One of the most critical elements making up a disk storage system is the air bearing interface located between the magnetic transducer and the disk surface on which data is stored and retrieved. The air film provided between the slider which houses the transducer and the disk serves to eliminate contact and wear of the solid surfaces and in addition must be extremely thin (on the order of 1/4 micron) in order to achieve a high density of data storage. Two of the most sought after properties of this lubricating film are a low generated load and high fluid stiffness. Low load allows the slider to be in contact with the disk when it is started and stopped, while high stiffness tends to provide a nearly uniform air bearing clearance which is important for reliable and efficient data storage and retrieval. The Zero-Load slider incorporates both low load and high stiffness in a single bearing. In this paper, the flying characteristics of the Zero-Load slider are discussed and its potential is explored. Numerical simulations which are presented for both static and dynamic operation indicate that the Zero-Load slider should be a strong contender for application in high performance disk files.
The effect of the film shape on the load carrying capacity of a hydrodynamically lubricated bearing has not been considered an important factor in the past. Flat-faced tapered bearing and the Raileigh’s step bearing of constant film thickness have been the primary forms of film shapes for slider bearing studies and design data developments. This article, by the computer aided numerical solution of the Reynolds equation for two dimensional incompressible lubricant flow, investigates hydrodynamically lubricated slider bearings having different film shapes and studies the effect of the film shape on the performance characteristics of finite bearings; and it shows that optimized bearing with film shapes having descending slope toward the trailing edge of the bearing has considerably higher load carrying capacity than the optimized flat-faced tapered bearing of the same properties. For example the truncated cycloidal film shape yields 26.3 percent higher load carrying capacity for Lz/Lx = 1 size ratio, and 44 percent higher for Lz/Lx = 1/2. The article then presents charts for the optimum designs of finite slider bearings having tapered, exponential, catenoidal, polynomial, and truncated-cycloidal film shapes, and illustrates their use in numerical bearing design examples. These charts also furnish information on flow rate, side leakage, temperature rise, coefficient of friction, and friction power loss in optimum bearings. Appended to the article are analytical solutions for infinitely wide bearings with optimum bearing characteristics. The computer aided numerical solution of the Reynolds equation in most general form is presented by which finite or infinitely wide hydrodynamically or hydrostatically lubricated bearings, externally pressurized or not, can be studied. A digital computer program is made available.
A theoretical analysis for the static characteristics of an aerostatic porous rectangular thrust bearing with an offset load, for both open and sealed end configurations, is presented considering three-dimensional flow in the porous matrix. Static characteristics for different operating parameters are calculated numerically and presented in dimensionless form as design charts. The effect of tilt is discussed.
In this work the authors give a method to determine the influence of the inertial terms, in the case of a lubricated slider bearing, based on the solution of the “inverse problem.” That is to determine the geometry of a slider bearing given the pressure distribution, the lubricant flow rate and the slider velocity. Unlike other methods till now proposed (i.e., the Slezkin and Targ, Kahlert, Constantinescu methods), in this work no simplifying assumptions have been made. A sensible influence of the inertial forces, already at the value of the modified Reynolds number Re* = 0.1, has been shown. At the same load capacity, the minimum thickness of the film decreases because of the inertia (at values of the parameters Re* = 0.1 and a = 0.2 the decreasing is about five percent). At a given geometry of the lubricant film, the inertial forces increase the load capacity, the friction forces and the friction coefficient if they are compared with the linear case. A comparison between the results of the present study and the results obtained solving the “direct problem” with approximate methods shows a good agreement relating to quality.
The stability characteristics of the porous disk thrust bearing was studied experimentally and compared with a theoretical prediction [3]2. The discrepancy between them was found to be small in the low supply pressure region and large in the high supply pressure region. At instability the pressure in the plenum was found to oscillate in response to the oscillation of the upper thrust plate. Thus the plenum volume emerged as an important factor affecting stability. This finding, which appears to be the first ever reported, led to a preliminary theoretical investigation of the plenum volume effect. A satisfactory explanation of the observed stability characteristics was obtained by using a circuit analogy. In addition to the porous disk bearing, two other types of thrust bearings, viz, the plain disk with a porous insert and the plain disk with a recess and orifice restriction, were also included in the experimental study. The three types of bearings were compared with reference to their stability. Within the same range of the gas flow rate, the porous insert bearing displayed an equal or larger region of stable operation in comparison with the orifice bearing. Besides, the porous insert bearing exhibited a feature which could simplify its construction and make its application very attractive.
