Improved differential shaft

Abstract

The present invention relates to a differential shaft (1) comprising a substrate having at least one flat surface (20) and including a coating layer deposited on said substrate. According to the invention, said machined flat surface (20) defines an edge (21) adjacent to a bearing surface (11) of said substrate, and said edge (21) has a radius of 0.05 mm or greater.

Description

[Technical Field]

[0001] The present invention relates to the technical field of motor vehicles, and in particular to differential shafts provided in motor vehicles. [Background technology]

[0002] In a motor vehicle, a differential allows wheels on the same axle to rotate at different speeds during cornering. The differential includes shafts drivingly mounted to differential pinions, each pinion meshing with a differential side gear rigidly connected to a driven shaft of an axle.

[0003] When a vehicle turns a corner, the differential side gears rotate at different speeds, while the differential pinions rotate freely about the differential shaft, which experiences significant friction during vehicle use and is particularly susceptible to wear.

[0004] To eliminate wear on the differential shaft or its associated differential pinion, it is known to provide flat surfaces on the shaft that migrate oil to the interface between the differential shaft and the pinion.

[0005] It is also known to deposit a friction-resistant coating on the outer surface of the differential shaft, which is intended to reduce wear on the shaft and its associated pinion.

[0006] Patent document 1 discloses the deposition of this type of anti-friction coating. However, anti-friction coatings are not always suitable for use on differential shafts because they tend to flake off, especially in the case of differential shafts with flat surfaces.

[0007] Furthermore, the advent of electric vehicles has significantly changed the conditions under which automotive gearboxes are used: electric motors behave quite differently from heat engines in that their nominal torque is reached at much lower motor speeds, which increases the dynamic loads on the components and causes the differential shaft to deteriorate more quickly. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2015 / 11361 Summary of the Invention [Problem to be solved by the invention]

[0009] SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an improved differential shaft which overcomes the above-mentioned drawbacks of the prior art and has improved durability against deterioration of the coating layer.

[0010] In this regard, a differential shaft has been developed that is constructed from a substrate having at least one machined flat surface and includes a coating layer deposited on the substrate. [Means for solving the problem]

[0011] According to the invention, the edge of the substrate is defined by a machined flat surface, adjoins the bearing surface and has a radius of at least 0.05 mm, preferably less than 5 mm. Preferably, said radiuses are tangent.

[0012] This allows the differential shaft shape to be optimized to have no sharp edges that would encourage peeling of the painted coating.

[0013] The proposed solution is simple and inexpensive, since only known and proven means are required to implement the method, eliminating the need for complex modifications that would otherwise be required, e.g., regarding the chemistry or deposition method of the coating.

[0014] Additionally, in order to further reduce stress concentrations occurring at the edges, the radius may be greater than 0.1 mm, preferably greater than 0.25 mm, more preferably greater than 1 mm, and even greater than 1.5 mm.

[0015] In one embodiment, the coating layer comprises amorphous carbon of the DLC type, which has been shown through trial and error to have excellent wear resistance and coefficient of friction.

[0016] The present invention relates to a method for manufacturing a differential shaft, which includes the steps of obtaining a cylindrical substrate, machining a flat surface, and depositing a coating layer on the substrate.

[0017] According to the invention, the method includes, prior to the depositing step, a step of grooving an edge defined by machining the flat surface.

[0018] This method allows for the groove machining step to achieve the desired radius, thereby providing a differential shaft with the above-mentioned advantages.

[0019] To promote adhesion of the deposited coating to the substrate, the method can include a first polishing step on the substrate prior to the depositing step, which also provides better adhesion to the material that forms the deposited coating.

[0020] Advantageously, said first polishing step also involves obtaining a roughness Ra of less than 0.1 μm.

[0021] To reduce the roughness peaks created by the depositing step, the method can include a second polishing step on the coating layer after the depositing step. If the coating peaks break off during use of the shaft, abrasive particles can be generated that can accelerate shaft degradation.

[0022] Preferably, the groove machining step and / or the first grinding step and / or the second grinding step are carried out by means of a centerless grinding wheel or a vibratory bowl, both of which grinding means are easy to implement.

[0023] In order to be able to parameterize the deposition of the coating layer in a simple manner, the coating layer is deposited by physical vapor deposition, preferably by plasma-enhanced physical vapor deposition. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 illustrates a vehicle differential in which the differential shaft, differential pinion, and differential side gears are visible. [Figure 2] FIG. 1 illustrates a prior art differential shaft in a deteriorated state. [Figure 3] FIG. 3 is an enlarged view of a deteriorated portion of the shaft in FIG. 2. [Figure 4] 4 is an enlarged view of a deteriorated portion of the shaft of FIG. 2, different from FIG. 3. FIG. [Figure 5] FIG. 1 is a plan view of a shaft according to the present invention without an anti-friction coating. [Figure 6] FIG. 1 is a front view of a shaft according to the present invention. [Figure 7] FIG. 3 is a cross-sectional view of the shaft as seen from the side. [Figure 8] FIG. 8 is a diagram showing an enlarged view of the cross section of FIG. [Figure 9] 9 shows a cross section similar to that of FIG. 8, illustrating a shaft according to the invention having a vapor-deposited anti-friction coating. [Figure 10] FIG. 2 is an enlarged view of the interface between the flat surface and the bearing surface of the differential shaft before being rounded. [Figure 11] FIG. 10 is an enlarged view of the interface between the flat surface of the differential shaft and the bearing surface after being rounded by grinding. [Figure 12]9 shows a cross section similar to that of FIG. 8, but showing a shaft according to the invention further comprising a chamfer between the flat surface and the bearing surface before being radiused. DETAILED DESCRIPTION OF THE INVENTION

