Air Spindle Research Articles

Modelling approach of hybrid air herringbone grooved journal bearing considering the angular misalignment and centrifugal expansion

Performance improvements can be achieved after introducing herringbone grooves into conventional plain aerostatic bearings at high speeds. However, the existing literature lacks a comprehensive model that accurately calculates the static and dynamic characteristics of this hybrid air bearing. To address this gap, this paper proposes a modeling method for high-speed hybrid air journal bearings with herringbone grooves, considering the effects of angular misalignment and centrifugal expansion. Based on the established model, effect of the grooves on the bearing performances is systematically studied. Furthermore, influences of the bearing angular misalignment and the rotor centrifugal expansion on the rotor’s vibration characteristics are illustrated through an unbalance response analysis for an industrial air spindle. The presented model provides an analysis tool for structure design and performance study of the herringbone grooved hybrid air journal bearings.

Research Developments of Aerostatic Thrust Bearings: A Review

In aerostatic thrust bearings (ATBs), a high-pressure gas film with a certain bearing capacity and stiffness is formed by passing high-pressure gas between the moving surface and the static surface. Aerostatic bearings have outstanding advantages in the following aspects: high precision, high speed, and long service life, etc. They are widely used in many fields, such as high-speed air spindles, precision machine tools, air-bearing guideways, turbine machinery, and high-speed drills. With the pursuit of higher efficiency and high-precision machining machinery, there is an increasing demand for high-performance ATBs. Much effort has been spent on the study of ATBs, such as improvements in load capacity and stiffness, and the enhancement of stability. Some significant progress has been achieved. In this paper, the research developments of ATBs are summarized from several aspects, such as theoretical models and experimental methods, static performance, dynamic performance, and applications. In addition, insights on the breakthrough and development trends of ATBs are put forward. It is hoped that this paper can provide some guidance for the design and application of ATBs.

Measurement method of spindle motion error based on composite laser target

The primary component of ultra-precision machining machines and measuring instruments is a high-precision spindle, and spindle error affects the accuracy of these machines. Traditional spindle error measurement methods often require a standard device as a reference, which inevitably reduces the accuracy of spindle error measurement, while the error separation technique is tedious and time-consuming. This study proposed a spindle error measurement method based on a composite laser target, which can measure the motion error of the spindle, including axial, radial, and tilt error in terms of five degrees of freedom. In this method, a composite laser target containing a laser point source and laser collimated beam was constructed and mounted on the spindle as a reference frame to mark the position of the spindle during rotation. Consequently, the error of the spindle was obtained via measurements of the position and angle of the composite laser target. Differential confocal microscopy technique was employed for measuring the axial position of the laser point source to obtain the axial error of the spindle, whereas the radial position was measured using a conventional microscope optical path to obtain the radial error of the spindle. Further, the angle of laser collimation was measured using a collimation measurement optical path to obtain the tilt error of the spindle. The measurement resolution of the proposed method for spindle error was verified through suitable experiments, yielding resolutions of 4 and 2 nm for axial and radial errors, respectively, and 0.2 μrad in case of tilt error. In addition, the method was tested on an air spindle to demonstrate the feasibility of using the method on machines. Thus, it was demonstrated that the proposed method can be used to obtain the spindle error by optical standard devices without additional error separation techniques, particularly for a spindle in ultra-high precision measurement equipment. Further, it facilitates real time monitoring or evaluation and calibration of the spindle error. This method is expected to serve several important practical applications in the field of ultra-precision machining and measurement. • Spindle error measurement method based on a composite laser target proposed. • Method eliminates reliance of accuracy on standard device used. • Motion error of spindle: axial, radial, tilt in terms of five degrees of freedom. • Resolution of proposed method verified through suitable experiments. • Practical applications in the field of ultra-precision machining and measurement.

Non-Linear Dynamic Analysis on Hybrid Air Bearing-Rotor System under Ultra-High Speed Condition.

