DE102013213041A1 - A method of reducing white etching crack susceptibility of a bearing element - Google Patents

Abstract

A method is disclosed for reducing susceptibility to white etching cracks in a bearing element, which can in particular be a swivel cradle, a bearing shell or a rolling element between the swivel cradle and the bearing shell. The bearing element here has a bearing material that can be brought into contact via its bearing surface with another bearing element for the purpose of transmitting bearing forces. In particular, a rolling contact with possibly additional slip components is provided here. In the method, the bearing material is heat-treated in a region of the bearing element that is spaced apart from the bearing surface. As a result of the heat treatment, a defined nitrogen content is set in the bearing material in this area through nitrogenization. Additionally or alternatively, the heat treatment can set a comparatively low hardness in the area. Furthermore, a bearing element is disclosed which is produced using the method shown. Furthermore, an axial piston machine with such a bearing element is provided.

Description

The invention is based on a method for reducing a white etching crack (WEC) susceptibility of a bearing element. Furthermore, the invention relates to a bearing element and an axial piston machine with such a bearing element.

In the case of bearing elements, for example in the automotive sector (wheel bearings), in transmissions of wind power plants, in hydraulic machines (axial piston machines, injection pumps) or in rail technology (railroad tracks), damage has been consciously perceived by so-called white-etching cracks for about 15 years. Such WECs are for example in the document EP 2 573 195 A1 described. These are damage to bearing elements that form in the metallic bearing material and become visible through metallographic white etching areas. These occur in the vicinity of discontinuities, for example inclusions under an upper or bearing surface in the region of a rolling contact of the bearing element. These WECs are also referred to as White Etching Areas (WEA), White Structure Flaking (WSF) or Butterfly, for example. In the event of damage, an expected service life of the bearing element decreases such that it can amount to only about 10% of the usually expected service life. A mechanism of WEC damage is not well known in the cause and not sufficiently researched. Various macroscopic influences are described.

The EP 2 573 195 A1 proposes to increase a WEC robustness of a rolling bearing to provide a bonding layer on the bearing surface of the bearing. The connecting layer in this case has a lower yield stress than the rest of the material of the rolling bearing. This is done by heating the rolling bearing at a certain temperature in a certain time and bringing the bearing surface into contact with a chemical additive. The chemical additive is, for example, a gear oil with a water content of 500 ppm.

In contrast, the invention has for its object to provide an alternative method for reducing a WEC susceptibility of a bearing element and thus to increase the robustness of the bearing element. Another object of the invention is to provide an over WECs more robust bearing element. In addition to be provided with a robust storage with the invention, an axial piston.

This object is achieved in terms of the method according to the features of claim 1, with regard to the bearing element according to the features of claim 13 and with respect to the axial piston machine according to the features of claim 14.

Other advantageous developments of the invention are the subject of further subclaims.

According to the invention, a method for reducing a white etching crack (WEC) susceptibility of a bearing element is provided. The bearing element in this case has a bearing material and a bearing surface, via which the bearing element can be brought into contact with another element for transmitting bearing forces, wherein the bearing surface is, in particular, a roller bearing surface which has a rolling load, optionally with additional slip portions, and others Operating stresses, such as vibration, may be exposed in rolling contact. In the method, the bearing material is heat-treated in a region of the bearing element which is at a distance from the bearing surface. By the heat treatment, a defined nitrogen content is set by nitriding in the bearing material. Alternatively or additionally, by means of the heat treatment, a hardness of, for example, 57 to 61 HRC, which is comparatively low for rolling bearing elements, can be set in the area of the bearing element which is spaced from the bearing surface. Typical hardnesses for rolling bearing elements are 60 to 64 HRC.

The method according to the invention relates to a targeted influencing of the atomic formation mechanism of WECs. It has surprisingly been found that when using the bearing elements, which are treated by the method according to the invention, a formation of WECs is prevented or at least significantly delayed. Thus, by preventing or delaying the occurrence of WECs, the reduction in the life of the bearing member caused by WEC damage is advantageously prevented or reduced. Furthermore, the life of bearing elements with potential WEC risk increases and reduces the probability of failure of the bearing elements. The setting of a comparatively low hardness in the region below the bearing surface advantageously leads to a more favorable local plasticizing capability of an atomic lattice of the bearing material.

On the other hand, it has been found that, on the one hand, nitriding on the one hand and the setting of a low surface hardness, on the other hand, can independently of one another bring about a contribution against the occurrence of WECs. However, surprisingly, in a combination of the nitriding with the setting of the lower hardness occurs a greater effect against the occurrence of WECs as a single measure.

