Hollow piston for an axial piston machine

Abstract

Hollow piston (10) for an axial piston machine (1), which has a cylindrical main body (36) with a first end portion (37) and with a second end portion (38), wherein a cover (39) is provided at the second end portion (38) and a connecting portion (40) to a head portion (41) is provided at the first end portion (37), wherein the cylindrical main body (36) has an annular cavity (42), wherein a first inner profile (47) is formed between a first end face (45) of the annular cavity (42) and a jacket wall (43) of the main body (36), and wherein a second inner profile (48) is formed between the first end face (45) of the annular cavity (42) and a mandrel (44), characterized by that the first inner profile (47) is rounded, wherein the rounding corresponds to an elliptical section, so that an elliptical rounding profile is formed, that the cavity (42) is formed between the jacket wall (43) of the main body (36) and the mandrel (44) extending along a longitudinal axis (L) of the main body (36), wherein the annular cavity (42) has the first end surface (45) adjacent to the connecting section (40) of the main body (36) and a second end surface (46) opposite the first, that the first inner profile (47) forms a first corner in the cross-section of the hollow piston (1), wherein the radius of the elliptical rounding profile of the first corner in the axial direction of the cylindrical main body (36) is twice as large as the radius of the elliptical rounding profile in the radial direction of the cylindrical main body (36), that the second inner profile (48) forms a second corner in the cross-section of the hollow piston (10), and that the second inner profile (48) has an elliptical rounding profile.

Description

[0001] The invention is based on a hollow piston according to the preamble of the main claim.

[0002] Engine pistons in axial piston engines are subject to high loads caused by centrifugal forces. The solid pistons typically used in axial piston engines limit operation at higher speeds. At higher speeds, strength problems arise for the cylinders due to the high centrifugal forces and for the piston retaining device due to the large inertia forces, as well as thermal problems at the contact surfaces between piston and cylinder due to the frictional forces resulting from the centrifugal forces. To operate axial piston engines at higher speeds and to save weight, lightweight pistons with a hollow space are used. These are manufactured by welding two individual parts. The inner contour of the prefabricated parts is currently manufactured using machining processes.

[0003] Various designs of hollow pistons are already known, but the hollow-bore piston design has the disadvantage that the resulting cavity in the piston is filled with fluid, which leads to a deterioration in efficiency. To overcome this problem, a cover is applied to the main piston body, which is connected to the main body using various methods.

[0004] For example, DE 196 20 167 C1 discloses a hollow piston with a radially welded cover, wherein the hollow piston and the cover are firmly joined together by electron beam welding. The inner contour of the hollow piston, produced by a machining process, has an annular space formed between the main body of the piston and a mandrel extending along the longitudinal axis of the main body. At the end of the annular space facing a head portion of the piston, this mandrel has circular arc-shaped rounded portions, which make the production of this contour by processes other than machining difficult or impossible.

[0005] However, hollow pistons manufactured using machining processes have the disadvantage that the required material properties, in particular fracture toughness, surface quality and cost-effectiveness, are not satisfactory.

[0006] The invention is therefore based on the object of creating a hollow piston with increased fracture resistance.

[0007] The object is solved by the features of claim 1. The subclaims contain advantageous developments of the invention.

[0008] EP 1 134 412 A2 discloses a method and apparatus for producing a hollow piston for a compressor, wherein the piston has a head portion slidably fitted in a cylinder bore of the compressor and an engagement portion that engages a reciprocating drive device of the compressor for reciprocating the piston, wherein at least the head portion of the piston is hollow. Although this document describes a hollow cylinder having a first portion (head portion) and a second portion (engagement portion), this document nevertheless remains silent about the hollow piston being intended for use in an axial piston machine. Instead, this document explicitly describes the production of a hollow cylinder for a compressor.

