US 20080089796 A1
A drive unit for a linear compressor comprising a frame and an oscillating body. The oscillating body is mounted in the frame via at least one diaphragm spring and can be moved back and forth in one direction. The diaphragm spring comprises a plurality of limbs, fastened with one end to the frame and with the other end to the oscillating body. Two limbs each have inversely curved sections between the two ends.
13. A drive unit for a linear compressor comprising a frame and an oscillating body mounted in the frame by means of at least one diaphragm spring and moving back and forth, the diaphragm spring having a plurality of limbs engaging the frame with a first end and the oscillating body with a second end, and a curved path between the two ends, wherein two of the limbs have inversely curved sections.
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24. A linear compressor comprising:
a working chamber;
a piston being movable back and forth in the working chamber for compressing a working fluid; and
a drive unit coupled to the piston for driving the back and forth movement and comprising a frame and an oscillating body mounted in the frame by means of at least one diaphragm spring and moving back and forth, the diaphragm spring having a plurality of limbs engaging the frame with a first end and the oscillating body with a second end, and a curved path between the two ends, wherein two of the limbs have inversely curved sections.
This invention relates to a linear compressor, in particular for use for compressing refrigerants in a refrigerating device, and in particular a drive unit for driving an oscillating linear piston movement for such a linear compressor.
U.S. Pat. No. 6,506,032 B2 discloses a linear compressor whose drive unit comprises a frame and an oscillating body mounted in the frame by means of one diaphragm spring. The oscillating body comprises a permanent magnet, a piston rod rigidly connected to the permanent magnet and a piston articulated to the piston rod, which piston can be moved back and forth in a cylinder. The movement of the piston is driven by an electromagnet arranged around the cylinder, which electromagnet interacts with the permanent magnet. A disc-shaped diaphragm spring is screwed centrally to the piston rod, and the outer edge of the diaphragm spring is connected to a yoke which surrounds the cylinder, the electromagnet and the permanent magnet.
The oscillating body and the diaphragm spring form an oscillating system whose natural frequency is determined by the mass of the oscillating body and the diaphragm spring, as well as by the stiffness of the diaphragm spring. The diagram spring only permits small oscillation amplitudes because any deflection of the oscillating body is associated with an expansion of the diaphragm spring. Due to the low oscillating amplitude it is difficult to reduce the dead volume of the cylinder reliably. However, the higher the dead volume the lower the efficiency of the compressor. The short stroke also necessitates designing the cylinder with a diameter that is proportional to the length in order to achieve a given throughput. It is expensive to seal the correspondingly large circumference of the piston sufficiently.
Since the oscillating body is only retained in the radial direction by its connection to the spring, it is possible that the head of the piston rod supporting the piston may oscillate back and forth and grind against the cylinder wall. To prevent this a compressed gas bearing is provided for the piston, i.e. the cylinder wall covered by the piston has openings which are connected to the high pressure outlet of the linear compressor to form a gas cushion between the inner wall of the cylinder and the piston. However, such a compressed gas bearing only functions if the required excess pressure is present at the outlet of the linear compressor, i.e. not when the compressor starts or stops. At these times there is a risk that the piston will grind against the cylinder wall, resulting in premature wear of the compressor.
A linear compressor is disclosed in U.S. Pat. No. 6,641,377 B2. In this double-piston linear compressor each piston is retained by two two-armed diaphragm springs.
Due to the curvature of the limbs a longer piston stroke is possible, but each diaphragm spring exerts a torque on the piston when deflected. If this torque is not exactly compensated for, the piston performs a rotary oscillation in addition to its linear oscillating movement, and wobble movements of the piston may be excited which may result in contact between the piston and cylinder and consequently to increased wear.
The object of this invention is to provide a low-wear drive unit for a linear compressor with a frame and an oscillating body mounted by means of a diaphragm spring, in which the diaphragm spring permits a long stroke of the oscillating body and which is able to achieve a high throughput with a small piston diameter.
The object is achieved in that the plurality of limbs of the diaphragm spring each engage with one end on the frame and with the other end on the oscillating body, and in that the limbs have pairs of sections with opposing curvature between the two ends. The limbs do not therefore extend along the shortest path between the two ends, so that when the oscillating body is deflected, they stretch and my approach the rectilinear shape without the material of the limbs having to be expanded for this purpose. Within the same workpiece it is very easy to produce the limbs so that their torques exactly compensate each other; if, as described in U.S. Pat. No. 6,641,377 B2, two diaphragm springs are provided with limbs covered in different directions, deviations in material strength from one spring to another may prevent such compensation or at least render it extremely difficult.
The diaphragm spring preferably has pairs of limbs with sections curved in opposite directions.
In the simplest case each limb has an individual section curved in one direction. Each such limb also exerts a torque on the oscillating body supported by it when deflected, but this is compensated for by the limb paired with it and curved in the opposite direction.
Each limb preferably has two sections curved in different directions. Since the differently curved sections also generate torques in opposite directions in this case too, the torque of each individual limb may therefore be made very small or caused to disappear altogether.
