Improved differential shaft

Abstract

The invention relates to a differential shaft (1) consisting of a substrate having at least one flat region (20) and comprising a coating layer deposited on the substrate. According to the invention, the machined flat area (20) defines an edge (21) adjoining the bearing surface (11) of the substrate, and the edge (21) has a rounded portion with a radius greater than or equal to 0.05 mm.

Description

Technical field

[0001] The present invention relates to the technical field of motor vehicles, and more particularly to a differential shaft provided in a vehicle. Background art

[0002] In a vehicle, a differential allows the wheels on the same axle to rotate at different speeds during turning. The differential includes a shaft pivotally mounted relative to differential pinions, each of which meshes with a differential side gear rigidly connected to a driven shaft of the axle.

[0003] When the vehicle makes a turn, the differential side gears rotate at different speeds, and the differential pinions rotate freely about the axis of the differential. Therefore, the axis of the differential is subject to significant friction during vehicle use; the axis of the differential is a particularly wear-prone part.

[0004] To compensate for wear on the differential shaft or corresponding differential pinions, it is known to provide a flat surface on the shaft for moving oil to the interface between the differential shaft and the pinions.

[0005] It is also known to deposit an anti-friction coating on the outer surface of the differential shaft. These anti-friction coatings are intended to reduce wear on the shaft and corresponding pinions.

[0006] The document WO201511361 in the name of the applicant discloses depositing this kind of anti-friction coating. However, anti-friction coatings are not always compatible with use on differential shafts because they may have a tendency to flake. This phenomenon is particularly evident in the case of differential shafts having flat surfaces.

[0007] In addition, with the emergence of electric vehicles, the operating conditions of vehicle transmissions are changing significantly. Electric motors behave very differently from heat engines, and the rated torque of an electric motor is reached at a very low motor speed. The dynamic stress on the parts thus increases, and the differential shaft deteriorates faster. Summary of the invention

[0008] Therefore, an object of the present invention is to propose an improved differential shaft that remedies the drawbacks of the prior art and improves the resistance to deterioration of the coating layer.

[0009] In this regard, a differential shaft has been developed that consists of a substrate having at least one machined flat surface and includes a coating layer deposited on the substrate.

[0010] According to the invention, the machined flat surface defines an edge adjacent to the bearing surface of the substrate, and the edge has a chamfer with a radius greater than or equal to 0.05 mm and preferably less than 5 mm. Preferably, the chamfer is tangential.

[0011] This optimizes the geometry of the differential shaft so that it does not have sharp edges that would cause the deposited coating to peel off.

[0012] The proposed solution is simple and inexpensive because implementing it only requires known and proven means and avoids complex improvements that may have been sought, such as chemical reactions or methods regarding the deposited coating.

[0013] Advantageously, and also for the purpose of reducing the stress concentration that may occur at the edges, the radius is greater than 0.1 mm, preferably greater than 0.5 mm, even more preferably greater than 1 mm or even greater than 1.5 mm.

[0014] In one configuration, the coating layer includes amorphous carbon of the DLC type. This type of coating has been tested and proven to produce good results in terms of coefficient of friction and wear resistance.

[0015] The present invention also relates to a method for manufacturing a differential shaft, the method comprising the following steps:

[0016] - Obtaining a cylindrical substrate;

[0017] - Machining a flat surface;

[0018] - Depositing a coating layer on the substrate.

[0019] According to the present invention, the method includes a step of grooving the edges defined by the machined flat surface before the deposition step.

[0020] This method enables the differential shaft with the aforementioned advantages to be obtained, and the grooving step provides the desired rounding.

[0021] To promote the adhesion of the coating deposited on the substrate, the method includes a first step of polishing the substrate before the deposition step. This method also gives better cohesion to the material forming the deposited coating.

[0022] Advantageously, the first polishing step includes obtaining a roughness Ra of less than 0.1 μm.

[0023] To reduce the rough peaks generated during the deposition step, the method includes a second step of polishing the coating layer after the deposition step. If the coating peaks peel off during the use of the shaft, this may generate abrasive particles that will accelerate the deterioration of the shaft.

[0024] Advantageously, the grooving step and / or the first polishing step and / or the second polishing step are carried out by means of a centerless grinding wheel or a vibratory bowl. Both of these polishing methods are easy to implement.

[0025] 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 enhancement. Description of the Drawings

[0026] Figure 1 A vehicle differential is shown, on which a differential shaft, differential pinions and differential side gears can be seen.

[0027] Figure 2 A prior art differential shaft in a deteriorated state is shown.

[0028] Figure 3 is Figure 2 an enlarged view of the deteriorated area of the shaft in

[0029] Figure 4 is Figure 2 another enlarged view of the deteriorated area of the shaft in

[0030] Figure 5 is a plan view of the shaft according to the invention, which has not yet received an anti-friction coating.

[0031] Figure 6 is a front view of the shaft.

[0032] Figure 7 is a cross-sectional view taken through the same shaft from the side.

