Ron A.J. van Ostayen

A Multiscale Method Modeling Surface Texture Effects

In this paper a multiscale method is presented that includes surface texture in a mixed lubrication journal bearing model. Recent publications have shown that the pressure generating effect of surface texture in bearings that operate in full film conditions may be the result of micro-cavitation and/or convective inertia. To include inertia effects, the Navier-Stokes equations have to be used instead of the Reynolds equation. It has been shown in earlier work [2] that the coupled 2D Reynolds and 3D structure deformation problem with partial contact resulting from the soft EHL journal bearing model is not easy to solve due to the strong nonlinear coupling, especially for soft surfaces. Therefore, replacing the 2D Reynolds equation by the 3D Navier-Stokes equations in this coupled problem will need an enormous amount of computing power that is not readily available nowadays. In this paper, the development of a micro-macro multiscale method is prescribed. The local (micro) flow effects for a single surface pocket are analysed using the Navier-Stokes equations and compared to the Reynolds solution for a similar smooth piece of surface. It is shown how flow factors can be derived and added to the macroscopic smooth flow problem, that is modelled by the 2D Reynolds equation. The flow factors are a function of the operating conditions such as the ratio between the film height and the pocket dimensions, the surface velocity and the pressure gradient over a surface texture unit cell. To account for an additional pressure build up in the texture cell due to inertia effects, a pressure gain is introduced at macroscopic level. The method also allows for micro-cavitation. Micro-cavitation occurs when the pressure variation due to surface texture is larger than the average pressure level at that particular bearing location. In contrast with the work of Patir and Cheng [4], where the micro-level is solved by the Reynolds equation, the Navier-Stokes equations are used at the micro-level. Depending on the texture geometry and film height, the Reynolds equation may become invalid. A second pocket-effect occurs when the pocket is located in the moving surface. In mixed lubrication, fluid can become trapped inside a pocket and squeezed out when the pocket is running into an area with higher contact load. To include this effect, an additional source term that represents the average fluid inflow due to the deformation of the surface around the pocket is added to the Reynolds equation at macro-level. The additional inflow is computed at micro-level by numerical solution of the surface deformation for a single pocket that is subject to a contact load. The pocket volume is a function of the contact pressure. It must be emphasized that before ready-to-use results can be presented, a large number of simulations to determine the flow factors and pressure gain as a function of the texture parameters and operating conditions have yet to be done. Before conclusions can be drawn, regarding the dominanant mechanism(s), the flow factors and pressure gain have to be added to the macro bearing model. In this paper, only a limited number of preliminary illustrative simulation results, calculating the flow factors for a single 2D texture geometry, are shown to give insight into the method.Copyright

Oil-Bleeding Model for Lubricating Grease Based on Viscous Flow Through a Porous Microstructure

One of the criteria in selecting lubricating grease for rolling-element bearing applications is its ability to bleed oil, sometimes called “grease bleeding.” Oil bleeding is assumed to be the dominating mechanism supplying new oil to the rolling track for lubrication. In this study, a physical model has been developed to understand the relation between parameters that control oil bleeding. In the model, lubricating grease is described as a porous network, formed by the thickener fibers, that contains the base oil. This type of structure is confirmed by SEM and AFM images of a lithium complex grease showing a matrix of rigid fibers with random orientation. A relatively simple flow model based on Darcy’s law for viscous flow in porous media and an anisotropic microstructure deformation model was developed. The model relates the pressure gradient, oil viscosity, thickener structure deformations, and permeability to the volumetric oil flow out of the thickener network. The permeability depends strongly on the thickener microstructure. The model was verified with experiments at a wide variety of temperatures and rotational speeds.

Film Height Optimization of Hydrodynamic Slider Bearings

The film height distribution which yields the maximum load carrying capacity for the rectangular slider is determined using a PDE-constrained optimization solver. The gradient of the objective function, i.e. the load, with respect to the design variable, i.e. the film height distribution, is calculated using a discrete adjoint approach. It is shown that, in contrast to the linear slider, the load carrying capacity per unit length and at constant width of the optimized slider steadily increases with increasing length over width ratio.Copyright

The application of fractional order control for an air-based contactless actuation system

