AXIAL-PISTON MACHINE HAVING AN ANTIWEAR LAYER

Abstract

An axial piston machine having a control plate and a cylinder drum rotating relative to the control plate. The control plate rubs or slides on a second sliding side of the cylinder drum via a first sliding side. One of the sliding sides has a carbon-containing layer.

Description

FIELD OF THE INVENTION

The present invention relates to a hydrostatic machine configured as axial-piston machine.

BACKGROUND INFORMATION

An axial-piston machine having a control plate and a cylinder drum sliding on the control plate is described in German Patent Application Serial No. DE 102 23 844 A1, for instance. Situated on one side of the paired surfaces rubbing against each other is a plastic layer, and on the other side is a carbon-containing layer.

Due to the two-axle loading in the axial and radial direction, and because of cavitation, the regions of the surfaces of the control plate and the cylinder drum facing each other are subjected to increased loading. This includes the normally present notches to avoid rapid pressure fluctuations at the control openings of the control plate. In particular, the surfaces facing each other are also stressed by rebounding of the cylinder drum or the pressure plate on to the control plate.

A disadvantage of the aforementioned axial piston machine is that in the presence of oil-containing flow media, the plastic layer may react to the flow medium in a disadvantageous manner, which subjects the plastic layer to increased wear. In particular in rapid load changes and at high operating pressures, vapor bubbles can form, and subsequent pressure pulses, so-called cavitation, may occur in the hydraulic medium, as well as the already mentioned rebounding of the cylinder drum or the pressure plate; the plastic layer has only insufficient resistance to such stresses and thus wears quickly, the plastic layer detaching only regionally, which results in uneven loading and faster wear.

From the related art it is also known to fuse a bronze layer onto the sliding side of the cylinder drum.

In addition, a production method for producing a diamond-like, amorphous carbon layer is described in International Application No. WO 98/54376 A1.

Furthermore, the publication “Helena Roukainen, Tribological properties of hydrogenated and hydrogen-free diamond-like carbon coatings”, VTT Publications, 2001, describes the ta-C or CVD method, specifically on pages 3 and 4 and pages 26 to 28.

Both the plastic layer and the bronze layer increase the production time and expense and are disadvantageous as a result.

SUMMARY

It is an object of the present invention to provide an axial piston machine that is able to be manufactured in a less complicated manner, may be operated with a multitude of flow media and is less susceptible to wear, especially under heavy loading.

An axial piston machine according to an example embodiment of the present invention, may provide the advantage of higher wear resistance, in particular with respect to knocking loading of the parts sliding along each other, with respect to cavitation and to the two-axle loading. Furthermore, the axial piston machine according to the example embodiment of the present invention is easier to produce and, in addition to water, may also be operated using oil-containing flow media.

In particular, it may be advantageous if the respective other sliding side not coated by the layer is made of steel which is hardened by nitration. This allows an easier manufacture of the sliding side lying opposite the carbon layer. The use of environmentally damaging nonferrous metals may be dispensed with.

Furthermore, it may be advantageous if the layer is applied on the first sliding side or on the sliding side of the control plate. Since the control plate of an axial piston machine has smaller dimensions than the cylinder drum, the layer is able to be applied on the control plate in a simpler and more cost-effective manner.

In addition, it may be advantageous if the control plate and/or the cylinder drum are/is generally made of metal in the region of their respective sliding sides, and if the layer is applied directly onto the metal surface of the sliding sides. The layer then adheres to the respective sliding side in a more durable manner. The functional layer may be applied either directly or preferably with the aid of an adhesive layer.

In addition, it may be advantageous if the layer is a diamond-like, amorphous carbon layer, in particular a tetrahedral, hydrogen-free amorphous carbon layer, ta-C. Compared to conventional carbon layers, the ta-C layer has especially advantageous characteristics when used in an axial piston machine. For instance, the fatigue resistance of the friction- and wear-reducing layer with respect to adhesive chipping and cohesive erosion, in particular, is increased considerably, especially with respect to the stresses by cavitation and the impact forces that occur at increased operating pressure and in rapid, heavy load changes, in particular. The wear resistance is improved, especially given increased loading and contaminated flow media.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described below in greater detail by way of example on the basis of an exemplary embodiment and specific example embodiments. Identical components have been provided with matching reference numerals.

FIG. 1 shows a schematic representation of an axial piston machine according to an example embodiment of the present invention.

FIG. 2 shows cut-away portion II shown in FIG. 1, in enlarged form.

FIG. 3 shows a preferred specific embodiment of a control plate of the axial piston machine according to the present invention, in a plan view of the sliding slide of the control plate.

FIG. 4 shows a preferred specific embodiment of a cylinder drum of the axial piston machine according to the present invention, in a plan view of the sliding side of the cylinder drum.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The axial piston machine illustrated in FIG. 1 is configured as swash plate with an adjustable displacement volume and one flow direction; in the conventional manner it includes a hollow-cylindrical housing 1 having an end that is open at the end face (upper end in FIG. 1); a connection plate 2, which is secured to housing 1 and seals its open end; a cam plate or swash plate 3; a control plate 4, which is also known as control body or control mirror; a drive shaft 5 and a cylinder drum 6.