The non-linear dynamic behavior of a hybrid air bearing-rotor system is very complicated and requires careful attention when designing to avoid spindle failure, especially under ultra-high speed condition. In this paper, the rotor trajectory of a hybrid air bearing-rotor system is obtained by solving the unsteady Reynolds equation and motion equations simultaneously. The typical non-linear behavior of hybrid air bearing-rotor systems is illustrated with the analysis of the rotor trajectory, the phase angle, time domain vibration and power spectral density. Furthermore, the influences of the rotor mass, external load, rotating speed and unbalanced mass on the non-linear behavior are investigated. Finally, the effect of structure parameters on the rotor trajectory is studied and the phenomenon under ultra-high speed condition is illustrated, which provides some new guidelines on the ultra-high speed air spindle design.

Study on vibration characteristics of motorized spindle based on multi-physics fields

Ceramic motorized spindle plays an important role on CNC machine tool. Its dynamic characteristic determines the machining quality of spindle. This paper established a dynamics model of ceramic bearing, and explored the relationship of bearing stiffness and vibration response. The temperature field model of ceramic motorized spindle was established to analyze the temperature field. The motorized spindle air gap influence on unbalanced magnetic pull was studied. The dynamic model of ceramic motorized spindle was established through considering the multiple physical fields influence factors, the changing law of dynamic characteristics was studied. It has been verified by experiments. The average error of ceramic spindle decreased from 5.1% to 2.1%. The establishment of multi-fields dynamic model provided an important guidance for spindle design.

Simulation in design air spindle with orifice and distribution grooves

This paper presents some results of simulation using ANSYS CFX software in design air spindle which has the structure in the form of orifice and grooves to ensure uniform pressure distribution and avoiding air hammer; it is also easy to manufacture and reduces costs. The results of simulation are good in investigating operation of air-spindle which is changed some parameters to find the optimal working mode for the air spindle with speed in the range of 5000–20,000 rpm.

Analysis and experimental verification of dynamic characteristics of air spindle considering varying stiffness and damping of radial bearings

The dynamics of a radial bearing and aerostatic spindle system in an ultraprecision fly-cutting machine cause mid-spatial waviness errors, which have a significant effect on surface topographies. Air bearings, which are externally pressurized, comprise two journal bearings and two thrust bearings with orifice restrictors. The dynamic coefficients of the radial bearings are obtained by adopting the perturbation and finite difference methods with the MATLAB® software. In addition, the influences of the spindle speed and tilt angle on the dynamics of the radial bearings are investigated in detail. The simulation results show that the rotating speed has a little impact on stiffness but having a great impact on damping, and the theoretical values of static stiffness are almost the same as the experimental values. The dynamic responses of the spindle system supported by pressurized air film are obtained, and the simulation analysis of spindle responses under cutting forces is also well matched to the experiment results.

Material Flow in Lap Joint of Dissimilar Metallic Foils by Friction Stir Diffusion Bonding with Micro Indentation

Friction stir diffusion bonding (FSDB) has been applied to lap dissimilar foils joints of aluminum alloy on top of commercial purity (CP) titanium. FSDB of the foils was successfully performed through stirring only the upper aluminum foil by the air spindle with an extremely-high rotation speed of 105,000 rpm. By using smooth surface of the tool tip, FSDB with micro indentation less than or equal 10μm could be carried out to improve the surface condition of the joint and the indentation depth required for joining was reduced. The material flow of the upper aluminum alloy foil in the FSDB was investigated by deposited platinum-palladium as the tracer material on the surface of the lower titanium foil. Then it was confirmed that the vestiges of vigorous stirring appeared in the region given at a distance from tool center while little vestiges was found in the other region. These regions seemed to be bounded by a radius of about 0.6 mm from the tool center. Moreover, the bonding strength distribution as a function of the distance from the tool center was likely correspondent to the stirring state.

