Thin film lubrication problems frequently involve the use of lubricants with non-Newtonian characteristics, and a relatively simple viscosity model that can describe several non-Newtonian fluids is the Herschel–Bulkley relation. This relation can model solid-like properties of a lubricant at low shear stress using a yield stress, while at higher shear stress values shear-thinning or thickening can be included. In literature, this viscosity model has been combined with various governing equations to solve the non-Newtonian thin film problem, resulting in models that range from full 3D CFD simulations, to 1D Reynolds equation based methods. However, something that all of these approaches have in common is that they are either computationally expensive, can only be used for 1D geometries, or use non-exact, regularised versions of the Herschel–Bulkley model for reasons of numerical stability. This paper therefore introduces a method for solving a thin film problem with a non-regularised Herschel–Bulkley lubricant using the 2D generalised Reynolds equation, and this approach is shown to be fast without compromising on accuracy. The increased speed will allow the model to be used more efficiently in complex simulations or design optimisation scenarios.

@en

Contactless handling systems for substrates hold significant potential in enhancing chip manufacturing yields by allowing the use of thinner and larger substrates, eliminating the risks associated with physical contact. This article introduces a novel contactless force actuator, employing the active air-bearing working principle, designed with a compact structure to effectively actuate substrates. The actuator features a continuous deformable air-bearing surface composed of compliant-based actuation unit cells, ensuring ease of fabrication to meet tight air-bearing tolerances. A modular design with seven unit cells is designed and manufactured to validate the performance. The results confirm that the proposed contactless actuator can be used to levitate and actuate the substrate simultaneously, in which case the maximum actuation force in the x -axis is determined to be 90 mN and a 42.5- μ m fly height in the z -axis is achieved.@en

The rolling-sliding dynamics of large-scale cam-roller contacts are strongly influenced by the inertia of the roller, particularly when slippage occurs. Slippage can potentially impact the reliability of these rolling interfaces. This study introduces an approach to replicate the rolling-sliding dynamics of cam-roller contacts in a large-scale hydraulic drivetrain, on a small scale. For that, we have upgraded our two-roller tribometer to enable cyclic loading, allow the application of resisting torques, and generate inertia torques. These are three essential elements required to mimic the dynamics observed at large scales. A method has been proposed for scaling the roller inertia accordingly. Furthermore, we have implemented a modeling framework from previous work to make predictions under various dynamic conditions. The results show that our small-scale approach can replicate five key characteristics anticipated at a large scale, including those linked to slippage. Small increments in the resisting torque significantly increased the slide-to-roll ratio (SRR) and peak traction force, among others. The simulations also predicted these effects, capturing trends and producing reasonable predictions of the magnitude and relevant features of key parameters. The use of cyclic loading, extra inertia, and adjustable resisting torques, effectively generated repeatable and controllable dynamic rolling-sliding conditions. Our work is significant for the design and development of novel large-scale hydraulic drivetrains. Our findings highlight the importance of reducing slippage at low contact forces to prevent the brusque change in the rolling conditions during the high contact force phase. By doing so, surface damage and detrimental dynamic effects can be prevented.@en

The frictional performance of water-lubricated UHMWPE is influenced by the combination of structural parameters and operating conditions. To improve the efficiency of optimal design of surface texture aimed at improving frictional performance, a novel integration of the Orthogonal Array method (OAM), machine learning (ML) prediction, and Particle Swarm Optimization (PSO) is proposed for predicting and optimizing the coefficient of friction (COF) of copper ball-textured UHMWPE surfaces using a small dataset. In order to reduce manufacturing and testing cost, decrease required training samples for ML algorithm, OAM which could efficiently acquire data set with comprehensive feature information is used to determine the parameters of test samples to generate a small but effective dataset. 25 textured samples based on L16 (4⁴) and L9 (3⁴) are fabricated, with the parameter set determined using OAM. COFs of the samples are tested using RTEC tribo-tester. Trend analysis is conducted to investigate the influence of force, frequency, depth and ellipse axis ratio on COF. Multi-linear Regression (MLR) and Gaussian Process Regression are employed. MLR exhibits better prediction accuracy and is integrated with PSO to minimize COF. The error between the experimental and the theoretical results obtained by the integration method of MLR and PSO is only 1.04%, demonstrating the feasibility of predicting COF and optimizing surface texture using the integrated method with a limited dataset determined by OAM.