Swash plate 3 is designed as so-called tilting cradle having a semi-cylindrical cross section and, via two bearing surfaces that extend with mutual clearance parallel to the tilting direction, is supported with hydrostatic destressing at two correspondingly formed bearing shells 8, which are mounted on the inner surface of housing end face 9 situated opposite connection plate 2. The hydrostatic destressing is implemented in a conventional manner via pressure pockets 10 formed in bearing shells 8 and supplied with pressure medium via connections 11. An actuating device 13, which is accommodated in a bulge of a cylindrical housing wall 12, engages with swash plate 3 via an arm 14 that extends in the direction of connection plate 2 and is utilized to tilt the same about a tilting axis that is perpendicular to the tilting direction.

Control plate 4 is centered on the outer ring of bearing 18 and, positioned in the circumferential direction, rests against the inner surface, facing the housing interior, of connection plate 2. Control plate 4 is provided with two straight-through openings 15 in the form of kidney-shaped control slots, which are connected to, respectively, a pressure and suction line (not shown) via a pressure channel 16D or suction channel 16 in connection plate 2. Pressure channel 16D has a smaller flow cross section than suction channel 16S. The control surface of control plate 4 facing the housing interior and having a spherical design is used as bearing surface for cylinder drum 6.

Drive shaft 5 projects into housing 1 through a through-hole in housing end face 9 and is rotatably supported in connection plate 2 with the aid of a bearing 17 in this through-hole, and with the aid of another bearing 18 it is supported in a more narrow bore section of a blind hole bore 19 widened at the end face. Furthermore, in the interior of housing 1, drive shaft 2 penetrates centrical through-hole bore 20 in control plate 4, a centrical through-hole bore 21 in swash plate 3, as well as a centrical through-hole bore in cylinder drum 6 having two bore sections.

One of these bore sections is formed in a sleeve-shaped extension 23, which is premolded on cylinder drum 6, projects beyond its end face 22 facing swash plate 3 and is utilized to connect cylinder drum 6 to drive shaft 5 in a torsionally fixed manner, with the aid of a wedge-groove connection 25. The remaining bore section has a conical design. It tapers from its cross section having the largest diameter, close to the first bore section, to its cross section having the smallest diameter, close to the end or bearing surface of cylinder drum 6 resting against control plate 4. The annular space defined by drive shaft 5 and this conical bore section is denoted by reference numeral 25.

Cylinder drum 6 has stepped cylinder bores 26, which generally extend in the axial direction and are evenly disposed on a graduated circle that is coaxial with respect to the axis of the drive shaft. Cylinder bores 26 discharge directly at cylinder drum end face 22 and, via end channels 27, at the cylinder drum bearing surface facing control plate 4, on the same graduated circle as the control slots. One cylinder sleeve 28 in each case is inserted in the cylinder bore sections having a larger diameter and discharging directly at cylinder drum end face 22. Cylinder bores 26 including cylinder sleeves 28 are denoted as cylinders here. Pistons 29, disposed within these cylinders so as to be displaceable, have spherical heads 30 at their ends facing swash plate 3, which are mounted in slide shoes 31 and are hydrostatically supported on an annular sliding pad 32 mounted on swash plate 3 via these spherical heads 30.

On its sliding surface facing sliding pad 32, each slide shoe 31 is provided with its own pressure pocket (not shown), which is connected to a stepped axial through channel 34 in piston 29 via a through hole 33 in slide shoe 31 and is thereby connected to the working chamber of the cylinder delimited from piston 29 in cylinder bore 26. A throttle is formed in each axial through channel 34 in the region of the assigned spherical head 30. A holding-down clamp 36, which is situated on drive shaft 5 so as to be axially displaceable, utilizing a wedge-groove connection 24, and which is acted upon in the direction of swash plate 3 by a spring 35, retains slide shoe 31 in contact with sliding pad 32.

The function of afore-described axial piston machine 1 is generally conventionally, which is why the following description of its use as pump is limited to the main portions.

Axial piston machine 1 is intended for an operation using oil as flow medium. Cylinder drum 6 together with pistons 29 is made to rotate via drive shaft 5. When an activation of actuating device 13 has brought swash plate 3 into a tilted position relative to cylinder drum 6, all pistons 29 execute translational movements. When cylinder drum 6 is rotated about 360°, each piston 29 executes an aspiration and a compression stroke during which corresponding oil flows are generated whose conveyance and evacuation is implemented via end channels 27, control slots 15 and pressure and suction channel 16D, 16S, respectively. During the compression stroke of each piston 29 pressurized oil flows from the individual cylinder into its pressure pocket via axial through channel 34 and through hole 33 in associated slide shoe 31 and generates a pressure field between sliding pad 32 and respective slide shoe 31, which is utilized as hydrostatic bearing for the latter. Furthermore, via connections 11, pressurized oil is conveyed to pressure pockets 10 in bearing shells 8 to support swash plate 3 hydrostatically.

