US20080000348A1 - Linear Compressor - Google Patents

Abstract

A linear compressor comprising a cylinder, a hydraulically borne piston that can be longitudinally oscillated inside the cylinder, and a drive unit driving the movement of the piston. The compressor is characterized in that at least one of the lateral faces of the piston and the cylinder that face each other is provided with an abrasion-resistant bearing coating.

Description

• The present invention relates to a linear compressor, especially for use in order to compress refrigerants in a refrigeration device.
• A linear compressor is known from U.S. Pat. No. 6,506,032 B2. In this linear compressor a circumferential hollow space is formed in the wall of the cylinder and is supplied with hydraulic fluid, and communicates with the internal space of the cylinder in which the piston moves by a plurality of openings distributed over the lateral wall of the cylinder. Hydraulic fluid penetrating from these openings into the cylinder chamber forms a cushion on which the piston slides without contact with the lateral wall. Since the cushion prevents friction contact between piston and cylinder, such a linear compressor can work for a long time in continuous operation, without friction wear resulting in a noticeable diminution in efficiency.
• However, when the compressor is used in practice in a refrigeration device, an unexpectedly fast diminution in efficiency is apparent.
• A linear compressor is known from U.S. Pat. No. 6,641,377 B2 in which the piston is not hydraulically borne. In order to achieve a precise orientation of the piston in the cylinder, it is proposed to apply a coating of a solid lubricant such as PTFE or a DLC layer to the surface of the piston or the cylinder before inserting the piston into the cylinder chamber, so that piston and cylinder can be fitted together and a clearance between them required for continuous operation is obtained by eroding the coating in a start-up phase of the compressor.
• The object of the present invention is to develop the linear compressor according to the preamble so that it has a constant efficiency even during long-term operation in a practical application such as a refrigeration device.
• The object is achieved in that at least one of the lateral faces of the piston and of the cylinder that face each other is provided with an abrasion-resistant bearing coating.
• It is apparent in fact that signs of friction wear unexpectedly occur on cylinder and piston during operation of a linear compressor of the type referred to in the preamble in a refrigeration device after lengthy operation despite the hydraulic bearing, said signs of friction wear resulting in an escape of fluid, increasing over time, from the cylinder chamber through the gap between piston and cylinder wall. If in fact the hydraulic fluid required for maintenance of the cushion is branched from a high-pressure connection of the compressor, the consequence is that at the start and end of each operating phase of the compressor, if the pressure at the high-pressure connection of the compressor is lower than during continuous operation, the cushion is not sufficiently effective to prevent friction contact between the lateral faces of piston and cylinder. In order to prevent wear in these operating phases, the abrasion-resistant bearing coating is necessary.
• Whereas in the case of the compressor from U.S. Pat. No. 6,641,377 B2 the DLC or PTFE bearing coating is not said to be abrasion-resistant, so that it is eroded, if a clearance between piston and cylinder which is desired for operation of the compressor is produced, it is important for the present invention that the bearing coating is also present if the clearance between the lateral faces of piston and cylinder required for efficient operation of the hydraulic bearing is present. Surprisingly it was found that the bearing coating does not reduce efficiency and operation of the compressor at optimum efficiency is achieved over its service life thanks to the bearing coating.
• Further features and advantages of the invention are given in the following description of an exemplary embodiment, and with reference to the enclosed figures. These show:
• FIG. 1 a perspective view of a linear compressor, and
• FIG. 2 a section through the cylinder of a linear compressor.
• The linear compressor shown inFIG. 1 in perspective view has a rigid frame, somewhat U-shaped in planar view, which is composed of three parts, namely twoflat wall pieces1 and anarch2. Afirst membrane spring3 is stretched between end sides of thearch2 facing each other and the twowall pieces1; a second membrane spring4 of the same construction as themembrane spring3 is attached to the end sides of thewall pieces1 facing away from the arch. The membrane springs3,4 punched from the spring plate each have twoelongated edge strips7 which cover the end sides of thewall pieces1 or of thearch2, and fourspring arms5 which extend in a zigzag fashion from the ends of theedge strips7 to acenter section6, on which they meet. Thecenter section6 has in each case three drill holes, two external ones, at which a permanent magnetic oscillatingbody8 is suspended with the aid of screws or rivets, and a center drill hole through which projects, in the case of themembrane spring3, a rod section10 which is attached to the oscillatingbody8 by, for example, being screwed to it.
• The rod section10 is connected to atransmission rod9 via a flexibly bendable tapered section11. A secondtapered section12 connects the transmission rod11 in one piece to apiston rod14 which engages in a pump chamber supported by thearch2, is routed through a drill hole in an end wall of the pump chamber, and in thepump chamber15 is connected to a piston16 (seeFIG. 2) which can move therein.
• Two electromagnets with an E-shaped yoke and a coil wound around the center arm of the E are in each case arranged between the oscillatingbody8 and thewall pieces1 with pole shoes facing the oscillating body and serve to drive an oscillating motion of the oscillatingbody8.
• Since thepiston rod14 rigidly connected to the piston16 is routed in the end-side drill hole in thepump chamber15, the piston16 is protected against canting, even if it expands only slightly in the direction of the backward and forward movement. The piston16 hence occupies little space in thepump chamber14, so that a large effective volume is achieved with small external dimensions.
• Thepump chamber15 is surrounded in annular fashion by ahollow space17, which communicates with thepump chamber15 through a plurality ofopenings18 in the side wall of the latter and which is supplied via a through hole19 with compressed gas tapped from apressure connection20 of the pump chamber. The compressed gas penetrating through theopenings18 into thepump chamber15 forms a cushion on the side wall, on which the piston16, whose diameter is slightly less than the free diameter of thepump chamber15, slides essentially friction-free.
• The shell of the piston16 sliding along the wall of thepump chamber15 is coated with a DLC (diamond-like carbon)layer21. TheDLC layer21 can in particular be a tetrahedral carbon layer (ta-C) or an amorphous hydrogenous carbon layer (a-C:H). Such layers represent highly effective protection against friction wear in the case of extremely small thickness. This means that the thickness of thelayer21 can be significantly smaller than the radial clearance between piston and cylinder, so that the layer can be applied to a completed piston16 whose dimensions are adapted to thepump chamber15, without disadvantageous machining being required in order to adjust the dimensions. This permits an especially low-cost implementation of the invention, since the application of theDLC layer21 to the piston16 represents a single additional step in the manufacture of the compressor, which can be inserted easily into established production processes for such compressors, as it does not require any follow-up adjustments such as, for example, changes to dimensions in other manufacturing steps.

Claims (6)

1-5. (canceled)

6. A linear compressor comprising:

a cylinder including a cylinder lateral face;

a hydraulically borne piston which moves in an oscillating fashion in a longitudinal direction of the cylinder and includes a piston lateral face; and

a drive unit driving the movement of the piston, wherein at least one of the lateral faces of the piston and of the cylinder that face each other is provided with an abrasion-resistant bearing coating.

7. The linear compressor as claimed inclaim 6, wherein sufficient clearance exists between the at least one lateral face of the piston or of the cylinder bearing the bearing coating and the surface of the piston or of the cylinder that faces said lateral face, to permit a movement of the piston in the longitudinal direction without mutual touching of the lateral faces.

8. The linear compressor as claimed inclaim 6, wherein the lateral face provided with the bearing coating is a shell of the piston.

9. The linear compressor as claimed inclaim 6, wherein the bearing coating includes a DLC layer.

10. The linear compressor as claimed inclaim 6, wherein a branch for hydraulic fluid for the bearing of the piston is formed at a high-pressure connection of the compressor.

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