The Linearity of Clearance Seal Suspension Systems

Abstract

The classic ‘Oxford’ cryocooler has a clearance seal between the piston and the cylinder which is maintained by the use of spiral disc springs. In a typical compressor this clearance is about 12 microns, and therefore the spring suspension system must have a linearity of no more than 3 or 4 microns to avoid contact. It has always been assumed that to maintain this linearity, the surfaces between which the springs are clamped must be very flat and very parallel to each other. It has also been assumed that the flatness and parallel-ness of the clamping at the inside of the spring is more important than at the outside.

Under a research and development effort with the Air Force Research Laboratory, some work was carried out to investigate how the linearity of motion is dependent on the clamping conditions of the springs. Tests were carried out on a typical suspension system which was deliberately assembled between non-parallel clamping surfaces, and the linearity of the resulting motion was measured. A simple theoretical model was developed which gives good agreement with the experimental results.

The results have several useful implications for the manufacture of cryocoolers. It was found that the normal method used to verify the linearity of motion by measuring the run-out at a single axial position could be highly misleading — at least two, and preferably more measurement points are needed. It was also concluded that the clamping surfaces around the outside of the spring are as important as those on the inside of the spring in defining the linearity of motion.