Gas bearing and method of making a gas bearing for a free...

Expansible chamber devices – With lubricating means

Reexamination Certificate

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C384S012000

Reexamination Certificate

active

06293184

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to free piston machines, and more particularly to a gas bearing apparatus, and a method of making the gas bearing apparatus, for a free piston machine.
2. Description of the Related Art
Pistons in many machines are connected to a rigid, mechanical link, such as a connecting rod connected to a crank shaft. These pistons are confined within predetermined positions, such as end limits. However, many machines are known which use one or more free pistons.
A free piston reciprocates in a cylinder without a mechanical connection. Such free pistons may be driven by an electromagnetic, linear motor and used, for example, as a gas or other fluid compressor or pump Free pistons are also found in free piston Stirling cycle machines, such as free piston Stirling cycle engines, coolers and cryocoolers.
Free pistons sealingly reciprocate in a cylinder formed in a housing, with a very small gap formed between the cylinder wall and the piston wall. The housing typically encloses a work space bounded by one end of the piston and a second space, or back space, bounded by the opposite end of the piston. A working gas, such as helium, fills the workspace, back space and other regions of the machine within the housing.
Because of the close proximity of the piston wall and cylinder wall during operation, the gap formed between the walls must be lubricated to prevent rapid wear. The most effective lubrication has been found to be a thin layer of the working gas forming a gas bearing. Such gas bearings are described in U.S. Pat. Nos. 4,412,418, 4,802,332 and 4,888,950, all to Beale.
In order to lubricate the piston, gas must be directed into the gap at three or more points around the circumference of the piston after being routed from the workspace or back space. However, transporting and releasing the gas into the gap requires a complex network of passages and ports. Such passages and ports are not easily formed, because the parts into which gas-transporting structures must be formed are small, delicate and made to close tolerance.
It is known to form a shrink fit annular valve sleeve assembly as described in U.S. Pat. No. 5,184,643 to Raymond. Such assemblies will not work for the purpose of forming a gas bearing on a free piston machine due to a lack of control over gas pressures, and a lack of passages for directing the gas against a cylinder wall.
Therefore, the need exists for a gas bearing structure, and a method of making the same, for a free piston machine.
BRIEF SUMMARY OF THE INVENTION
The invention is an improved free piston machine having a gas bearing. In a preferred embodiment, an improved piston includes two parts: an inner, cylindrical core and an outer, cylindrical sleeve. The inner core has a radially outwardly facing surface that abuts a radially inwardly facing surface of the outer sleeve when the core is positioned within a passage formed in the sleeve. A circumferential reservoir groove preferably extends around the core, and a passage with a one-way valve permits fluid to flow into the reservoir. A longitudinal groove extends from the reservoir to at least one, and preferably four circumferential, fluid metering grooves formed in the radially outwardly facing surface of the core. The fluid metering grooves thereby form fluid passages when the inwardly facing surface of the sleeve bridges over and covers the groove. At least three radial passages are formed through the sidewall of the sleeve in fluid communication with the groove to direct fluid in the groove into the gap at the sidewall.
When the improved piston is reciprocating in the cylinder, gas flows from the momentarily higher pressure work space into the reservoir, through the longitudinal groove, through the fluid metering grooves and into the radial sleeve passages that empty the gas into the gap between the piston and the cylinder. This forms a gas bearing that reduces wear on the facing piston and cylinder walls. Such a structure is formed by fitting the core into the sleeve after forming the groove on the outer surface of the core.
In a preferred embodiment, a gas bearing is also formed between a displacer rod and a core passage through which the displacer rod extends. A displacer rod extends through a cylindrical passage through the core, and at least three radial passages are formed through the core's sidewall. The radial passages are in fluid communication with the fluid metering groove, causing gas to flow from the metering groove through the radial passages and into a gap between a radially inwardly facing surface on the core and an exterior displacer rod surface.
In a preferred method of making the piston, the sleeve is heated to expand it, and the core is aligned coaxially with the cylindrical passage. The core is pushed into place within the sleeve's passage, and the two parts equalize in temperature. A very tight seal is formed between the outwardly facing surface of the core and the inwardly facing surface of the sleeve, preventing fluid from passing therethrough except where grooves are formed.


REFERENCES:
patent: 3127955 (1964-04-01), Macks
patent: 4412418 (1983-11-01), Beale
patent: 4802332 (1989-02-01), Beale
patent: 4888950 (1989-12-01), Beale
patent: 5184643 (1993-02-01), Raymond

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