[0009] DE 26 53 867 A1 discloses a hollow piston for an axial piston engine with a spherical head, a piston skirt connected to the spherical head via an end plate, a central column penetrating the hollow piston, and an end face cover. According to this document, the spherical head, end plate, and piston skirt are manufactured from a single workpiece, and the central column and end face cover are manufactured from a separate workpiece. DE 26 53 867 A1 explicitly states that the spherical head, end plate, and piston skirt consist of a single workpiece. According to this document, a separate workpiece is described which contains the central column and the end face cover. The combination of these two workpieces to form a hollow piston is intended to create a piston that can withstand high mechanical stresses and is nevertheless inexpensive to manufacture using simple means.However, this leads the person skilled in the art away from producing a hollow piston from a workpiece by means of an optimized cold extrusion process and in no way suggests to the person skilled in the art to provide a specifically pronounced first inner profile which makes the optimized cold extrusion process possible in the first place without further machining steps for producing the inner contour.

[0010] DE 85 09 011 U1 discloses a hollow piston for an axial piston engine with a spherical head, a piston skirt connected to the latter via an end plate, a central column, and an end face cover. The spherical head, the end plate, and the piston skirt consist of a workpiece into which a central column consisting of a tube is integrated. Even though this document shows that an annular space is formed between the central column and the piston skirt, and this document describes a first internal profile between a first end face of the annular cavity and a skirt wall of the main body, and even though this document shows a second internal profile between the first end face of the annular cavity and a mandrel, this document nevertheless in no way shows that a first internal profile is rounded, wherein the rounding corresponds to a section of an ellipse, so that an elliptical rounded profile is formed.

[0011] GB 855 582 A discloses a hollow piston having a tube with a small diameter, the tube extending axially within the hollow piston skirt. Furthermore, this document describes that the hollow piston has a spherical head and a cover. Although this document states that an annular space is formed between the tube and the hollow piston skirt and that the Fig. 2 and Fig. 3 of this document describe a first internal profile between a first end face of the annular cavity and a shell wall of the main body, and also if Fig. 2 and Fig. 3 show a second internal profile between the first end face of the annular cavity and the tube, this document nevertheless does not in any way indicate that a first internal profile is rounded, the rounding corresponding to a section of an ellipse, so that an elliptical rounding profile is formed.

[0012] The hollow piston according to the invention with the characterizing feature of the main claim has the advantage that it can be produced using a cost-effective process and provides improved material properties and surface quality compared to hollow pistons produced using machining processes in the prior art. According to the invention, a strength-optimized lightweight piston for hydrostatic machines is provided, which overcomes the disadvantages of the prior art, since the specific inner contour of the hollow piston according to the invention makes it possible to produce the piston using a cold extrusion process, thus eliminating the need for machining steps to produce the inner contour.

[0013] To achieve this, the fillet at the end of the annular space facing the piston head section is designed as a first internal profile, which forms a first rounded corner between the first end surface of the annular space and the jacket wall of the main body in the cross-section of the hollow piston. The rounding of the first corner corresponds to an elliptical section, resulting in an elliptical fillet profile instead of a circular arc-shaped fillet. This special geometry of the fillet allows for the use of optimized cold extrusion tools and a correspondingly optimized cold extrusion process, which is not the case with a circular arc-shaped fillet.

[0014] It is particularly preferred if the radius of the elliptical fillet profile in the axial direction of the hollow cylinder is twice as large as the radius of the elliptical fillet profile in the radial direction of the hollow cylinder, with the preferred dimensions being approximately 2.8 mm for the axial radius and approximately 1.4 mm for the radial radius. This design achieves a particularly smooth transition from the cylindrical outer or shell wall to the bore bottom. This leads to significantly reduced component stress at this point.

[0015] An exemplary embodiment of a hollow piston according to the invention and of an axial piston machine with a hollow piston according to the prior art is shown in simplified form in the drawing and is explained in more detail in the following description. They show: Fig. 1 shows a schematic cross-section through an axial piston machine according to the prior art; Fig. 2a a hollow piston according to the invention; Fig. 2b an enlarged section of the rounding profile of the hollow piston of the Fig. 2a in area IIb and Fig. 3 a schematic representation of the rounding.

