Pneumatic Hammer Instability Research Articles

Experimental Force Coefficients of a Hydrostatic Thrust Bearing and Comparisons to Predictions

Abstract Reliable fluid film bearings for high-performance turbomachinery provide significantly extended operating life with controllable stiffness and damping force coefficients at low cost. Hydrostatic thrust bearings support thrust loads and control axial motions. This paper reports measurements and predictions of static and dynamic load characteristics of a hydrostatic thrust bearing fed with pressurized air. The current bearing model uses dynamic axial periodic motion amplitudes, that is, models as a function of the initial film thickness, to predict the dynamic force coefficients. Note that, presently, the recorded frequencies and their amplitudes of pneumatic hammer instability along the axial direction are implemented as an external excitation source. The measured and predicted static stiffnesses increase as the film thickness decrease and supply pressure into the bearing increase. The predicted dynamic force coefficients show a favorable agreement with the test data. Analysis reveals the negative damping coefficients and damping ratios as a distinctive onset condition of the present pneumatic hammer instability of the test bearing. A careful recess design and adequate operation conditions along with an accurate prediction can avoid or prevent undesirable pneumatic hammer instability in hydrostatic thrust bearings. This work extends the knowledge database of hydrostatic thrust bearings with pneumatic hammer instability and provides their design guideline for various operating conditions.

On the Pneumatic Hammer of Hybrid Gas Bearings: Measurements and Predictions

Abstract Hybrid bearings integrating an external pressurizing source are widely used in high-speed turbomachinery due to their notable advantages, including low friction and wear, enhanced reliability and durability, and accurate rotor positioning, as well as large static stiffness and load carry capacity even lubricated with low viscosity liquids. This paper reports experimental data and predictions to identify the characteristics of pneumatic hammer of hybrid gas bearings, 60 mm in diameter, with increasing feed gas pressures. Pneumatic hammer characteristics with static load stiffnesses and flow rates of the test bearing are recorded for increasing static loads with various shaft center positions. A computational program for modeling of hybrid gas bearings predicts static load characteristics of the test bearings. Predictions show a remarkable agreement with measurements. Comparisons of measurements and predictions reveal that calculated reduced damping factors and damping coefficients of hybrid bearings, relying on volume ratio between recess and fluid film, are reliable indicators to estimate the onset condition of pneumatic hammer instability.

Pneumatic hammer instability in the aerostatic journal bearing–rotor system: A theoretical and experimental analyses

In this study, the pneumatic hammer instability phenomena in the aerostatic journal bearing–rotor system is analysed and discussed for different feeding hole configurations theoretically and experimentally. The influences of the configuration of the feeding holes on the nonlinear dynamics of the system are also investigated. The air flow between the surfaces is modelled with Reynold’s equation and it is numerically solved with differential transform and finite difference hybrid method. Three different aerostatic bearings are modelled and simulated to investigate the ınfluences of the configuration of the holes for different angular speeds. An experimental test rig is designed and tested for different rotor speeds to validate the obtained numerical results. The dynamic response of the system is analysed using waterfall plots, bifurcation diagrams, orbit plots, phase portrait and Poincaré map, which are drawn to determine the pneumatic hammer instability region of the modelled system. The results reveal a nonlinear dynamic response of the rotor centre. In addition, the analysis shows that the feeding hole configuration affects the rotor dynamics and the pneumatic hammer instability region.

Synthesis of Experimental and Theoretical Analysis of Pneumatic Hammer Instability in an Aerostatic Bearing

The present work is focused on the pneumatic hammer instability in an aerostatic bearing with shallow recesses and orifices of four different diameters. Operating conditions were zero rotation speed, zero load, and different supply pressures. The diameters of the tested orifices were large compared to the usual practice and correspond to a combined inherent and orifice restriction. The theoretical analysis was based on the computational fluid dynamics (CFD) evaluation of the ratio between the recess and the feeding pressure and on the “bulk flow” calculation of the rotordynamic coefficients of the aerostatic bearing. Calculations showed an increase of the direct stiffness with decreasing the orifice diameter and increasing the supply pressure and, on the other hand, a decrease toward negative values of the direct damping with decreasing the orifice diameter. These negative values of the direct damping coefficient indicate pneumatic hammer instabilities. In parallel, experiments were performed on a floating bearing test rig. Direct stiffness and damping coefficients were identified from multiple frequency excitations applied by a single shaker. Experiments were performed only for the three largest orifices and confirmed the decrease of the direct damping with the orifice diameter and the supply pressure. The identification of the rotordynamic coefficients was not possible for the smallest available orifice because the aerostatic bearing showed self-sustained vibrations for all feeding pressures. These self-sustained vibrations are considered the signature of the pneumatic hammer instability. The paper demonstrates that aerostatic bearings with shallow recesses and free of pneumatic hammer instabilities can be designed by adopting orifice restrictors of large size diameter.

