Hollow connecting rod

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of double-jointed connecting rods and more particularly those used in aeronautical engines or any other product containing connecting rods and needing to control mass. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Today these connecting rods are metallic or solid composite (with round or non-round cross-section, constant or non-constant).

[0003] These connecting rods are readily available and inexpensive, withstand fairly high temperatures (>300°C), are generally made of metal (steel, stainless steel) and have a simple geometry (solid round bar with a constant cross-section), but are relatively heavy, especially when the distance between the two interfaces increases. Indeed, as Euler's formula indicates, for constant resistance (buckling / compression, tension), the mass of solid round connecting rods increases with the cube of the distance between the attachment points.

[0004] However, when the fixing points are increasingly further apart, for example from 331mm to 440mm, one of the objectives was therefore to find a connecting rod concept that was both cheaply manufacturable and lighter to limit the mass gain of the engine or the product containing this connecting rod.

[0005] To address these issues, there are tubular metal or composite connecting rods such as those described in patents EP 2 643 600 and EP 1 864 027. Document DE 103 24 981 A1 discloses the characteristics of the preamble of claim 1.

[0006] However, the weight of the connecting rod remains significant and it is not possible to reduce the walls without degrading the performance of these connecting rods. SUMMARY OF THE INVENTION

[0007] The invention offers a solution to the problems mentioned above, by reducing the amount of material, which makes it possible to lighten the connecting rod, while increasing its inertia, hence a reinforcement against buckling, especially in the central area.

[0008] The connecting rod according to the invention comprises a hollow body with a ring at each end and a center, a wall of thickness e, the wall defining an outer perimeter pe and an inner perimeter pi, a surface area s being contained between the outer and inner perimeters, it is characterized in that the outer perimeter pe increases from the ends of the hollow body towards the center of the connecting rod, the hollow body keeping a constant surface area s, the thickness e decreasing from the end towards the center of the connecting rod.

[0009] The geometry of the new connecting rod is hollowed out, with a perimeter that increases from the edges towards the center. The wall thickness is also adjusted to equalize the working sections along the entire hollow body, so the minimum thickness is located at the center of the connecting rod. The constant cross-section maintains a certain rigidity in the connecting rod. This results in a reduction in mass, an increase in buckling resistance, and an increase in the frequencies of the first vibration modes. This profile provides both significant mass savings (approximately -40%) and twice the strength. The increase in strength is so significant that the mass reduction can be further amplified either by reducing the wall thickness or by using a lighter and / or less stiff material.

[0010] According to a first variant, the hollow body is cylindrical of revolution, which allows both symmetry and balance of the connecting rod.

[0011] According to a second variant, the hollow body is oval or rectangular. This allows the overall size to be reduced in a given direction, for example if the connecting rod is transverse to a gas or liquid flow.

[0012] It is also possible to deflect the "trajectory", or deform the line or axis of the connecting rod, to avoid a (hot) area or object more easily than in the solid version because the hollow profile has better inertia and better stiffness at equal mass.

[0013] Advantageously, a reinforcing ring is positioned in the center of the connecting rod on the inner side of the hollow body. This reduces, or even eliminates, the risk of localized buckling, as the wall is thinner in the center of the connecting rod, which would otherwise risk folding back on itself or being damaged during handling because the central area is the most "exposed".

[0014] Advantageously, a hole is provided in the extension of the hollow body at an interface with each ring. The presence of these holes at the edges of the interfaces serves, in particular, to make the connecting rod "powder-removable," that is, to allow the powder to be evacuated from the central cavity after additive manufacturing by laser beam melting (LBM), also known as powder bed fusion, without weakening it. This also makes it possible to lighten the connecting rod.

[0015] Advantageously, the connecting rod length is greater than or equal to 440mm. The profile is particularly suited to long connecting rod lengths.