[0016] Fig. Figure 1 shows an axial piston machine 1 according to the prior art. The axial piston machine 1 is designed as a swash plate with adjustable displacement and comprises, as essential components, a hollow cylindrical housing 2, a connection block 3 attached to the housing 1, a swash plate 4, a control body 5, a drive shaft 6, and a cylinder drum 7 in which the cylinder bores 8, 9 are arranged with radially even distribution. The hollow pistons 10, 11 are displaceably arranged in the cylinder bores 8, 9, with the rod ends of the hollow pistons 10, 11, designed as spherical heads 12, 13, being supported on the swash plate via sliding shoes 14, 15. According to the prior art, the hollow pistons 10, 11 have circular arc-shaped roundings, as indicated by the dashed line.

[0017] An adjusting device 17 accommodated in a recess 16 of the housing 2 engages the swash plate 4 via an arm 18 extending in the direction of the connection block 3 and serves to pivot the same about a pivot axis perpendicular to the pivoting direction.

[0018] The control body 5 is attached to the inner surface of the connection block 3 facing the housing interior and is provided with two through openings in the form of kidney-shaped control slots 19, 20, which are connected to a pressure or suction line (not shown) via a pressure channel 21 and a suction channel 22 in the connection block 3. The spherically shaped control surface of the control body 5 facing the housing interior serves as a bearing surface for the cylinder drum 7.

[0019] The drive shaft 6 extends into the housing 2 through a through-bore in the housing end wall 23 and is rotatably mounted in this through-bore by means of a bearing 24 and by means of another bearing 25 in the connection block 3. The cylinder drum 7 is connected to the drive shaft 6 in a rotationally fixed manner by means of a keyway connection 26.

[0020] The cylinder bores are provided with outlet channels 27, 28, which open out on the same pitch circle as the control slots 19, 20 of the control body 5. A bushing 29, 30 is inserted into each of the cylinder bores 8, 9. Each sliding block 14, 15 is provided with a pressure pocket (not shown) on its sliding surface facing the sliding plate 31 of the swash plate 4. This pressure pocket is connected via a through hole 32, 33 in the sliding block 14, 15 to a stepped, axial through channel 34, 35 in the associated piston 10, 11 and is thus connected to the working chamber of the cylinder delimited by the piston 10, 11 in the cylinder bore 8, 9. A throttle is formed in each axial through channel 34, 35 in the area of the associated spherical head 12, 13.

[0021] For a detailed description of an axial piston machine of this type, please refer to DE 44 23 023 A1.

[0022] Fig. 2a shows a hollow piston 10 for hydrostatic machines, in particular for the Fig. 1, according to the invention. The hollow piston 10 is preferably manufactured using a cold extrusion process. In this process, the material is forced to flow under the influence of high pressure. A punch presses the workpiece blank through a forming tool opening with a reduced cross-section - a die. The forming process during cold extrusion takes place at room temperature, thereby achieving high dimensional accuracy and a remarkably high surface quality.

[0023] The hollow piston 10 has a cylindrical main body 36 with a first end section 37 and a second end section 38. In this exemplary embodiment, the cylindrical main body 36 has a length of approximately 88 mm. A cover 39 is provided on the second end section 38. The cover 39 is firmly connected to the second end section 38 of the cylindrical main body 36, for example by welding. Adjoining the first end section 37 is a connecting section 40 which connects the first end section of the cylindrical main body 36 to a head section 41. The head section 41 is as in the Fig. 1, a joint head is designed as a spherical head. The cylindrical main body 36 further comprises an annular cavity or ring space 42 formed between a jacket wall 43 of the main body 36 and a mandrel 44 arranged centrally in the main body 36. In this exemplary embodiment, the ring space 42 has a diameter of approximately 22 mm.

[0024] The mandrel 44 extends along a longitudinal axis L of the cylindrical main body 36. The annular space 42 thus formed is defined by the jacket wall 43, the mandrel 44, a first end surface 45 adjacent to the connecting portion 40, and a second end surface 46 on the cover 39. The first end surface 46 is opposite the second end surface 47.

[0025] A through-channel 49 extends along the longitudinal axis L of the cylindrical main body 36 through the mandrel 44, the cover 39, and the head portion 41. The through-channel 49 in the head portion 41 can be provided with a throttle point 50. The through-channel 49 creates a connection between the working volume of the cylinder bores 8, 9 and the through-bores 32, 33 in the slide shoes 14, 15.

