Linear compressor and drive unit therefor

The present invention relates to a linear compressor, in particular for use in compressing refrigerant in a refrigerating device, and a drive unit for driving an oscillating piston movement for such a linear compressor.

U.S. Pat. No. 6,596,032B2 discloses a linear compressor whose drive unit comprises a frame and an oscillating body mounted in the frame via a diaphragm spring. The oscillating body comprises a permanent magnet, a piston rod rigidly connected to the permanent magnet, and, connected by an articulated joint to the piston rod, a piston that can move with a reciprocating motion in a cylinder. The movement of the piston is driven by an electromagnet disposed all around the cylinder that interacts with the permanent magnet. A disc-shaped diaphragm spring is screwed onto the piston rod in the center, and the outer edge of the diaphragm spring is connected to a yoke that surrounds the cylinder, the electromagnet and the permanent magnet.

Compared with many other types of spring, the diaphragm spring has the advantage that it can only be deformed with difficulty at right angles to the oscillation direction. Hence the oscillating body can only move with one degree of freedom, unlike an oscillating body suspended from a coil spring, for example, which always has three degrees of freedom of translational motion, and requires a guide if the motion is to be restricted to a single degree of freedom. Such a guide is not required for an oscillating body supported on a diaphragm spring. Hence the movement of such an oscillating body can be converted with low friction losses into the movement of a piston in a compressor, which is necessarily guided along a strictly linear path.

The oscillating body and the diaphragm spring form an oscillatory system whose resonant frequency is determined by the mass of the oscillating body and of the diaphragm spring, and the stiffness of the diaphragm spring. The diaphragm spring permits only small oscillation amplitudes because each deflection of the oscillating body is associated with an extension of the diaphragm spring. The small oscillation amplitude means it is difficult to make the dead volume of the cylinder reliably small. The larger the dead volume, however, the lower the efficiency of the compressor. The short travel also compels the cylinder to be designed with a large diameter relative to the length in order to achieve a given capacity. It is costly to seal the correspondingly large piston circumference.

Another option for increasing the capacity is to make the diaphragm spring very stiff in order to increase the resonant frequency thereby. The stiffer the diaphragm spring, however, also means that there is a greater risk of the spring suffering fatigue for a given oscillation amplitude. This means that, in order to avoid fatigue, the amplitude must be made smaller the stiffer the spring, so that again a satisfactory increase in capacity cannot be achieved in this way.

The object of the present invention is to create a drive unit for a linear compressor having a frame and an oscillating body mounted in the frame via a diaphragm spring, in which the diaphragm spring allows large travel of the oscillating body without the risk of fatigue, so that a high capacity can be achieved for a small piston diameter.

The object is achieved in that a coil spring, in addition to the diaphragm spring, is attached to the oscillating body and the frame and can be extended and compressed in the direction of movement. It is thereby possible to split the functions of guiding the oscillating body and of temporary storage of its kinetic energy. The coil spring is only slightly suited to constraining the oscillating body along an exactly defined straight line, but it is not difficult to dimension it to sustain both a desired amplitude of movement and a desired frequency of movement of the oscillating body without the risk of material fatigue. The diaphragm spring must have only a small material thickness in order to achieve a desired large oscillation amplitude. Such a diaphragm spring would only permit a low resonant frequency of the oscillating body were it the sole mechanism having to perform the function of temporary energy storage. By connecting the two types of springs in parallel, however, all three requirements can be achieved simultaneously, namely the requirements for strict guidance of the oscillating body, a large amplitude and a high oscillating frequency.

Ideally, the springs should only exert forces but no turning moments on the oscillating body. For this purpose, the coil spring is preferably disposed around an imaginary straight line along which the center of gravity of the oscillating body can perform a reciprocating motion. The straight line preferably coincides with a longitudinal axis of the coil spring.

In order to prevent the diaphragm spring exerting a turning moment, or in order to minimize such a turning moment, the diaphragm spring preferably has an axis of symmetry that coincides with the straight line, or a plane of symmetry on which the straight line lies.

In order to transfer the force from the coil spring into the oscillating body without any turning moment, one end of the coil spring preferably acts on the circumference of a spring plate to whose center the oscillating body is attached.

In order to make the diaphragm spring slightly deformable in the direction of movement, it preferably has a plurality of bent arms, one end of each arm being fixed to the frame and another end to the oscillating body.

In order to improve the accuracy with which the oscillating body is guided along the straight line, at least two diaphragm springs are preferably provided, which act on areas of the oscillating body that are set apart in the direction of the oscillating movement.

The subject of the invention is also a linear compressor having a working chamber, a piston performing a reciprocating motion in the working chamber in order to compress a working fluid, and a drive unit as defined above, which is coupled to the piston to drive the reciprocating motion. In order to make such a linear compressor compact, it can be advantageous for the working chamber to be surrounded at least partially by the coil spring.