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 machining of the flat region (20) defines edges (21) bordering a bearing surface (11) of the substrate, and the edges (21) have a rounded portion with a radius greater than or equal to 0.05 mm.

Description

Description Title of the invention: improved differential axle Technical field

[0001] The invention relates to the technical field of motor vehicles, and in particular to differential axles fitted to vehicles. Prior art

[0002] On a vehicle, a differential allows the wheels on the same axle to rotate at different speeds when turning. A differential consists of an axle pivotally mounted relative to planet gears, each of which meshes with a sun gear secured to the axle's driven shafts.

[0003] When the vehicle is cornering, the planetary gears rotate at different speeds, and the satellite gears rotate freely around the differential axle. The differential is therefore subject to significant friction during vehicle operation. It is a part that is particularly prone to wear.

[0004] In order to compensate for wear on the differential shaft or the opposing satellite pinion, it is known to provide flats on the shaft to allow oil to circulate to the interface between the differential shaft and the pinion.

[0005] It is also known to deposit anti-friction coatings on the external surface of the differential axle. These anti-friction coatings are intended to reduce wear on the axle and the opposing pinion.

[0006] It is known from document WO201511361, in the name of the applicant, the deposition of such an anti-friction coating. However, anti-friction coatings are not always compatible with use on a differential shaft, because they can tend to flake. This phenomenon is particularly encountered in the case of differential shafts with a flat surface.

[0007] Furthermore, with the emergence of electric vehicles, the conditions of use of vehicle mechanics are changing significantly. Indeed, the behavior of an electric motor is very different from that of a thermal engine: the nominal torque of an electric motor is reached at very low engine speeds. The dynamic stress on the parts is therefore increased, and the differential axles deteriorate more quickly. Statement of the invention

[0008] The aim of the invention is therefore to propose an improved differential shaft, overcoming the drawbacks of the prior art, and for which the resistance to degradation of a coating layer is improved.

[0009] For this purpose, a differential axle was developed consisting of a substrate having at least one machining of a flat and comprising a coating layer deposited on the substrate.

[0010] According to the invention, the machining of the flat defines edges bordering a surface of the substrate, and the edges have a rounded radius greater than or equal to 0.05 mm, and preferably less than 5 mm. Preferably, the roundedness is of the tangent type.

[0011] In this way, the geometry of the differential axle is optimized so as not to have sharp edges, which would promote flaking of the deposited coating.

[0012] The proposed solution is simple and inexpensive, because its implementation only requires known and proven means, and does not require complex improvements that could have been sought, for example, concerning the chemistry or the method of depositing the coating.

[0013] Advantageously and always with the aim of reducing the stress concentrations which may occur at the edges, the radius is greater than 0.1 mm, preferably 0.5 mm, and even more preferably 1 mm or even 1.5 mm.

[0014] In one mode, the coating layer includes amorphous carbon called "DLC". This type of coating is proven and gives good results in terms of friction coefficient and wear resistance.

[0015] The invention also relates to a method of manufacturing a differential axle comprising the following steps: - obtaining a cylindrical substrate; - machining of a flat; - depositing a coating layer on the substrate.

[0016] According to the invention, the method comprises a step of radiusing an edge defined by machining the flat, prior to the deposition step.

[0017] This process allows to obtain a differential axle with the aforementioned advantages, the radiusing step providing the desired rounding.

[0018] In order to promote the adhesion of the coating deposited on the substrate, the process includes a first step of polishing the substrate, prior to the deposition step. This method also gives better cohesion to the materials constituting the deposited coating.

[0019] Advantageously, the first polishing step consists of obtaining a roughness Ra of less than 0.1 pm.

[0020] In order to reduce roughness peaks generated during the deposition step, the method comprises a second step of polishing the coating layer, after the deposition step. The removal of the peaks of the coating, during use of the spindle, could generate abrasive particles accelerating the deterioration of the spindle.

[0021] Advantageously, the radiusing step and / or the first polishing step and / or the second polishing step is carried out by means of a pointless type grinding wheel or a vibrating bowl. Both of these polishing methods are simple to implement.

[0022] In order to easily configure the deposition of the coating layer, this is carried out by physical vapor deposition, preferably with plasma assistance. Brief description of the figures

[0023] [Fig.1] is an illustration of a vehicle differential showing a differential shaft, planetary gears and planetary gears.

[0024] [Fig.2] is an illustration of a degraded prior art differential shaft.

[0025] [Fig.3] is an illustration of an enlargement of the degraded area of ​​the axis of Figure 2.

[0026] [Fig.4] is another illustration of the enlargement of the degraded area of ​​the axis of Figure 2.

[0027] [Fig.5] is a diagram of an axle according to the invention, seen from above, and not yet having received an anti-friction coating.

[0028] [Fig.6] is a diagram of this axis, seen from the front.

[0029] [Fig.7] is a section of this same axis, seen from the side.

[0030] [Fig.8] is an enlargement of the section of figure seven.

[0031] [Fig.9] is a section similar to that of Figure 8, showing an axle according to the invention and having received the anti-friction coating.