@en

This work introduces two innovative rolling pair concepts to minimize slippage and reduce mass in cam-roller systems of large-scale hydraulic drivetrains: The variable contact length and the Shifting Contact Geometry concepts. Both aim to improve traction in the low contact force phase in cyclically loaded rolling contacts. The shifting contact geometry concept was validated using three custom rolling contacts: a line contact, a double elliptical contact, and a combination of both (i.e., shifting contact geometry). The tests were conducted under synchronized cyclic loading to mimic the conditions in a hydraulic drivetrain. Furthermore, a model from previous work was implemented to make predictions and compare them against the experimental results. During preliminary tests, the double elliptical contact displayed superior tractive behavior than the line contact under the same load thanks to higher contact pressures. Under synchronized cyclic loading, the line contact displayed high sensitivity to applied resisting torques at low contact forces, leading to high slide-to-roll ratios and traction force peaks. In contrast, the rolling pair with shifting contact geometry exhibited minimum slippage even under high resisting torques, resulting in substantially lower (and in most cases negligible) slide-to-roll ratio and traction force peaks. The simulations also captured this behavior, proving the validity of the model for predicting and comparing the rolling-sliding dynamics of these two different rolling pairs. This study demonstrates that rolling pairs with shifting contact geometry can not only improve the tribological performance of cam-roller contacts in large-scale hydraulic drivetrains but also yield a more favorable dynamic behavior.

@en

This work presents the development of a numerical model for the elastohydrodynamic lubrication of roll-to-plate nanoimprinting with flexible stamps. Roll-to-plate nanoimprinting is a manufacturing method to replicate micro- and nanotextures on large-area substrates with ultraviolet-curable resins. The roller is equipped with a relatively soft elastomeric layer, which elastically deforms during the imprint process. The elastic deformation is described by linear elasticity theory. It is coupled to the pressure build-up in the liquid resin film, which is described by lubrication theory. The flexible stamp, which is treated as a tensioned web, is pre-tensioned around the roller. The elastic deformation of the tensioned web is described by the large-deflection bending of thin plates equations, considering its non-negligible bending stiffness. A Fischer–Burmeister complementarity condition captures the contact mechanics between the tensioned web and the roller. The governing equations combine in a coupled elastohydrodynamic lubrication model, which is fully described by a set of non-dimensional numbers. These are used in a parameter study to investigate the effect on the pressure and film height distributions. It is shown that the bending stiffness of the tensioned web results in an additional hydrodynamic pressure peak and corresponding minimum in the film height, near the inlet of the roller contact. An increase of the bending stiffness corresponds to a decrease in film height. The numerical results are compared with benchmarks from literature and experimentally validated with layer height measurements from flat layer imprints. Good agreement is found between the numerical and experimental results.

@en

A journal bearing test bench is used to find the transition speed between the hydrodynamic and mixed lubrication regimes for a modified magnetorheological (MR) fluid. It is shown that the transition speed of the bearing can be reduced by applying a local magnetic field near minimum film when it is lubricated with the MR fluid, and that this will only marginally increase friction. The lubricating performance of the MR fluid is compared to that of a reference oil, and all experimental results are compared with a Finite Element model based on the Reynolds equation.

@en

Compliant joints have significant advantages compared to rigid-body hinges due to a monolithic design and the absence of friction, which prevents effects like wear, backlash, and stick-slip behavior. However, the loading capability is often limited and the support stiffness generally decreases during rotation. A new design principle called closed form pressure balancing has been proposed as a solution to improve these limitations. By using an incompressible fluid as the main compliant element, the support stiffness becomes independent of rotation and buckling no longer limits the loading capability. This work analyzes the fundamental working principle behind closed form pressure balancing and introduces a 2D design model to determine stiffness properties. The design model is validated with a finite element model and used to construct an optimization strategy for optimum joint performance. Additionally, a conversion model and some practical considerations are presented for the transition to a 3D design model.@en

Generally speaking, excessive side thrust and roller slippage are two different aspects affecting cam-roller mechanisms. In novel large-scale hydraulic drivetrains for offshore wind turbines, the highly dynamic nature of these mechanisms combined with the interplay of cyclic loads, frictional torques and inertia promote slippage at the cam-roller interface. At larger scales, the effects of roller inertia become much more pronounced, as the inertia escalates exponentially with the roller’s radius. This study presents a comparative analysis between radial and offset roller followers in novel large-scale hydraulic drivetrains, where offset followers are incorporated to minimize the side thrust. The framework encompasses a comprehensive kinematic and force analysis, to provide the inputs for two lubrication models integrated into the torque-balance equation, where the possibility of slippage is allowed. The findings reveal that the equivalent side thrust can be reduced by 51% with offset followers. Both configurations experience slippage during the low-load phase, but it rapidly diminishes during the high-load phase. This sudden transition in rolling conditions results in a sharp increase in surface temperature and traction force, emphasizing the importance of minimizing sliding at the interface. However, besides the substantial side thrust reduction, offset followers showed superior tribological performance, mitigating undesirable thermal and frictional effects. @en

Transverse vibrations can induce the non-linear compression of a thin film of air to levitate objects, via the squeeze-film effect. This phenomenon is well captured by the Reynolds' lubrication theory; however, the same theory fails to describe this levitation when the fluid is incompressible. In this case, the computation predicts no steady-state levitation, contradicting the documented experimental evidence. In this Letter, we uncover the main source of the time-averaged pressure asymmetry in the incompressible fluid thin film, leading the levitation phenomenon to exist. Furthermore, we reveal the physical law governing the steady-state levitation height, which we confirm experimentally.

