Patent ID: 7942651

Claim:
A cryocooler system for cooling an element comprising: a refrigeration gas compressor having a crankcase for forming a compression cylinder, a compression piston supported to reciprocate within the compression cylinder and a cylinder head attached to the crankcase for sealing the compression cylinder; a rotary DC motor comprising a solid rotor shaft assembly coupled to the compression piston by a drive coupling wherein the rotor shaft assembly comprises a solid shaft portion, made substantially from a ferromagnetic material with a magnetic permeability from about 2000 to about 75000 as compared to vacuum and supported for rotation about a longitudinal rotation axis, a hollow tubular shaped flywheel mass disposed concentric with and radially distal from the rotation axis, a wall portion radially disposed between the shaft portion and the flywheel mass, wherein the shaft portion, the wall portion and the flywheel mass are integrally formed as a unitary element.