Balanced Common Module Coolers

The practice of cooling an IR detector by mounting it on the cold finger of a small mechanical cryogenic cooler can cause some degradation of system MTF and other problems due to vibrations produced by the cooler. A balancing method has been devised, involving simple internal design changes and no external configuration changes, which reduces these vibrations by more than an order of magnitude. The principal vibrations are caused by the reciprocating masses of the piston and regenerator and by the unbalanced rotating masses on the eccentric shaft. The vector sum of vibratory forces caused by two equal reciprocating masses in the same plane, driven in simple harmonic motion with 90° phase difference by the same eccentric shaft, is equivalent to the force produced by a single mass equal to one of them mounted on the eccentric shaft. To balance the cooler, the regenerator assembly is modified to make its mass equal to the piston assembly mass and balance weights are added to the eccentric shaft at 180° from the eccentric center to balance both the rotating and reciprocating masses. Coolers have been built incorporating these changes.