Cylindrical spring with integral dynamic gas seal

#The present invention relates generally to springs and flexure seals and, more specifically, to a spring design that may include an integral dynamic gas seal.

#The present invention falls generally into the fields of springs and seals. However, one of the preferred applications of the present invention is in a thermoacoustic engine or refrigerator. One such thermoacoustic device is shown in FIG. 1. This thermoacoustic device 20 is the subject of U.S. provisional patent application Serial No. 60/372,008, filed Apr. 10, 2002, and a co-pending patent application entitled “Compliant Enclosure for Thermoacoustic Devices,” filed Apr. 9, 2003, the entire contents of both of which are incorporated herein by reference. Another thermoacoustic device is shown in U.S. provisional patent application Serial No. 60/371,967, filed Apr. 10, 2002, and a co-pending patent application entitled “Thermoacoustic Device,” filed Apr. 9, 2003, the entire contents of both of which are incorporated herein by reference.

#The thermoacoustic refrigerator 20 includes an outer pressure vessel 50 containing substantially all of the components of the refrigerator, including a compliant enclosure. The compliant enclosure, in turn, houses the thermal components or thermal core 40. These thermal components 40 include a cold heat exchanger 14, a regenerator 16, and a hot heat exchanger 18. These components 40 are supported by a thermally insulating plate 22 that accommodates the passage of heat exchanged fluids through tubes 24, which communicate with thermal sinks or loads outside the pressure vessel 50. A piston 26 is spaced below the thermal components 14-18, with a bellows 28 extending between and interconnecting the piston 26 and the thermally insulated plate 22. A linear motor or actuator 30 is interconnected with the piston 26 by a moving portion 32 of the motor 30. Therefore, the motor 30 is operable to move the piston. In the illustrated embodiment, the moving portion 32 of the motor 30 is interconnected with the piston 26 by a rigid tube 36. A cylindrical spring 34 may be provided between the piston 26 and some stationary portion of the device, such as the insulating plate 22, to adjust the mechanical resonance frequency of the system.

#FIG. 1 is a cross-sectional view of a thermoacoustic device;

#FIG. 2 is a perspective view of first embodiment of a spring according to the present invention;

#FIG. 3 is a cross-sectional view of the spring of FIG. 2 taken along lines 3-3;

#The description and operation of the present invention is based on two figures that serve to illustrate the geometry of the spring (FIGS. 2 and 3) and additional figures that allow for the simplified calculation of the spring's stiffness (FIGS. 4a and 4b). A design example will be presented to demonstrate how considerations of the desired spring compression and expansion (excursion), available spring material properties (Young's modulus and endurance limit), the stiffness of an individual “beam,” and the dimensions of the spring interact to constrain the parameter space available for exploitation of the desirable features of this invention.

#FIG. 2 provides a perspective view of a cylindrical spring according to the present invention that can be thought of as having been machined from a uniform tube of material. The inner radius of the tube is R and the tube's wall thickness is b. The upper (moving) portion 72 of the spring 70 is called the upper collar (which has a length u). This upper collar 72 may be used for joining the top of the spring 70 to the moving piston and electromechanical transducer of a thermoacoustic device. The upper collar is shown with twelve threaded holes to accept threaded fasteners, though other attachment means may be used. The lower (fixed) portion of the spring 70 (which as a length v) is called the lower collar 74. The lower collar 74 serves the same mounting function at the spring's bottom, and is considered to be immobile. The threaded mounting holes in the lower collar 74 are not visible in this Figure. Since the position of the lower collar 74 is fixed, v can be as long as necessary for purposes of attachment or for passage of transport fluids or electrical cables through the spring 70. The length of the upper collar, u, is preferably as short as possible, since the upper collar moves with the piston. Lowering of the total moving mass, mo, which includes the mass of the upper collar, reduces the total stiffness requirement, ktot, to resonate the electromechanical transduction system at a given frequency,fo=ωo/2π. 1 f o = ω o 2 ⁢ π = 1 2 ⁢ π ⁢ k tot m o ( 1 )

#The length of the spring (excluding the upper and lower mounting collars) is composed from the sum of the thickness of the m beams 76, each with a thickness t, and the (m+1) gaps 78 of height g. The total length of the spring, comprised by the spring portion plus the upper and lower collars is Ltot=u+v+mt+(m+1)g. The gaps 78 are illustrated at FIG. 2 as being filled with a sealing material, which is an optional feature of the present invention, and will be discussed in more detail hereinbelow.