@en

This paper aims to investigate the combined effects of working condition and structural parameters of groove texture on the dynamic characteristics, stability and unbalance response of a water-lubricated hydrodynamic bearing–rotor system to avoid instability and excessive vibration of the rotor. The Navier–Stokes equation, standard K-ε model with enhanced wall treatment and Zwart–Gerber–Belamri cavitation model are considered using the commercial software Fluent to calculate the stiffness and damping coefficients of a groove-textured, water-lubricated bearing based on the dynamic mesh method; the critical mass to express the stability and the unbalance response solved by the fourth order Runge–Kutta method of the rotor are calculated based on dynamic equations. The results indicate that shallower and longer groove textures can improve the direct stiffness along the load direction (Formula presented.), weaken the stiffness in the orthogonal direction (Formula presented.), improve stability and decrease the unbalance response amplitude of the water-lubricated bearing–rotor system at a greater rotational speed and smaller eccentricity ratio; however, the impact of grooves on damping parameters is not as great as it is on stiffness—there exists an optimum groove width to achieve a best dynamic performance.

@en

Compliant mechanisms (CM) with adaptive stiffness have been widely used in robotics and machine design applications. This paper proposes adapting the endpoint stiffness of a spatially curved compliant beam using a movable torsional stiffener and a new graphical characterization method for the resulting anisotropic stiffness of the endpoint for large deflections. A slender clamped-free cruciform beam with a predetermined spatial shape was utilized as the main compliant part, and a shorter sliding bellow was served as the torsional stiffener. The beam's endpoint displacements are mainly determined by its bending and torsional deformations. Therefore, the relocation of a bellow stiffener with high torsion and low bending stiffness along the described beam with relatively low torsion and high bending stiffness led to notable changes in the kinetostatic behavior at the endpoint. The share of bending and torsional stiffness of elements along the beam to endpoint stiffness varies depending on the direction. Experiments with arbitrarily chosen parameters of the current design reveal an anisotropically adaptive stiffness with 21.5 times more stiffness variations in one direction compared to the other. Effective characteristics for this behavior, such as the length and position of the bellow, were explored in an effort to improve it. To capture the effect of these parameters, the Isoforce Displacement Closed Surface (IDCS) was introduced as a new characterization method to visualize the nonlinear kinetostatic behavior of a CM throughout its three-dimensional range of motion. The IDCS was further used to elucidate how individual components of the current mechanism contribute to the system's overall kinetostatic behavior. Experiments were done on prototypes to confirm the changes in endpoint stiffness that were predicted by simulations.

@en

Roll-to-plate nanoimprinting with flexible stamps is a fabrication method to pattern large-area substrates with micro- and nanotextures. The imprint consists of the preferred texture on top of a residual layer, of which the thickness and uniformity is critical for many applications. In this work, a numerical model is developed to predict the residual layer thickness (RLT) as a function of the imprint parameters. The model is based on elastohydrodynamic lubrication (EHL) theory, which combines lubrication theory for the pressure build-up in the resin film, with linear elasticity theory for the elastic deformation of the roller material. The model is extended with inextensible cylindrical shell theory to capture the effect of the flexible stamp, which is treated as a tensioned web. The results show that an increase in the tension of the web increases the effective stiffness of the roller, resulting in a reduction in the RLT. The numerical results are validated with layer height measurements from flat layer imprints. It is shown that the simulated minimum layer height corresponds very well with the experimental results for a wide range of resin viscosities, imprint velocities, and imprint loads@en

The usually high eigenfrequencies of miniaturized oscillators can be significantly lowered by reducing the stiffness through stiffness compensation. In this work, a mechanical design for a compliant ortho-planar mechanism is proposed in which the stiffness is compensated to such a degree that it can be identified as statically balanced. The mechanism was fabricated using laser micro-machining and subsequently preloaded through packaging. The statically balanced property of the mechanism was experimentally validated by a measurement of the force-deflection relation. A piezoelectric version of the design was fabricated for the purpose of energy harvesting from low-frequency motion. For a sub 1 Hz excitation, the device demonstrated an average power output of 21.7 μW and an efficiency that compares favorably to piezoelectric energy harvesters reported in the literature. Therefore, it was found that stiffness compensation is a promising method for the design of piezoelectric energy harvesters for low-frequency motions.