As bearing material in particular iron materials, such as, for example, tempered steels, case hardened steels, nitriding steels, free cutting steels, stainless steels, microalloyed steels, steel and cast iron, etc., are considered in a wide variety of structural states.

The bearing element can be a typical bearing component, such as a rolling element or a ring, or a component in which the function provides rolling contact, optionally with additional slip component, such as drive shafts, camshafts or gears. As a bearing element also near-net prefabrication parts of bearing elements can be considered.

Nitrous oxide is a thermochemical treatment for nitrogen enrichment.

According to the invention, a bearing element is provided of a bearing material which can be brought into contract via its bearing surface with another element for transmitting bearing forces. The bearing material is in this case heat treated in a space spaced from the bearing surface area of the bearing element. In the heat treatment, a defined nitrogen content is set by nitriding in the bearing material. Additionally or alternatively, the heat treatment sets a comparatively low hardness in the area of the bearing element which is spaced from the bearing surface.

The area of the bearing element which is at a distance from the bearing surface is preferably the WEC-critical load range. The nitriding thus takes place at least in this WEC-critical load range. It is conceivable that the nitriding can also take place in areas of the bearing element which are not critical for the formation of WEC. Furthermore, it is conceivable that for smaller bearing elements, the nitriding may affect the entire bearing element.

In a further embodiment of the invention, the nitrogen content is adjusted in terms of quantity and position such that in the region of the bearing element which is to be embroidered, chromium nitrides and / or other nitrides selectively precipitate as submicroscopic particles within the atomic lattice of the bearing material or the metal matrix.

Preferably, the enrichment of the bearing element with nitrogen by carbonitriding occurs within or subsequent to a carburizing of the bearing material or by mere nitridation when using a material having a corresponding carbon content. When carburizing, the bearing element is provided with a higher carbon content, wherein the carburizing is a thermochemical process. If the carbon is enriched with nitrogen, carbonitriding is used in the carburizing process.

Preferably, the nitriding of the bearing element takes place above the austenitizing temperature of the bearing material in a nitrogen-emitting atmosphere which is decarburizing, carburizing or carbon neutral. This is also called gas nitridation.

Alternatively, it is conceivable that the nitriding of the bearing element takes place above the austenitizing temperature in nitrogen-giving molten salts which are decarburizing, carburizing or carbon neutral. This is commonly referred to as Salzbadaufstickung.

As a further alternative, it is conceivable that the nitriding of the bearing element takes place above the austenitizing temperature in a nitrogen-emitting plasma which is decarburizing, carburizing or carbon neutral. This is commonly referred to as plasma nitriding.

Advantageously, the nitriding of the bearing element can be carried out above the austenitizing temperature of the bearing material before hardening or bainitization.

Alternatively, the nitriding of the bearing element can also take place by means of a nitriding or nitrocarburizing process below the austenitizing temperature of the bearing material. After such nitriding, austenitization may be carried out followed by hardening or bainitization.

Advantageously, a hardness or surface hardness of the bearing surface of the bearing element is approximately in the range of 56 to 62 HRC or the same range of a comparable hardness scale. Preferably, the range can also be selected to be smaller and set, for example, between 56 to 59.9 HRC.

The adjustment of the hardness of the bearing surface of the bearing element is preferably also carried out by a heat treatment, such as tempering.

The bearing element is preferably a roller bearing element of a rolling bearing, in which case the bearing surface is a roller bearing surface. The bearing element may be a machine element with a rolling bearing surface, roll on the rolling elements directly along. In this case, the rolling elements in another machine element be stored over a plain bearing. An example of this is a hydrostatic radial piston engine in which rollers are slidably mounted in radial pistons that roll along an outer lift curve. However, the rolling elements can also roll along the further machine element - then this machine element would at the same time also be a bearing element - or roll along a bearing element inserted into it. An example of the latter would be the rolling bearing of the adjustable swash plate of a hydrostatic axial piston machine. There, the rolling elements can roll on the one hand directly on the swash plate and on the other hand on a bearing shell used in the housing of the axial piston. It can also be the rolling elements according to the invention bearing elements. Of course, the term bearing element and the outer ring or the inner ring or each individual rolling element of a conventional rolling bearing to understand.

