Analysis and optimization of dynamic response of air bearing sliders to disk waviness

Flying stability has been becoming more critical for air bearing sliders with extremely low flying height (FH). Therefore, the effects of disk waviness on flying height modulation (FHM) cannot be neglected. This paper presents an analytical study on the mechanism of FHM of air bearing sliders due to disk waviness, and a design optimization for increasing waviness following ability of sliders. An analytical three-degree-of-freedom (3-DOF) model is developed, where the air bearings are modeled as six lumped linear springs and dampers. The purpose of this model is to develop a quantitative understanding of how air bearing sliders respond to disk waviness. The dynamic characteristics of the slider-air bearing system are then analyzed, and the closed-form frequency resonance function (FRF) of FHM to disk waviness is derived. The impact of disk surface features and the positions of the trailing pad, the side pads, the leading pads and the negative pressure center on FHM are also investigated using parametric analysis. The analysis results show that the improvement of the roll-off characteristics of the disk surface waviness can also decrease the FHM. In addition, shortening the distance between the trailing pad pressure center and the head position, moving backward the side pads and leading pads and forward the negative pressure center can increase waviness following ability of the slider. Finally, an air bearing slider is designed according to the proposed design strategies for reducing the FHM due to disk waviness.     

图案化介质磁盘-磁头气体润滑动态特性分析

在小扰动条件下,提出应用偏导数法在复数域内求解可压缩气体润滑的动态Reynolds方程,以计算任意磁盘表面形状的磁盘-磁头气膜润滑的动态刚度和阻尼系数。以圆柱形和立方形比特图案化介质(BPM)磁盘为研究对象,研究了磁单元尺寸参数对气膜动态系数的影响,并比较了两种BPM对应的动态性能。数值结果表明,与光滑磁盘相比,图案化介质磁盘对应的气膜润滑动态刚度系数和阻尼系数大大减小,且随磁单元高度增大,动态系数逐渐减小。相比圆柱形BPM,立方形BPM对应的气膜具有较大动态刚度和阻尼系数,计算结果可为图案化介质磁盘的设计提供重要的理论依据。     

A Numerical Model of a Rock Bit Bearing Seal

The mechanical seal with metal faces, used to seal the bearings in rock drill bits, plays a critical role in ensuring a long service life of the drill bit. A numerical model of such a seal has been developed for use as a design tool. The axisymmetric model encompasses a mixed lubrication elastohydrodynamic analysis, and predicts such quantities as leakage rate, film thickness distribution, fluid pressure and contact pressure distributions, interface temperature and face deformation. Model results show that the behavior of this seal, unlike that of conventional industrial seals, depends strongly on the ambient pressure, not just on the pressure drop across the seal.     

Effects of flying height deviations on glide height tests for manufacturing hard disks

In this study, theoretical analysis and experiments were carried out to investigate the effects of avalanche-point deviation and the deviation between the calibration and test-flying heights during the glide head calibration in the glide height tests for manufacturing hard disks. To obtain accurate glide height test results and improve the reproducibility of the test, flying height control should be carried out by limiting the acceptable deviation range of the flying heights of the glide heads. When the avalanche-point deviation is zero, the test results using different calibrated rails (or heads) are the same when they are used to detect the same defect. To avoid wrong test results due to avalanche-point deviation, the test-flying height should be the same as the calibration flying height because the difference of the output voltages of any two rails (or heads) is zero in this case. If these two deviations cannot be eliminated completely, the calibration and test-flying heights should be carefully selected because the error still can be minimised depending on the selection of the flying heights.     

Development of flying type head/slider for optical recording technology

Flying-type sliders with the integration of optical components were developed for Near-Field Recording (NFR) and Far-Field Recording (FFR) technologies. The key design issue was the integration of the optical components with the slider. Due to the size of the lenses mounted on it, the slider had to be relatively large, corresponding to the size of a micro-slider. Also, the non-uniform distribution of the slider body density was incorporated in the design. As for the optical disk substrate, a plastic material such as polycarbonate was investigated because of its manufacturing convenience and cost effectiveness. The flying and tribological performance of the prototype optical sliders on various media were assessed. The results showed that the tribological characteristics of the slider/disk interface were sensitive to several factors including the properties of the disk. Adequate flying characteristics of the optical sliders on glass (NFR) and plastic (FFR) disks could be attained by optimizing these parameters.     

