Low friction linear clearance seal

This invention provides a low friction linear clearance seal for use in closed cycle cryogenic coolers. The clearance seal comprises a cylinder, a piston mounted within the cylinder for reciprocating movement and a self lubricating polymer liner bonded to the piston which forms an effective seal around the piston. The clearance seal improves the sealing capability between the piston and the cylinder and reduces heat generated by the motion of the piston along the cylinder wall. The clearance seal also improves the performance and reliability of the cryogenic cooler.

FIELD OF THE INVENTION 
This invention relates in general to clearance seals used in cryogenic 
coolers and more particularly to a low friction self lubricating linear 
clearance seal used in closed cycle cryogenic coolers. 
BACKGROUND OF THE INVENTION 
Infra-red detectors are used for night vision and heat seeking weapons. In 
order to operate properly, infra-red detectors must be cooled to very low 
temperatures approaching absolute zero. Such temperatures are referred to 
as cryogenic temperatures. Cryogenic coolers are used to provide a means 
for cooling infrared sensors so as to avoid temperature-induced "noise" 
and to improve the operating efficiency of the sensors. Earlier coolers 
operated on the Joule-Thompson principle of cryogenic refrigeration. 
Closed loop helium refrigerators were later developed and were based on a 
Stirling Cycle using a Rotary Drive Compressor which continuously produced 
cryogenic temperatures at a cold tip. More recently linear drive cryogenic 
coolers have been developed which have improved reliability, extended 
life, reduced vibration, and less operating noise compared to the previous 
types of cryogenic coolers used. 
A cryogenic cooler has a compressor section and a expander section combined 
in a single package. The expander and compressor use reciprocating pistons 
which are mechanically or pneumatically driven according to a proper phase 
relationship so that no valves are required in the system. Helium gas is 
semi-hermetically sealed within a compressor volume and a working volume 
of the system. The compressor volume surrounds the drive motor, and space 
behind the compressor pistons. The working volume consists of a 
compression space between the compressor pistons, an interconnecting gas 
passage between the compressor and expander, and all other space in the 
expander including voids in the porous regenerator matrix, an annular 
clearance around a displacer, and an expansion space below the displacer. 
The compressor volume is isolated from the working volume by compressor 
piston clearance seals. Since the swept volume of the compressor pistons 
constitutes the major portion of the working volume, moving the pistons 
reciprocally in a sinusoidal manner within the cylinders generates a 
sinusoidally varying pressure throughout the entire working volume. The 
compressor volume stabilizes at a pressure level essentially equal to the 
average value of the fluctuating working volume pressure. 
A complete cycle involves the flow of gas from the compression space 
through the interconnecting gas passage and the regenerator to the 
expansion space. The gas is then returned from the expansion space to the 
compression space by reversing the gas flow through the same flow-path. 
Refrigeration is produced in the expansion space at the tip of the cold 
finger. The regenerator is a porous matrix of fine wire mesh having a 
large heat capacity which is able to maintain a temperature gradient along 
its length spanning a range from about ambient to -321.degree. F. Helium 
gas cools as it passes through the regenerator matrix from the compressor 
to the expander and warms up on the return pass. 
In order for the cycle of gas flow and cooling to continue smoothly and 
efficiently it is important to have a clearance seal in both the 
compressor and expander sections of the cryogenic cooler which is both 
long wearing and low friction. Clearance seals typically consist of a 
piston with a precision machined liner bonded to it. The piston/liner 
assembly is matched to a precision ground cylinder with diametral gaps 
between the two which are controlled to very close tolerances. The liner 
materials currently being used are Fluorogold.TM., Rulon.TM., and ceramic. 
These are selected for minimal wear against the mating cylinder to provide 
a long life clearance seal. 
