Piston unit and hydrostatic radial-piston machine

Abstract

A piston unit includes a piston which has a rolling element seat configured to rotatably support a rolling element. The rolling element seat has a bearing surface with a surface coating composed of a polymer compound or with a ceramic surface coating.

Claims

1. A piston unit of a hydrostatic machine, comprising: a piston including a rolling element seat configured to rotatably support a rolling element, the rolling element seat including a bearing surface, wherein the bearing surface has a surface coating composed of a polymer compound, wherein the polymer compound is applied in at least two layers, and wherein the polymer compound in a first layer differs from the polymer compound in a second layer.

2. The piston unit as claimed in claim 1, wherein the polymer compound has a layer thickness of approximately 40 to 80 m.

3. The piston unit as claimed in claim 1, wherein the polymer compound is a dispersion before it is applied.

4. The piston unit as claimed in claim 1, wherein the surface coating is applied to the bearing surface in the form of an aerosol.

5. The piston unit as claimed in claim 1, wherein the rolling element is supported directly on the bearing surface.

6. The piston unit as claimed in claim 1, wherein the surface coating of the bearing surface extends so far in a longitudinal direction of the piston that the rolling element is at least partially supported in a radial direction of the piston.

7. A piston unit, of a hydrostatic machine, comprising: a piston including a rolling element seat configured to rotatably support a rolling element, the rolling element seat including a bearing surface, wherein the bearing surface has one of a surface coating composed of a polymer compound and a ceramic coating, and wherein the bearing surface is integrally formed with the piston.

8. The piston unit as claimed in claim 7, wherein the ceramic surface coating has a layer thickness of approximately 25 to 35 m.

9. A hydrostatic radial-piston machine, comprising: at least one piston unit including a piston, the piston having a rolling element seat configured to rotatably support a rolling element, wherein the rolling element seat includes a bearing surface having a surface coating composed of a polymer compound, wherein the polymer compound is applied in at least two layers, wherein the polymer compound in a first layer differs from the polymer compound in a second layer, and wherein the at least one piston unit is guided in a cylinder bore of a cylinder block.

10. The piston unit as claimed in claim 9, wherein the polymer compound has a layer thickness of approximately 40 to 80 m.

11. The piston unit as claimed in claim 9, wherein the polymer compound is a dispersion before it is applied.

12. The piston unit as claimed in claim 9, wherein the surface coating of the bearing surface extends so far in a longitudinal direction of the piston that the rolling element is at least partially supported in a radial direction of the piston.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawing:

(2) FIG. 1 shows a section through a part of a hydrostatic radial-piston machine having a piston unit according to the disclosure,

(3) FIG. 2 shows a perspective top view of a piston from FIG. 1 with a bearing surface, coated according to the disclosure, of a rolling element seat and

(4) FIG. 3 shows a perspective side view of a piston from FIG. 1 with a bearing surface, coated according to the disclosure, of a rolling element seat.

(5) In the figures the same components are provided with the same reference numerals throughout.

DETAILED DESCRIPTION

(6) FIG. 1 shows an exemplary embodiment of a hydrostatic radial-piston machine 1 according to the disclosure in a section perpendicular to its axis of rotation. In this exemplary embodiment the hydrostatic radial-piston machine 1 is a radial-piston motor having an outer piston support. The basic working principle of such a hydrostatic radial-piston machine is described, for example, in DE 40 37 455 C1.

(7) The hydrostatic radial-piston machine 1 comprises a centrally arranged shaft 2, which serves to drive an element (not represented here) affixed thereto. The shaft 2 is driven by a rotating rotor 4, in which multiple cylinders 6 are formed. In the exemplary embodiment the hydrostatic radial-piston machine 1 comprises precisely eight cylinders 6, of which only three are fully represented in FIG. 1, together with halves of two further cylinders. The cylinders 6 are arranged radially or in a star-shaped pattern around the shaft 2. Each cylinder 6 moreover has a main bore 8 and a radially back-stepped guide bore 10, in each of which a piston unit 12 is moveably accommodated.

