Experimental and Numerical study of Air thrust bearing controlled by diaphragm valve

Aero-static thrust bearings offer some distinct advantages in comparison with contact bearings as there is no contact between the moving and stationary parts, thus there is no wear, limited heat generation and long life can be achieved. Moreover, linear positioning control is not affected by the highly nonlinear friction forces, and high precision positioning or high speed can be achieved. The main drawbacks of aerostatic bearings are low stiffness and load capacity and sometimes poor damping in comparison to contact bearings, so that their application in some sectors, e.g. machine tools, is less popular. Active compensation methods can be used as a solution for this problem. Electronic control with piezo actuators give generally good results in both static and dynamic conditions but they are expensive do to use of many sensors and piezo actuators for each pad. In addition in this type of methods, special design is necessary for each air bearing and normal commercial air bearings cannot be used. Pure pneumatic control is an interesting alternative solution in terms of performances and cost. This thesis presents designing, modelling and test of pneumatic valve which is an upstream restrictor and can control the air flow to improve the stiffness of any commercial thrust bearing. The static experiments show that the active system can improve the static stiffness significantly but it was in cost of losing the load carrying capacity. Different improvement have been made to improve the valve design in order to improve the load carrying capacity. In the final design it is possible to use the active system instead of wide range of commercial thrust air bearing with higher load carrying capacity and stiffness. In this study the numerical simulation of grooved feeding thrust air bearings is done for the first time. These type of bearing was selected as a passive commercial pad due to high stiffness of these type of air pads in compare with other feeding methods. The lumped numerical model is used to simulate the active system. The experimental results show that the proposed model can adequately describe the performance of grooved pad and pneumatic valve. Two different type of dynamic experiment is performed in order to identify the dynamic characteristics of the system like damping and dynamic stiffness in order to find the stability range which is important especially for pneumatic control.