Sliding surface

Abstract

For friction reduction in sliding bearing, it is known to structure the sliding surface (1) by ECM by introducing a plurality of microscopic small indentations (27). According to the invention it is proposed in particular in the same process step to smoothen also the intermediary spaces (6) between the indentations (27), thus to remove the tips of the surface profile.

Claims

1. A rotation symmetrical sliding bearing surface for a sliding movement along an opposite surface, wherein a surface of the sliding surface (1) is structured by microscopic indentations (27), which comprises: a roughness (Rz) is reduced compared to non structured portions in a structured portion (11) in entire intermediary spaces (5) between the indentations (27), wherein the roughness (Rz) in the portion between the indentations (27) increases with increasing distance from the indentations (27) when a distance (21) between two adjacent indentations (27) is either twice the size of the lateral extension of the electrical scatter field (29) of one of the protrusions (26) beyond an edge of the protrusion (26), or greater than the greatest extension (E) of the indentation (27) viewed in top view, otherwise the roughness (Rz) decreases inversely with increasing distance from the indentations (27).

2. The sliding surface according to claim 1, characterized in that the intermediary spaces (5) in the structured portion (11) between the indentations (27) have a roughness (Ra) of 0.2 m at the most and/or a roughness (R.sub.pk) of 0.16 m at the most.

3. The sliding surface according to claim 1, characterized in that the roughness (Rz) is at least 10% lower in the intermediary spaces (5) between the indentations (27) of the structured portion (11) than in the non-structured portion.

4. The sliding surface according to claim 1, characterized in that in the portion between the indentations (27), the surface has a roughness (Rz) which is less than the depth (t) of the indentations (27) and/or a contact portion of at least 50%.

5. The sliding surface according to claim 1, characterized in that the tips of the microscopic surface profile that are removed in the structured portion during reduction of the roughness (Rz) are respectively rounded enough so that the removed tips do not represent a plateau anymore, but a convex cambered surface.

6. A rotation symmetrical sliding bearing surface for a sliding movement along an opposite surface, wherein a surface of the sliding surface (1) is structured by microscopic indentations (27), which comprises: a roughness (Rz) is reduced compared to non structured portions in a structured portion (11) in entire intermediary spaces (5) between the indentations (27), wherein the intermediary spaces (5) in the structured portion (11) between the indentations (27) have a roughness (Ra) of 0.2 m at the most and a roughness (R.sub.pk) of 0.16 m at the most.

Description

IV. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(1) Embodiments of the invention are subsequently described in more detail with reference to drawing figures, wherein:

(2) FIG. 1a illustrates a top view of a structured portion of a sliding surface;

(3) FIG. 1b illustrates an enlarged representation of a bearing of a crank shaft;

(4) FIG. 2 illustrates a cut through indentations in the sliding surface;

(5) FIG. 3 illustrates an enlarged view of the tool used at the sliding surface.

(6) Friction in a hydro dynamic sliding bearing in which a lubricant, typically oil is arranged between two sliding surfaces of the tribological pairing, wherein the lubricant is distributed over the sliding surface through the relative motion of the sliding surfaces and forms a sliding film in the bearing gap facilitates reducing friction when indentations 27 are introduced into the sliding surface 1 as illustrated in FIG. 1a in a top view of sliding surface 1. Depending on the lubricant used, the material and surface properties of the sliding surface and a plurality of additional parameters this effect can be reinforced by an optimized shape, size, depth, distance and other parameters of the indentations 27.

(7) FIG. 1b illustrates a typical application of a structured sliding surface 1 represented by a bearing of a crank shaft or workpiece 2 in which typically indentations 27 are only introduced in a structured portion 11, namely in a circumferential portion 11a and typically also only in a particular width portion 11b of the total width of the bearing 1.

(8) In order to be able to produce such indentations 27 in the range with a defined shape, size, depth and distance from each other in a reproducible manner in a large economical quantity electrochemical manufacturing ECM will be used.

(9) As illustrated in FIG. 3, thus an electrode which typically represents the negative shape of the sliding surface 1 to be produced thus which has protrusions 26 at its effective surface is brought into a very close distance of a few m from the sliding surface 1 that is to be processed. Metal ions are released from the surface of the work piece through an electrical current flowing from the tool 25 to the work piece 2 through an electrically conductive fluid 4, the electrolyte in the operating gap 3 there between and the protrusions of the tool 25 are imaged as indentations 27 onto the surface 1 of the work piece 2.

(10) The surface portion of the indentations within the structured portion should thus be in a range of 15% to 40%.

(11) The surface portion of the intermediary spaces 5 between the indentations 27 in the structured portion is thus significantly larger than the surface portion of the indentations 27.

(12) The friction reducing effect of the indentations 27 is caused by the depot effect of the lubricant in that the lubricant is pulled out of the indentations 27 due to the plurality of indentations 27 which have a small absolute distance from each other in particular at a beginning of the relative movement in the sliding bearing and wherein the lubricant is distributed in the intermediary spaces 5 between the indentations 27 in the bearing gap. As illustrated in FIG. 2 at least the flank 9 of the indentation through which the lubricant is pulled out during operations of the sliding bearing is configured slanted. Typically in particular when there are circular indentations 27 all flanks 9 are configured identical and have a round shape at a transition to the intermediary spaces 5.