(8) Each of the piston units 12 comprises a piston 14, which in the exemplary embodiment shown is a stepped piston. At an outer end in a radial direction of the hydrostatic radial-piston machine 1, each piston 14 has a cylindrical rolling element 16, which extends over an angle of more than 180 degrees and in which a rolling element 18 embodied as a roller is rotatably supported and held. For directly supporting the respective rolling element 18, each rolling element seat 16 has a respective bearing surface 20, which merely for illustrative purposes in this representation is shown to be relatively thick in its build-up or application. This is because in principle the bearing surface 20 is a relatively thinly coated surface, which cannot readily be represented on a drawing, and in particular not drawn true to scale. The rolling element seat 16 and the bearing surface 20 are integrally formed with the piston by a metal-cutting or chip-forming production process. Each piston 14 further comprises a circumferential groove 22 in which a piston ring 24 serving as sealing element is received.

(9) The hydrostatic radial-piston machine 1 further comprises a fixed stroke ring 26, which represents a part of a housing and has a profiled cam track 28. This is arranged so that it rotates about the rotor 4 and therefore also about the cylinder 6 together with the piston unit 12. This arrangement allows the rolling elements 18 to be supported on the cam track 28 of the stroke ring 26 and to roll on the stroke ring 26 during a rotational movement of the rotor 4. Consequently, the piston units 12 perform an oscillating movement inside the cylinders 6.

(10) The optimal-friction bearing surface 20 is described below with reference to FIGS. 2 and 3, which each show a perspective view of a piston 14 from FIG. 1. The bearing surface 20, as described above, is integrally formed with the rolling element seat 16 and therefore also integrally formed with the piston 14, using a metal-cutting or chip-forming production process. To improve the surface condition of the rolling element seat 18 or the bearing surface 20 integrally formed therewith, these are broached by a broaching tool following the shaping process. This integral formation of the bearing surface 20 with the rolling element seat 16 or the piston 16 dispenses with a bearing shell conventionally used as an intermediate component arranged between the rolling element seat 16 and the respective rolling element 18.

(11) In order to reduce the (boundary) friction between the rolling element 18 to be seated and the bearing surface 20, the bearing surface 20 has a surface coating, which in this exemplary embodiment is composed of a polymer compound. A polyetherketone (PEEK)-based coating has been selected here. This is available from Messrs. Victrex under the tradename VICOTE. This is a VICTREX PEEK polymer-based coating product which initially comes as a dispersion and as such is applied to the rolling element seat 16 in a single coating operation. The layer thickness of the surface coating of the bearing surface 20 is between 40 to 80 m, preferably 50 to 75 m. The bearing surface 20 is formed together with the surface coating up to an upper end of the rolling element seat 16 in FIG. 2 or 3. That is to say the bearing surface 20 extends so far in the axial direction of the piston 14 that the rolling element 18 can also be at least partially supported in a radial direction of the piston 14.

(12) Starting from the exemplary embodiment represented, the piston unit 12 according to the disclosure and the hydrostatic radial-piston machine 1 equipped therewith can be modified in a variety of ways.

(13) For example, the bearing surface 20 need not necessarily be formed with a surface coating composed of a polymer compound. An optimal-friction ceramic surface coating is also possible as an alternative to the polymer compound. In addition, the respective surface coating may also be applied gradually in multiple coating operations, in two or more layers. It is also possible to vary the precise material composition of the surface coating, particularly that of the polymer compound. With multiple layers of the surface coating, different material compositions of the individual layers are also possible.

(14) In addition, the piston unit 12 need not necessarily be used in a hydrostatic radial-piston machine 1 with outer piston support, but may also advantageously be used in other technical applications. In particular, an application in hydrostatic radial-piston machines of another design type having no outer piston support is also feasible. Use in technical systems which are not radial-piston machines is also feasible.

(15) A piston unit is disclosed, having a piston which comprises a rolling element seat for rotatably supporting a rolling element, together with a hydrostatic radial-piston machine constructed with at least one such piston unit. According to the disclosure the rolling element seat has a bearing surface with a surface coating composed of a polymer compound or with a ceramic surface coating.

LIST OF REFERENCE NUMERALS

(16) 1 radial-piston machine 2 shaft 4 rotor 6 cylinder 8 main bore of the cylinder 10 guide bore of the cylinder 12 piston unit 14 piston 16 rolling element seat of the piston 18 rolling element 20 bearing face with surface coating 22 circumferential groove of the piston 24 piston ring 26 stroke ring 28 lifting cam