Ferrofluid can be used effectively as a liquid seal for rotary shafts to seal off gasses. However, this technology tends to be less effective when sealing off a liquid. In such applications, one major problem is the degradation of the fluid when it comes into contact with the liquid. This paper presents details of a method for replacing the ferrofluid in the seal without pressure loss, in order to improve the lifetime of the seal. A test set-up has been designed to comply with all contemporary knowledge covering ferrofluid–water interface seals and to improve on it by adding a replenishment method. Using a finite element analysis model to predict the pressure capacity, the results show the measured capacity conforms to the model. In addition, the set-up has produced promising results on the stability of the seal, when water is used as a medium in the pressure chamber@en

The objective of this research is to demonstrate the capability of a long stroke linear ferrofluid (FF) stage. This stage is a passive alternative to existing linear aerostatic stages and can be used in low loaded CNC devices, pick and place machines, microscopy or scanner applications. To compete with aerostatic stages the bearing must be repeatable and achieve sufficient stiffness for the application. The effects of ferrofluid trail formation are countered with the use of a ferrofluid reservoir located on the mover. To increase stiffness a specially designed magnet configuration is used. A linear guidance was built with outer dimensions of 180x600x80 mm (WxLxH), a mover of 1.8 kg without actuator and payload having a 430 mm stroke. The load capacity of the stage was measured to be 120 N, with a stiffness of 0.4 N/μm. The maximum height delta after a stroke with 1 kg payload and a mover velocity of 0.25 m/s was measured to be less than ±3μm, and with 1.75 kg payload and a velocity of 0.5 m/s the height delta was within ±7μm. Using a rheometer, it was shown that the effects of evaporation in ferrofluid can be reversed, within certain limits of mass loss, by adding carrier fluid. The damping is shown to be a function of payload and velocity and was measured to be between 2 and 4 N⋅s/m for velocities between 0.2 and 0.5 m/s. In comparison to a linear aerostatic stage it can be concluded that while the linear ferrofluid stage is outperformed in stiffness and out-of-plane repeatability, the ferrofluid stage does not require a continuous supply of air and has lower fabrication tolerances due to the higher fly height. Thus, the linear ferrofluid stage is a cost-effective alternative to a linear aerostatic stage when the stiffness and straightness are of less importance.

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The novel contribution of this research is insight into the influence of different parameters in the magnet configurations on the load and stiffness of a ferrofluid pressure bearing. It is shown that magnets with a small cross-section magnetized alternatively up and downwards combine a high load capacity and moderate stiffness while being low on material cost and complexity. The configuration where magnets are placed alternatively in left and right direction magnetized inter spaced with iron yields the highest load capacity and stiffness, albeit at the cost of weight and complexity. It is shown that an increase in the number of magnets is beneficial for the stiffness in both magnetization configurations, as is an increase in remanent flux density of the magnet. A metal bottom plate made of iron reduces the necessary height of the magnet in the up-down magnetization configuration. The model was validated using a bearing pad arranged in the up-down configuration. The force-displacement curve of this pad was measured in a load frame, using the APG 513 ​A ferrofluid from Ferrotec. A load capacity of 1.75 ​N/cm² was achieved, this exceeds previous pressure bearing implementations and performs comparable or better than implementations of single seal ferrofluid pocket bearings. These results show that the ferrofluid pressure bearing is a passive alternative in motion systems where the designer otherwise would have needed to use an active bearing.

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Magnetic fluids have been around since the 1940s. They come in different forms: magnetorheological fluids (MR fluids) and ferrofluids. MR fluids characterise themselves by having a large change in viscosity under the influence of a magnetic field. Ferrofluids have a significantly smaller change in viscosity, however ferrofluids are colloidal suspensions. After their discovery many applications followed, such as the MR clutch, magnetic damper and bearing applications, in which the fluids are subjected to ultra high shear rates. Little information is available on what happens to the rheological properties under these conditions. In general, the characteristics determined at lower shear rates are extrapolated and used to design new devices. Magnetic fluids have potential in the high tech and high precision applications and their properties need to be known in particular at shear rates around 10 ⁶ s ^-1. Commercially available magnetorheometers are not able to measure these fluids at ultra high shear rates and are limited to 10 ⁵ s ^-1. Therefore a new magnetorheometer is required to measure ultra high shear rates. In this paper the physical limitations of current measuring principles are analysed and a concept is designed for ultra high shear rate rheometry in combination with a magnetic field. A prototype is fabricated and the techniques used are described. The prototype is tested and compared to a state of the art commercial rheometer. The test results of the prototype rheometer for magnetic fluids show its capability to measure fluids to a range of 10 ⁴ s ^-1 s ^-1 and the capability to measure the magnetorheological effect of magnetic fluids.

