The load-stiffness relationship of a roller sandwiched between two flat surfaces is experimentally investigated. This study provides a comprehensive and well-documented dataset on roller stiffness in bearing applications. The measurements, obtained using hammer impact tests on a variety of rollers are both accurate and precise. Comparing the experimental results with the predictions from the commonly used Tripp model, we observe that the stiffness of the roller under relatively light load conditions is significantly lower than what is predicted by the Tripp model. This discrepancy is primarily attributed to surface roughness, a factor not accounted for in the Tripp model. For practical engineering applications, we propose simple formulas that predict the stiffness of real-world roller-rail contacts with an accuracy of ±10%.

This research aimed to study the wear amount and mechanism of a hydrodynamic journal bearing lubricated with magnetorheological (MR) fluid. The effects of bearing load, hard particle content in the MR fluid, magnetic field activation, and bearing sleeve material were experimentally investigated. Results revealed that the standard bronze sleeve experienced extreme wear with MR lubrication, two orders of magnitude higher than for oil lubrication, while the friction coefficient was almost 6 times higher, probably due to severe third-body abrasion. The least amount of wear among all tested materials was observed with a more flexible polymer sleeve, which showed ∼3.5 times more wear than the oil-lubricated bearing and a smaller increase in friction coefficient, around 2.6 times, as well as the formation of a possibly protective layer of crushed particles in the converging region of the film. The results suggest that polymers, and possibly also softer materials such as rubber, are a promising alternative for bearings lubricated with MR fluids under low-speed and high-load conditions.

The applications where fluid film bearings are used to guide high loads over wavy surfaces are limited. This because current designs of fluid film bearings often consist either of rigid embodiments that are unable to adapt to varying surface curvatures to form the required thin fluid film, or of compliant designs that have been designed to allow only for small deformations. This work discusses the requirements to design highly deformable fluid film bearings and introduces two metrics to compare their performance. Additionally, it introduces a compliant cell that is filled with an incompressible fluid as a design element to obtain both a high load capacity and sufficient deformability for such bearings. This closed fluid cell is also implemented in a 2D axi-symmetric hydrostatic bearing concept, that is numerically modelled and validated by experiments with a prototype. The simulations and prototype show that it is able to operate on surfaces with a hundred times higher curvature than has been analysed in previous studies.

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.

Micro-textured water-lubricated thrust bearings exhibit significant potential in motorized spindles due to their low friction, high heat dissipation efficiency, and superior damping performance. However, existing numerical methods for evaluating the lubrication performance of such bearings face challenges in balancing computational efficiency with accuracy, along with limitations in macro-micro cross-scale coupling capabilities. To address these issues, this study proposes a novel approach combining the flow factor model with a machine learning algorithm. First, the average Reynolds equation based on the average flow model (AF-ARE) is formulated using flow factors. However, results indicate that when dealing with high-density textures with small diameters, AF-ARE still suffers from computational inefficiency as it requires individual calculation of flow factors for each texture element. To address this limitation, machine learning-based prediction models are subsequently developed using three algorithms: Gaussian process regression, support vector machine, and extreme learning machine. The prediction models enable rapid estimation of flow factors for all texture elements, and their predictive performances are systematically compared and evaluated. Subsequently, by integrating these flow factor prediction models with the average Reynolds equation, a novel average flow-machine learning-averaged Reynolds equation (AFML-ARE) method is proposed. The proposed AFML-ARE multiscale numerical method offers a novel approach to overcome current research limitations in cross-scale numerical analysis of lubrication characteristics for high-density, small-diameter micro-textured bearings, enabling efficient performance evaluation from microscopic texture effects to macroscopic bearing behavior.

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.

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.

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.

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.

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.

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.

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.

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.

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.