Verification of Long Life Operation through Real Time Dynamic Alignment Tracking

Abstract

Oxford style mechanical cryocoolers rely on clearance seals to prevent wear between the pistons and cylinders. The support structure that maintains this clearance has to handle the large pneumatic forces that occur during operation in addition to the armature’s weight. It is therefore more difficult to develop clearance seals that remain non-contacting during full power operation than when at rest.

A standard stiction (“static friction”) test only confirms that a seal does not drag in quasi-static operation. To measure the clearance during operation requires a precise measurement of the piston location at high speeds. During the early development of long life space cryocoolers, Ball developed a unique system using proximity sensors that allowed for the measurement of the clearance gaps in real time under dynamic conditions. This system was critical to the development and verification of long life design features in Ball’s space cryocooler product line.

This paper briefly describes that system and discusses measurements from the AFRL/Ball 35/ 60 K Stirling cycle mechanical cooler that has been in life test at the Air Force Research Lab (AFRL) Cryogenic Technological Laboratory. The data indicate that the cooler’s clearance seals and alignment have remained essentially unchanged during more than 22,000 hours of continuous operation.