(13) Since the intermediary spaces 5 between the indentations 27 still primarily bear the mechanical load of the sliding bearing. The load bearing capacity of the bearing and other properties depend from the microscopic configuration of the surface in the intermediary spaces 5, in particular their roughness and contact portion as illustrated in FIG. 2 in the enlarged cut out.

(14) This was accomplished so far by respective mechanical processing by grinding and finishing until the desired roughness and in particular a sufficient contact portion were provided and the indentations 27 were subsequently introduced into the surface.

(15) According to the invention introducing the indentations 27 by ECM is used in particular simultaneously to reduce the roughness Rz in the intermediary spaces 5 between the indentations 27.

(16) FIG. 3 illustrates an option how this can be achieved by a respective configuration of the tool 25 for electrochemical material removal.

(17) For this purpose the protrusions 26 formed on the operating surface of the tool 25 which are to be imaged into indentations 27 in the surface of the tool 2 have a substantially greater height h than a depth t of the indentations 27 to be produced therewith.

(18) This has the effect that even for maximum approximation of the protrusions 26 to the surface of the work piece 2 even when the indentations 27 have already been partially imaged therein, also into the indentations 27 as illustrated in the left image half of FIG. 3, a much larger distance 3 remains between the protrusions 26 between the tool 25 and the work piece 2, than in the portion of the protrusions 26.

(19) Since the material removal effect of electro chemical removal is among other things a function of the size of this distance 3 the material removal through the current flowing in the intermediary spaces 5 between the protrusions 26 from the tool 25 to the work piece 2 is lower than in the portion of the protrusions 26 but still provided.

(20) The controlled selection of the distance 3 during maximum approximation of tool 25 and work piece 2, typically during electro chemical removal and oscillation approximation of tool and work piece is used, is coupled with a synchronous pulsating current loading, the amount of material removal in the gaps 5 can be predetermined.

(21) The material removal in this portion can thus be adjusted so that only the tips are removed from the microscopic surface profile of the surface of the work piece, thus in particular the removed tips have a convex, in particular semi spherical contour, thus according to the enlarged representation in FIG. 2 the contact portion is increased and the roughness Rz and/or Ra is reduced.

(22) In this context other factors are worth considering.

(23) On the one hand side the current flow between the tool 25 and the work piece 2 does not only occur perpendicular to the macroscopic contact plane between both components but from the corners of the protrusions 26 the current also flows in a directed manner perpendicular to the surface of the tool 25, for example from its protrusions 26 in the form of a so called scatter field 29 and reaches the surface of the work piece 2, thus also in the corner portions 6 of the intermediary spaces 5 as illustrated in FIG. 2 in the right image half. Since the distances 21 between the indentations 27 as illustrated in FIG. 1a typically amount to many times the diameter of the indentations 27 the entire surface of the intermediary spaces 5 is not processed that way.

(24) Simultaneously a current flows from the flat portions of the effective surface 24 of the tool, thus in the portion between the protrusions 26 in a direction of the intermediary spaces 5 onto the surface of the work piece 2 and overlaps in the edge portions 6 of the intermediary spaces 5 with the scatter filed 29 from the protrusions 26.

(25) In case excessive material is thereby removed in the edge portions 6 this can be mitigated by a specific configuration of the effective surface 24 of the tool 25 in the portion between the protrusions 26, for example in that the effective surface 24 in the edge portion 6 is lowered around the protrusions 26 and includes a convex bulge 7 in the center portion there between.

(26) It has to be furthermore considered that the microscopic structure of the surface of the tool 25 is imaged on the surface of the work piece 2 also in a portion between the protrusions 26, though with a slightly reduced imaging precision due to the larger distance.

(27) Therefore care has to be taken that also the microscopic structure of the surface of the tool 25 in view of the provided imaging precision has predetermined parameters with respect to roughness and contact portion, thus in particular in the portion between the protrusions 26 as illustrated in FIG. 3.

(28) Namely excessive roughness in these portions of the effective surface 24 of the tool 25 would generate a surface structure with insufficiently low contact portion and impermissibly high roughness through the electro chemical material removal in a portion where the load bearing capability of the sliding bearing namely in the intermediary spaces 5 of the surface of the work piece 2 depends from a sufficient contact portion and a required maximum roughness.

REFERENCE NUMERALS AND DESIGNATIONS

(29) 1 bearing, sliding surface 2 crankshaft, work piece 3 distance, operating gap 4 fluid, electrolyte 5 intermediary space 6 edge portion 7 convex bulge 8 round surface 9 flank 10 axial direction, rotation axis 11 structured portion 11a circumferential portion 11b width portion 24 effective surface 25 tool, electrode 26 protrusion 27 indentation 28 movement direction, rotation direction 29 scatter field t depth h height