METHOD FOR SURFACE TREATMENT OF A PISTON ROD

Abstract

METHOD FOR SURFACE TREATMENT OF A PISTON ROD One aspect of the invention relates to a method for surface treatment of a piston rod (21), the rod being made of a high-strength alloy with a minimum hardness greater than 45 HRC, characterized in that it comprises the following steps: a grinding step and a polishing step until an average roughness Ra ≤ 0.2 µm is obtained. This provides a surface finish that meets the dynamic sealing requirements of the piston without requiring chrome plating. Figure to be published with the abbreviation: Figure 2

Description

Title of the invention: METHOD FOR SURFACE TREATMENT OF A PISTON ROD TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of the surface treatment of mechanical parts such as piston rods and rods obtained.

[0002] The present invention relates in particular to piston rods equipping the braking systems of aeronautical trains. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] The pistons used in braking systems compress a set of discs mounted in the wheel and on the axle. This assembly, called a heat sink, consists of stators attached to the axle and rotors attached to the wheel. Compressing the discs slows the relative rotational movement between the rotors and stators. The pistons involved in the present invention are hydraulic pistons, set in motion by pressurized hydraulic fluid.

[0004] The pistons are directly screwed into a ring attached to the braking system and must fulfill the following main functions: • apply the pressure force on the heat sink, • automatically compensate for wear of said heat sink, • retract the piston to avoid residual braking in the braking system, • ensure the sealing function. The rod is one of the major components of the piston. It contributes to the wear compensation function of the heat sink by progressively deforming a deformation tube using a sphere screwed to its end.

[0005] The rod requires sufficient mechanical properties in order to withstand the loads necessary for the deformation of the deformation tube but also to resist the prestressing forces related to the tightening of the sphere.

[0006] The rod must also provide a sealing function between the pressurized environment (hydraulic fluid side) and the unpressurized environment (brake disc side) by means of a rod seal. The rod requires a specific roughness to limit seal wear while promoting seal sliding, and sufficient hardness to prevent this roughness from being altered by friction against the seal. The seal can be made of a single piece of elastomer or in several parts: for example, an elastomer seal with a harder PTFE (Polytetrafluoroethylene) component.

[0007] Currently, these rods are made of stainless steel, for example of the type 15-5PH, which has the mechanical properties necessary to ensure the wear compensation function.

[0008] The body of the rod in interface with the seal is coated with a hard chrome deposit. This deposit is ground to obtain the roughness suitable for the sealing function, and its high hardness allows it to resist friction against the rod seal.

[0009] However, this technical solution poses several problems: • Hard chrome plating may exhibit porosity that could promote hydraulic leakage. • The range of chrome plating and grinding processes is complex and expensive, • The chrome plating range does not meet environmental requirements.

[0010] A solution for a ground, hard chrome-free 15-5PH stainless steel rod also exists from other aircraft brake suppliers, but this solution is not applicable for rod seals that require a minimum rod surface hardness greater than 45 HRC, i.e., a two-part seal: elastomer and PTFE. Summary of the invention

[0011] The invention offers a solution to the problems mentioned above, by making it easier to produce rods that provide satisfactory sealing and do not require additional surface treatment.

[0012] Thus, a material is used which has both the mechanical properties necessary for the wear compensation function (Rm > 1100 MPa), both the ability to be machined and ground to achieve a roughness satisfactory for the sealing function (Ra value < 0.2 pm, defined limit), under the stress conditions described above and specific to the application, and both sufficient hardness to resist friction with the seal and guarantee the integrity of the surface condition (< 45 HRC).

[0013] One aspect of the invention relates to a method for surface treatment of a piston rod, the rod being made of a high mechanical strength alloy, characterized in that it comprises the following steps: • a grinding step and, • a finishing step until an average roughness Ra < 0.2 pm is obtained.

[0014] Advantageously, the average roughness Ra < 0.1 pm.

[0015] This results in a surface finish that meets the dynamic sealing requirements of the piston without requiring chrome plating. For example, a maximum height Rp < 0.4 pm and a maximum gap Rz < 2.0 pm can be achieved.

[0016] Advantageously, the alloy has a minimum hardness greater than 45 HRC. This allows the use of a wider variety of seals. The alloy may be a metallic alloy.

[0017] Advantageously, the alloy is a nickel-based superalloy. It will be possible by for example, use an IN718 type alloy.

[0018] Advantageously, the alloy has a tensile strength Rm > 1100 MPa. This strength allows the rod to deform the deformation tube.

