Improved differential shaft

Abstract

The present invention relates to a differential shaft (1) comprising a substrate having at least one flat surface (20) and a coating layer deposited on the substrate. According to the present invention, the machining of the flat surface (20) defines an edge (21) that forms a boundary with the bearing surface (11) of the substrate, and the edge (21) has a rounding with a radius of 0.05 mm or more.

Description

Technology Field

[0001] The present invention relates to the field of technology for differential shafts equipped in automobiles, particularly vehicles. Background Technology

[0002] In a vehicle, a differential causes wheels on the same axle to rotate at different speeds during cornering. The differential includes a shaft mounted to pivot relative to different pinions, each pinion meshing with a differential side gear that is rigidly connected to the driven shaft of the axle.

[0003] When a vehicle turns a corner, the differential side gear rotates at different speeds, and the differential gear rotates freely around the shaft of the differential, and therefore this shaft experiences significant friction when the vehicle is in use; it is a particularly worn part.

[0004] It is known that, in order to compensate for wear on the differential shaft or the mating differential gear, a flat surface is provided on the shaft to move oil far to the boundary between the differential shaft and the gear.

[0005] In addition, it is known that an anti-friction coating is deposited on the outer surface of the differential shaft. These anti-friction coatings are intended to reduce wear on the shaft and the mating gear.

[0006] The applicant's document WO 2015 / 011361 A1 discloses the deposition of this type of anti-friction coating. However, anti-friction coatings may have a tendency to flake, so they are not always compatible with use on differential shafts. This phenomenon is observed particularly in the case of differential shafts having a flat surface.

[0007] Furthermore, with the advent of electric vehicles, the operating conditions of vehicle gearings are changing significantly. Electric vehicles behave very differently from thermal engines—the nominal torque of electric motors reaches very low motor speeds. Consequently, dynamic stress on components increases, and differential shafts degrade more rapidly. The problem to be solved

[0008] Therefore, the objective of the present invention is to propose an improved differential shaft to overcome the disadvantages of the prior art and improve the resistance of the coating layer to degradation. means of solving the problem

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

[0010] According to the present invention, machining of a flat surface defines an edge that borders the bearing surface of a substrate, and the edge has a rounding having a radius of 0.05 mm or more, preferably less than 5 mm. Preferably, the rounding is tangential.

[0011] This optimizes the geometric shape of the differential shaft so as not to have sharp edges that may have promoted peeling of the deposited coating.

[0012] The proposed solution is simple and low-cost because its implementation requires only known and proven means, avoiding complex improvements that might have been required, for example, regarding coating deposition methods or chemical reactions.

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

[0014] In one configuration, the coating layer contains DLC-type amorphous carbon. This type of coating has been tried and tested and yields excellent results in terms of wear resistance and coefficient of friction.

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

[0016] - Step of obtaining a cylindrical substrate;

[0017] - Step of machining a flat surface;

[0018] - Includes the step of depositing a coating layer on a substrate.

[0019] According to the present invention, the method comprises a step of grooving an edge defined by machining a flat surface prior to the deposition step.

[0020] Through this method, a differential shaft having the aforementioned advantages can be obtained, and the groove cutting step provides the desired rounding.

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

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

[0023] To reduce roughness peaks generated during the deposition step, this method includes a second step of polishing the coating layer after the deposition step. If the coating peaks are torn off while the shaft is in use, this could generate abrasive particles that accelerate the degradation of the shaft.

[0024] Advantageously, the grooving step and / or the first grinding step and / or the second grinding step are performed by a centerless grinding wheel or a vibrating bowl. All of these grinding methods are simple to implement.

[0025] In order to simply parameterize the deposition of the coating layer, the coating layer is deposited by physical vapor deposition, preferably by plasma-enhanced. Brief explanation of the drawing

[0026] FIG. 1 is a drawing illustrating a vehicle differential in which a differential shaft, a differential gear, and a differential side gear can be seen. FIG. 2 is a drawing illustrating a differential shaft of the prior art in a deteriorated state. Figure 3 is an enlarged example of the deteriorated area of ​​the shaft in Figure 2. Figure 4 is another example of an enlarged view of the deteriorated area of ​​the shaft in Figure 2. FIG. 5 is a plan view of another shaft of the present invention that has not yet received an anti-friction coating. Fig. 6 is a front view of this shaft. Figure 7 is a drawing showing a cross-section through this shaft from the side. Figure 8 is a drawing illustrating an enlarged cross-section of Figure 7. FIG. 9 is a drawing illustrating a shaft according to the present invention that has received an anti-friction coating, and a cross-section similar to that in FIG. 8. Figure 10 is an example of an enlarged view of the interface between the flat surface of the differential shaft and the bearing surface before rounding is created. Figure 11 is an example of an enlarged interface between the flat surface of the differential shaft and the bearing surface after rounding is created by grinding. FIG. 12 is a drawing illustrating a shaft according to the present invention having a chamfer between a flat surface and a bearing surface before rounding is created, and a cross-section similar to that in FIG. 8. Specific details for implementing the invention

[0027] Referring to FIG. 1, the present invention relates to a differential shaft (1) for a vehicle. It should be noted that the differential shaft (1) contains two differential gears (S) and undergoes frequent deterioration.

