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 behaviour. However, the loading capability is often limited and the support stiffness generally decreases during rotation, caused by the use of solid leaf flexures. Previously, a new design principle called closed form pressure balancing was introduced, which uses an incompressible fluid as the main compliant element in the joint. Although it showed great potential in terms of stiffness performance, theoretically not much is known about this design principle. This thesis analyses the fundamental working principle of closed form pressure balancing and introduces a design model to analyse the characteristic behaviour and to provide a practical design tool. This design model has been validated with a finite element method model, which shows a quantitative agreement. Additionally, a solution for the limited shear stiffness characteristic for pressure balanced joints is proposed. The potential of this solution is shown with the use of a case study on the design of compliant piston-slipper mechanisms, for which a prototype has been designed, built and tested. ... Read more

A LED induced fluorescent technique based test setup is introduced , which is used to measure the water film thickness of compliant hydrostatic bearings. Fluorescent particles have the characteristic of absorbing light of a certain wavelength and emitting light of a higher wavelength. A LED with a wavelength of 540 nm (green light) is used to excite the Rhodamine B fluorescent particles, which have a excitation wavelength of 545 nm. The emission wavelength of the particles is 570 nm (red light) and this colour shift is captured by a RGB colour camera where the red and green channels have been used as an optical filter. The increase of intensity due to the excitation of the thin film can be coupled to the thickness of the film. The performance of the technique is validated by comparing the film height to a laser sensor measured film height for a rigid bearing containing features with known dimensions. A film thickness measurement technique is presented that is able to measure 15-1400 $\mu$m and an average accuracy of 6.4\% is achieved. ... Read more

Compliant mechanisms consist of a monolithic body and obtain motion through elastic deformation. Multiple compliant flexure designs are known but their range of motion and load capacity are often limited. When considering rotational hinges, improving the operational range requires reducing the rotation stiffness while increasing normal stiffness. This work introduces a novel compliant hinge design with increased stiffness ratio compared to the state of the art compliant hinges. The design makes use of a so-called closed form pressure balancing principle to obtain this high ratio. This principle utilizes incompressible fluid inside a closed body. A 2D parameter sweep is performed to identify the highest performing hinge design. Subsequently, a computational 3D analysis is performed and the resulting design is realized as a demonstrator. The performance is compared to conventional compliant hinges based on the ratio of the normal- and rotation-stiffness. This showed an increase of at least a factor 30 on the stiffness ratio. ... Read more

Hydrostatic bearings are superior in terms of performance to contact based bearings, but non-usable in applications with non-constant curvature counter surfaces. A solution to this would be the introduction of deformable hydrostatic bearings components to cope with these required deformations. To reduce the required deformation of a bearing pad, multiple pads are connected through a so-called whiffletree support system. In this work, the design approach for such a whiffletree based hydrostatic bearing system is introduced. The approach includes determining the load capacity of all individual slippers. Design considerations are given regarding the joint rotational-, normal- and shear stiffness of each individual joint. ... Read more

Hydrostatic bearings are well known for having low friction and low wear in combination with high loading capacity.Traditionally, the running surfaces for a hydrostatic bearing are plane, rigid and smooth, ensuring a parallel lubricating film in the full range of movement. In this paper, we consider a new type of compliant, hydrostatic bearing where the running surfaces have a surface waviness with a large amplitude resulting in a varying curvature for the hydrostatic bearing to follow in its range of movement. Design choices and subsequent modeling of these types of bearings are presented in this work. The introduced bearing concept is based on a compliant whiffletree mechanism able to distribute load over multiple hydro-static slippers. This increases deformability of the complete bearing system. In a case study the limitations of a large deforming hydrostatic bearing are modeled to understand the impacts on performance and implications. ... Read more

The modeling method for the performance of positive displacement pumps of hydraulic pumps is based on collecting operational data of the pump for its operational envelope and using equations with empirical fitting parameters and fit these equations to the data. This way values for the fitting parameters are obtained, with which the equations can be used as model for the operating performance of this specific pump. By implementing this modelling method no information about the contribution of each component present in the pump to the performance losses is obtained. A model which analyzes each coponent and estimates its contribution to the total power losses would be beneficial in the design of pumps. Therefore the answers to the following research questions are sought after in this work:

- What are the components critical to performance within hydraulic pumps?
- To what accuracy can a component-wise, analytical model for pump performance be created?
- How sensitive to certain parameters used in the equations is the model?
- What are the possibilities of implementing this model into pump design?