An Integrated Approach to the Design and Analysis of an Ultra-High Speed Air-Bearing Spindle

This paper presents an integrated approach to the design and analysis of an ultra-high speed air bearing spindle, by integrating the structural design, performance analysis and system optimization in a virtual design environment. Firstly, the ultra-high speed air bearing spindle is designed, including grooved hybrid air bearing, helical water cooling channel and built-in motor, etc; Subsequently, pneumatic hammer instability and whirl instability of air bearing are studied; The thermal-structural behaviors of the spindle system at ultra-high speeds are investigated by using structural FEA coupled CFD; Static and dynamic performance of spindle is studied to predict the stiffness, modes and natural frequencies of the spindle; Lastly, system optimizations are conducted to obtain optimal performance and dynamic behaviors of the spindle. The proposed integrated approach can be used to design an optimal ultra-high speed air bearing spindle.

Dynamic Characteristics of Aerostatic Porous Journal Bearings With a Surface-Restricted Layer

Aerostatic porous bearings have been successfully applied to various precision devices such as machine tools and measuring equipment to achieve a higher accuracy of motion. However, aerostatic porous bearings have a disadvantage in that they are prone to cause pneumatic hammer instability. Therefore, to avoid this instability, a surface-restricted layer that has permeability smaller than the bulk of the porous material is usually formed on the bearing surface. In this paper, the dynamic characteristics of aerostatic porous journal bearings that have a surface-restricted layer are investigated numerically and experimentally. The effects of permeability in bulk porous materials and of a surface-restricted layer on the bearing characteristics are discussed using two kinds of porous material: graphite and metal. It was confirmed that aerostatic porous metal bearings with relatively large permeability could achieve large values of dynamic stiffness and damping coefficients using a low permeability, surface-restricted layer.

Whirling instability of a rotor supported by aerostatic porous journal bearings with a surface-restricted layer

Recently, aerostatic porous bearings have been used in ultra precision machine tools and precision-measuring equipment because they have higher stiffness and larger damping coefficients than other types of aerostatic bearings. However, an aerostatic porous bearing has a disadvantage in that it is prone to cause pneumatic hammer instability. Therefore, it is considered that reducing the permeability of the porous material only near the bearing surface in an aerostatic porous bearing is a very effective means of avoiding pneumatic hammer instability. In this article, the whirling instability of a rotor supported by aerostatic porous journal bearings with a surface-restricted layer is investigated theoretically and experimentally. It is found that a surface-restricted layer can greatly improve the static stiffness and the whirling stability of a rotor at high speeds.

Static characteristics of aerostatic porous journal bearings with a surface-restricted layer

Graphite porous materials have been successfully used in an aerostatic bearing. This is because graphite porous materials have a lower permeability than porous metal, which makes it possible to achieve a higher stiffness in aerostatic bearings. However, aerostatic porous bearings have a disadvantage in that they are prone to cause pneumatic hammer instability. Therefore, a restricted layer which has a permeability lower than the bulk of the porous material is usually formed on the bearing surface to avoid this instability. In this paper, the static characteristics of aerostatic porous journal bearings with a surface-restricted layer are investigated theoretically and experimentally. In addition, the effects of a surface-restricted layer on the static characteristics are discussed for two types of loads, symmetrical and coupled.

108 表面目詰まりを持つ金属多孔質静圧空気軸受の特性

Recently, brittle porous materials such as graphite and advanced ceramics have been often applied to aerostatic bearings because gas flow through these porous materials can be limited to a small amount. However, aerostatic bearings using these porous materials are prone to cause pneumatic hammer instability. Therefore, reducing the permeability of porous material only near the bearing surface is usually adopted as a very effective means for avoiding pneumatic hammer instability. In this paper, the static and dynamic characteristics of aerostatic circular thrust bearings with a restricted layer are investigated, theoretically and experimentally. In the theoretical calculations, several calculation models are presented and theoretical results obtained are discussed.

