overhung rotor rig at NASA Lewis. The test results also show the effectiveness of the control system for suppressing the unbalance response of two modes using active stiffness and active damping.

Constrained output feedback control of flexible rotor-bearing systems

The design of an optimal constrained output feedback controller for a rotor-bearing system is described, based on a reduced order model. The aims are to stabilize the unstable or marginally stable motion and to control the large build-up of periodic disturbances occurring during operation. The reduced order model is constructed on the basis of a modal model and singular perturbation, retaining the advantages of the two methods. The onset of instability due to spillover is prevented by the constrained optimization, and the robustness and pole assignability are improved by designing not merely a static output feedback but a dynamic compensator. The periodic disturbances, usually caused by rotation, are reduced by using the disturbance observer and feed-forward compensation. The efficiency of the proposed method is demonstrated through two simulation models, a rigid shaft supported by soft bearings at its ends and an overhung rotor system with a tip disk, under both transient vibration and sudden imbalance situations.

Experiments and Stability Predictions of Two Sets of Tilting Pad Bearings on an Overhung Rotor

An overhung rotor was experimentally tested in two sets of five shoe tilting pad bearings. The preload factors of the two sets were 0.1 and 0.5, respectively. The rotor had critical speeds at 3000 and 6800 rpm. When mounted on the first set of bearings, the rotor remained stable for speeds over 11 000 rpm. However, when mounted in the second set of bearings the system went unstable due to whip at 10 400 rpm. A theoretical analysis was performed on the stability of the rotor system to determine if the bearings were the cause of the whip. The analysis indicates that the system is marginally stable, and that small cross couplings generated by the disk-shaft mountings destabilized the system. Also, pad inertia has little affect on the stability of this particular rotor. The system was predicted to be the most stable when mounted on the bearings with a preload of 0.1, so that the predicted trends with varying preload were correct. The predictions also indicate that subsynchronously reduced bearing coefficients...

Experiments on the Stability of an Overhung Rotor in Pressure-Dam and Multilobe Bearings

Four sets of bearings were tested on a three-mass overhung flexible rotor: axial-groove, pressure-dam, and two sets of preloaded (0.75 and 0.40) multilobe bearings. The first and second critical speeds were observed at 3200 and 6800 rpm; predicted values were 2960 and 7420, respectively. The instability thresholds were observed to depend strongly on the load orientations for the axial-groove and multilobe bearing. The maximum instability thresholds (optimum load direction for the axial-groove, multilobe (m = 0.75), and multilobe (m = 0.40) bearings were 6700 rpm, 9200 rpm, and 6800 rpm, respectively. For the pressure-dam bearings, it was 6400 rpm and was predicted to be 5800 rpm. The lowest instability threshold for the axial-groove and multilobe bearings were 5750 rpm, 7600 rpm, and 6200 rpm, respectively. When known unbalances were added to the rotor in the pressure-dam bearings, the stability threshold was increased to above the safe running range of the rotor and a brief instability was observed as th...

Synchronous Unbalance Response of an Overhung Rotor with Disk Skew

This paper deals with the influence of disk skew on the synchronous unbalance response of flexible rotors in damped bearings. A simple overhung rotor is treated to illustrate the effects of various combinations of unbalance and disk skew on the amplitude and phase angle response at the disk and bearings. The paper shows that it is impossible to balance the rotor at all speeds by single plane balancing even if three correction planes are employed. The presence of disk skew may be best detected by monitoring the far bearing for a rapid phase angle decrease after passing through the first critical speed.

Friction Induced Rotor Whirl—A Study in Stability

Of deep concern to designers of high speed, close clearance rotating machinery is that type of self-excited rotor whirl induced by rubbing friction between rotor and stator. Since the probability of transient or continuous rubbing is high, it is important to determine whether the consequent whirl will be sustained, whether it will decay, or whether it will increase destructively. This paper develops, for a simple flexible overhung rotor, an analysis showing that regimes of stability, dependent upon damping and stiffness characteristics, do indeed exist. A stability chart is computed for the selected model and this may be used to determine whether a specific disturbance deflection will decay, grow, or remain constant during the disturbance or after its removal.

Investigation of a rotary regenerator

The rotor drum 6 consists of two concentric shells; the space between them is baffled by radial ribs 7. The resulting compartment chambers are filled with packing 8 only partially down the length of the drum barrel. The unfilled portions act as channels to aid uniform distribution of the coolant over the front of the packings. The packing can be corrugated sheet metal, mesh, shot, or chips. The drum is mounted on a special flange 9 made of heat-insulating material(Textoli te, Teflonwith glass reinforced plastic filler, etc.). The flange features an extension machined to match the bearing 10, and gear toothing to drive the rotor shaft. The drum and flange are bolted together to form the shaftless overhung rotor of the regenerator. The shaftless rotor design makes it possible to minimize the amount of heat flowing in over the body of the drum (by reducing the area presented by the cross section}, while the overhung design and the heatinsulating flange set up favorable thermal conditions for the operation of the rotor bearings. In our case, the regeneratorbeari ngs function at room temperature, so that standard bearings can be used in the assembly. Sealing and gas distribution over the rotor packing in the low-pressure and high-pressure streams are achieved with the aid of packing plates in the fixed shaft-end seals. There are two of these shaft-end seals, one on the warm side, the other on the cold side of the rotor. The end plates have contoured ports allowing the high-pressure and low-pressure streams to pass through. These ports communicate with the