Numerical and experimental optimizations of nozzle distance in minimum quantity lubrication (MQL) milling process

Minimum quantity lubrication (MQL) is the efficient and environmentally friendly technology, which is desirable to achieve sustainability during machining process. The nozzle distance has its significance in dominating the MQL spray and the related droplet transportation and penetration. In this paper, the nozzle distance has been optimized for MQL milling process through numerical and experimental methods. A two-way computational method has been employed to solve for the comprehensive flow field and particle trajectories, with the wall condition established on the spray impingement theory. The interactions of air flow rates and spindle rotational speeds on droplet penetration are investigated in details. The optimal ranges of nozzle distance setup obtained by simulation for different conditions were verified via slot milling tests, in which the nozzle distances were selected according to the key points obtained in the numerical process. The cutting force and surface roughness were recorded for the verification of adhesion ability and the related cutting performance. The comparison between numerical simulations and milling experiments has shown great consistency. This paper has achieved better understanding of the nozzle orientation setup and device development in MQL milling process, especially for external MQL. The theoretical basis and scientific instruction have been provided for the optimization of MQL operating parameters in industrial applications.

A novel modeling method to investigate the performance of aerostatic spindle considering the fluid-structure interaction

The performances of aerostatic spindle are highly affected by the fluid-structure interaction (FSI) between air film and solid structure. This paper proposes a novel modeling method to investigate the FSI of aerostatic spindle system, by which the structure deformation included by air film force can be acquired. Furthermore, a virtual weight loading method is proposed to estimate the stiffness of spindle with consideration of structure deformation and gravitational eccentricity. The reliability of the proposed method is verified by experiments. Based on the proposed FSI model, the influences of air film geometrical parameters and structure dimension on the performance of aerostatic spindle are further investigated and discussed to guide the design of air spindle.

Research on a Hierarchical and Simultaneous Gravity Unloading Method for Antenna Pointing Mechanism

Abstract. This paper presents a hierarchical and simultaneous gravity unloading method. An air bearing gravity unloading facility for two-orthogonal-axis antenna pointing mechanism (APM) is designed based on this method. This method is proposed based on the characteristics analysis of the two-orthogonal-axis APM and air bearings. The mechanism of the hierarchical and simultaneous gravity unloading method is described in detail. It solves the coupling problem of two axes and unloads the gravity of both joints hierarchically and simultaneously. The air bearing gravity unloading facility which is a structure mechanism with two layers based on this method is designed with planar air bearing and air spindle. The structure of the facility is described in detail. The dynamic models of the APM with its load in space environment and on the air bearing gravity unloading facility are derived respectively. With the analysing of the driving torques and vertical forces of the APM joints in both models, the results demonstrate that the gravity unloading facility can simulate the microgravity environment successfully. This conclusion is also proved by the dynamic simulation with ADAMS software. The simulation also provides some optimization targets for the gravity unloading facility. At last, the gravity unloading facility is established and some experiments are done. The dynamic models, the simulation results and the experiments all show the effectiveness of the hierarchical and simultaneous gravity unloading method.

Thermal Error Modeling Method with the Jamming of Temperature-Sensitive Points’ Volatility on CNC Machine Tools

Aiming at the deficiency of the robustness of thermal error compensation models of CNC machine tools, the mechanism of improving the models’ robustness is studied by regarding the Leaderway-V450 machining center as the object. Through the analysis of actual spindle air cutting experimental data on Leaderway-V450 machine, it is found that the temperature-sensitive points used for modeling is volatility, and this volatility directly leads to large changes on the collinear degree among modeling independent variables. Thus, the forecasting accuracy of multivariate regression model is severely affected, and the forecasting robustness becomes poor too. To overcome this effect, a modeling method of establishing thermal error models by using single temperature variable under the jamming of temperature-sensitive points’ volatility is put forward. According to the actual data of thermal error measured in different seasons, it is proved that the single temperature variable model can reduce the loss of forecasting accuracy resulted from the volatility of temperature-sensitive points, especially for the prediction of cross quarter data, the improvement of forecasting accuracy is about 5 μm or more. The purpose that improving the robustness of the thermal error models is realized, which can provide a reference for selecting the modeling independent variable in the application of thermal error compensation of CNC machine tools.