Sliding surfaces 44, 45, shown in greater-detail in FIG. 2, are formed on the facing sides of control plate 4 and cylinder drum 6. Control plate 4 has first sliding side 44, and cylinder drum 6 has second sliding side 45. A friction- and wear-reducing layer 46, which is shown in greater detail in FIG. 2, has been applied on first sliding side 44, between the two sliding sides 44, 45.

FIG. 2 shows cutaway II illustrated in FIG. 1 in an enlarged view, the region around layer 46, control plate 4, and cylinder drum 6, as well as around first sliding side 44, second sliding side 45, and opening 15 being shown in enlarged form as a result. In the exemplary embodiment, layer 46 is situated on first sliding side 44, i.e., on control plate 4. Layer 46 is preferably made of a tetrahedral, hydrogen-free amorphous diamond-like carbon, which is known by the abbreviation ta-C, from the group of DLC layers (diamond like carbon layers).

Layer 46, which reduces friction and protects against wear, is evenly applied on first sliding side 44 by a PVD method (physical vapor deposition), for instance, or by the specialized arc-PVD method or a CVD method (chemical vapor deposition), preferably, however, with the aid of a PECVD method (plasma enhanced chemical vapor deposition). Furthermore, layer 46 is formed as so-called thin layer, at a thickness of up to approximately 15 micrometers, a range of 1 to 3 micrometer being endeavored. A metallic adhesion layer is normally used, in particular made of Cr, Ti, Zr.

Second sliding side 45 of cylinder drum 6, which lies opposite layer 46 and is likewise made completely of steel, is hardened, preferably by nitration.

During operation of axial piston machine 1 designed according to the present invention by way of example, the rotational movement causes a hydrodynamic sliding film to form between layer 46 and second sliding side 45, which is made up of the flow medium. However, in startup operation, layer 46 rubs against second sliding layer 45. Due to the hydraulic pressure fluctuations that occur in axial piston machine 1, especially when the axial piston machine is subjected to heavy loading, second sliding side 45 may briefly and rapidly drop back onto layer 46 again. This causes knocking loading, whose intensity is a function of the working pressures and the pressure fluctuation profile, in particular.

FIG. 3 shows a preferred specific embodiment of a control plate 4 of axial piston machine 1 according to the present invention, in a plan view of sliding side 44 of control plate 4. Easily visible in this view are the approximately kidney-shaped openings 15, which are used to control the filling and evacuation of cylinder bores 26. In the exemplary embodiment, both openings 15 have a notch 47 at each of their ends disposed in the direction of rotation. Notches 47 cause a soft reversing and are known as such from the related art. Sliding side 44 of control plate 4 is completely coated by layer 46, the surfaces of notches 47 and the regions of the inner sides of openings 15 being included within the meaning of the present invention. In other exemplary embodiments, in order to have the reversing occur in a softer manner, notches (not shown) may be disposed on the end situated counter to the direction of rotation, or bores may be introduced into control plate 4; the surfaces of these notches (not shown) and the inner surfaces of the bores (not shown) would likewise be coated by layer 46 according to the present invention.

In axial piston machines material stresses caused by cavitation tend to occur in a region 48, which has been outlined by a circular ring by way of example.

FIG. 4 shows a preferred specific embodiment of a cylinder drum 6 of axial piston machine 1 according to the present invention, in a plan view of sliding side 45 of cylinder drum 6, which is denoted as second sliding side 45. Regions 48, which are outlined by circular rings in this figure and mark the regions at risk by cavitation, lie approximately between end channels 27 formed as elongated holes, the ends of end channels 27 being outlined as well. According to the present invention, second sliding side 45 instead of first sliding side 44 may be coated by layer 46, the inner surfaces of end channels 27 preferably also being coated by layer 46 in this case.

The present invention is not restricted to the exemplary embodiments and specific embodiments. The features of the exemplary embodiment and the specific embodiments may be combined with each other as desired.

Claims

1-8. (canceled)

9. An axial piston machine, comprising: a control plate; anda cylinder drum which rotates relative to the control plate, the cylinder drum and the control plate rubbing or sliding against each other, the control plate rubbing or sliding on a second sliding side of the cylinder drum via a first sliding side of the cylinder drum, wherein one of the first sliding side and the second sliding side has a carbon-containing layer, and the other of the first sliding side and the second sliding side is made of metal.

10. The axial piston machine as recited in claim 9, wherein the sliding side made of metal is made of steel hardened by nitration.

11. The axial piston machine as recited in claim 9, wherein the layer is applied on the first sliding side.

12. The axial piston machine as recited in claim 9, wherein at least one of the control plate and the cylinder drum is made of metal in a region of the first and second sliding sides, and the layer is applied directly on a metal surface of the one of the first or second sliding side.

13. The axial piston machine as recited in claim 9, wherein the layer is a diamond-like amorphous carbon layer.

14. The axial piston machine as recited in claim 13, wherein the layer is a tetrahedral, hydrogen-free amorphous carbon layer (ta-C).

15. The axial piston machine as recited in claim 9, wherein the layer is a diamond layer, the diamond layer being one of a nanocrystalline, microcrystalline or doped CVD diamond layer.

16. The axial piston machine as recited in claim 9, wherein one of the first and second sliding sides spherically projects into the other sliding side.