Dynamics of transient events at the head/disk interface

Slider/disk contacts of nano and pico sliders are investigated using an acoustic emission sensor and a high bandwidth laser Doppler vibrometer (LDV). The following cases are studied: (a) influence of scratch impact on the airbearing stiffness; (b) influence of lubricant thickness on slider dynamics for single bump impacts; (c) influence of lubricant thickness on slider vertical stick–slip vibrations; (d) dynamics of take-off and landing. Linear time frequency analysis is applied to study simultaneously the impact response of the airbearing and the slider torsional and bending modes. The contact dynamics of single bump impacts is examined as a function of disk velocity and lubricant thickness. Increased slider vibrations are found for thick lubricant films both for sliding contacts as well as for single bump impacts. During the transition from sliding to flying a change of the bending mode frequency is observed.     

Numerical Simulation of the Head/Disk Interface for Patterned Media

The use of patterned media is a new approach proposed to extend the recording densities of hard disk drives beyond 1 Tb/in.². Bit-patterned media (BPM) overcome the thermal stability problems of conventional media by using single-domain islands for each bit of recorded information, thereby eliminating the magnetic transition noise (Albrecht et al., Magnetic Recording on Patterned Media, 2003). Considering steady state conditions, we have transferred the pattern from the disk surface onto the slider surface and have investigated the pressure generation due to the bit pattern. To reduce the numerical complexity, we have generated the bit pattern only in the areas of the slider near the trailing edge, where the spacing is small. Cylindrical protrusions were modeled using very small mesh size on the order of nanometers to obtain the flying characteristics for the entire slider air bearing surface (ABS) using the “CMRR” finite element Reynolds equation simulator (Duwensee et al., Microsyst Technol, 2006; Wahl et al., STLE Tribol Trans, 39(1), 1996). The effect of pattern height, pattern diameter, slider skew angle, and slider pitch angle on flying height of a typical slider is investigated. Numerical results show that the flying height decreases for a patterned slider and the change in flying height is a function of the pattern height and ratio of the pattern diameter to the pattern pitch. In comparison to discrete track media, the flying height loss is larger for a patterned slider disk interface for the same recessed area of pattern.     

航空发动机轴承腔内两相流动数值模拟

主轴承腔作为航空发动机润滑系统油气两相流的重要区域,腔内的高温高乐及其回油特性对润滑系统的性能都有很大影响.利用VOF数值计算模型对某航空发动机轴承腔简化模型内油气两相流进行数值计算,将两相之间表面张力作为源项添加到动量方程中,并依据实际情况添加壁面黏附模型,计算结果与现有实验数据符合良好.分析几种工作参数下润滑油相界面的差异及其因为,研究腔压及回油油气体积比随转子转速及润滑油流量的变化规律.结果表明:腔内的压力沿周向在出口处附近较低,并且随着转子转速或润滑油流量的增加而增大;回油孔出口处油气体积比随润滑油进口流量增加而增大;当润滑油进口流量一定时转子转速增大不利于回油.     

Experimental investigation of AE and friction signals related to the durability of head/disk interface

The durability of a hard disk drive is one of the most critical issues that must be optimized for best performance. Especially as the flying height of the head slider of a hard disk drive decreases over the years, the concern for surface damage and head contamination continues to grow. In this paper the characteristics of AE and friction signals for various operating conditions using CSS and drag tests were investigated from the durability point of view. Also, the wear characteristics of the laser bumps on a magnetic disk were compared between the CSS and drag tests. The general shapes of the AE and friction signals during a single CSS test were quite similar even under less than ideal operating conditions. However, it was found that the AE signal was more sensitive than the friction signal in assessing the damage of the slider/disk interface. Finally, a correlation was established between the CSS and drag testing methods with respect to the laser bump wear. This outcome suggests that the drag test may be used to accelerate the surface damage effect of head/disk system.     