Research on surface topography in ultra-precision flycutting based on the dynamic performance of machine tool spindle

The dynamic performance of ultra-precision cutting machine tools is an important factor for its machining accuracy. To improve the precision of the machine tool, a surface topography model is built in this paper and the tool path during the cutting process is achieved by dynamic finite element analysis (DFEA) of the air spindle in ANSYS, surface topography is obtained using MATLAB simulation by combing the model with the tool path, and an experiment is carried out for validation. Results show the simulation surface is highly consistent with the experimental result. Dynamic performance of the spindle is the main factor for texture generation, and optimization of spindle is investigated, DFEA of the optimized spindle shows better performance, and the connection mode between diamond tool and cutter head should be further studied.

Magnetoresistive Sensors for the Condition Monitoring of High-frequency Spindles

The magnetoresistive (MR) sensing principle was first applied industrially in the read-heads of hard disc drives. However, in the past decade the MR effect has also been used successfully for sensors to measure position, current and magnetic fields. MR sensors offer a number of benefits compared to other technologies used for condition monitoring of machine spindles. They are more compact and offer a higher bandwidth than inductive or capacitive sensors. They are more robust than optical encoders, being largely insusceptible to oil, water and other contaminants [1]. MR sensors can be applied over a wider temperature range and with less demanding assembly tolerances. This makes them particularly well suited to applications with limited available space and demanding operating conditions, as is often the case in machine tool sensing applications. This paper explains the basics of magnetoresistive sensor technology and outlines the advantages compared to other sensor principles. Furthermore the results of a joint R&D project between Sensitec GmbH and the PTW Institute of the Technical University Darmstadt, funded by the Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke e.V. (AiF), to investigate the application of MR sensors to the condition monitoring of high-frequency air spindles will be described [2]. Position sensors based on the giant magnetoresistive (GMR) effect have been specially adapted to measure directly the radial and axial displacement of the spindle shaft. Importantly only minor modifications have been made to the spindle shaft to enable this measurement and the sensors are completely integrated within the spindle housing. Tests demonstrate that this solution can resolve axial and radial displacements of less than 0.5μm with high repeatability under typical operating conditions.

Influence of Spindle Air Pressure and its Direction on the Quality Characteristics of Polyester/Cotton Vortex Yarn

The influence of spindle air pressure and its direction on the properties of Polyester/Cotton vortex yarn was studied. The spindle air pressure direction was changed in both Z and S directions on the Z twist yarn of Ne 40s Polyester/Cotton vortex yarn (50:50 polyester: cotton) and these yarns were then tested for their properties such as tensile, unevenness, and hairiness. It was found that unevenness was lower with normal spinning condition and increased when the spindle air pressure increased in both directions. Imperfections were also found to be minimum with normal spinning condition and increased with increase in spindle air pressure. The yarn tenacity and elongation at break found to be lower at normal spinning condition. When the spindle air pressure increased yarn tenacity and elongation at break also increased with increase in spindle air pressure. At the same time spindle air pressure in the same direction of basic yarn twist gave less increase in tenacity and elongation than opposite direction of basic yarn twist. Hairiness index, H and hairiness in different length classes continuously increased as the spindle air pressure increased in opposite direction of basic yarn twist.

Study of variable depth air pockets on air spindle vibrations in ultra-precision machine tools

The low vibration of air spindle is a very important thing to achieve nanometer accuracies in the products of machined parts in nano-machining. Compared to the common air pockets with constant depth used in air cells of ultra-precision machines, this paper studied some special air pockets including five variable depth modes: flat, conical, pyramidical, spherical with two sphere radii. Considering rotational speed as well as these parameters, 18 experiments have been committed using a lathe machine. From experiment results, the air spindle with air pockets of pyramidical depth at a low rotational speed has minimum vibrations.