[0028] FIG. 2 illustrates a conventional differential shaft (1) in which a flat surface (20) is provided to place an oil tank in communication with the friction surface between the cylindrical bearing surface (11) of the shaft (1) and the pore of the differential gear (S). The shaft (1) is coated with an anti-friction coating (30), such as a deposit of DLC-type amorphous carbon.

[0029] In this drawing, the shaft (1) is deteriorated; and the coating (30) can be seen torn off in the scratch (E) area.

[0030] Referring to FIGS. 3 and 4, it can be seen that tearing occurs in the region of the edge (21) that borders the flat surface (20) of the material. Due to these observations, the applicant investigated the properties and behavior of the coating (30) near the edge (21).

[0031] The machining of the flat surface (20) defines a sharp edge (21) at the joint with the cylindrical bearing surface (11) of the shaft (1), and it appears that the peeling of the coating (30) and the resulting rapid deterioration of the shaft (1) are accelerated by the sharpness of the edge (21).

[0032] In addition, scratches generated during machining of the flat surface (20) exacerbate this phenomenon.

[0033] Referring to FIGS. 5 through 12, an important part of the present invention is that the sharp edge (21) generated during the machining of the flat surface (20) is rounded. The appearance of machining scratches and material tearing is measured to obtain a value between 50 µm and 100 µm. The rounding must therefore be at least 0.05 mm. In this way, the edge (21) no longer shows incipient flaking from the substrate (10).

[0034] Depending on the roughness of the substrate (10) forming the differential shaft (1), the rounding may have a value of, for example, 0.1 mm, 0.25 mm, 0.3 mm, 0.5 mm, or even a higher value of more than 1 mm; as the rounding increases, it may be able to compensate more for local defects corresponding to significant roughness.

[0035] Selecting a rounding radius of at least 0.1 mm eliminates deterioration that could have resulted from machining scratches. Selecting a rounding radius of at least 0.25 mm or 0.3 mm ensures a safety factor against such deterioration.

[0036] In fact, the upper limit of the rounding value is not mandatory; only one dedicated part must be retained so that oil can move between the shaft (1) and the hole of the gear (S). Excellent results are obtained with a rounding of 1.3 mm or 2.2 mm.

[0037] In the case of rounding obtained by polishing, a higher value is associated with longer polishing; therefore, a balance must be struck between the surface condition of the substrate (10) after machining the flat surface (20) and the desired radius.

[0038] In particular, when grinding is performed using a belt and / or grinding wheel, in order to grind two edges equally, the part must be ground in the direction of rotation around its axis (a) and then ground in the opposite direction of rotation.

[0039] When rounding is achieved by CNC (Computer Numerical Control) machining, the difficulties related to this balance are alleviated.

[0040] In addition, it is possible to polish the substrate (10) to adjust its surface condition before the coating (30) is deposited. In particular:

[0041] - The arithmetic roughness (Ra) of the substrate (10) may be 0.1 μm or less, preferably less than 0.07 μm, and more preferably less than 0.04 μm;

[0042] - The maximum roughness (Rz) may be 0.1 μm or less, preferably 0.08 μm or less, and more preferably less than 0.04 μm;

[0043] - The reduced depth of the peak (Rpk) may be 0.07 μm or less, preferably 0.05 μm or less.

[0044] Different roughness measurements are indirectly correlated with one another insofar as grinding reduces all roughness values; however, different measurement methods do not reflect the same characteristics of the surface:

[0045] - Arithmetic roughness (Ra) reflects the average roughness of the substrate. This is not significantly affected by scuffing or contamination.

[0046] - Maximum roughness (Rz) reflects the maximum amplitude between the trough and the peak on the surface. Since this depends on the peak value, it is very sensitive to scuffing and also to contamination.

[0047] - The reduced depth of the peak (Rpk) reflects the presence of localized peaks prone to tearing during the initial use phase of the shaft (1); such tearing will generate wear particles at the interface. This value is suitable for addressing friction and wear.

[0048] Rounding is created by polishing, so the following can be obtained in a single step:

[0049] - Suitable roughness of the substrate (10) for the deposition to properly adhere to the substrate (10); and

[0050] - Rounding of the edge (21) sufficient to avoid the phenomenon of material peeling or tearing.

[0051] More specifically, referring to FIG. 5, the edge (21) defined by the flat edge (20) comprises two straight sections (21a) parallel to the rotation axis (a) of the differential shaft (1) and two elliptical sections (21b). Rounding must exist at the interface with the hole of the gear (S); generally, this relates to the straight section (21a), and rounding is also preferably present at least on the straight section (21a).

[0052] FIG. 10 illustrates this type of longitudinal edge (21a) before rounding is created.