[0016] The invention also relates to a method for manufacturing a connecting rod with at least one of the preceding characteristics; it is characterized in that the connecting rod is produced by additive manufacturing. This manufacturing method allows for a cost saving of approximately 20% compared to conventional profiles. Additive manufacturing can be performed by laser powder bed fusion or by direct energy deposition (DED).

[0017] According to one manufacturing variant, the connecting rod is made in a foundry.

[0018] According to another manufacturing method, the connecting rod is produced by machining. It is possible to combine casting with machining.

[0019] Advantageously, the manufacturing process includes the following steps: fabrication of a first half-connecting rod, fabrication of a second half-connecting rod, assembly of the two half-connecting rods.

[0020] The two connecting rod halves can be joined by welding, brazing, screwing or any other means of assembly. BRIEF DESCRIPTION OF THE FIGURES

[0021] The figures are presented for illustrative purposes only and are in no way limiting to the invention. [ Fig. 1 ] is a perspective view of different sizes and shapes of connecting rods; [ Fig. 2 ] is a perspective view of a connecting rod according to the invention; [ Fig. 3 ] is a cross-sectional view of the connecting rod of the figure 2 ; Fig. 4 ] is a cross-section of half a connecting rod according to the invention; [ Fig. 5 ] shows the detail of the interface between the hollow body and a ring according to a variant of the invention; [ Fig. 6 ] shows the detail of the center of the connecting rod; ] Fig. 7 ] is a cross-section of the hollow body; [ Fig. 8 ] illustrates the finite element simulation results of the buckling of the connecting rod with a conventional profile; [ Fig. 9] illustrates the finite element simulation results of the buckling of the connecting rod with a hollow profile according to the invention. DETAILED DESCRIPTION

[0022] The figures are presented for illustrative purposes only and are in no way limiting to the invention.

[0023] Unless otherwise specified, the same element appearing on different figures has a unique reference.

[0024] We can see at the figure 1 Conventional connecting rods 1 and 2 in the prior art are increasingly longer, resulting in an increase in the diameter of the body 10, 20 that connects the two rings 11, 21 located at one end 36, and therefore in their weight. They are generally metallic, made of steel or stainless steel. Connecting rod 3 according to the invention, unlike connecting rods 1 and 2 in the prior art, does not have a constant diameter. All these connecting rods 1, 2, and 3 have a longitudinal axis X.

[0025] Connecting rod 3 can be seen in more detail at the figure 2 The hollow body 30 is made of two parts forming two connecting rod halves 300 and 301, but it could also be made of a single part without departing from the scope of the present invention. The body 30 terminates in two rings 31, each intended to be connected to a part and serving as a pivot with it. An interface 310 connects each end 36 of the hollow body 30 to a ring 31.

[0026] As we can see figure 3 The hollow body 30 defines an internal space 32. The two connecting rod halves 300 and 301 are joined together by a joint 33 made by welding, brazing, or any other means of joining. We can see at the figure 6that a reinforcing ring 4 is placed at the center 35 of the connecting rod 3, that is to say here on the connection 33 between the two half-connecting rods 300 and 301. This reinforcing ring 4 is located inside the hollow body 30 on the inner part 340 of the wall 34. It makes it possible to reduce or even eliminate the risk of local buckling of the wall 34 which is thinner at the center 35.

[0027] The wall 34 of the hollow body 30 has a thickness e. As visible at the figure 4The thickness decreases from interface 310 to the center 35 of the connecting rod 3, while the distance between the X-axis and the wall 34 increases. The cross-sectional area s is circular; it is the difference between the area defined by the outer wall 341 and the area defined by the inner wall 340 for a given cross-section, i.e., the area between the outer perimeter pe of the wall 34 and the inner perimeter pi of the wall 34. The cross-sectional area s remains constant along the entire length of the hollow body 30, so the greater the distance between the X-axis and the wall 34, the smaller the thickness e becomes. The cross-section of the wall 34 could also be oval or rectangular, or even change shape from interface 310 to the center 35 of the connecting rod 3, while always remaining closed. The X-axis is straight here, but it can be deformed to avoid an area (a hot zone, for example) or an object.