After the pump system was analyzed and the internal interactions found, the components which effect pump performance the most were identified. These include bearings, rollers, seals, springs, valves and leakage flow which is dependent on gap size, piston movement and viscosity of the hydraulic fluid. For these components analytical equations are presented which are incorporated in the model.

The accuracy of the model is tested by comparing its results to test data of an experimental camring driven radial piston pump. The model predicts leakage flow to an average difference of 3% relative to the data, but outliers up to 49% for high pressures and -35% for low pressures are observed. Input torque is predicted to an average difference of -19% relative to the data. This difference is mainly caused by the inaccuracy of the predictions below 30 bar operating condition, where outliers up to -61% are observed. These results are compared to an empiric modelling method and are found to be more accurate for leakage flow for the entire operational envelope, yet on average less accurate for input torque.
The sensitivity of the model to certain parameters is tested. The parameters investigated are the rolling resistance coefficient of rollers and bearings, bearing lubricant viscosity, hydraulic fluid temperature, piston-cylinder alignment and the corresponding forces resisting piston motion and the assumption of laminar leakage flow.

To show the possibilities of the model in pump design the rollers and appurtenant bearings present in the pump are replaced with hydrostatic bearings. The design is discussed and a hydrostatic bearing with an orifice restrictor is designed of which implementation will approximate the total efficiency of the pump with rollers. The Reynolds number of the flow necessary is found to be too high for capillary restrictors. To approximate the same total efficiency for each operating condition the average bearing flow is found to be 0.0016 L/s, which corresponds to an average fluid film height of 12 μm. The orifice restrictor diameter which allows this flow is found to equal 0.24 mm.

This work shows the possibilities of such a model, yet there is plenty room for improvement of the accuracy of the predictions made. Recommendations to this end are given in the conclusion and recommendations of the report. ... Read more

This work introduces a new method to deal with design dependent pressure loads in Topology Optimisation (TO) using the SIMP material model. A pronounced focus is on optimising pressure actuated compliant mechanisms. The difficulty herein is the interpretation of the pressure boundary in a TO design. In TO the boundaries are blurry, because of the filtering of the design variables, which is necessary to prevent checkerboarding. Another reason why the boundary is poorly defined is that the optimisation starts from an equally distributed grey, where black and white respectively are solid and void, so the boundary is either the domain boundary or not defined in early iterations of the optimisation.
The methods proposed in literature often try to find the void-solid interface exposed to the pressure source to apply the loading from a pressure line directly. The method proposed in this work, appropriately called the Darcy method, first calculates the pressure field by using Darcy's law governing the flow through porous media and the associated pressure drop. A flow coefficient is introduced that decreases if the virtual element density increases. This results in a design dependent pressure field that can be solved using the Finite Element Method (FEM), which can then be translated to consistent nodal forces that are applied to the TO problem. A drainage coefficient has also been introduced to make sure that the pressure is drained entirely to the environment pressure over the first encountered void-solid interface exposed to the pressure source. The Darcy method has proven to function well in several test cases. The method has been thoroughly tested using several parameter sweeps on a clamping problem objective. The parameters whose influence is examined are: the initial condition, the density threshold value, flow coefficient gradient at the threshold, output spring stiffness and the volume fraction.
Subsequently, some alternative TO problems are solved showing the diversity of the method. In perspective of future research, the Darcy method can function as a great tool to research load sensitivities by tuning the pressure field control parameters. The extension of the Darcy method to 3D or to several load cases comes naturally but has not been tested in this work, this is also recommended for future research. ... Read more