Improvement of Pneumatic Hammer Instability of Aerostatic Thrust Bearings with Compound Restrictors.

It is well known that aerostatic bearings with compound restrictors tend to cause pneumatic hammer instability at the optimum bearing clearance for the maximum stiffness. In this paper, the improvement of such an instability in the aerostatic thrust bearings are discussed. An orifice restrictor put in series into an air supply line is used as a stabilizer. The static and dynamic characteristics of aerostatic thrust bearings with this stabilizer are investigated experimentally and theoretically, and the usefulness of this method for improving the stability of this kind of bearings is confirmed. It is consequently found that the proposed method is very efficient in avoiding the pneumatic hammer instability.

305 多孔質静圧空気ジャーナル軸受の動的特性

Aerostatic porous journal bearings are very suitable for precision devices such as ultra-precision machine tools and precision measuring equipment. However, it is said that aerostatic porous bearings are prone to cause pneumatic hammer instability, unless the surface restricted layer is formed on the bearing surface. This paper theoretically investigates the dynamic characteristics of aerostatic porous journal bearings with a surface restricted layer. In addition, the influences of various design parameters on the dynamic characteristics are discussed and the effects of the surface restricted layer on pneumatic hammer instability are clarified.

フルイドダンパを付加した複合絞り型静圧気体ジャーナル軸受の軸の応答特性

Hydrostatic journal gas bearing with a large pocket in the bearing surface and with fluid damper has high stiffness without causing pneumatic hammer instability. But the settling time cannot be lower than two or three times of natural period time owing to the large pocket volume even if the damping coefficient of fluid damper can be set high. In this paper, the settling time of hybrid restrictor type hydrostatic journal gas bearings is investigated. The stiffness of hybrid type is about 60% less than that of pocket type, and the pocket volume of this type, where pocket depth is about two or three times as deep as bearing clearance, is about 2-5% of the pocket volume of the pocket type. The following conclusion is obtained : the settling time of hybrid type can be set to one natural period time (critical damping) by the use of the appropriate damping coefficients of fluid damper.

Static and Dynamic Characteristics of Cirular Aerostatic Porous Thrust Bearings.

Recently brittle porous materials such as graphite and advanced ceramics have been used in the fabrication of aerostatic bearings because gas flow through these porous materials can be limited to a small amount. However, aerostatic bearings consisting of these porous materials are prone to cause pneumatic hammer instability. Therefore, it is considered that reducing the parmiability of porous material only near the bearing surface is a very effective means for avoiding pneumatic hammer instability. In this study, the static and dynamic characteristics of aerostatic circular thrust bearings with such a restrained layer near their surfaces are investigated, theoretically and experimentally. Furthermore, the effects of the restrained layer on the bearing characteristics are clarified.

フルイドダンパを付加したポケット型静圧気体ジャーナル軸受の軸の応答特性

Pneumatic hammer instability has been the greatest problem in realizing a high stiffness gas bearing with a large pocket in the bearing surface. The authors have already succeeded in realizing a high stiffness and heavy damping hydrostatic thrust gas bearing with a large pocket by the use of fluid damper. In this paper, the shaft response characteristics and static performances in a hydrostatic pocket journal gas bearing are investigated both experimentally and theoretically. The following conclusions are obtained. (1) Fluid damper has a good influence on the improvement of the trangent motion and the prevention of pneumatic hammer instability. (2) When an impact force acts on the shaft, it is possible to make the amplitude of rigid body as small as that of elastic body. (3) A high stiffness journal gas bearing with a large pocket can easily be realized by the use of fluid damper. (4) As the settling time is greatly affected by the pocket size, the pocket depth must be made as small as the bearing clearance in order to realize the critical damping condition.