[0028] The wall thickness is thus adjusted to equalize the working sections along the entire length of the hollow body 30, the minimum thickness is located at the center of the connecting rod.

[0029] The interface 310 between the ring 31 and the hollow body 30 is here pierced with a hole 320 arranged in the extension of the inner space 32. It is intended to allow the evacuation of the powder remaining in the hollow body 30 during additive manufacturing.

[0030] Connecting rod 3 can indeed be manufactured using additive manufacturing via laser powder bed fusion. It can also be produced by concentrated energy deposition. These manufacturing processes allow for the desired wall thicknesses. Connecting rod 3 is then manufactured vertically.

[0031] Connecting rod 3 can also be constructed as a single piece or as two half-rods assembled by welding, brazing, or bolting. This is especially true if it is manufactured by casting or machining.

[0032] Finite element simulations were performed to evaluate the buckling of connecting rod 3 ( figure 9 ) compared to a connecting rod with a conventional profile ( figure 8 ) of the same length. Three types of buckling have been defined: buckling A, buckling B, buckling C.

[0033] A load multiplier (MC in Tables 8 and 9) has been calculated for each type of buckling; it corresponds to the factor by which the load applied to the connecting rod must be multiplied to obtain the buckling considered.

[0034] For buckling A, we observe that the load applied to connecting rod 3 is more than twice as high as that for the conventional connecting rod, 2.354 instead of 1.0985.

[0035] For buckling B, the difference is more than 50% between the two connecting rods, 6.7314 for 4.351.

[0036] For buckling C also the difference is more than 50%: 7.0982 for connecting rod 3 and 4, 6821 for the conventional connecting rod.

[0037] It can thus be seen that far from diminishing the connecting rod's properties, particularly its inertia, the connecting rod profile according to the invention improves its properties while significantly reducing its weight, by up to 40%. The resulting increase in strength is such that the weight reduction can be further enhanced either by further reducing the wall thickness or by switching to a lighter and less stiff material, such as a TA6V titanium alloy.

Claims

1. A connecting rod (3) comprising a hollow body (30) with a bushing (31) at each end (36) and a centre (35), a wall (34) having thickness e, the wall (34) defining an external perimeter pe and an internal perimeter pi, a cross-sectional area s being included between the external and internal perimeters, characterised in that the external perimeter pe increases from the ends (36) of the hollow body (30) towards the centre (35) of the connecting rod (3), the hollow body (30) keeping a constant cross-sectional area s, the thickness e decreasing from the end (36) towards the centre (35) of the connecting rod (3).

2. The connecting rod (3) according to claim 1 characterised in that the hollow body (30) is rotationally cylindrical.

3. The connecting rod (3) according to claim 1 characterised in that the hollow body (30) is oval or rectangular.

4. The connecting rod (3) according to one of the preceding claims characterised in that a reinforcement ring (4) is disposed in the centre (35) of the connecting rod (3) on the inner side of the hollow body (30).

5. The connecting rod (3) according to one of the preceding claims characterised in that a hole (320) is provided as an extension of the hollow body (30) at an interface (32) with each bushing (31).

6. The connecting rod (3) according to one of the preceding claims, characterised in that its length is greater than or equal to 440 mm.

7. A method for manufacturing a connecting rod (3) according to one of the preceding claims, characterised in that the connecting rod (3) is made by additive manufacturing.

8. The method for manufacturing a connecting rod (3) according to one of claims 1 to 6, characterised in that the connecting rod (3) is made in foundry.

9. The method for manufacturing a connecting rod (3) according to one of claims 1 to 6, characterised in that it is made by machining.

10. The method for manufacturing a connecting rod (3) according to one of claims 7 to 9 characterised in that it comprises the following steps of: - making a first half-connecting rod (300, 301), - making a second half-connecting rod (301, 300), - assembling the two half-connecting rods (300, 301).