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The bearing and the seal are two commonly used tribological components since nearly all moving machinery relies on them for proper operation. Even a small improvement in these components can have a big impact on both the market and the environment. The two main problems of these components are wear and friction. In addition, seals suffer from the problem of leakage which is fundamental to both their functioning as well as their performance. The application of magnetic fluids has the potential to be beneficial for these systems. Magnetic fluids consist of a suspension of magnetic particles in a carrier fluid. This gives them the unique properties of being attracted to a magnetic field and changing their rheological behaviour in the presence of a magnetic field. These special properties can give these bearing and sealing systems unique behaviour, potentially improving their performance. Therefore, this thesis has two main objectives. The first objective is to further investigate the potential of magnetic fluids in bearing and sealing system. This part consists of exploratory, fundamental and early stage research. The unique properties of magnetic fluids are eventually used in ferrofluid bearings, ferrofluid seals, bearings with rheological textures and self-healing bearings. The second objective is to develop the necessary knowledge to bring these concepts to society in the form of applications. The research on ferrofluid bearings as described in this thesis consists of different experimentally validated models for the load capacity, torque capacity, out of plane stiffness, rotational stiffness, friction and operational range for both ferrofluid pocket bearings and ferrofluid pressure bearings. These models are suitable to determine the most important bearing parameters which makes it possible to design a bearing system according to desired specifications. The most recent demonstrator shows specifications that are competitive with conventional bearing systems. The research on ferrofluid seals as described in this thesis has resulted in the first seal concept that does not show any leakage over time. The concept relies on a replenishment system that makes sure that that degraded ferrofluid is removed and replaced by fresh ferrofluid. This gives the system a theoretical infinite lifetime, as long as the replenishment system continues to work. The research on bearings with magnetorheological fluids as described in this thesis has resulted in the new design concept of rheological textures. A rheological texture is defined to be a local change in the rheological behaviour of the lubricant in the lubricating film such that a local change in lubricant transport and flow resistance occurs. The idea is to replace geometrical surface textures of more traditional bearings with these rheological textures to enhance the performance of the bearing. This work has in addition provided a new lubrication theory for Bingham plastics that properly models the behaviour. Furthermore, a new experimental method is developed to characterize the rheology of magnetic fluids at high shear rates. The last significant result of this research is the new concept of a self-healing bearing using a lubricant with a suspension of magnetic particles: the application of a local magnetic field with high gradients in the lubricating film causes the particles to settle at locations where the magnetic field gradient is high. This creates geometrical surface textures that will regrow when worn away. In this way we have a bearing with a surface texture that has a theoretical infinite lifetime.@en

Ferrofluid rotary seals are mechanical contact-free magnetic liquid seals that are characterised by their simple structure, low friction and ability to hermetically seal. Although ferrofluid rotary seals for sealing vacuum and gases are part of a well established industry, the sealing of liquids has not been implemented yet. Literature learns that degradation of the ferrofluid seal over time when it dynamically contacts a liquid results into premature seal failure. This paper presents a new type of ferrofluid rotary seal in which a ferrofluid replenishment system is implemented that renews the ferrofluid in the sealing ring while sealing capacity is maintained. By replacing the degraded ferrofluid in the seal at a sufficient rate, service life of the ferrofluid rotary seal can theoretically be unlimited. An analytical model and FEM analysis are used to design the ferrofluid sealing device and to predict its sealing capacity. An experimental test setup has been built on which the sealing capacity and service life of the device has been tested for different sealing conditions. It is demonstrated that the ferrofluid replenishment system successfully extends and controls the service life of the ferrofluid rotary seal that dynamically seals pressurised water.

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Introduction: The current COVID-19 pandemic has caused large shortages in personal protective equipment, leading to hospitals buying their supplies from alternative suppliers or even reusing single-use items. Equipment from these alternative sources first needs to be tested to ensure that they properly protect the clinicians that depend on them. This work demonstrates a test suite for protective face masks that can be realized rapidly and cost effectively, using mainly off-the-shelf as well as 3D printing components. Materials and Methods: The proposed test suite was designed and evaluated in order to assess its safety and proper functioning according to the criteria that are stated in the European standard norm EN149:2001+A1 7. These include a breathing resistance test, a CO₂ build-up test, and a penetration test. Measurements were performed for a variety of commercially available protective face masks for validation. Results: The results obtained with the rapidly deployable test suite agree with conventional test methods, demonstrating that this setup can be used to assess the filtering properties of protective masks when conventional equipment is not available. Discussion: The presented test suite can serve as a starting point for the rapid deployment of more testing facilities for respiratory protective equipment. This could greatly increase the testing capacity and ultimately improve the safety of healthcare workers battling the COVID-19 pandemic.

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This paper discusses a new type of hybrid journal bearing in which a magnetorheological fluid is used in combination with local magnetic fields, such that the hydrodynamic and hydrostatic working regimes are not compromised. This demonstrates the potential of using the concept of rheological texture in bearings. The performance of this new type of bearing is assessed via Finite Element Modelling (FEM) in which the behaviour of the fluid film is described by the ideal Bingham plastic fluid model. Both the yield stress and the viscosity increase as a function of the magnetic field.