Industry pushes towards ever faster and more accurate production of thin substrates. Contactless positioning offers advantages, especially in terms of risk of breakage and contamination. A system is considered designed for contactless positioning by floating a silicon wafer on a thin film of air. This paper focuses on the design of a control system, including actuators, sensors and control method, suitable for this purpose. Two cascaded control loops, with decoupled SISO controllers, are implemented for this moving mass controlled on a mass-spring system, which can be modelled as a fourth order system. The SISO controllers are first designed with classic loopshaping tools, which are then modified using fractional control. Two arguments based on examples in this system are given for the application of fractional control. Firstly, to increase the bandwidth of a regular mass-spring system, and secondly to control a plant which behaves fundamentally fractional, such as the moving mass in this cascaded fourth order system. By merely the application of fractionality, the bandwidths are extended by 14.6 % and 62 %, for the inner and outer loop respectively. A closed-loop positioning bandwidth of the wafer of 60Hz is achieved, resulting in a positioning error of 104nm (2σ value), which is limited by sensor noise and pressure disturbances. This paper shows how the extension of classic loopshaping tools with fractional control can directly improve the performance, without adding to the complicatedness of the control system. Moreover it demonstrates a working concept of a novel type of contactless actuator.

Planar Flat Product Transport Using Viscous Traction

In this paper a new contactless transport system for thin, flat products, such as glass substrates and silicon wafers, is introduced. The transport function is realized using viscous traction on the product surface. After an explanation of the operating principles, a mathematical model is presented. Preliminary computations indicate that the generated acceleration is approximately 2.5 m/s2 for a glass substrate with a surface area of 2 m2 and a thickness of 0.7 mm. Furthermore, the vertical bearing stiffness for the considered actuator geometry of the system is 4.2·106 N/m, depending on the properties of the inflow and outflow points of the actuator cells.Copyright

Multilobe Floating Ring Bearings for Automotive Turbochargers

Floating ring bearings are commonly applied in automotive turbocharger machinery because they are inexpensive and are able to reduce unbalance-induced vibrations. This type of bearings, however, can become a source of noise due to oil whirl-induced sub-synchronous vibrations. This study examined whether the concept of a floating ring bearing with a lobed clearance might be a solution to diminish oil whirl induced vibrations. A numerical model is developed to evaluate the dynamic behavior of the coupled rotor-bearing system. The resulting run-up simulations have been validated with experimental results of turbocharger run-up measurements. The results of the plain floating ring design and the multilobe floating ring design are compared, showing that the multilobe floating ring bearing design is superior in suppressing both synchronous and sub-synchronous vibrations at the cost of a slight increase in friction losses. Hence, multilobe floating ring bearings are an attractive alternative to plain floating ring bearings for automotive turbocharger applications.

Thermal analysis of an LPV test-rig

Abstract A material property which is often used in the comparison and selection of polymer based materials used for dry rubbing journal bearings is the Limiting Pressure-Velocity (LPV) value. In a large pressure and velocity range this value is determined by the maximum temperature and compressive strength in the contact area. The temperature is related to the amount of heat generated in the contact area between shaft and bearing and the total thermal resistance from that contact area to the ambient atmosphere. This thermal resistance is therefore not only determined by the bearing itself but also by the assembly as a whole in which that bearing has been used. The LPV-value for a particular material measured on a particular test-rig is therefore dependent on the thermal resistance of that test-rig and cannot be compared directly with an LPV curve for that same material measured on another test-rig which has a different thermal resistance. In this paper a thermal model is used to calculate the heat generation and transport in an LPV test-rig and the results of this model are verified with some experiments. Furthermore it is shown how the material properties such as friction coefficient and melting temperature obtained by another material testing method viz the so-called Finger Print (FP), in combination with the calculated thermal resistance of the test-rig, can be used to predict LPV-values for a material on that test-rig.

Effects of Magneto-Mechanical Coupling on Structural Modal Parameters

Structures that are exposed to a magnetic field experience magnetic forces. As these forces are geometry dependent they vary with the displacement of the structure that can result in an additional stiffness. Furthermore eddy currents induced by the movement of the structure can lead to an increased dissipation resulting in a higher damping value for the mechanical part of the system. This paper introduces calculation techniques for predicting these effects and validates them with measurements done on a simple set up in the lab.

Prototype Design of a High-Speed Viscous Air Spindle

In this paper an innovative air driven spindle for micro cutting applications is presented. The spindle uses a viscous traction concept which has the advantage that the viscous traction forces can act directly on the cylindrical part of the tool, which makes the tool-holder redundant. Furthermore, the tool can be actuated in the axial direction within the housing. In this paper the concept of the viscous turbine, a design of a prototype spindle along with the traction and load-capacity of the spindle are discussed.Copyright

Thermal network modelling of an LPV test-rig

A material property which is often used in the comparison and selection of polymer based materials used for dry rubbing journal bearings is the limiting pressure-velocity (LPV) value. In this paper the thermal network method (TNM) is used to model an LPV test-rig and the results of this model are verified with some experiments.