Expansible chamber devices – Piston
Patent
1998-04-28
1999-10-26
Denion, Thomas E.
Expansible chamber devices
Piston
92 71, 92127, F16J 100
Patent
active
059708450
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a piston for a hydrostatic piston machine, in particular of swash plate construction having a cylinder drum, axial cylinder bores arranged therein, and a swash plate on which the pistons are supported. The pistons consist of a piston shaft with a piston end face and a cylindrical skirt surface moving back and forth in the cylinder bore, and a piston head supported on the swash plate.
2. Discussion of the Prior Art
Such pistons for axial piston machines are known for example from DE-A 23 20 554 and DE-A 36 09 892. At least the end regions of the cylindrical skirt surface towards the end pressure surface of these pistons are chamfered or rounded for various reasons. It is known to the skilled person that with such machines the piston play and the kinematics of such machines inevitably lead to a changing oblique positioning of the piston in the cylinder bore, which brings about increased friction and wear, in particular in the case of soft cylinder material, e.g. bronze. During the piston movement there arises at the wall of the cylinder bore a chafing effect due to the end edges of the piston/skirt surface. So that no chafing effect arises during the piston movement at the wall of the cylinder bore, the cylinder bores are in this region partially recessed. In order to reduce the wear of the walls of the cylinder, with the pistons according to DE A 23 20 554 the inner end section of the piston is bevelled towards the end pressure surface. With this configuration with bevelling (chamfering) wear still occurs in the absence of a recessing of the wall of the cylinder bore, at the entry and exit point of the piston shaft, since the bevelling has edges of its own.
It has been determined that these edges of the bevelling, or also edges chamfered with uniform, small radius of curvature, do not avoid the mentioned chafing effect. This chafing effect prevents the build up of the leakage oil lubrication film needed to discharge the heat of friction, and tends to jamming of the pistons in the cylinder bores (drawer effect).
SUMMARY OF THE INVENTION
The object of the invention is to provide a piston for a hydrostatic axial piston machine with which the wear at the entry and exit point of the piston shaft is further reduced and at the same time the hydraulic-mechanical and volumetric efficiency of the hydrostatic machine is improved.
For attaining this object, the piston according to the present invention is characterised in that the skirt surface of the piston shaft has a bevel with continuously varied radius of curvature in its forward end region where it transforms into the piston end face and/or in its rear end region lying towards the piston head. Thereby, not only a reduction of the friction at the piston shaft ends is achieved in that the bevel profile is adapted to the different degrees of oblique positioning of the piston in its back and forth movement, but also a lesser tolerance for the permitted piston play can be assumed. The invention makes possible an improved lubricating film build-up for the discharge of the heat of friction of the cylinder bore walls. Finally, there is attained a surprising improvement of the hydraulic-mechanical and volumetric efficiency--up to 3% has been measured--in that the bevel profile enlarges the effective end pressure surface of the piston.
Expedient configurations are indicated in the subclaims.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary configurations of the invention are described below in more detail with reference to the drawings, which show:
FIG. 1 an axial piston machine of swash plate construction, schematically in section,
FIG. 2 schematically in a sectional view, cylinder drum, swash plate and pistons, with oblique positioning of the pistons in the cylinder bores illustrated greatly exaggerated,
FIG. 3 a piston with slipper, in section,
FIG. 4 the region Y in FIG. 3, as a detail to an enlarged scale,
FIG. 5 the region X in FIG. 3, as a detail to an enlarged scale,
FIG. 6 the region
REFERENCES:
patent: 4776260 (1988-10-01), Vincze
patent: 5070765 (1991-12-01), Parsons
Brueninghaus Hydromatik GmbH
Denion Thomas E.
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