[0032] [Fig.10] is an illustration of an enlargement of the interface between a flat and a differential axle bearing surface, before rounding is carried out.

[0033] [Fig.11] is an illustration of an enlargement of the interface between a flat and a differential axle bearing surface, after rounding by polishing.

[0034] [Fig. 12] is a section similar to that of Figure 8, showing an axis according to the invention and also having a chamfer between the flat and the bearing surface, before the rounding is carried out. Detailed description of the invention

[0035] With reference to Figure 1, the invention relates to an axle (1) for a vehicle differential. It can be seen that the axle (1) of the differential, which carries the two satellite pinions (S), is subject to frequent damage.

[0036] Figure 2 illustrates a differential shaft (1) of the prior art, on which a flat (20) is provided to connect an oil reserve and a friction surface between the cylindrical bearing surface (11) of the shaft (1) and the bore of the satellite pinion (S). The shaft (1) is coated with an anti-friction coating (30), such as an amorphous carbon deposit called “DLC”.

[0037] We can see in this figure that the axis (1) is degraded: the coating (30) has been torn off at the level of a groove (E).

[0038] With reference to Figure 3 and Figure 4, material tearing can be observed to have occurred at the edge (21) bordering the flat (20). These observations led the applicant to investigate the nature and behavior of the coating (30) in the vicinity of the edge (21).

[0039] It appears that the machining of the flat (20) defines a sharp edge (21) at the junction with the cylindrical bearing surface (11) of the axis (1), and that it is the fact that the edge (21) is sharp which facilitates the flaking of the coating (30), and thereby the rapid degradation of the axis (1).

[0040] In addition, the striations generated during machining of the flat (20) accentuate this phenomenon.

[0041] With reference to Figures 5 to 12, the invention resides mainly in that the sharp edge (21) generated during the machining of the flat (20) is rounded. The measurement of the machining striations as well as the surfaces of the material tears provide values ​​between 50 pm and 100 pm. The rounding must therefore be at least 0.05 mm. In this way, the edge (21) no longer has any incipient flaking of the substrate (10).

[0042] Depending on the roughness of the substrate (10) which constitutes the differential axis (1), the rounding may have a greater value, for example 0.1 mm, 0.25 mm, 0.3 mm, 0.5 mm, or even more than 1 mm: the greater the rounding, the more it will be able to compensate for local defects corresponding to significant roughness.

[0043] Choosing a rounding radius of at least 0.1 mm helps prevent damage that could be caused by machining grooves. Choosing a rounding radius of at least 0.25 mm or 0.3 mm ensures a safety factor against such damage.

[0044] In practice, the upper limit of the rounding value is not essential: it is only necessary to keep a portion dedicated to the oil path between the axis (1) and the pinion bore (S). Good results have been obtained with roundings of 1.3 mm, or even 2.2 mm.

[0045] In the case of a rounding obtained by polishing, a higher value is synonymous with longer polishing: it is therefore necessary to make a compromise between the surface condition of the substrate (10) after machining the flat (20) and the desired radius.

[0046] It may be necessary to polish the part in one direction of rotation around its axis (a) and then in the opposite direction of rotation in order to polish both edges in the same way, especially when polishing is done with belts and / or wheels.

[0047] In the case of a rounding obtained by numerically controlled machining ("CNC" for computer numerical control), the difficulty of this compromise is reduced.

[0048] It is further possible to polish the substrate (10) in order to adapt its surface condition prior to the deposition of the coating (30). In particular: - the arithmetic roughness Ra of the substrate (10) may be less than or equal to 0.1 pm, preferably less than 0.07 pm, and even more preferably less than 0.04 pm; - the maximum roughness Rz may be less than or equal to 0.1 pm, preferably less than or equal to 0.08 pm and even more preferably less than 0.04 pm; - the reduced depth of the Rpk peaks may be less than or equal to 0.07 pm, or preferably less than or equal to 0.05 pm.

[0049] The different roughness measurements are indirectly correlated with each other in that polishing decreases all roughness values, however the different measurement methods do not illustrate the same surface characteristics: - Arithmetic roughness Ra illustrates the average roughness of the substrate. It is not significantly influenced by scratches or contamination. - Maximum roughness Rz illustrates the maximum amplitude between peaks and valleys of the surface. It is very sensitive to scratches, but also to contamination due to its dependence on peak values. - The reduced depth of the Rpk peaks illustrates the presence of local peaks, likely to be torn off during an initial phase of the use of the axis (1), which would constitute abrasive particles at the interface. This is a value suitable for the evaluation of friction and abrasion.

[0050] Carrying out the rounding by polishing therefore makes it possible to obtain in a single step: - a roughness of the substrate (10) adapted so that the deposit adheres correctly to the substrate (10); and - a rounded edge (21) sufficient to avoid the phenomena of chipping or tearing of material.

[0051] With more particular reference to Figure 5, the edge (21) defined by the flat (20) comprises two rectilinear portions (21a) parallel to the axis of revolution (a) of the differential axis (1), and two elliptical portions (21 b). The rounding must be present at the interface with the bores of the pinions (S): in general these are the rectilinear portions (21a), so the rounding is preferably present at least on the rectilinear portions (21a).