[0033] Figure 8 shows Figure 7 an enlarged view of the cross-section in

[0034] Figure 9 shows Figure 8 a cross-section similar to the cross-section in

[0035] Figure 10 is an enlarged view of the interface between the flat surface and the bearing surface of the differential shaft before rounding is produced.

[0036] Figure 11 is an enlarged view of the interface between the flat surface and the bearing surface of the differential shaft after rounding is produced by polishing.

[0037] Figure 12 shows Figure 8 a cross-section similar to the cross-section in Detailed Description

[0038] Reference Figure 1, the present invention relates to a shaft (1) for a vehicle differential. It should be noted that the shaft (1) of the differential that supports two differential pinions (S) typically deteriorates.

[0039] Figure 2 A prior art differential shaft (1) is shown, on which a flat surface (20) is provided so that an oil groove is placed in communication with the friction surface located between the cylindrical support surface (11) of the shaft (1) and the holes in the differential pinions (S). The shaft (1) is coated with an anti-friction coating (30), such as a deposit of amorphous carbon of the DLC type.

[0040] It can be seen in this figure that the shaft (1) has deteriorated; the coating (30) has peeled off in the area of the scratch (E).

[0041] Reference Figure 3 and Figure 4 show that the material has peeled off in the area of the edge (21) adjacent to the flat surface (20). These observations have guided the applicant to study the nature and behavior of the coating (30) near the edge (21).

[0042] It seems that the machined flat surface (20) defines a sharp edge (21) at the junction with the cylindrical support surface (11) of the shaft (1), and the sharpness of the edge (21) has promoted the peeling of the coating (30), and thus has promoted the rapid deterioration of the shaft (1).

[0043] In addition, the scratches generated during the machining of the flat surface (20) have exacerbated this phenomenon.

[0044] Reference Figures 5 to 12 shows that the main key of the present invention is that the sharp edge (21) generated during the machining of the flat surface (20) is rounded. Measuring the phase of the machining scratches and material tearing gives values between 50 μm and 100 μm. Therefore, the rounding must be at least 0.05 mm. In this way, the edge (21) will no longer show peeling starting from the substrate (10).

[0045] Depending on the roughness of the substrate (10) forming the differential shaft (1), the rounding can have higher values, such as 0.1 mm, 0.25 mm, 0.3 mm, 0.5 mm or even greater than 1 mm; the larger the rounding, the more it will be able to compensate for local defects corresponding to significant roughness.

[0046] Selecting a rounding radius of at least 0.1 mm excludes deterioration that may result from machining scratches. Selecting a rounding radius of at least 0.25 mm or 0.3 mm can ensure a safety factor against such deterioration.

[0047] In practice, the upper limit of the chamfer value is not important; only a dedicated section should be left for the oil to travel between the holes in the shaft (1) and the pinion (S). Good results have been obtained with a chamfer of 1.3 mm or 2.2 mm.

[0048] In the case of obtaining the chamfer by polishing, higher values are closely related to longer polishing; therefore, a balance must be achieved between the surface state of the substrate (10) after machining of the flat surface (20) and the desired radius.

[0049] It may be necessary to polish the part in one rotational direction around the axis (a) of the part and then in the opposite rotational direction to polish both edges in the same way, especially when using a belt and / or grinding wheel for polishing.

[0050] When the chamfer is obtained by CNC machining (Computer Numerical Control), the difficulty associated with this balance is reduced.

[0051] The substrate (10) can also be polished before depositing the coating (30) in order to adjust its surface state. In particular:

[0052] - The arithmetic roughness Ra of the substrate (10) can be less than or equal to 0.1 μm, preferably less than 0.07 μm, and even more preferably less than 0.04 μm;

[0053] - The maximum roughness Rz can be less than or equal to 0.1 μm, preferably less than or equal to 0.08 μm, and even more preferably less than 0.04 μm;

[0054] - The reduced peak depth Rpk of the peaks can be less than or equal to 0.07 μm, preferably less than or equal to 0.05 μm.

[0055] The different roughness measurement results are indirectly related to each other because polishing reduces all the roughness values; however, different measurement methods do not reflect the same surface features:

[0056] - The arithmetic roughness Ra reflects the average roughness of the substrate. It is not significantly affected by scratches or contamination.

[0057] - The maximum roughness Rz reflects the maximum amplitude between the peaks and valleys on the surface. Due to its dependence on the peaks, it is very sensitive to scratches and contamination.

[0058] - The reduced peak depth Rpk of the peaks reflects the presence of local peaks that are easily shed during the initial stage of use of the shaft (1); this will generate abrasive particles at the interface. This value is suitable for evaluating friction and wear.

[0059] Therefore, generating the chamfer by polishing makes it possible to obtain the following effects in a single step:

[0060] - A suitable roughness of the base (10) to enable the deposit to adhere properly to the base (10); and

[0061] - A sufficient rounding of the edge (21) to avoid the phenomenon of material chipping or tearing.

[0062] More specifically referring to Figure 5 , the edge (21) defined by the flat surface (20) includes two straight portions (21a) parallel to the axis of rotation (a) of the differential shaft (1) and two elliptical portions (21b). The rounding must be present at the interface with the hole in the pinion (S); generally speaking, this concerns the straight portion (21a), and the rounding is also preferably present at least on the straight portion (21a).