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The rheological behavior of many lubricants used in hydrodynamic bearings can reasonably be modeled using the Bingham plastic material model. This behavior is characterized by a strong discontinuity, from a pure solid state to a viscous fluid state depending on the local shear stress. In literature three methods have been presented to model a Bingham plastic lubricated film. A full CFD and thus expensive, numerical simulation has been used. A general Reynolds equation based simulation has been used, however with a less accurate numerical regularization of the material discontinuity. Or a general Reynolds equation based simulation has been used, but with a severe reduction of the geometric complexity. In this paper, an ’exact’ thin film lubrication simulation for a Bingham plastic fluid is presented. The model is said to be exact in the sense that it requires no additional approximations to those used in the derivation of the general Reynolds equation, and requires no numerical regularization of the Bingham plastic fluid model and can still be used on any lubricating film geometry. Simulations on both infinite and finite journal bearings shows that the results of this new method are in good accordance with literature, demonstrating the validity of the method.

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Magnetorheological fluids can be used as a smart lubricant as a result of the fact that its properties can be changed with the use of a magnetic field. Local flow resistance and local pressure can be generated by applying a local magnetic field. This work presents a hydrostatic bearing in which the pressure profile of a conventional hydrostatic bearing is recreated with solely the use of a magnetic field and a magnetorheological fluid. The magnetic field is applied only locally at the outer edges of the bearing with the use of an electromagnet. The principle is demonstrated with the use of an experimental setup and a model from literature.

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Ferrofluid pocket bearings are interesting for fast and precise positioning systems thank to the absence of stick-slip, the low viscous friction and their cost-effective nature. However, the characteristics of the bearing change due to over(de)compression since air escapes out of the enclosed pocket. This article presents an experimentally validated model that includes the air mass inside the pocket in the calculation of the equilibrium position of the ferrofluid bearing. Moreover, a simple and efficient way to obtain the operational range of the bearing is presented and a sensitivity analysis was performed. The sensitivity analysis showed that ferrofluid pocket bearings are always self-aligning and that the tilt stiffness increases when the fly height decreases or the tilt angle increases.@en

Using a smart fluid, for example, magnetorheological or electrorheological, in a hydrostatic bearing gives the possibility to actively change the bearing properties during operation. This work presents an analytical model to predict the load and stiffness of a planar hydrostatic bearing lubricated with a Bingham plastic fluid. The model is validated with the use of a numerical model that uses the Bingham–Papanastasiou regularization to achieve convergence. The model gives insight into the size of the operational range of the bearing and the load characteristic.@en

Ferrofluid pocket bearings are a type of bearing that are able to carry a load using an air pocket encapsulated by a ferrofluid seal. Previously designed ferrofluid bearings show the great potential of the stick-slip-free and low viscous friction bearings, however until now the load capacity is limited. In this article a method is presented to increase the load capacity in a simple and cost effective way by the addition of ferromagnetic material around the magnet. First, a mathematical model of the bearing is presented and is validated by experiments using an axially magnetized ring magnet surrounded by two steel rings. The model is used to optimize the dimensions of the added ferromagnetic material for maximum load capacity. Depending on the fly height, the load capacity has been increased by a factor three to four by the addition of steel rings to the ferrofluid pocket bearing configuration.

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Ferrofluid bearings have been demonstrated to be very interesting for precision positioning systems. The friction of these bearings is free of stick-slip which results in an increase of precision. More knowledge on the friction behaviour of these bearings is important for there application in precision positioning systems. This paper demonstrates that the friction of a ferrofluid bearing can be modelled by a viscous damper model and provides a basic model to predict the friction behaviour of a bearing design. The model consists of a summation of a Couette flow with a Poiseuille flow such that there is no net fluid transport under the bearing pads. The model is experimentally validated on a six degrees of freedom stage using ferrofluid bearings. A stiffness in the form of a closed-loop control gain is introduced in the system to create a resonance peak at the desired frequency. The damping coefficient can be identified from the peak height of the resonance, since the peak height is the ratio of total energy to dissipated energy in the system. The results show that the newly derived model can be used to make an estimate of the damping coefficient for small(∼1mm) stroke translations. Furthermore, the model shows that the load capacity of a ferrofluid pocket bearing is affected during sliding.

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Ferrofluid bearings have recently been successfully implemented in precise positioning systems. This article gives an overview of systems built in the Department of Precision and Microsystems Engineering at Delft University of Technology in the Netherlands. Most applications are in microscope stages, where high accuracy is important while stroke and speed attain only moderate levels. Furthermore, some basic theory is provided on ferrofluid bearings that can help engineers incorporate ferrofluid bearings in their designs.@en

A ferrofluid pocket bearings is a type of hydrostatic bearing that uses a ferrofluid seal to encapsulate a pocket of air to carry a load. Their properties, combining a high stiffness with low (viscous) friction and absence of stick-slip, make them interesting for applications that require fast and high precision positioning. Knowledge on the exact performance of these types of bearings is up to now not available. This article presents a method to model the load carrying capacity and normal stiffness characteristics of this type of bearings. Required for this is the geometry of the bearing, the shape of the magnetic field and the magnetization strength of the fluid. This method is experimentally validated and is shown to be correct for describing the load and stiffness characteristics of any fixed shape of ferrofluid pocket bearing.

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