Probability Model for the intermolecular force with surface roughness considered

This paper studies the intermolecular force considering both the roughness of the air-bearing surface and the disk surface by simulation. A model is developed to deal with the intermolecular force, the contact force and the air-bearing force based on the probability distributions of the roughness of the surfaces. The intermolecular force is linked with the contact force when its repulsive term is stronger than its attractive term. In such a case, all the intermolecular force, the contact force and the air-bearing force can be extended to the various flying height regions. Some interesting results are observed and discussed. It is found that both the Hamaker constant and the surface roughness have significant influences on the intermolecular pressure. Compared with the intermolecular pressure with smooth surfaces, that with the surface roughness considered shows greater attractive pressure at the flying height higher than 0.7 nm approximately, but much smaller values between 0.26 and 0.7 nm approximately. A negative stiffness region exists when the minimum flying height is between −0.2 and 1.2 nm for the case studied in this paper. It shows that the Probability Model is suitable for the intermolecular force calculation with the surface roughness considered.     

Study of a Spherical-Pad Head Slider for Stable Low-Clearance Recording in Near-Contact Regime

A slider with an air-bearing surface that has a small spherical pad around the read/write elements on the trailing center pad was developed to reduce the meniscus and friction forces and slider clearance loss. While the slider which has a spherical pad of a radius less than 10 mm, the meniscus force is reduced to less than 10% of the air-bearing lift force, thus, flying height modulation is decreased and instability are suppressed. Evaluation using an air-bearing surface model with a spherical pad showed that slider clearance was increased by 1 nm at a 5-nm nominal flying height. Evaluation using touchdown-takeoff testing of a spherical pad slider fabricated by depositing carbon on the center pad air-bearing surface by means of the lift-off resist technique showed that the spherical pad slider had a very small friction force and acoustic emission output up to a 5-nm interference height. It thus provides instability-free sliding in the near-contact regime.     

Study on the cyclotriphosphazene film on magnetic head surface

Stable lubrication is very important to the slider/disk interface with the increasing demand on the life of computer hard disk drive (HDD). The inert lubricant perfluoropolyether (PFPE) on the surface of magnetic hard disk is still prone to be catalyzed to decomposition by the slider material Al₂O₃. The properties of a partial fluorinated hexaphenoxy cyclotriphosphazene, X-1P, are investigated and its function to reduce the catalytic decomposition of PFPE is discussed. The results of contact start–stop (CSS) tester indicate that the thermal stability of the lubricant was greatly improved in the presence of X-1P, and its film thickness has a great influence on the lubrication properties of the HDD.     

Ultra-low-flying-height design from the viewpoint of contact vibration

The design of a head-disk interface for ultra-low flying height has been studied from the viewpoint of contact vibration. It is known that a super-smooth disk is necessary for a slider to fly at an ultra-low flying height; however, such a disk increases the friction force, which potentially increases the vibration of the slider. To solve this problem, the head-disk interface must be optimized to reduce this increased vibration. It has been shown that a large pitch angle and center-pad-mounted read/write elements have advantages in terms of slider/disk contact. It has also been found that a micro-texture on the air bearing surface can prevent contact vibration. Moreover, a frequency-shift-damping slider was found to damp the vibration effectively. To further investigate these findings, numerical simulation and modeling of slider dynamics during contact have been performed. Their results revealed two zones of contact vibration: a stable zone and an unstable zone.     

Process Transfer Function Development for Optical Media Manufacturing

The injection moulding of digital video disk substrates requires the selection of over forty process parameters to ensure the satisfaction of over twenty quality attributes. The work described characterised the behaviour of this complex moulding process through a formal set of design of experiments (DOE) consisting of: 1. Main effects DOE. 2. Multiple decoupled DOE. 3. Optimisation DOE. The described characterisation methodology identified critical process parameters and modelled linear, interaction, and non-linear effects of multiple quality attributes with regression coefficients varying from 65% to 98%. The developed empirical models are being used to facilitate advanced material development and production optimisation of digital video disks, but the described characterisation methodology can also be applied to other complex manufacturing processes.     