高圧給気による気体軸受の高剛性化 第２報

One of the simplest way to improve the stiffness of externally pressurized gas bearings is to increase the supply pressure. The purpose of this study is to examine the possibility and practicality of high supply pressure. In the preliminary test with simple circular thrust bearings with a slot restrictor, it proves that the bearing stiffness increases almost linearly with supply pressure as predicted by the theoretical calculation. Bearing stiffness is far improved with a stepped clearance. Pneumatic hammer instability and gas flow rate are not so critical problems for the high stiffness bearings with a small clearance. The effect of deformation of the bearings by high supply pressure can not be recognized in those experiments. It is confirmed that the increase in supply pressure is the simplest and most practical way to improve the stiffness of externally pressurized gas bearings.

Effects of fluid compressibility on the dynamic response of hydrostatic journal bearings

A theoretical analysis for the dynamic performance characteristics of laminar flow, capillary and/or orifice compensated hydrostatic journal bearings is presented. The analysis considers in detail the effect of fluid compressibility in the bearing recesses. At high frequency excitations beyond a break frequency the bearing hydrostatic stiffness increases sharply and is accompanied by a rapid decrease in direct damping. Also, the potential of pneumatic hammer instability (negative damping) at low frequencies is likely to occur in hydrostatic bearing applications handling highly compressible fluids. Useful design criteria for avoiding undesirable dynamic operating conditions at low and high frequencies are determined. The effect of fluid recess compressibility is brought into perspective, and found to be of utmost importance to the entire frequency spectrum response and stability characteristics of hydrostatic and hybrid journal bearings.

フルイドダンパを付加した静圧気体スラスト軸受の軸の応答特性

In this paper, the shaft response characteristics of hydrostatic thrust gas bearings with fluid damper are described. In hydrostatic gas bearings, the both clearances at which stiffness reaches its maximum value and damping reaches its minimum value are well known to be at almost the same. Therefore, when an impact force acts on the shaft, the shaft response behavior of vibration generally shows under damping, and it takes several times of the natural period in convergence. So, the possibility of reducing convergence time is examined by the use of fluid damper. Following conclusions are obtained : (1) The shaft convergence behavior of vibration can be improved (from under damping to critical damping) by changing the damping coefficient of fluid damper. (2) The shaft convergence can be achieved in about one natural period. (3) The optimum damping coefficient of fluid damper is set at 0.6-1.0 of damping factor in the bearing system. (4) Fluid damper can be of great help to make high stiffness gas bearings with large pocket in the bearing surface without causing pneumatic hammer instability.

多孔性セラミック気体軸受の特性 （第１報）

Porous gas bearings show high load capacity and bearing stiffness. Conventional metal sintered porous gas bearings are the surface choked type and there are some difficulties in controlling gas permeability. Recently, porous ceramics with low and homogeneous permeability can be easily obtained and machined with high accuracy without any surface chok-ing. In this paper, analysis of porous ceramic gas bearings are presented and steady state characteristics are calculated for the cases of externally pressurized, self acting, and hybrid type operation. Pressure distribution in porous ceramics is also calculated when the surface is slightly choked to improve pneumatic hammer instability.

ポケット付きオリフィス絞りを有する静圧気体ジャーナル軸受の静特性

Recently, gas bearings have been used in high technology industry. But, conventional bearings have obliged mean clearance to be as wide as 20-30 μm by the use of inherently compensated restrictors. In this paper, an externally pressurized gas journal bearing is made as an experiment, and load capacity, stiffness and gas flow rate are measured. The test bearing is 30 mm in length, 30 mm in diameter, 3 mm in pocket diameter and 0.05, 0.1 and 0.2 mm in orifice diameter respectively. Following conclusions are obtained. (1) By the use of pocketed orifice restrictors, the stiffness is the highest at 7-13 μm in mean radial clearance without generating pneumatic hammer instability. (2) As the bearing can be operated with a narrow clearance, gas flow rate is about 4 L/min with 0.1 mm in orifice diameter and 10 μm in radial clearance at supply pressure of 0.6 MPa.

Design of aerostatic flat pad bearings using annular orifice restrictors

Design charts are presented for use in the design of aerostatic thrust bearings which use annular orifice restrictors. Annular orifices are preferred when pneumatic hammer instability is a potential problem. The design procedures presented are based upon analyses produced using finite difference equations to the compressible form of the Reynolds equation. The method provides the designer with a graphical technique that enables him to take into account bearing stiffness, clearance, number of orifices and bearing dimensions in obtaining a satisfactory solution to his problem.