The prior art liners in general, function adequately to provide a seal 
within the compressor and expander sections of cryogenic coolers. However, 
prior art clearance seals have high friction losses and therefore, cause 
the cryogenic cooler to be less efficient, consume more power, run hotter, 
and suffer from reliability problems. The high friction losses are due to 
the piston-liner materials used. The Fluorogold.TM. and Rulon.TM. 
materials, in order to wear properly, must transfer a small layer of 
material to the mating cylinder surface. This transfer of material may 
occur unevenly due to variations in surface finish, side loads on the 
piston, and variations in concentricity of the parts. This uneven transfer 
leads to a high build up of friction, causing the piston to drag or stick. 
The ceramic liner material does not have the transfer problems of 
Rulon.TM. or Fluorogold.TM.. However, due to both mating surfaces being 
hard, the ceramic liner material cannot absorb any loose particles or 
debris which may enter the seal gap, and this results in scoring of the 
surfaces. This scoring causes high friction and eventual seizing of the 
piston. 
Another disadvantage of using prior art liner materials is the difficulty 
in machining the piston liner materials. Rulon.TM. and Fluorogold.TM. will 
deform during machining, making it necessary to remove material slowly in 
thin layers to obtain the tolerances required. Likewise, ceramic liners 
must be ground and lapped by a slow and expensive process. 
It is therefore an object of an aspect of the present invention to provide 
a low friction linear clearance seal for use in cryogenic coolers which 
overcomes the limitations of the clearance seals used in the prior art. 
SUMMARY OF THE INVENTION 
In accordance with a first aspect of the present invention there is 
provided a novel low friction linear clearance seal for use in closed 
cycle cryogenic coolers. The clearance seal comprises a cylinder, a piston 
mounted within the cylinder for reciprocating movement and a self 
lubricating polymer liner bonded to the piston which forms an effective 
seal around the piston. The new seal both improves the sealing capability 
between the piston and the cylinder and reduces heat generated by the 
motion of the piston along the cylinder wall. 
The novel clearance seal significantly improves performance and reliability 
of the cryogenic cooler. The present invention is also less costly to 
produce than prior art clearance seals. 
In accordance with another aspect of the present invention there is 
provided a low friction clearance seal using a new polymer material 
comprised of Ryton.TM. polyphenylene sulfide (PPS). The Ryton.TM. PPS is 
combined and blended with 10% carbon, 10% graphite and 10% 
polytetrafluoroethylene (PTFE). In another embodiment the Ryton.TM. PPS is 
blended with 15% polytetrafluoroethylene (PTFE) and 30% carbon. 
In yet a further embodiment of the present invention, a dry lubricant of 
molybdenum disulfide is applied to both the liner and the mating surface 
of the cylinder. The lubricant is polished to provide additional lubricity 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A low friction linear clearance seal for use in cryogenic coolers according 
to the present invention is shown with reference to FIG. 1 in which 
similar reference numbers denote similar elements throughout the drawing. 
A compressor section 7 of a cryogenic cooler is shown in FIG. 1 in which a 
low friction linear clearance seal is illustrated. The clearance seal 
comprises a cylinder 10 and a piston 12 which is reciprocally mounted 
within the cylinder 10. A liner 14 of polymer material is bonded to the 
outer concentric surface of the piston. The piston is typically machined 
from tungsten. 
FIG. 1 also shows an expander section 9 of a cryogenic cooler incorporating 
a low friction linear clearance seal. As in the compressor, the clearance 
seal comprises a cylinder 11 and a piston 13 which is reciprocally mounted 
within the cylinder 11. In the embodiment of FIG. 1, the piston is 
machined from aluminum. A liner 15 of polymer material is bonded to the 
outer concentric surface of the piston 13. The liner 15 is machined to a 
diametral clearance fit of about 0.0009 inches to the coldfinger assembly 
18. 
In both the compressor and expander sections, the piston/liner assembly is 
machined to very close tolerances in order to match the precision ground 
cylinders 10 and 11. The diametral gap between the parts is typically 
about 0.0005 inches. The mating bore surface finish is held to a finish of 
6 RMS (Root Mean Squared) or better. The concentricity of the mating 
surfaces are held to about 0.0001 inch. 