旋流式反循环钻头孔底流场模拟分析

贯通式潜孔锤反循环钻头结构经过不断的改进,已改善了反循环效果,但是在钻进坚硬、破碎地层和仰孔钻进作业中,仍存在反循环形成不彻底的问题。采用Fluent软件,结合无底喷孔的旋流式反循环钻头结构,分析了引射孔数量与孔径对引射孔仰角为60°,中心偏距为10.33mm的旋流式反循环钻头孔底流场的影响,为优化旋流式反循环钻头结构参数提供理论依据。分析结果表明,当两个直径9mm引射孔均匀分布时,旋流式反循环钻头可形成最佳的反循环效果。     

Microwear mechanism of head carbon film during head disk interface sliding contact

Thermomechanical sliding contact of head disk interface (HDI) causes critical wear on the carbon film of a head slider. An improved contact model accounting for both asperity and substrate deformation is applied to analyze the HDI contact behavior, while theories of frictional heat generation and heat transfer are used to investigate the change in HDI temperature. Based on actual HDI design and operation parameters, parametric study of thermomechanical HDI contact has been performed. It was found that severe wear of head carbon film would be significantly attributed to thermal degradation of carbon material during its sliding contact.     

Sliding-pin shapes and lubricant thickness change on a thin-film magnetic disk

In developing hard disk drives, it is necessary to keep lubricant as thick as possible during operations. For this purpose, we studied lubricant loss under different-shape contact-sliders on thin-film magnetic disks by using transparent-pin sliding tests with a built-in ellipsometer. We compared sliding pins with spherical, flat circle, flat square, and double-flat-rail surfaces.We found that lubricant loss was smaller under flat pins than under the spherical pin, and the smallest under the double-flat-rail pin among flat sliders. The results show that the horizontal and vertical shapes of sliders must carefully be selected for contact recording systems.     

牙轮钻头滑动轴承材料的力学性能试验

牙轮钻头滑动轴承副可用20CrNiMo钢和20Ni4Mo钢配对,二者经热处理以后,表面较内部材料的性能有很大的差异。为此各对20CrNiMo钢和20Ni4Mo钢的三种试样进行了力学性能测试。结果表明,滑动轴承副经渗碳热处理后表面材料比内部材料的抗压强度提高了近50％,其表面材料的应力一应变几近直线关系,破坏前没有明显的屈服变形阶段。这种表面强度硬度高、中间韧性好的特性,正好满足了滑动轴承对表面耐磨和心部抗冲击、抗振动的要求。     

Experimental and analytical studies of dynamic friction at the head/disk interface

Friction is an important parameter that critically impacts the tribological performance of a head/disk interface. The head/disk interface with laser zone texture affords a model system for the study of dynamic friction by virtue of its precisely-controlled contact geometry. By using two types of head sliders, i.e. the conventional slider and the padded slider, and a matrix of hard disks with a wide range of laser zone texture parameters, head/disk contacts involving a small number as well as a large number of bumps are realized. A rich variety of dynamic friction behaviors are observed with respect to bump height and bump density variations. To shed new light on the nature of HDI dynamic friction, an analytical model that treats both the deformational and the adhesive friction components on equal footings is formulated. It is shown that, based on the model analysis, the friction is deformation-dominated for HDIs involving a small number of contacting bumps and adhesion-dominated for HDIs involving a large number of contacting bumps. In the former case the friction decreases with bump density, whereas in the latter the friction increases with bump density.     

硬盘驱动器电磁轴承结构设计

介绍硬盘驱动器电磁轴承设计方法，径向轴承采用8极均匀分布结构，根据体积受限磁力最大原则，求出关键尺寸的最佳数值或近似表达式，轴向轴承根据承载力一定体积最小原则，设计各主要尺寸。