Hydrostatic piston engine, in particular hydrostatic axial piston engine

Abstract

Hydrostatic piston engine with a cylinder drum (4) in which a plurality of cylinder bores (7) are formed in which axially movable displacer pistons (8) are arranged, wherein the cylinder drum (4) is made of a steel without any heat treatment following its manufacture, wherein the steel is a tempered steel, wherein the displacer pistons (8) are made of steel and have a hard surface layer, wherein the displacer pistons (8) are nitrided or nitrocarburized, wherein the displacer pistons (8) are post-oxidized after nitriding or nitrocarburizing.

Description

[0001] The invention relates to a hydrostatic piston machine which can be used as a pump or as a motor and is designed in particular as a hydrostatic axial piston machine and which has a cylinder drum in which a plurality of cylinder bores are formed in which axially movable displacement pistons are arranged.

[0002] Hydrostatic axial piston pumps are known that use a cast iron cylinder drum and gas-nitrided pistons. To ensure this combination works well, a so-called compound layer (also called the white zone or white layer), also known as the VS for short, is created on the outermost surface of the cylinder bores of the cylinder drum through heat treatment, which usually consists of nitriding or nitrocarburizing. This layer consists of iron nitrides, and a so-called diffusion layer is created underneath. After heat treatment, the cylinder drums are checked for dimensional accuracy and, if necessary, remachined to ensure dimensional accuracy. If the compound layer is missing or is removed during remachining, seizure can occur between the displacement pistons and the cylinder drum, leading to the failure of the hydrostatic piston machine.

[0003] Hydrostatic axial piston pumps are also known, which use a sintered cylinder drum and gas-nitrided and post-oxidized displacement pistons. By using post-oxidized displacement pistons, a running-in process ensures the functionality of the piston machine. The sintered cylinder drums are not heat-treated and therefore do not have a bonding layer in the cylinder bores.

[0004] Hydrostatic axial piston pumps are also known, which utilize a steel cylinder barrel whose surface layers are hardened by nitriding, and nitrided and post-oxidized displacement pistons. This heat-treated cylinder barrel again has a bonding layer in the cylinder bores due to the nitriding process.

[0005] DE 28 12 417 A1 discloses a cylinder drum for a hydrostatic piston engine made of gas-nitrided steel. The displacement pistons run directly in the associated cylinder bores of the cylinder drum.

[0006] From DE 19 63 769 A an axial piston machine with a cylinder drum is known, the cylinder bores of which are tenifer-treated, the cylinder drum being made of steel or spheroidal cast iron.

[0007] EP 1 310 577 A1 discloses a fuel pump in the form of a piston engine. In the sliding pairing between the displacement piston and the cylinder bore, one sliding partner is to be nitrocarburized, while the other is to be coated with a "carbon group film," the latter being produced by heat treatment.

[0008] An axial piston machine with a cylinder drum is known from FR 1 159 428 A. The displacement pistons are provided with a metal coating, while the cylinder drum is made of steel.

[0009] The contact points between the displacement pistons and the cylinder barrel of a hydrostatic piston machine are tribologically complex contact points. These contact points also generate, among other things, the greatest mechanical and hydraulic losses during corner power. If these contact points do not function properly, this will very quickly lead to the failure of the hydrostatic piston machine. Any materials can be used for the contact points, i.e., the displacement pistons and the cylinder barrel, as long as proper function is to be ensured.

[0010] On the other hand, users are demanding ever-higher operating pressures to achieve higher performance in hydrostatic piston engines within a given installation space. Above certain operating pressures, currently used materials such as cast iron or sintered iron can no longer be used, as the materials can no longer withstand the high loads and fail.

[0011] The object of the invention is to create a hydrostatic piston machine of the type mentioned above with a well-functioning, cost-effective and low-friction pairing / combination of the materials of the displacement piston and cylinder drum.

[0012] This object is achieved according to the invention with a hydrostatic piston machine having the features of patent claim 1.

[0013] According to the invention, the cylinder drum is made of a tempered, i.e. relatively soft, steel.

[0014] According to the invention, the displacement pistons are heat-treated, namely nitrided or nitrocarburized, and then post-oxidized. They can, in particular, be gas nitrided.

[0015] According to the invention, the displacement pistons are made of steel.

[0016] It is also possible to provide the displacement pistons with a DLC layer (diamond-like carbon layer).

[0017] The cylinder barrel made of quenched and tempered steel can be used without further heat treatment and the creation of a hard surface layer on the cylinder bores. Quenched and tempered steel is steel that is first hardened and then tempered at temperatures of 500 degrees Celsius and above. After such treatment, the risk of cracking and brittle fracture is greatly reduced. Tempering hardened steel generally reduces hardness and strength, while increasing formability.

[0018] A steel grade that is well suited for the cylinder drum is, for example, steel 42 CrMoS4+QT, where the abbreviation QT stands for “quenched and tempered”.

