This application claims priority under 35 U.S.C. § 119 to application no. DE 10 2019 215 159.0, filed on Oct. 2, 2019 in Germany, the disclosure of which is incorporated herein by reference in its entirety.

The disclosure relates to an actuating piston for a hydrostatic adjustment installation and to an adjustment installation.

BACKGROUND

Actuating pistons which are received so as to be longitudinally displaceable in an actuating cylinder and thus configure a hydrostatic adjustment installation which can generate high actuating forces even in the case of small dimensions are known from the prior art. If actuating members which are pivotable by the actuating piston are eccentrically articulated, the actuating piston in terms of the latter being guided in the actuating cylinder is subject not only to an axial force but also to a radial force or transverse force that acts transversely to said axial force. Accordingly, the actuating piston must be guided in its cylinder in a reliable manner so as to be stable in terms of tilting. At the same time, there is the object of reliably sealing a pressurized actuating chamber which is delimited by the actuating piston in the actuating cylinder.

Multi-part solutions for a sealing and guiding installation which meets the above-mentioned requirements for a positive-displacement machine having an adjustable displacement volume are known in the prior art. For example, the Bosch-Rexroth data sheet RDE 92003-84-P/03.2016 thus shows an axial piston pump in a swash-plate construction mode having an adjustable displacement volume and an adjustment installation in which a dual-action actuating piston is disposed in an actuating cylinder. The sealing of the actuating piston and the guiding of the latter in the actuating cylinder in terms of functioning herein takes place separately by way of individual elements which are received in an internal shell face of the cylinder and in a sealing and guiding manner contact the external shell face of the piston. The sealing element herein is “pressure-active”, that is to say that a lip which is deformable in a radially inward manner is pressed against the external shell face of the actuating piston by the actuating pressure prevalent in the pressurized actuating chamber. The guide element has a comparatively large axial extent such that the piston is imparted stability in terms of tilting. Alternatively, the sealing and guiding installation can also be disposed so as to be proximal to the piston or the rod, respectively. The respective elements are assembled so as to be disposed in suitable grooves which hold said respective elements in an axial manner. It is disadvantageous in a solution of this type that up to three elements per sealing and guiding installation have to be assembled, this meaning a high level of complexity in terms of material and assembly. Moreover, an assembly error is possible on account of the number of parts. Post-assembly calibration is necessary when sintered elements, in particular with a Teflon content, are used.

An actuating piston of the generic type for a hydrostatic adjustment installation is known from DE 10 2017 211 750 A1. The actuating piston has an external shell face that is encompassed by a sealing and guiding installation which is embodied as a sealing collar and by way of which the actuating piston is guided in an actuating cylinder so as to be stable in terms of tilting and displaceable in an axial manner, a gap between the actuating piston and the actuating cylinder being able to be sealed. The sealing and guiding unit is held on the actuating piston and is configured so as to be integral, therefore having a dual function, that is to say that the sealing and guiding unit guides the actuating piston in the actuating cylinder and seals the actuating piston in relation to the actuating cylinder.

The production of the above-mentioned sealing collar is described in DE 10 2017 219 361 A1. Accordingly, the sealing collar which configures a sealing and guiding unit is initially present as an annular disk and is then expanded by means of an expansion device, wherein the annular disk folds inward so as to assume a conical shape. The internal diameter of the sealing collar in further expansion stages is subsequently expanded until the desired internal diameter has been reached and the annular disk has been formed so as to assume the shape of a cone or a sleeve. The sealing collar is then applied to the actuating piston by means of a compression device, wherein the sealing collar is brought to the nominal diameter of the latter.

It is disadvantageous in the afore-described actuating pistons that the absorption of transverse forces on the actuating piston is restricted and the friction between the actuating piston and the actuating cylinder is increased.

SUMMARY

In contrast, the disclosure is based on the object of achieving an actuating piston in which the introduction of transverse forces is improved, and of achieving an adjustment installation having an actuating piston of this type.

The object is achieved by an actuating piston having the features described herein, or by an adjustment installation having the features set forth herein.

An actuating piston according to the disclosure for a hydrostatic adjustment installation, in particular for adjusting a displacement volume of a hydrostatic positive-displacement machine having a variable displaced volume, has an external shell face which is encompassed by a sealing and guiding unit. The sealing and guiding unit is disposed in a region of a circumferential recess of the external shell face of the actuating piston. The actuating piston by way of the sealing and guiding unit is able to be guided in an actuating cylinder of the hydrostatic adjustment installation so as to be stable in terms of tilting and displaceable in an axial manner. The sealing and guiding unit is conceived for sealing a gap between the external shell face of the actuating piston and an internal shell face of the actuating cylinder. According to the disclosure, the circumferential recess of the actuating piston is divided into two regions, wherein a first region is configured for absorbing transverse forces and a second region is configured for absorbing axial forces.

As opposed to the cited prior art, a base of the circumferential recess is configured such that protrusions are configured in the second region, while the first region is embodied in such a manner that the latter is optimized in terms of absorbing transverse forces. Moreover, particularly low-friction sealing and guiding is enabled on account of the subdivision into two regions. It is particularly advantageous that the hysteresis is substantially improved in comparison to the prior art.

