Optimisation of flat pad aerostatic bearings with the aid of finite element analysis

This paper demonstrates how a multi criteria optimisation code can be combined with a finite element code to design and select flat pad aerostatic bearings. The type of flat pad bearings design that have been chosen for optimisation are bearings with an open ended, double row of orifices (commonly referred to as the Rank L bearings). Additional complexity has been introduced by allowing the spacing between orifices to vary. Investigations on the performance of flat pad aerostatic bearings have shown that their design is a multi-variable, multi-criteria problem. The criteria used to judge the performance of rectangular flat pad bearings are load capacity, flow rate and stiffness while the complex interrelationship between the dimensions of the bearing, the size and position of the orifices and the bearing gap height form the basis of the multi-variable problem. The multi-criteria optimisation program has been used to generate a uniformly distributed sequence of points within the design space, each trial point corresponding to a particular bearing design. Suitable functional constraints have been applied so as to eliminate those designs that are impractical for manufacturing. The performance of the feasible points within the design space have been calculated and suitable criteria constraints applied to find a set of flat pad bearings that are suitable in terms of both performance and manufacturability. The performance of this set of flat pad bearings based on load capacity, stiffness and flow rate has then been compared using the Pareto optimal technique. Unlike other optimisation techniques the Pareto optimal technique does not weight or give preference to any of the criteria, hence this multi-criteria method will often produce many solutions. The problems of interpreting this solution set are also discussed in this paper.