WO1997001035A1

WO1997001035A1 - Piston compressor

Info

Publication number: WO1997001035A1
Application number: PCT/EP1996/002522
Authority: WO
Inventors: Helmut Andresen
Original assignee: Danfoss Compressors GmbH
Current assignee: Danfoss Deutschland GmbH
Priority date: 1995-06-23
Filing date: 1996-06-11
Publication date: 1997-01-09
Anticipated expiration: 1997-12-23
Also published as: AU6302296A; DE19522210C1

Abstract

A piston compressor (1) is disclosed, in particular for domestic refrigeration machines, having at least one cylinder (2), in which a piston (3) is arranged to move axially, which piston is connected by way of a bolt (7) to a connecting rod (5), the bolt (7) being secured in the piston (3) by means of a press fit (10). It is desirable for the efficiency in such a piston compressor to be improved. To that end, the press fit (10) comprises a pairing of seating surfaces which is under reduced normal stress circumferentially of the piston (3).

Description

Piston compressor

The invention relates to a piston compressor, in particular for domestic refrigeration machines, having at least one cylinder, in which a piston is arranged to move axially, which piston is connected by way of a bolt to a connecting rod, the bolt being secured in the piston by means of a press fit.

In the case of piston compressors of that kind, it is important for efficiency that the losses owing to leakage between piston and cylinder are as small as possible. On the other hand, it is also desirable for the frictional losses to be kept as small as possible. Both requirements can be combined with each other when the form of the piston matches the form of the cylinder very exactly.

In the case of small piston compressors, such as are used, for example, in hermetically encapsulated domestic refrigeration machines, there is a particular problem with the connection between the connecting rod and the piston. It is desirable to ensure that the movement is effected exclusively between the connecting rod and the bolt, that is, the bolt is held fixedly in the piston εo that it does not rotate. There is generally not sufficient space available to absorb sliding bearing forces between the bolt and the piston. Since it is difficult for reasons connected with space to secure the bolt in the piston by means of additional fastening means, for example, pins, in many cases a press fit between the bolt and the piston is used. Since the material that is used for the bolt is generally stronger, on account of the stresses occurring, than the material used for the piston, this press fit leads to a deformation, which predominantly affects the piston. This piston deformation is only relatively small, but produces an irregular gap between the piston and the cylinder. This gap is necessary to keep the frictional losses small or even to prevent jamming between piston and cylinder. On the other hand, however, the gap leads to increased losses owing to leakage, as mentioned above, which has an adverse effect on the efficiency of the piston compressor. Deformation of the piston also causes unreliability in the exact diameter of the piston and an unreliable determination of the air gap.

The invention is based on the problem of improving the efficiency of a piston compressor.

In a piston compressor of the kind mentioned in the introduction, that problem is solved in that the press fit comprises a pairing of seating surfaces which is under reduced normal stress circumferentially of the piston.

With a normal press fit, such as occurs, for example, between a cylindrical bolt and a bore of circular cross-section, there are normal stresses which run radially in relation to the bolt and the bore. In relation to the piston, there is thus a normal stress component that runs axially, that is, in the direction of movement of the piston, and a normal stress component that runs transversely thereto. This transverse direction corresponds at least in the closer vicinity of the fixing of the bolt in the piston substantially to the circumferential direction. The axial component leads to a length change in the piston, which is less critical. The transverse component, however, leads to a change in diameter of the piston, which is to be observed at least locally, which causes the change in the uniform gap. Through the change in the geometry of the press fit it is possible for the normal stress, which acts circumferentially or transversely, to be reduced. Deformations of the piston, which result in a deviation from the desired cylindrical shape, can be kept correspondingly smaller. Securing of the bolt in the piston by means of forces which act substantially axially is sufficient, because this is also the main direction in which force is applied between the connecting rod and the piston. Accordingly, a secure seat of the bolt in the piston can be effected without detrimental deformation of the piston in a radially or circumferentially. The form of the piston can therefore be kept closer to the desired piston form, so that the gap between piston and cylinder can be kept correspondingly uniform and small, with no need to fear increased frictional losses or jamming. The smaller is the gap between piston and cylinder, the smaller are the losses owing to leakage and the greater is the efficiency.

It is preferred for at least one of the seating surfaces to have, compared with a basic form, a recess which is arranged transverse to the axial direction. The basic form is here the form of the relevant seating surface where there is a substantially uniform normal stress distribution over the circumference of the bolt and the bore. In the region of the recess the normal stress is reduced. If the recess is suitably configured, the normal stress can even be made to disappear. At any rate, with the bolt inserted there is no need for a gap between bolt and piston to form in the region of the recesε. In the region of the recess the piston and bolt can slide on each another or bear against each other under a slight pressure or slight stress. The normal stress must be decreased only to the extent that no undesirable deformation of the piston occurs.