The piston liners 14 and 15 are preferably made from a new polymer material 
known as Ryton.TM. polyphenylene sulfide (Ryton.TM. PPS) which is a 
semi-crystalline, high-performance polymer which has an excellent chemical 
resistance, chemical stability and high strength. It has a low water 
absorption value and a very low coefficient of linear thermal expansion 
providing for excellent dimensional stability during and after machining. 
This makes it ideal to use as a liner material. The Ryton.TM. PPS resin is 
preferably combined and blended with 10% carbon, 10% graphite, and 10% 
polytetrafluoroethylene (PTFE). The blended material is manufactured by 
the EGC Corporation of 11718 McGallion, Houston, Tex. 77076 and is 
referred to as EGC Alloy X-655. The Ryton.TM. PPS can also be blended with 
15% PTFE and 30% carbon to provide a second liner material which is 
manufactured by LPN Corporation of 1831 East Carnegie St., Santa Ana, 
Calif. 92705 and is referred to as OCL-4036. 
The Ryton.TM. PPS is a self lubricating material which does not require the 
transfer of material to operate as does Fluorogold.TM. or Rulon.TM. 
materials. The self lubricating nature of this material creates a 
clearance seal that is inherently lower in friction than other prior art 
seals. The Ryton.TM. PPS has a greater stiffness than other known liner 
materials and as such, the Ryton.TM. PPS machines very easily and holds 
its shape and dimensions, thereby reducing manufacturing costs. The 
Ryton.TM. PPS can absorb loose particles that can enter the seal area, 
thereby preventing scoring of the mating surfaces and extending the life 
of the seal. In general, the Ryton.TM. PPS is a self-lubricated, high 
moduli, flame retardant, wear, heat and chemical resistant material. The 
specific physical properties of the Ryton.TM. PPS are summarized in Table 
One. 
In other embodiments of the present invention, a dry lubricant of 
molybdenum disulfide is applied to both the outer surface of the liner and 
the surface of the cylinder for the compressor and expander. The 
molybdenum disulfide is polished into the surfaces in order to provide 
additional lubricity. 
It is understood by those skilled in the art that the low friction linear 
clearance seal of the present invention can be used in different types of 
cryogenic coolers. It is also to be understood by one skilled in the art 
that the low friction self-lubricating linear clearance seal of the 
present invention can also be used in other types of equipment which 
require the characteristics provided by the seal. 
In summary, a novel low friction linear clearance seal is provided for use 
in cryogenic coolers. The low friction linear clearance seal has a self 
lubricating liner material which does not require the use of additional 
lubricants. Furthermore, the liner material is such that it prevents 
scoring of the mating surfaces of the piston and cylinder, thereby 
prolonging the life of the seal. The nature of the liner material provides 
for a more efficient cryogenic cooler system. The clearance seal is easy 
and cost efficient to machine and manufacture. 
While embodiments of the present invention have been illustrated and 
described, it will be evident to those skilled in the art that variations 
and modifications may be made therein without departing from the scope of 
the invention as defined by the claims appended hereto. 
TABLE One 
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Filler/Reinforcement Carbon Fiber 
Flexural Strength 36.00k 
Percentage of Filler 30% 
Flexural Modulus 3.00 M 
Processing Method Injection 
Compressive Str. (PSI) 
19.00k 
Ratings UL 
Izod RT (ftib/in) 1.0 
Processing Temperature 
610.degree. F. 
Hardness (Test) R123 (Rockwell) 
Mold Pressure (PSI) 17.50k 
Therm. Cond. BTU in/hr ft2 .times. F 
2.50 
Mold Shrinkage (in/in) 
1.00 m 
Thermal Exp. (In/in .times. F) 
9.00 u 
Density (lb/ft3) 96.1 
HDT @ 264 PSI (.times.F) D648 
500 
Tensile @ Break (PSI) 25.00k 
HDT @ 66 PSI (.times.F) D648 
500 
Elongation @ Break (%) 
0 
Volume Res. (Ohm-cm) D257 
40.0 
Tensile Modulus (PSI) 3.80 M 
Water Absorption (%) D570 
0.020 
UL Standard 94 V-O 
Injection Mfg Yes 
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