The maximum diameter of the actuating piston incl. sealing/guiding is reduced in comparison to the prior art. The diameter of the high circumferential recesses of the second region is less; the compression tool can thus be smaller. After the calibration of the sealing/guiding element and after the relaxation of the sealing/guiding material a sealing diameter is created which is smaller in comparison to the prior art, this leading to substantially lower friction and thus to a reduced hysteresis.

The first region of the circumferential recess is preferably disposed on a side of the base of the circumferential recess that faces away from a pressurized chamber and is configured by a comparatively wide guide region for absorbing transverse forces, said guide region being radially elevated in relation to a base of the circumferential recess. On account of said guide region, the transverse force can be directed from the actuating piston into the housing bore by way of a large area. On account thereof, the load per area unit in the guide region is significantly reduced in comparison to solutions known from the prior art.

On account of the comparatively wide guide region in the first region, the radial deformation of the sealing and guiding unit under load and the wear during operation are significantly reduced in comparison to the known solutions from the prior art.

The radial dimension in the first region is preferably such that a centric gap of 0.01% to 0.25% of the actuating piston diameter is created after the sealing and guiding unit has been applied.

The second region is preferably disposed on the side of the circumferential recess that faces the pressurized chamber.

In one refinement of the disclosure, the axial extent of the guide region in the first region is larger by a multiple than the axial extent of the protrusions in the second region.

It has proven particularly advantageous for the sealing and guiding unit to be configured so as to be integral.

The protrusions in the second region are advantageously configured in a stepped manner. In other words, a first protrusion which is closest to the first region, when viewed in the radial direction of the actuating piston, is substantially smaller than a last protrusion which is closest to a pressurized-chamber-proximal end of the circumferential recess.

Proceeding from a separation region between the first and the second region, the diameter of the protrusions preferably increases toward the pressurized-chamber-proximal end of the circumferential recess.

The sealing and guiding unit at least in portions preferably engages directly in regions of the circumferential recess. The sealing and guiding unit engages in a form-fitting manner in at least one of the protrusions of the second region, in particular in all protrusions, and particularly preferably not in the guide region in the first region. The protrusions thus partially engage behind a circumferential face of the sealing and guiding unit, said circumferential face facing the external shell face of the actuating piston, in that said protrusions are pushed into the sealing and guiding unit and/or engage in recesses of the sealing and guiding unit, said recesses being disposed so as to be opposite the protrusions.

In one refinement the protrusions are configured in such a manner that said protrusions absorb axial forces acting on the sealing and guiding unit, thus guaranteeing sealing between the actuating piston and the actuating cylinder. End face portions of the protrusions engage in the sealing and guiding unit and thus absorb the forces, in particular axial forces, acting on the sealing and guiding unit, and dissipate said forces into the actuating piston.

It is particularly preferable herein for the protrusion having the largest radial extent, when viewed from the actuating piston, hereunder also referred to as the largest protrusion, to invade the sealing and guiding unit by at least more than 40%, preferably by 42%, of the initial thickness of said sealing and guiding unit in the freshly assembled state.

On account of such a shaping of the second region which substantially absorbs axial forces, in conjunction with the afore-described design embodiments of the first region which substantially absorbs transverse forces, a gap in the sealing region between the actuating piston and the actuating cylinder is avoided when interacting with the sealing and guiding unit, said gap potentially leading to leakages. By reducing the deformation under load, for example by virtue of the acting transverse force, and by reducing the wear, it is also prevented that the largest protrusion on the side facing the pressure invades the sealing and guiding unit by more than 55% of the material thickness of the latter and thus damages said sealing and guiding unit.

In one refinement the rebound of the sealing and guiding unit for material-related reasons is embodied in such a manner that a low-friction sealing diameter is configured after the sealing and guiding unit has been applied, which is also referred to as calibrating. In terms of low friction between the actuating piston and the actuating cylinder it is relevant that the maximum diameter of the actuating piston in conjunction with the sealing and guiding unit is only slightly larger than the bore diameter of the actuating cylinder. Sufficient tightness in association with lower friction is thus achieved. This sealing diameter is, for example, at most 0.5% larger than a bore diameter of the actuating cylinder. The sealing and guiding installation is conceived such that the sealing and guiding diameters are created when calibrating at room temperature.

A calibration sleeve can be used for applying the sealing and guiding unit to the actuating piston. Said calibration sleeve is at least 0.3% smaller than a bore diameter of the actuating cylinder. In order for the material-friendly calibrating of the sealing and guiding unit at room temperature to be enabled, a guide diameter is formed mainly by the dimensions of the diameter of the external circumferential recess in the first region and the material thickness, or the wall thickness, respectively, of the sealing and guiding unit and not by the compression when calibrating. The guide diameter thus bears on the first region and is less than the sealing diameter in the second region.

It is particularly preferable for the diameter of the actuating piston in end portions of the first and the second region of the circumferential recess to be in each case smaller than in the guide region of the first region. A space which extends between the sealing and guiding unit and the actuating piston is thus configured between the sealing and guiding unit and the base of the circumferential recess. The internal stress in the material of the sealing and guiding unit draws the latter into the space thus created and on the ends of the sealing and guiding unit, i.e. the end portions that face the pressurized chamber and face away therefrom, forms in each case a chamfer, the latter causing the configuration of a lubricating wedge in the case of a moving actuating piston, this further reducing the friction between the actuating piston and the actuating cylinder.

