Improvement of the seizure resistance of orifice-type aerostatic bearings (case study)

In this case study, professional equipment installed at several different sites was suffering from recurring seizure of the aerostatic thrust bearings of a linear motor, causing unacceptable downtime and production losses. It was necessary to find a rapid and cost-effective solution that could be applied to equipment already installed in the field, as well as an optimum design solution for new equipment. Analysis of the field failures indicated that failure was due to either the magnetic attraction and trapping of contaminant particles between the thrust pads and bearing track surfaces, initiating adhesive wear and material transfer, or to (temporary) air supply failures or overloading, resulting in contact between the bearing surfaces. The pre-selection of potential solutions to the problem was aimed at reducing both the particle contamination and the tendency for adhesive wear behaviour. Pin-on-disk tests were carried out in order to rapidly identify potential solutions. These tests were followed by actual-scale system tests simulating the crashing of air bearings and the influence of contaminant particles introduced into the bearing gap.The most favourable results in terms of performance and costs were achieved by applying a thermo-chemical carbon diffusion surface treatment (Kolsterising®) to thrust bearings made of austenitic stainless steel, and applying a pulse plasma nitriding treatment to the bearing track material. This was chosen as the preferred solution for new equipment design. For equipment already installed in the field, a satisfactory solution was provided by replacing the existing thrust pads with pads equipped with partially stabilised zirconia (PSZ) plates on the bearing surfaces.