@en

This paper presents the development of a contactless sensing system and the dynamic evaluation of an air-bearing based precision wafer positioning system. The contactless positioning stage is a response to the trend seen in the high-tech industry, where the substrates are becoming thinner and larger to reduce the cost and increase the yield. Using contactless handling it is possible to avoid damage and contamination. The system works by floating the substrate on a thin film of air. A viscous traction force is created on the substrate by steering the airflow. A cascaded control design structure has been implemented to the contactless positioning system, where the Inner Loop Controller (ILC) controls the actuator which steers the airflow and the Outer Loop Controller (OLC) controls the position of the substrate by controlling the reference of the ILC. The dynamics of the ILC are evaluated and optimized for the performance of the positioning of the substrate. The vibration disturbances are also handled by the ILC. The bandwidth of the system has been improved to 300 Hz. For the OLC a linear charge-coupled device has been implemented as a contactless sensor. The performance of the sensing system has been analyzed. During control in steady state, this resulted in a position error of the substrate of 12.9 (Formula presented.) m RMS, which is a little more as two times the resolution. The bandwidth of the OLC is approaching 10 Hz.

@en

Vibrations are a promising source for powering wireless sensors, for example in low-frequency environments like human motion. These environments suffer from unpredictable vibration spectra and their low-frequency and large amplitude characteristics offer great possibilities for mechanisms with double well potential energy characteristics. The dynamical output performance of a bistable mechanism depends on the oscillation in the large amplitude trajectory between the two potential wells. However, requires enough force to overcome potential energy barrier. This work aims to improve the occurrence of interwell oscillation by lowering the potential energy barrier between the two potential wells by the influence of hard mechanical travel limits. A bistable mechanism is numerically modelled and experimentally tested to investigate the influence of the mechanical travel limits for low-frequency excitations. An axial loaded buckling beam was used to introduce bistability and combined with a parallel guidance mechanism to compensate for the strong negative stiffness. A single-degree of freedom model is used to model the bistable characteristics and is expanded with a coefficient of restitution model to represent the mechanical characterization of the travel limits. This combination resulted in a decrease in required force for the oscillation in the desired large amplitude trajectory by constraining the oscillator motion with travel limits. Furthermore, the results from the numerical bistable model in combination with mechanical characteristics of the travel limits at impact, proves to be in good agreement with the experimentally obtained results.

@en

In this paper a method is demonstrated for tuning the stiffness of building blocks for statically balanced compliant ortho-planar mechanisms. Three post-buckled mechanisms are proposed where the flexural rigidity can be manipulated over a part of their length in order to tune the ratio between the first two critical loads. A sensitivity analysis using finite element simulation showed that the best balancing performance is obtained in these mechanisms when this ratio was maximized. The results were validated experimentally by capturing the force-deflection relations.

@en

In tribotronic bearing design active components are used to adapt bearing performance to operating conditions. The principle of self-adaptive bearings has also been presented in literature in which a passive modification of geometry was used for a variation in conditions. This work presents an alternative design approach for self-adaptive bearings. This approach is focused on the shift between two known bearing geometries, where each of them is the preferred solution in a part of the operating regime. Using compliant elements in the bearing design allows for passive shape shifting. Four examples are presented which present this behavior for variable velocity and load conditions. The design approach could possibly provide a cheaper alternative for simple active bearing designs, or could be combined with active components in a tribotronics design to improve existing performance.

@en

Water lubricated bearings have been named in literature as a sustainable alternative to their oil-based counterparts. In order to clarify when water lubricated bearings are or are not a sustainable alternative, and inform design decisions, an analytical design tool is introduced based on anticipatory Life Cycle Assessment (LCA). This model is based on data from the bearing geometries, materials, and material combinations that have been the subject of research attention in the past 20 years in water lubricated bearing design. The model provides simple equations, fed with data from literature on materials, production and their tribological combination to provide initial insight on the sustainability of these types of bearings and future designs. A case study illustrates that quantifying environmental impacts can help determine when lubricant loss is more important than material choice, or vice-versa. The method aids bearing designers towards more sustainable designs.

@en

Compliant ortho-planar mechanisms can be fabricated from a single sheet of material and they can move out of the plane in which they are fabricated. In this work, a design of such a mechanism is proposed in which stiffness compensation is applied. With a finite-element model, the force-deflection relation of the buckled mechanism was modelled and it was shown that by changing the design parameters, the stiffness of the mechanism can be tuned such that it can be positive, negative or even close to zero. The mechanism was prototyped and the force-deflection relation was measured to validate the simulations.@en