[0019] Axial piston machines with a pair of post-oxidized displacement pistons and a relatively soft steel cylinder barrel have been tested over long operating times without failure. This innovative combination allows the cylinder bores to be manufactured very precisely and quickly in terms of dimensions and shape, as distortion of the cylinder bores due to the lack of subsequent heat treatment is avoided. Due to the precision of the cylinder bores in terms of dimensions and shape, good mechanical-hydraulic efficiency of the hydrostatic piston machine is achieved. The running-in behavior is improved. Eliminating heat treatment reduces manufacturing costs. Finally, the use of steel for the cylinder barrel enables high pressures.

[0020] An embodiment of the invention is illustrated in the drawing and described in more detail below. The sole figure of the drawing shows a longitudinal section through an adjustable hydrostatic axial piston machine with a swash plate design.

[0021] The axial piston machine shown has a drive shaft 1 which is rotatably mounted via two rolling bearings 2 and 3 in a housing which essentially consists of a housing pot and a connecting plate closing the housing pot.

[0022] The drive shaft 1 has a first shaft section which is radially enclosed by a recess of a cylinder drum 4.

[0023] A second shaft section of the drive shaft 1 has an external toothing area 5, on which the cylinder drum 4 is arranged with an internal toothing bore 6. The drive shaft 1 and the cylinder drum 4 are rotationally coupled to each other via the toothing.

[0024] The cylinder drum 4 has a plurality of cylinder bores 7 located on a common pitch circle, in each of which a piston 8 is slidably guided. These pistons 8, together with the cylinder bores 7, define working chambers 9, the volume of which changes with the stroke of the pistons.

[0025] A joint ball 10 of each displacement piston 8, remote from the working chamber 9, is pivotally connected to a piston guide shoe 11. This piston guide shoe 11 rests against a swash plate 12 pivotally mounted on the housing, also called a pivoting cradle due to its pivotability. The angle of the support surface 13, against which the piston guide shoes 11 rest and can slide, determines the piston stroke performed during one revolution of the cylinder drum 4 and thus the displacement volume (pump operation) or displacement volume (motor operation) of the machine.

[0026] A retraction ball 18 is arranged on the drive shaft 1, which acts on a retraction plate 19 in the direction of the pivoting cradle 12, wherein the retraction plate 19 in turn holds the piston sliding shoes 11 in contact with the support surface 13 of the pivoting cradle 12.

[0027] On the side facing away from the pivoting cradle 12, the cylinder drum 4 has an end wall 14 with which it rests on a rotationally fixed control plate 15 supported on the housing, which has a pressure kidney 16 and a suction kidney 17 on the same pitch circle as the cylinder bores 7. The pressure kidney 16 and the suction kidney 17 are connected to the high-pressure channel and the low-pressure channel of a connecting plate 21, respectively. In the figure, the pressure kidney and the suction kidney are drawn on the control plate for better understanding. In the section shown, which lies in a plane perpendicular to the pivot axis of the pivoting cradle and in which the axis of the drive shaft 1 lies.

[0028] The pivoting cradle 12 is adjustable by an adjusting device 20.

[0029] According to the invention, the cylinder drum 4 is made of a tempered steel. A steel grade well suited for the cylinder drum is, for example, steel 42 CrMoS4+QT, where the abbreviation QT stands for "quenched and tempered." The displacement pistons 8 are also made of a steel. After the final machining, the displacement pistons are gas nitrided, in particular gas nitrocarburized, and post-oxidized. Due to the post-oxidation, the top layer of the displacement pistons is an oxide layer.

[0030] In gas nitrocarburizing, the surface of the displacer pistons is enriched with nitrogen and carbon. This occurs in a gas atmosphere containing ammonia and carbon-releasing additive gases, at a treatment temperature of, for example, 570°C to 580°C.

[0031] The resulting nitrocarburizing layer consists of a bonding layer of, for example, about 20 µm and an underlying diffusion layer.

[0032] The displacer pistons are then post-oxidized in an oxygen-releasing medium. This creates a Fe304° oxide layer with a thickness of 1 to 4 µm, usually between 2 and 3 µm, in the outer region of the bonding layer.

[0033] A hydrostatic axial piston machine according to the invention is particularly advantageously used as a component of a hydraulic hybrid drive in a motor vehicle, in particular in a passenger car. In vehicles, small space requirements and low costs of the individual components while maintaining high performance are particularly important.

Claims

[1] Hydrostatic piston machine with a cylinder drum (4) in which a plurality of cylinder bores (7) are formed, in which axially movable displacement pistons (8) are arranged, wherein the cylinder drum (4) is made of a steel without any heat treatment following manufacture, wherein the steel is a tempered steel, wherein the displacement pistons (8) consist of steel and have a hard surface layer, wherein the displacement pistons (8) are nitrided or nitrocarburized, wherein the displacement pistons (8) are post-oxidized after nitriding or nitrocarburizing. [2] Hydrostatic piston machine according to claim 1, characterized by that it is an axial piston machine with a swash plate design. [3] Hydrostatic piston machine according to one of the preceding claims, wherein the cylinder drum is made of steel 42 CrMoS4+QT.

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