According to the invention, an axial piston machine is provided, which has a pivoting cradle, which is mounted on rolling elements in a bearing shell. The pivoting cradle and / or one or more rolling elements and / or the bearing shell are in this case designed according to the bearing element according to the invention. Due to the or the treated bearing elements / bearing element such axial piston machine is extremely low maintenance and robust.

In the following, a preferred embodiment of the invention is explained in more detail with reference to drawings. Show it:

1 in a longitudinal section an axial piston machine according to the invention with a bearing element according to the invention,

2 in a further longitudinal section of the axial piston machine 1 and

3 in a diagram, a concentration ratio of nitrogen and carbon in a bearing element treated according to the invention.

According to 1 has the axial piston machine 1 a cylinder drum 2 on a shoot shaft 4 arranged. About a first rolling bearing 6 and a second, not shown rolling bearing is the drive shaft 4 stored in a machine housing, not shown. The axial piston machine is well known in terms of their construction from the prior art, see, for example, document DE 34 42 391 C1 Therefore, reference is made to the cited document for additional information regarding the basic structure.

In the cylinder drum 2 are a variety of cylinder bores 8th introduced, in which axially displaceable piston 10 are stored. The pistons 10 rely on sliding shoes 12 on a pivoting cradle 14 from. The pivoting cradle 14 in turn is about rolling elements 16 swiveling in two bearing shells 18 stored, which in turn are arranged in the machine housing, not shown. For pivoting the pivoting cradle 14 is a lever 20 provided, which cooperates with an adjusting device, not shown.

The sectional plane of the longitudinal section to 2 extends at a parallel distance to one of a longitudinal axis of the drive shaft 4 in 1 and the surface normal of the drawing plane of the 1 spanned level. The rolling elements 16 according to 2 have an approximately circular cylindrical bearing surface 22 , which is a rolling bearing surface. The rolling elements 16 be over a cage 24 held and guided. The pivoting cradle 14 then supports itself on the rolling elements 16 about two cross-sectionally approximately circular arc configured bearing surfaces 26 and 28 off, see 1 , Every bearing shell 18 also has two in cross section circular arc-shaped bearing surfaces 30 and 32 on. The pivoting cradle 14 , Rolling elements 16 and one bearing shell each 18 then form a rolling bearing 34 ,

Between the bearing elements 14 . 16 and 18 of the rolling bearing 34 occurs a rolling stress, with slip components can occur in the rolling contact. To prevent WEC formation, the bearing elements 14 . 16 and 18 of the rolling bearing 34 subjected to a heat treatment according to the invention. The pivoting cradle 14 and / or the rolling elements 16 and / or the bearing shell 18 are here in the critical for the WEC formation load range below its storage area 26 . 28 respectively 22 respectively 30 . 32 heat treated so that a defined nitrogen content is set by nitriding. In the nitriding thus the bearing material of the pivoting cradle 14 and / or the rolling element 16 and / or the bearing shell 18 Nitrogen supplied. This is according to 3 seen.

According to 3 is one weight percent of nitrogen (N) and carbon (C) of the swing cradle 14 from the 1 and 2 shown. On the abscissa of the diagram in the 3 This is a distance to the surface or storage area 26 . 28 mapped, where zero is the surface of the bearing surface 26 . 28 should be. The distance is in this case approximately in the normal direction to the bearing surface 26 . 28 measured and is given in mm. The ordinate shows weight percentages of nitrogen and carbon in the bearing material. The in the 3 lower curve 36 It can be seen that the weight fraction of nitrogen, starting from the bearing surface, where it is approximately between 0.3 and 0.4%, increases with increasing distance to the bearing surface, initially at approximately 0.3 to 0, 4% remains and afterwards From about 0.25 mm to about a distance of 1 mm to the bearing surface falls far below a weight fraction of 0.1%. According to 3 is an area 38 drawn, which is a WEC critical load range of the pivoting cradle 14 out 1 is. The area extends at a distance from the storage area 26 . 28 between about 0.05 to about 0.25 mm. In this WEC critical load range, in the present embodiment, the highest concentration ratio of nitrogen is approximately 0.4%.

The WEC critical load range may be present at other bearing elements at different distances to the surface of the rolling contact.

According to 3 is also the weight fraction of carbon (C) from the curve 40 in the swivel cradle 14 out 1 shown. The proportion by weight of the carbon is in the range of 0.6 to 0.7% up to a distance of about 0.6 mm. Thereafter, it falls with increasing distance to the storage area to about 0.22%.