It is also advantageous to provide at least a second diaphragm spring whose limbs engage on a region of the oscillating body which is distant from the region of engagement of the first diaphragm spring in the direction of the oscillating movement. The oscillating body is reliably guided linearly in the direction of the desired oscillating movement by the two diaphragm springs, and a lateral deflection movement, which could result in contact between a piston supported by the oscillating body and a cylinder surrounding the piston, can be avoided.
The limbs of the same diaphragm spring are preferably joined integrally together at their ends engaging on the frame and/or at their ends engaging on the oscillating body. The ends engaging on the frame may also be connected by a frame integral with the leaf springs.
To provide a long stroke without risk of material fatigue, the limbs of the at least one diaphragm spring should be produced from a very thin material. Its strength may be dimensioned so small that it is only sufficient to prevent lateral deflection of the oscillating body. However, such a weak diaphragm spring would result in a low natural frequency of the drive unit and hence, at a predetermined stroke, in a low throughput of a compressor driven by the drive unit. To achieve a natural frequency of the drive unit sufficient for the required throughput, each limb is preferably assigned a readjusting spring which counteracts deformation of the limb so that the diaphragm spring, together with the readjusting springs, form an elastic system whose stiffness is considerably higher than that of the diaphragm spring alone.
The effective spring constant of the correlation of diaphragm and readjusting spring may be made adjustable so that the natural frequency of the drive unit can be adapted as required. A helical spring is preferably used as the readjusting spring.
A further subject matter of the invention is a linear compressor with a working chamber, a piston that can be moved back and forth in the working chamber to compress a working fluid, and a drive unit of the type described above, coupled to the piston, for driving the back and forth movement.
Further features and advantages of the invention are evident from the following description of exemplary embodiments with reference to the attached figures.
The linear compressor shown in
Pipe section 4 is surrounded concentrically by a second pipe section 6 and is connected to it by a radial flange 7. The circumference of a diaphragm spring 8 is fastened to the end of pipe section 6 facing away from flange 7. An oscillating body 9, which is composed of a piston rod 10, to which piston 2 is articulated, a flange 11 fastened to piston rod 10 and a permanent magnet 12, which is fastened to flange 11 and projects into the interval between pipe sections 6, 4, is fitted in the centre of diaphragm spring 8. Electromagnets, also accommodated in the interval, for exerting a force in the direction of piston rod 10 on permanent magnets 12, are omitted in the figure.
Diaphragm spring 8 consists of spring steel or another elastically deformable, but essentially non-expandable material. The central region of diaphragm spring 8 can be elastically deflected with little force in a direction perpendicular to the plane shown in
A second design of diaphragm spring 8 is shown in
When central region 16 is deflected, this results in slight upward bending of curved sections 18, 19. Because of the opposite directions of curvature of the two sections 18, 19 of each limb, the upward bending gives rise to opposing torques, so that the torque exerted by each individual limb 14 on central region 16 is small. Moreover, the torques of adjacent limbs 14 are mutually compensating because each of them is the mirror image of the other and the torques exerted by them are therefore inversely the same. Central area 16, and consequently also a piston rod 10 fastened to it, are therefore guided exactly linearly and free from distortion.
The ends of oscillating mass 24 are fastened to central regions 16 of two diaphragm springs 8 of the shape shown in
Diagram springs 8 are retained on bridges 26 by screws or rivets 27, each of which intersect a foot piece 28 of an upper and lower yoke 29, 30 and one of bores 19 in the corners of frame 13, and engage in central chamber 21.
Lower yoke 30 supports two helical springs 31, each of which is positioned so that free head piece 32 of these springs each touch curved sections 18 of two limbs 14, as also denoted as a dash-dot outline in
Upper yoke 29 also supports a cylinder 33 in which a piston connected to oscillating mass 24 by means of a piston rod 10, not shown in the figure, is able to move back and forth. Since oscillating mass 24 is guided exactly linearly by the two diaphragm springs 8, piston rod 12, and with it the piston supported by it, cannot deviate transversely to the direction of movement and grinding of the piston against the inner wall of cylinder 33 can be avoided.
When oscillating mass 24 is located at one of the points of inversion on its trajectory, its entire kinetic energy is stored in the diaphragm springs 8 and the helical springs 31 in the form of deformation energy, the distribution of the energy among the spring types depending on their respective spring constants. The diaphragm springs may therefore be made very thin and easily deformable so that no material fatigue occurs even during protracted operation. For the energy which the diaphragm springs are unable to store due to insufficient stiffness may be absorbed by suitably dimensioned helical springs 31.
Moreover, compressors with different throughputs can be achieved with the same model of diaphragm spring if the diaphragm springs are each combined with helical springs with different spring constants, resulting in different natural frequencies of the oscillating system.
It is also conceivable to render the natural frequency of a drive unit adjustable by mounting helical springs 31 displaceably on yokes 29, 30. The closer the region of limbs 14 touched by head pieces 32 of helical springs 31 is to central region 16 of diaphragm springs 8, the stiffer will be the entire system, consisting of the diaphragm spring and helical springs, and the higher will be the natural frequency of the resultant drive unit.
In the extreme case it is possible to replace the two helical springs 31 of each yoke 29, 30 by a single helical spring which touches central region 16 directly.