[0025] 1, the present invention relates to a shaft 1 for a vehicle differential. It is known that the differential shaft 1, which supports two differential pinions S, frequently deteriorates.

[0026] Figure 2 shows a prior art differential shaft (1) having a flat surface (20) for locating an oil reservoir that interfaces with a friction surface between the cylindrical bearing surface (11) of the shaft (1) and a bore in the differential pinion (S). The shaft (1) is coated with a friction-resistant coating (30), such as a DLC-type amorphous carbon coating.

[0027] As can be seen in the figure, the shaft (1) has a torn and deteriorated coating (30) in the area of ​​the scratch (E).

[0028] 3 and 4, it can be seen that the material is torn off at the edge (21) where it meets the flat surface (20). These observations led applicant to investigate the nature and behavior of the coating (30) near the edge (21).

[0029] The machined flat surface (20) defines a sharp edge (21) at the junction with the cylindrical bearing surface (11) of the shaft (1), and the sharpness of the edge (21) is believed to promote peeling of the coating (30) and rapid deterioration of the shaft (1).

[0030] Additionally, scratches created during machining of the flat surface (20) exacerbate the problem.

[0031] 5 to 12, the crux of the present invention is to round off the sharp edges (21) that occur in the machined flat surface (20). Measurements of the machining scratches and material breaks yield values ​​between 50 μm and 100 μm. Therefore, the rounding must be a minimum of 0.05 mm. In this case, the edges (21) no longer show peeling from the substrate (10).

[0032] Depending on the roughness of the substrate (10) forming the differential shaft (1), the roundness can be as large as 0.1 mm, 0.25 mm, 0.3 mm, 0.5 mm, or even 1 mm or more, and the larger the roundness, the more easily it is possible to eliminate localized defects corresponding to larger roughness.

[0033] Selecting a radius of at least 0.1 mm eliminates possible deterioration due to machining scratches, while selecting a radius of at least 0.25 mm or 0.3 mm provides a safety margin against such deterioration.

[0034] In practice, the upper limit of the radius is not critical, as long as there is a dedicated area for oil to travel between the shaft (1) and the hole in the pinion (S). Good results were obtained with radiuses of 1.3 mm or 2.2 mm.

[0035] When achieving roundness by grinding, a larger value correlates with a longer grind, so a balance must be struck between the desired radius and the surface condition of the substrate (10) after machining of the flat surface (20).

[0036] In order to achieve the same polishing of both edges, particularly when polished using a grinding wheel and / or belt, it may be necessary to polish the part while rotating it in one direction about axis (a) and then polish the part while rotating it in the opposite direction.

[0037] When achieving roundness through CNC (computer numerical control) machining, the difficulty is related to the balance mentioned above.

[0038] Prior to depositing the coating 30, the substrate 10 may be polished to condition the surface, such that the substrate 10 has an arithmetic roughness Ra of 0.1 μm or less, preferably less than 0.07 μm, and more preferably less than 0.04 μm, a maximum roughness Rz of 0.1 μm or less, preferably less than 0.08 μm, and more preferably less than 0.04 μm, and a reduced peak height Rpk of 0.07 μm or less, preferably less than 0.05 μm.

[0039] Insofar as polishing reduces all roughness values, different roughness measurements are indirectly correlated with each other. However, different measurement methods do not reflect the same characteristics of the surface. The arithmetic roughness Ra reflects the average roughness of the substrate. The arithmetic roughness Ra is not significantly affected by abrasion or contamination. The maximum roughness Rz reflects the maximum magnitude between peaks and valleys on the surface. Rz depends on the peak value, and is therefore sensitive to both abrasion and contamination. The reduced height of the peaks Rpk reflects the presence of local peaks that tend to break off during the early stages of use of the shaft (1). This will generate abrasive particles at the interface. This value is suitable for evaluating friction and wear.

[0040] Therefore, creating a rounded edge by polishing can provide, in one step, the appropriate roughness of the substrate (10) for proper adhesion to the substrate (10) and sufficient roundness of the edge (21) to avoid material peeling or tearing.

[0041] Referring specifically to Figure 5, the edge 21 defined by the flat surface 20 includes two straight edge portions 21a parallel to the axis a of the differential shaft 1 and two elliptical edge portions 21b. A radius must be present at the boundary surface of the hole in the pinion S. Generally, this relates to the straight edge portions 21a, and preferably, the radius also exists at least tangent to the straight edge portions 21a.