The seating surface preferably has two recesses lying opposite one another. In this manner a certain symmetry of fixing is achieved. The bolt therefore remains in the middle of the bore receiving it. •

The recess is preferably formed by a gradual departure of the seating surface from its basic form. This gradual departure can also be described by the expreεsion "continuously differentiable". Such a configuration has the advantage that the normal stress is not abruptly decreased to the reduced value; on the contrary, there is a stress distribution with a correspondingly gradual reduction in the normal εtress. Such a gradual departure of the seating surface from the basic form is relatively complicated to produce, however.

In an alternative construction, the transition between the seating surface and its recess is therefore formed by a step. Such a recess is easier to create. There is admittedly a different distribution of the normal stress, but in an extreme case the step allows a certain displacement of the material of the corresponding seating surface, so that the forces occurring may lead to a deformation of the piston, but this deformation is restricted locally to the region of the step. A deformation of the piston which leads to a departure from the desired circular form can be virtually ruled out. Advantageously, the seating surface portionε in which a normal stresε sufficient to hold the bolt fixedly in the piston is present engage around the bolt in the axial direction for less than 33% of its height. For two thirds or more of its height no normal stresses having a component transverse to the axial direction, that is, in the circumferential direction, therefore occur. The risk that these normal stresses will lead to deformation of the piston iε accordingly low. In many cases it will even be sufficient for the seating surfaces to engage around the bolt only for 20 or 25% of its height, because the forces that in operation act transversely on the bolt are considerably smaller than the forces that act axially.

For receiving the bolt, the piston advantageously has a bore which has an enlarged diameter circumferentially of the piston. The bore therefore differs from the circular form transverse to the axial direction of the piston. The recesses which lead to the desired reduction in the normal εtreεs are arranged in the region of the enlarged diameter.

In an alternative or additional construction, at at least one end, the bolt can have a relieved area on at least one side. This relieved area of the bolt can also, of course, be uεed in conjunction with the diametral enlargement of the piston bore. The corresponding reduction in the normal stress will also occur in the region of the relieved area. The relieved area can be convex, flat, or concave. It can also be formed by a recess.

It is in that case especially preferred for the relieved area to extend substantially parallel to the axial direction. This facilitates assembly. There is a defined εurface of the bolt which can be used for aligning the bolt during assembly.

In place of the relieved area, in another preferred construction provision can be made for the bolt to have a slot at at least one end. This measure too creates a region of the surface pairing having reduced normal stress. In addition, the bolt is in that case somewhat more resilient, so that the risk that the piston will εuffer deformation becomeε less.

The invention is described hereinafter with reference to a preferred embodiment in conjunction with the drawings, in which: Fig. 1 is a diagrammatic side view of a piston compressor, partly in section, Fig. 2 is a view of a piston from direction II according to Fig. 1, Fig. 3 is a perspective view of a bolt and Fig. 4 is a perspective view of an alternative example of the bolt.

A piston compressor 1, as is used, for example, in hermetically encapsulated small-type refrigeration machineε for refrigerators or other domestic refrigeration appliances, has a diagrammatically illustrated cylinder 2 in which a piston 3 is arranged so aε to be movable axially, that iε, in the direction of a double arrow 4. The piston 3 is connected to one end of a connecting rod 5 which effects the drive of the piston. For that purpose, the connecting rod 5 iε connected at its other end 6 to a crank shaft, not illustrated.

The connection between the connecting rod 5 and the piston 3 iε effected by way of a bolt 7. On movement of the piεton 3 in the axial direction 4 there iε an angular movement between the piεton 3 and the connecting rod 5 with the bolt 7 as the pivot point. Here, the bolt 7 should be held non-rotatably (and, of course, also non-displaceably) in the piεton 3. A rotation εhould therefore be effected only between the bolt 7 and the connecting rod 5, and not between the bolt 7 and the piεton 3.

For that purpose, the bolt 7 is held in the piston 3 by means of a press fit clamping 10. Here, the bolt 7 is driven in the direction of the arrow 8 into the piston 3, εimultaneouεly paεεing through the connecting rod 5. Viewed from the assembly direction 8, this involves first of all a sliding fit 9 between piston 3 and bolt 7, and on the side of the connecting rod 5 furthest inwards in the direction of assembly 8, a press fit 10 between the bolt 7 and the piston 3. This press fit 10 holds the bolt 7 fixedly in the piston 3.