Experimental results are given for load capacity and whirl onset in journal bearings of circular, elliptical and offset halves bore shape. The general validity of the linearized model for predicting whirl is confirmed experimentally. Deviations between experimental results and the model, based on an isoviscous film, are attributed to the varying viscosity that occurs in practice, and to unavoidable excitation that gives rise to premature whirl. It is shown that increasing groove size has a destabilizing effect that can more than cancel the beneficial effect of preloading. This result is particularly relevant to the design of journal bearings in turbomachinery.
The conditions of film formation are examined theoretically when starvation occurs. The analysis is for two-dimensional Newtonian flow and includes surface tension effects. Using an integral equation method, stream function solutions, velocity fields, pressure and shear stress distributions are calculated along and across the inlet zone of a sliding contact. The effect of surface tension and feeding thickness on the meniscus shape and on pressure buildup is studied in correlation with hydrodynamic effects. In all cases, pressure value lower than the gas pressure acting on the free boundary is found along the sliding surface. This depression value increases with an increase in viscosity or surface velocity. Owing to these results, a new interpretation of some published experimental data on starved contacts is proposed.
An analysis is conducted and solutions are provided for the effect of centrifugal forces on hydrostatic misaligned thrust bearings. The results show that centrifugal forces reduce considerably the load capacity, the friction torque and increase the lubricant flow rate. It is found that the effect of centrifugal forces is decreased as tilting of the bearing is increased.
Using the results shown in some previous papers, the overall characteristics of bearings lubricated with ferrofluids are calculated. First, the load carrying capacity of infinitely long sliding and journal bearings is considered. The pressure center and the attitude angle are also determined. Then, similar characteristics of finite bearings are obtained. In both cases, the new boundary conditions and the actual film extent under magnetic stresses are taken into account. It is shown that friction forces are mainly affected by the change in viscosity due to magnetic particle suspensions, while magnetic stresses modify friction only under special conditions which usually are not met in lubrication. In addition, the flow rates and the side flow coefficients are calculated. It is shown that the lubricant leakage can be reduced and even avoided when proper measures are taken. For short bearings, diagrams are given for the film extent, the film boundaries, the load carrying capacity, and the attitude of the journal center.
A time-transient nonlinear dynamic analysis is presented to study the motion of statically unloaded journal-bearing tilting pads. The major finding is that unloaded pads can exhibit a strong sub-synchronous self-excited vibration. The frequency of this periodic motion is somewhat below half the rotational speed and bears a close relationship to self-excited oil-whip vibration of rotors on lightly loaded non-tilting pad journal bearings. The identification of this type of self-excited pad vibration has practical significance to the solution of problems in applications involving damage to unloaded pads. A comprehensive parametric study is presented and shows which tilting-pad journal bearing parameters are significant to self-excited pad vibration and its elimination.
The purpose of this paper is to derive lubrication equations suitable for constant-property fluids exhibiting inelastic non-Newtonian characteristics. The analysis results in a slightly modified form of Reynolds equation. Fluid characteristics show up in this equation through an equivalent power-law. Data are presented for journal bearing performance over a range of L/D’s and rheological exponents.
The experimental investigation discussed here gives experimental confirmation of the slip-flow theory for modeling hydrodynamic gas bearings with clearances below 0.25 microns. An interferometric technique employing two CW lasers is used to measure the small clearances with an accuracy of 0.025 microns. The effects of molecular rarefaction are studied by operating the slider bearing in different gas media of different mean free paths. Bearings operating at extremely high local Knudsen numbers are studied without approaching excessively high bearing numbers. Experimentally measured trailing edge clearances and pitch angles are compared with theoretical predictions using the modified Reynolds equation with velocity slip boundary conditions. Excellent agreement between experiment and theory is found for clearances as high as 1.60 microns to as low as 0.075 microns with corresponding ambient Knudsen numbers of 0.04 and 2.51, respectively.
It is shown that squeeze film damping cutoff frequencies can be computed directly from the lowest eigenvalue of the Helmholtz equation. A kinematic mode is proposed and analyzed for the computation of those frequencies and it is demonstrated that Griffin’s calculations may underestimate considerably those frequencies. New results are given for the squeeze film behavior between rectangular plates, annuli which are not necessarily thin and plate sectors.