Additionally or alternatively to the nitriding can in the WEC critical load range 38 out 3 the pivoting cradle 14 and also in the WEC-critical load ranges of the rolling elements 16 and / or the bearing shell 18 be adjusted by heat treatment a comparatively low hardness. This has also been shown to retard or prevent the formation of WECs in the WEC critical load ranges.

If a heat treatment at least of the WEC critical load range of the pivoting cradle 14 , the rolling element 16 and / or the bearing shell 18 With the nitriding in combination with the setting of a lower hardness, it has been shown that this leads to an increased improvement of the resistance to WECs as the nitriding or the setting of the relatively low hardness alone. It should be noted that the nitriding and the setting of the relatively low hardness in a combination of the two heat treatments need not be done simultaneously.

Disclosed is a method for reducing a white-etching cracks vulnerability of a bearing element, which may be in particular a pivoting cradle, a bearing shell or a rolling element between the pivoting cradle and the bearing shell. The bearing element in this case has a bearing material which can be brought over its bearing surface with another bearing element for transmitting bearing forces in contact. In this case, a rolling contact with optionally additional slip portions is provided in particular. In the method, the bearing material is heat-treated in a region of the bearing element which is at a distance from the bearing surface. Due to the heat treatment, a defined nitrogen content in the bearing material in this area is set by nitriding. Additionally or alternatively, a comparatively low hardness can be set by the heat treatment in the area.

Furthermore, a bearing element is disclosed, which is produced by the indicated method. Furthermore, an axial piston machine is provided with such a bearing element.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2573195 A1 [0002, 0003]
• DE 3442391 C1 [0030]

Claims (14)

Method of reducing a white etching crack (WEC) susceptibility of a bearing element ( 14 . 16 . 18 ), which has a bearing material and that via its bearing surface ( 22 . 26 to 32 ) with another bearing element ( 14 . 16 . 18 ) is brought into contact for the transmission of bearing forces, with the step: - heat treatment of the bearing material in one to the bearing surface ( 22 . 26 to 32 ) spaced area ( 38 ) of the bearing element ( 14 . 16 . 18 ), wherein in the heat treatment in the bearing material in the area ( 38 ) a defined nitrogen content is set by nitriding, and / or wherein by the heat treatment a comparatively low hardness in the range ( 38 ) is set. Method according to claim 1, wherein the area ( 38 ) is a WEC critical load range. Method according to claim 2, wherein additionally a WEC-uncritical load range or the entire bearing element ( 14 . 16 . 18 ) is subjected to the heat treatment. Method according to one of claims 1 to 3, wherein an adjustment of the nitrogen content in terms of quantity and position takes place such that chromium nitrides and / or other nitrides are precipitated as particles within a lattice of the bearing material. Method according to one of claims 1 to 4, wherein the nitriding takes place together with a carburizing by carbonitriding the bearing material or subsequently to the carburizing by carbonitriding the bearing material. Method according to one of claims 1 to 4, wherein the nitriding above an austenitizing temperature of the bearing material takes place in nitrogen-providing and in decarburizing, carburizing or carbon neutral atmospheres or molten salts. Method according to one of claims 1 to 6, wherein the nitriding above the Austenitisierungstemperatur of the bearing material takes place before curing or bainitization. A process according to any one of claims 1 to 4 wherein the nitriding is carried out below the austenitizing temperature of the bearing material by a nitriding or nitrocarbonating process. A method according to claim 8, wherein after the nitriding an austenitization followed by either a hardening or bainitization. Method according to one of the preceding claims, wherein a hardness of the bearing surface ( 22 . 26 to 32 ) is set to 56 to 61 HRC or the same range of a comparable hardness scale, or to a smaller range between 56 and 61 HRC. Method according to one of the preceding claims, wherein an adjustment of the hardness of the bearing surface ( 22 . 26 to 32 ) by a heat treatment. Method according to one of the preceding claims, wherein the bearing element ( 14 . 16 . 18 ) a roller bearing element of a rolling bearing ( 34 ). Bearing element that has a bearing surface ( 22 . 26 to 32 ) with another bearing element ( 14 . 16 . 18 ) can be brought into a rolling contact for transmitting bearing forces, characterized in that the bearing element ( 14 . 16 . 18 ) is treated according to the method according to one of the preceding claims. Axial piston machine with a pivoting cradle ( 14 ), which via rolling elements ( 16 ) in a bearing shell ( 18 ) is mounted, wherein a rolling element ( 16 ) and / or the pivoting cradle ( 14 ) and / or the bearing shell ( 18 ) is a bearing element / bearing elements according to claim 14 is / are.

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