[0026] A first inner profile 47 is formed on the first end face 45 and the jacket wall 43. The first inner profile 47 corresponds to a first rounded corner in the cross-section of the piston 10, but according to the invention has an elliptical rounding profile instead of a circular arc-shaped rounding profile as known from the prior art. Opposite the first corner 47 is a second inner profile 48, which corresponds to a second rounded corner 48 between the first end face 45 and the mandrel 44 in the cross-section of the piston 10. In the example shown, the rounding is circular arc-shaped with a radius of approximately 1.4 mm. In another embodiment, however, both corners can also be formed with an elliptical rounding profile.According to the invention, the radius of the elliptical rounding profile in the axial direction of the cylindrical main body 36 is approximately twice as large as the radius of the rounding profile in the radial direction of the main body 36 and is thus approximately 2.8 mm in the example.

[0027] As detailed in Fig. As shown in Figure 2b, in this embodiment, the axial radius of the elliptical fillet is approximately 2.8 mm and the radial radius is approximately 1.4 mm. The radius of the circular arc-shaped fillet is also approximately 1.4 mm. Since the first inner profile or the first corner 47 is designed with an elliptical fillet profile with the same radial radius but with approximately twice the axial radius, manufacturability by cold extrusion can be ensured while maintaining the radial dimension.

[0028] In Fig.Figure 3 shows an enlarged and schematic view of the elliptical rounding according to the invention. The elliptical rounding not only enables the production of the hollow piston 10 by cold extrusion, but also achieves a smooth transition from the cylindrical jacket wall 43 to the bore base at the first end surface 45, which leads to greater fracture strength, particularly under radial loads. As already mentioned, this design also leads to significantly reduced component stress at this point, which reduces piston wear.

[0029] The invention is not limited to the illustrated embodiment. Other radii, for example, are also possible. All described and / or illustrated features can be combined with one another in any way.

Claims

[1] Hollow piston (10) for an axial piston machine (1), which has a cylindrical main body (36) with a first end portion (37) and with a second end portion (38), wherein a cover (39) is provided at the second end portion (38) and a connecting portion (40) to a head portion (41) is provided at the first end portion (37), wherein the cylindrical main body (36) has an annular cavity (42), wherein a first inner profile (47) is formed between a first end face (45) of the annular cavity (42) and a jacket wall (43) of the main body (36), and wherein a second inner profile (48) is formed between the first end face (45) of the annular cavity (42) and a mandrel (44), characterized by , that the first inner profile (47) is rounded, wherein the rounding corresponds to an elliptical section, so that an elliptical rounding profile is formed, that the cavity (42) is formed between the jacket wall (43) of the main body (36) and the mandrel (44) extending along a longitudinal axis (L) of the main body (36), wherein the annular cavity (42) has the first end surface (45) adjacent to the connecting section (40) of the main body (36) and a second end surface (46) opposite the first, that the first inner profile (47) forms a first corner in the cross-section of the hollow piston (1), wherein the radius of the elliptical rounding profile of the first corner in the axial direction of the cylindrical main body (36) is twice as large as the radius of the elliptical rounding profile in the radial direction of the cylindrical main body (36), that the second inner profile (48) forms a second corner in the cross-section of the hollow piston (10), and that the second inner profile (48) has an elliptical rounding profile. [2] Hollow piston according to claim 1, characterized by that the axial radius is approximately 2.8 mm and the radial radius is approximately 1.4 mm. [3] Hollow piston according to claim 1 or 2, characterized by that the hollow piston (10) is manufactured by cold extrusion. [4] Hollow piston according to one of claims 1 to 3, characterized by that the cylindrical main body (36) is welded to the cover (39). [5] Hollow piston according to one of claims 1 to 4, characterized by that a through-channel (49) extends along the longitudinal axis (L) of the cylindrical main body (36) through the mandrel (44), the cover (39) and the head section (41). [6] Hollow piston according to claim 5, characterized bythat the through-channel (49) in the head section (41) is provided with a throttle point (50).

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