[0052] Figure 10 illustrates such an edge (21a), longitudinal, before the rounding is carried out.

[0053] With reference to figure 12, the machining of the flat (20) may have a chamfer, of angle (b). It is understood that the part of the axis (1) where the damage is generated in use is the interface with the bore of the pinion (S): in all cases, the edge (21a) is the line of intersection of the machining (flat (20) or chamfer) and the bearing surface (11) of the axis (1).

[0054] In practice, polishing means such as a centerless wheel or a vibrating bowl will polish the entire circumference of the edge (21) and thus provide the desired roundness around the entire circumference of the edge (21). Centerless wheels have a certain flexibility, which allows them to follow the entire circumference of the edge (21) during polishing. The same is true for polishing carried out with abrasive belts.

[0055] With reference to figure 11, the rounded edge (21a) makes it possible to obtain a progressive transition from the bearing surface (11) towards the flat (20).

[0056] This avoids stress concentrations within the coating material. Machining of the pinion bore (S) by the edge (21a) is also avoided.

[0057] The limit of the interface (li) between the axis (1 ) and the bore of the pinion (S) is located at the tangency between the rounding of the edge (21a) and the bearing surface (11 ). It can also be seen in this figure that machining grooves, present on the flat (20), are now set back from the limit of the interface (li): there is no longer any risk of tearing or chipping due to the geometry or roughness of the grooves.

[0058] Figure 8 illustrates a section of a substrate (10) of an axis (1), prior to the deposition step. The rounding according to the invention can be seen there. In this figure, the rounding is of the tangent type in order to reduce the 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 points of the rounding.

[0060] Figure 9 illustrates an axis (1) according to the invention. The coating layer (30) is deposited on the substrate (10), preferably by means of a vacuum vapor deposition technique, possibly with plasma assistance. This method allows the chemistry of the deposited material to be easily adjusted by the selection of a suitable target and / or suitable precursor gases, and the adjustment of the deposition and assistance parameters allows the nature of the deposited layer to be modified.

[0061] The coating layer (30) preferably comprises amorphous carbon known as “DLC”, but other coatings may be considered, such as a deposit of nickel by electrolysis. Nevertheless, DLC has the advantage of being more resistant to wear than other coatings.

[0062] The coating layer (30) is generally a few micrometers thick, typically between 0.5 pm and 5 pm. Thus, even on a finite axis (1), it is possible to estimate with sufficient precision the roundness that the substrate (10) had before the coating step, when measuring the radius of the edge (21) by profilometry.

[0063] The axis (1) and its manufacturing method may be shaped differently from the examples given without departing from the scope of the invention, which is defined by the claims.

[0064] The rounding may be of different value on different portions of the edge (21) of the axis. For example, a longitudinal edge (21a) on the left side of the flat (20) may have a rounding of 1.9 mm, while the longitudinal edge (21a) on the right side of the same flat (20) may have a rounding of 1.5 mm.

[0065] Furthermore, the technical characteristics of the different embodiments and variants mentioned above can be, in whole or in part, combined with each other. Thus, the axis (1) and its manufacturing method can be adapted in terms of cost, functionality and performance.

Claims

Claims [Claims 1] Differential axle (1) consisting of a substrate (10) having at least one machining of a flat (20) and comprising a coating layer (30) deposited on the substrate (10), characterized in that the machining of the flat (20) defines edges (21) bordering a bearing surface (11) of the substrate (10), and the edges (21) have a rounded radius greater than or equal to 0.05 mm, and preferably less than 5 mm. [Claims 2] Differential axle (1) according to claim 1, characterized in that the radius is greater than 0.1 mm, preferably 0.25 mm, and even more preferably 1 mm. [Claims 3] Differential axle (1) according to one of the preceding claims, characterized in that the coating layer (30) comprises amorphous carbon known as “DLC”. [Claims 4] Method of manufacturing a differential axle (1) comprising the following steps: - obtaining a substrate (10) of cylindrical shape; - machining of a flat (20); - depositing a coating layer (30) on the substrate (10); characterized in that it comprises a step of rounding an edge (21) defined by machining the flat (20), prior to the depositing step. [Claims 5] Method according to claim 4, characterized in that the radiusing consists of obtaining a rounding of radius greater than 0.05 mm. [Claims 6] Method according to claim 4 or 5, characterized in that it comprises a first step of polishing the substrate (10), prior to the deposition step. [Claims 7] Method according to claim 6, characterized in that the first polishing step consists of obtaining a roughness Ra of less than 0.1 pm. [Claims 8] Method according to one of claims 4 to 7, characterized in that it comprises a second step of polishing the coating layer (30), after the deposition step. [Claims 9] Method according to one of claims 4, 6 or 8, characterized in that the radiusing step and / or the first polishing step and / or the second polishing step is carried out by means of a pointless type grinding wheel or a vibrating bowl. [Claims 10] Method according to one of the preceding claims, characterized in that the deposition of the coating layer (30) is carried out by physical vapor deposition, preferably with plasma assistance.

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