[0063] Figure 10 This kind of longitudinal edge (21a) before the rounding is shown.

[0064] Referring to Figure 12 , the machined flat surface (20) can have a chamfer at an angle (b). It should be noted that the part of the shaft (1) that deteriorates during use is the interface with the hole in the pinion (S); in all cases, the edge (21a) is the intersection line between the machined part (flat surface (20) or chamfer) of the shaft (1) and the support surface (11).

[0065] In practice, polishing devices (such as centerless grinding wheels or vibratory bowls) polish the entire periphery of the edge (21), and thus provide the desired rounding around the entire periphery of the edge (21). Centerless grinding wheels have a certain flexibility, allowing them to surround the entire periphery of the edge (21) during polishing. This also applies to polishing using abrasive belts.

[0066] Referring to Figure 11 , the rounded edge (21a) enables a gradual transition from the support surface (11) towards the flat surface (20).

[0067] Therefore, stress concentration is avoided within the coating material. The influence of the machining of the edge (21a) on the hole in the pinion (S) is also avoided.

[0068] The boundary of the interface (li) between the shaft (1) and the hole in the pinion (S) is located at the tangency between the rounding of the edge (21a) and the support surface (11). It can also be seen in this figure that the machining scratches present on the flat surface (20) now recede from the boundary of the interface (li); due to the geometry or roughness of the scratches, there is no longer any risk of tearing or chipping.

[0069] Figure 8It is a cross-section of the substrate (10) passing through the shaft (1) before the deposition step. The rounding according to the present invention can be seen therein. In this figure, the rounding is tangential in order to minimize the initial fracture as much as possible.

[0070] The radius of the rounding can be measured by profilometry by reconstructing the circle (22) passing through the measurement points of the rounding.

[0071] Figure 9 The shaft (1) according to the present invention is shown. The coating layer (30) is preferably deposited on the substrate (10) by means of vacuum vapor deposition, optionally with plasma enhancement. This method makes it easy to adjust the chemical properties of the deposited material by selecting a suitable target and / or suitable precursor gas. In addition, adjusting the deposition and enhancement parameters allows the properties of the deposited layer to be modified.

[0072] The coating layer (30) preferably comprises amorphous carbon of the DLC type, but other coatings such as electroplated nickel can also be envisaged. However, the advantage of DLC is that it is more wear-resistant than other coatings.

[0073] The coating layer (30) is usually a few micrometers thick, generally between 0.5 μm and 5 μm. Therefore, even on the finished shaft (1), when measuring the radius of the edge (21) by profilometry, the rounding that the substrate (10) had before the coating step can be estimated accurately enough.

[0074] Without departing from the scope of the present invention defined by the claims, the shaft (1) and its production method can be designed in a different way from the examples given.

[0075] On different parts of the edge (21) of the shaft, the rounding can have different values. For example, the longitudinal edge (21a) located on the left-hand side of the flat surface (20) can have a rounding of 1.9 mm, while the longitudinal edge (21a) located on the right-hand side of the same flat surface (20) can have a rounding of 1.5 mm.

[0076] In addition, the technical features of the various embodiments and variants mentioned above can be combined wholly or only partly. Therefore, the shaft (1) and its production method can be adjusted in terms of cost, function and performance.

Claims

1. A differential shaft (1) consisting of a substrate (10) having at least one machined flat surface (20) and including a coating layer (30) deposited on said substrate (10), characterized in that, The machined flat surface (20) defines an edge (21) adjacent to the bearing surface (11) of the substrate (10), and the edge (21) has a chamfer with a radius greater than or equal to 0.05 mm and preferably less than 5 mm.

2. The differential shaft (1) according to claim 1, characterized in that, The radius is greater than 0.1 mm, preferably greater than 0.25 mm, and even more preferably greater than 1 mm.

3. The differential shaft (1) according to any one of the preceding claims, characterized in that, The coating layer (30) comprises amorphous carbon of the DLC type.

4. A method for producing a differential shaft (1), the method comprising the following steps: - obtaining a cylindrical substrate (10); - machining a flat surface (20); - depositing a coating layer (30) on the substrate (10); characterized in that the method comprises a step of grooving an edge (21) defined by the machined flat surface (20) before the deposition step.

5. The method according to claim 4, wherein The grooving comprises obtaining a chamfer with a radius greater than 0.05 mm.

6. The method according to claim 4 or claim 5, characterized in that, The method comprises a first step of polishing the substrate (10) before the deposition step.

7. The method according to claim 6, characterized in that, The first polishing step comprises obtaining a roughness Ra of less than 0.1 μm.

8. The method according to any one of claims 4 to 7, characterized in that, The method comprises a second step of polishing the coating layer (30) after the deposition step.

9. The method according to any one of claims 4, 6 or 8, characterized in that The grooving step and / or the first polishing step and / or the second polishing step are carried out by means of a centerless grinding wheel or a vibrating bowl.

10. The method according to any one of the preceding claims, characterized in that, The coating layer (30) is deposited by physical vapor deposition, preferably by plasma enhancement.

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