Air pocket effects on air spindle vibrations in nanomachining

In this study, factors that affect air spindle vibrations have been investigated experimentally. In order to achieve nanometer accuracies, the low vibration of air spindle is vital. Shape, size, depth, number of air pockets, and rotational speed of air spindles are important parameters influencing nanomachining vibrations. A total of 3 levels are selected for each parameter and in total 243 experiments have been performed. In this study, the rotor is considered externally which rotates about a central stator. Experiments were committed by the help of a lathe machine. For vibration measurements, the VibroTest 60 has been used. Results have been analyzed using the Design of Experiment method. From the results, it has been analyzed that the air spindle with 2 circular/rectangular air pockets and with 300 mm2 area and 3 mm depth at a low rotational speed has minimum vibrations.

Behavior analysis of machines and system air hemispherical spindles using finite element modeling

PurposeThe spindle behavior of machines and systems depends largely on the choice and design quality of the mechanical components used for the displacement between different parts. As far as very high technology is concerned, air bearings are suitable, for instance, for machining a telescope mirror or for systems used in medical applications that require a micro and nanometric resolution in displacement. Therefore, air bearings play a crucial role in ensuring spindle stability in machines and systems. The static and dynamic behavior of air spindles is dependent on several parameters, such as external load, dimensions, supply pressure, manufacturing capability and fluid properties.Design/methodology/approachThis paper presents a methodology for the calculation and analysis of the stability and reliability of machine and system spindles supported by air hemispherical bearings. The static and dynamic characteristics of air spindles are calculated using the finite element method (FEM). The stochastic Response Surface Method (SRSM) is used for the approximation of the performance function, and the reliability is assessed by Monte Carlo Simulation (MCS) and the First Order Reliability Method (FORM).FindingsThe static and dynamic characteristics of air spindles are calculated using the finite element method (FEM). Stochastic Response Surface Method (SRSM) is used for the approximation of the performance function, and the reliability is assessed by Monte Carlo Simulation (MCS) and First Order Reliability Method (FORM).Originality/valueThe article presents an original approach for the behavior analysis of machines and systems spindles supported by hemispherical fluid bearings. The methodology based on the finite element method and the principle of structural reliability, allows studying the influence of physical and geometrical parameters on the static and dynamic characteristics and the failure probability of a spindle. Thus, the optimum behavior of a spindle can be predicted for different configurations of a bearing design taking into account the reliability evaluation.

An experiment on the shape and depth of air pocket on air spindle vibrations in ultra precision machine tools

In order to achieve nanometer accuracies, low vibration in air spindle is vital. Some parameters affecting air spindle vibration could be mentioned as rotational speed, inlet hole diameter, air pocket geometry, air gap pressure and so on. In this study, the air pocket geometry and depth plus rotational speed are experimentally investigated. Three levels of quantity are selected for each parameter. Vibration movements were considered as experiment output. Then, experimental results were analyzed by design of experiment method. The results showed that the air spindle with an air pocket of rectangular shape and 3 mm depth at low rotational speed has minimum vibrations.

Experimental investigation on effect of number and size of rectangular air pockets on air spindle vibrations in nanomachining

In order to achieve nanometer accuracies, the low vibration of air spindle and air table is vital. Number and size of air pockets in air spindles and air tables are important parameters in nanomachining vibrations. In this article, rotational speed as well as these parameters has been studied. Three levels are selected for each parameter and, in total, 27 experiments have been committed. Also, in this work, air pockets were rectangular. In this study, rotor considered externally which rotates about stator. Experiments were conducted with the help of a lathe machine. For vibration measurements, the VibroTest 60 was used. The results were analyzed using design of experiment method. Experimental results show that the air spindle with two air pockets with 200 mm2 area at a low rotational speed has minimum vibrations.