The press fit 10 is formed by a seating surface 11 in an inner wall of a bore 12, which receives the bolt 7; the seating surface 11 co-operateε with a εeating εurface 17 (Fig. 3) on the circumference of the bolt 7. In the assembled state, the two seating surfaces 11, 17 lie against each other with a certain normal stress. As apparent from Fig. 2, the seating surface 11 is interrupted, however, by two opposing recesses 13 which are arranged transverse to the axial direction 4, that is, form a diametral enlargement of the bore 12 which, viewed in the direction of the arrow II in Fig. 1, extends in the circumferentially of the piston 3. The transition between the seating surface 11 and each of its recesseε 13 iε effected by a reεpective εtep 14. The bolt 7 is circular in cross-section. It is therefore of cylindrical form. At its end 15 which is introduced into the bore 12 and has the seating surface 17, it has on both sides a relieved area 16 which is aligned substantially parallel to the axial direction 4. The relieved area 16 can, as shown, be in the form of a flat surface. It may, however, also be convexly or concavely curved.

If the bolt 7 is now driven into the bore 12, a gap is formed between the -recesεeε 13 and the relieved areas 16 εo that in thiε region no normal stresses are able to form between the piston 3 and the bolt 7. Of course, it would also be sufficient either to use only the recesεes 13 or to use only the relieved areas 16. There need not even be a gap between the piston 3 and the bolt 7. In the region in which only small normal stresseε, or if possible no normal εtresεes at all, are to occur, it is quite possible for the bolt 7 to be held in a sliding fit in the piston 3. Even relatively εmall normal εtreεεeε can be allowed if theεe do not lead to a correεponding deformation of the diameter of the piεton 3.

Aε is apparent from Fig. 3, the transition between the relieved areas 16 and the external circumference of the bolt 7 can also be effected gradually, so that concentrations of the normal stress can be prevented. The same tranεition can alεo be provided in the case of the bore 12. Basically, any desired combination of recesεes 13 and relieved areaε 16 is possible. What matterε iε that there is reduced normal stress circumferentially in the presε fit 10.

Aε iε eεpecially apparent from Fig. 2, only a εmall part of the height of the bolt 7 iε engaged by the εeating εurface 11, or more accurately by the portion of the εeating εurface 11 in which normal stresses are preεent. Thiε portion of the height iε relatively small, in this particular example, less than 25%. This is sufficient, however, to secure the bolt 7 satisfactorily in the transverεe direction in the piεton 3. The higheεt forces between the bolt 7 and the piston 3 in any caεe occur in the axial direction 4.

Fig. 4 εhowε an alternative conεtruction of the bolt, in which the reference numerals for corresponding parts are the same as in Fig. 3, but provided with a prime sign. The relieved area 16 is replaced by a slot 16'. Where there is no material because of the slot, it is imposεible for normal εtress to occur. Moreover, the end 15' of the bolt 7' is now slightly resiliently deformable, since the two projectionε defining the slot 16' are able to yield slightly radially inwardε.

Claims

Patent Claims

1. Piston compressor, in particular for domestic refrigeration machines, having at least one cylinder, in which a piston is arranged to move axially, which piston is connected by way of a bolt to a connecting rod, the bolt being secured in the piston by means of a press fit, characterized in that the press fit (10) compriseε a pairing of εeating εurfaces (11, 17) which is under reduced normal stress circumferentially of the piston (3) .

2. Piεton compreεεor according to claim 1, characterized in that, compared with a baεic form, at leaεt one of the εeating surfaces (11, 17) has a recess (13, 16) which is arranged transverεe to the axial direction.

3. Piεton compreεεor according to claim 1 or 2, characterized in that the seating surface (11, 17) has two recesses (13, 16) lying opposite one another.

4. Piston compreεsor according to claim 2 or 3, characterized in that the recess (16) is formed by a gradual departure of the seating surface (17) from its basic form.

5. Piston compreεsor according to claim 2 or 3, characterized in that the transition between the seating surface (11) and its recess (13) is formed by a step (14).

6. Piston compressor according to one of claims 1 to 5, characterized in that the seating surface portions in which a normal stress sufficient to hold the bolt (7) fixedly in the piston (3) is preεent engage around the bolt (7) in the axial direction for leεε than 33% of its height.

7. Piston compressor according to one of claims 1 to

6, characterized in that, for receiving the bolt (7) , the piεton (3) haε a bore (12) which has an enlarged diameter circumferentially of the piston (3) .

8. Piston compressor according to one of claims 1 to

7, characterized in that, at at least one end (15), the bolt (7) has a relieved area (16) on at least one side.

9. Piston compressor according to claim 8, characterized in that the relieved area (16) extends substantially parallel to the axial direction (4) .

10. Piston compressor according to one of claims 1 to 7, characterized in that the bolt (7') has a slot (16') at at least one end (15').