[0042] FIG. 10 shows such a longitudinal edge (21a) before the radiusing is created.

[0043] Referring to Figure 12, the machined flat surface (20) has a chamfer with an angle (b). It should be noted that the area where deterioration of the shaft (1) occurs with use is the interface with the bore in the pinion (S). In all cases, edge (21a) is the intersection of the machined surface (flat surface (20) or chamfered surface) with the bearing surface (11).

[0044] Specifically, a polishing means such as a centerless grinding wheel or a vibrating bowl polishes the entire periphery of the edge 21, thereby providing an ideal roundness around the entire periphery of the edge 21. The centerless grinding wheel has a certain degree of flexibility, so that the centerless grinding wheel can surround the entire periphery of the edge 21 during polishing. The same applies to polishing using an abrasive belt.

[0045] Referring to FIG. 11, the rounded edges (21a) allow a gradual transition from the bearing surface (11) to the flat surface (20).

[0046] This prevents stress concentration inside the coating material, and also prevents the effect of the edge (21a) forming a bore in the pinion (S).

[0047] The interface (li) between the shaft (1) and the hole in the pinion (S) is located at the contact point between the radius of the edge (21a) and said bearing surface (11). In this view, the machining scratches present in the flat surface (20) are now recessed from the interface (li) and there is no longer any risk of tearing or peeling due to the shape and roughness of the scratches.

[0048] Figure 8 shows a cross section through the substrate 10 of the shaft 1 before the deposition process. The radii according to the invention can be seen in this figure, where the radii are tangent to minimize premature breakage.

[0049] The radius of the roundness is measured by profilometry by reconstruction of a circle (22) passing through the measurement point of the roundness.

[0050] Figure 9 shows a shaft (1) according to the present invention. The coating layer (30) is deposited on the substrate (10), preferably by vacuum deposition, optionally using plasma. This method allows for easy adjustment of the deposition chemistry by selecting the appropriate target and / or appropriate precursor gases. Furthermore, the properties of the deposited layer can be altered by adjusting the deposition and enhancement parameters.

[0051] The coating layer 30 preferably comprises amorphous carbon of the DLC type, although other coatings such as nickel electroplating are also contemplated, however DLC has the advantage of being more resistant to wear than other coatings.

[0052] The coating layer (30) is generally a few micrometers thick, typically between 0.5 μm and 5 μm, so that even for the finished shaft (1), when the radius of the edge (21) is measured by profilometry, it is possible to estimate with sufficient accuracy the radius of the substrate (10) before it is coated.

[0053] The shaft 1 and its manufacturing method may be designed differently from the above examples without departing from the scope of the claimed invention.

[0054] The radius can have different values ​​in different parts of the edge 21. For example, the left-hand longitudinal edge 21a of the flat surface 20 can have a radius of 1.9 mm, while the right-hand longitudinal edge 21a of the same surface can have a radius of 1.5 mm.

[0055] Furthermore, the technical features mentioned in the various embodiments and modifications above can be combined in whole or in part, so that the shaft 1 and its manufacturing method can be adapted in terms of cost, function, and performance. [Explanation of symbols]

[0056] 1 shaft 10 Base material 11 Bearing surface 20 flat surface 21 Edge 21a Straight edge section 21b Edge ellipse 22 yen 30 coating layers E Scratching part S Pinion

Claims

1. A differential shaft (1) comprising a substrate (10) having at least one machined flat surface (20), the substrate (10) including a coating layer (30) deposited on the substrate (10), The machined flat surface (20) is formed with an edge (21) that contacts the bearing surface (11) of the base material (10); A differential shaft (1) characterized in that the edge (21) has a roundness with a radius of 0.05 mm or more and preferably less than 5 mm.

2. the radius is greater than 0.1 mm, preferably greater than 0.25 mm, more preferably greater than 1 mm; A differential shaft (1) according to claim 1.

3. The coating layer (30) contains DLC type amorphous carbon. A differential shaft (1) according to claim 1 or 2.

4. Obtaining a cylindrical substrate (10); machining the flat surface (20); and depositing a coating layer (30) on the substrate (10), a step of machining the flat surface (20) into a groove at an edge (21) defined by machining the flat surface (20) prior to the step of depositing. A method for manufacturing a differential shaft (1).

5. The step of grooving includes grooving having a radius greater than 0.05 mm. The method of claim 4.

6. The method further comprises a first polishing step of polishing the substrate (10) before the depositing step. The method according to claim 4 or 5.

7. characterised in that the first polishing step comprises obtaining a roughness Ra of less than 0.1 μm. The method of claim 6.

8. a second polishing step of polishing the coating layer (30) after the depositing step; The method of any one of claims 4 to 7.

9. The groove machining step and / or the first grinding step and / or the second grinding step are carried out by means of a centerless grinding wheel or a vibrating bowl. The method of any one of claims 4, 6 and 8.

10. characterised in that the coating layer (30) is deposited by physical vapor deposition, preferably using plasma. The method of any one of claims 4 to 9.

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