[0053] Referring to FIG. 12, the machining of the flat surface (20) may have a chamfer of an angle (b). It should be noted that the part of the shaft (1) that deteriorates during use is the interface with the hole of the gear (S); in all cases, the edge (21a) is the intersection line between the machining (flat surface (20) or chamfer) and the bearing surface (11) of the shaft (1).

[0054] In fact, a grinding means such as a centerless grinding wheel or a vibrating ball grinds the entire circumference of the edge (21), thereby providing the desired rounding around the entire circumference of the edge (21). A centerless grinding wheel has a certain flexibility so that it can accommodate the entire circumference of the edge (21) during grinding. The same applies to grinding performed using a wear belt.

[0055] Referring to FIG. 11, due to the rounded edge (21a), a gradual transition from the bearing surface (11) toward the flat surface (20) can be obtained.

[0056] Accordingly, stress concentration is avoided within the coating material. The effect caused by the hole of the gear (S) being machined by the edge (21a) is also avoided.

[0057] The boundary of the interface surface (li) between the shaft (1) and the hole of the gear (S) lies at the tangent between the rounding of the edge (21a) and the bearing surface (11). In this drawing, it can also be seen that the machining scratch present on the flat surface (20) is now removed from the boundary of the interface surface (Ii); and there is no longer any risk of tearing or peeling due to the roughness or geometric shape of the scratch.

[0058] FIG. 8 is a cross-section through the substrate (10) of the shaft (1) before the deposition step. The rounding according to the present invention can be seen here. In this figure, the rounding is tangential to reduce incipient fractures as much as possible.

[0059] The radius of the rounding can be measured by profilometry by reconstructing a circle (22) passing through the measured point of the rounding.

[0060] FIG. 9 illustrates a shaft (1) according to the present invention. A coating layer (30) is deposited on a substrate (10) preferably by vacuum vapor deposition, optionally by plasma-enhanced deposition. Due to this method, the chemical reaction of the deposited material can be easily adjusted by selecting a suitable target and / or a suitable precursor gas. Furthermore, the properties of the deposited layer can be changed by adjusting the deposition and enhancement parameters.

[0061] The coating layer (30) preferably comprises DLC-type amorphous carbon, but other coatings such as nickel electroplating may also be considered. However, the advantage of DLC is that it has greater wear resistance than other coatings.

[0062] The coating layer (30) is generally several μm thick, typically between 0.5 μm and 5 μm. Accordingly, even on the finished shaft (1), when the radius of the edge (21) is measured by profilometrics, the rounding of the substrate (10) before the coating step can be estimated sufficiently accurately.

[0063] The shaft (1) and the method of manufacturing the same may be designed differently from the given example, provided that they do not deviate from the scope of the invention as defined by the claims.

[0064] The rounding may have different values ​​for different parts of the edge (21) of the shaft. For example, the longitudinal edge (21a) on the left side of the flat surface (20) may have a rounding of 1.9 mm, while the longitudinal edge (21a) on the right side of the same flat surface (20) may have a rounding of 1.5 mm.

[0065] Furthermore, the technical characteristics of the various embodiments and variations mentioned above may be combined wholly or partially. Accordingly, the shaft (1) and the method of manufacturing it may be adapted in terms of cost, function, and performance.

Claims

Claim 1 A differential shaft (1) comprising a substrate (10) having at least one machining of a flat surface (20) and a coating layer (30) deposited on the substrate (10), wherein the machining of the flat surface (20) defines an edge (21) that forms a boundary with a bearing surface (11) of the substrate (10), and the edge (21) has a rounding having a radius of 0.05 mm or more and less than 5 mm. Claim 2 A differential shaft (1) according to claim 1, characterized in that the radius is greater than 0.1 mm. Claim 3 A differential shaft (1) according to claim 1 or claim 2, wherein the coating layer (30) comprises DLC-type amorphous carbon. Claim 4 A method for manufacturing a differential shaft (1), comprising the following steps: obtaining a cylindrical substrate (10); machining a flat surface (20); and depositing a coating layer (30) on the substrate (10), wherein, prior to the deposition step, a groove is formed in an edge (21) defined by the machining of the flat surface (20). Claim 5 A method for manufacturing a differential shaft according to claim 4, wherein the step of making a groove comprises the step of obtaining a rounding having a radius greater than 0.05 mm. Claim 6 A method for manufacturing a differential shaft according to claim 4 or claim 5, characterized by including a first step of polishing the substrate (10) before the deposition step. Claim 7 A method for manufacturing a differential shaft according to claim 6, wherein the first polishing step comprises a step of obtaining a roughness (Ra) of less than 0.1 μm. Claim 8 A method for manufacturing a differential shaft according to claim 6, characterized by including a second step of polishing the coating layer (30) after the deposition step. Claim 9 A method for manufacturing a differential shaft according to claim 8, wherein the groove-cutting step and / or the first grinding step and / or the second grinding step are performed by a centerless grinding wheel or a vibrating bowl. Claim 10 A method for manufacturing a differential shaft according to claim 4 or claim 5, characterized in that the coating layer (30) is deposited by physical vapor deposition.

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