[0019] The final roughness can be obtained in two ways: with grinding followed by superfinishing by mechanical polishing (disc abrasion), or with grinding followed by finishing by tribofinishing.

[0020] Advantageously, the finishing step is a tribofinishing process. This technique makes it possible to obtain the desired level of roughness. Tribofinishing encompasses all industrial machining processes by abrasion that, by moving abrasive media around a workpiece in a tank, improve roughness (through abrasion, shearing, or impact). Examples include tumbling, linear and circular vibrators, centrifugal force machines, smuritropy, and surface finishing.

[0021] A first aspect of the invention relates to a piston rod obtained by the surface treatment process according to one of the preceding characteristics.

[0022] A second aspect of the invention relates to a piston comprising a piston rod obtained by the process according to the invention.

[0023] Advantageously, the piston rod includes a sealing gasket between a pressurized medium and an unpressurized medium. The surface characteristics of the rod limit wear of the gasket and ensure good sealing despite the pressure difference in the case of dry pistons.

[0024] Advantageously, the rod is arranged in a deformation tube and includes a sphere at one end cooperating with the deformation tube. The rod is sufficiently rigid to withstand the loads necessary for the deformation of the deformation tube and also to resist the preload forces associated with mounting the nut and the sphere.

[0025] A third aspect of the invention relates to a braking system comprising at least one piston according to the invention.

[0026] The invention and its various applications will be better understood by reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0027] The figures are presented for illustrative purposes only and are in no way limiting of the invention.

[0028] [Fig. 1] is a cross-section of an aircraft landing gear braking system,

[0029] [Fig.2] shows the detail of a piston equipping the braking system of the [Fig.1],

[0030] [Fig.3] shows the wear of a piston seal of [Fig.2],

[0031] [Fig.4] is a table showing the results of the wear tests as a function of the average roughness Ra of the rod surface,

[0032] [Fig.5] is a table showing the results of the wear tests as a function of the surface roughness parameter Rp of the rod,

[0033] [Fig.6] shows the appearance of the rod before and after testing with different roughnesses,

[0034] [Fig.7] represents the different roughness parameters. DETAILED DESCRIPTION

[0035] Unless otherwise specified, the same element appearing on different figures presents a unique reference.

[0036] The braking system 1 illustrated [Fig. 1] is intended for an aircraft landing gear. It consists of a ring 5 and several brake discs stacked one on top of the other, forming what is called a "heat sink" because of the temperature they can reach. Half of these discs are fixed to the wheel (or rim) with axis X and rotate with it; these are the rotor discs 11. The other half are fixed to the aircraft via the axle and do not rotate; these are the stator discs 10, 12. They are mounted alternately to form an assembly called a heat sink. The friction of the discs against each other provides the braking effect.

[0037] The first disc 12, on which pistons 2 act (only one shown), is a stator disc fixed to the axle. When the brakes are applied, piston 2 extends out of the sleeve 3 under the effect of hydraulic pressure and compresses the heat sink. When the pressure is released, a spring 20 pushes piston 2 back into the sleeve 3.

[0038] The piston 2 comprises a hollow body 25 and includes a guide 24 which holds the spring 20 between two bearing surfaces: a first surface 240 integral with the guide 24 and a second surface 250 integral with the hollow body 25.

[0039] The piston 2 also includes a rod 21 which serves as an automatic brake wear adjustment device. The rod 21 is housed within a deformation tube 22 and includes at one end 210 a sphere 23 whose diameter is larger than the diameter of the deformation tube 22. As the brake discs wear, the sphere 23 moves backward, deforming the deformation tube 22 and thus shifting the piston stroke to compensate for the wear. The distance between the stator discs 11 and the rotor discs 10, 12 is thus maintained at a constant minimum by the movement of the sphere 23 within the deformation tube 22, ensuring a consistent brake pedal travel regardless of disc wear.

[0040] The operation of the piston 2 is as follows: a hydraulic fluid 4, such as oil, enters one side of the piston 2 and pushes it with a force F to bring it into contact with the first stator disc 12 until the stator discs 12, 10 and the rotor discs 11 are in contact. If the discs show signs of wear, the piston 2 will As the piston advances further, rod 21 will move backward within the deformation tube 22, thus lengthening the stroke of piston 2 by the thickness of the disc wear. When the brakes are released, piston 2 is pushed back by spring 20 located on the opposite side to the hydraulic fluid inlet 4.

[0041] The rod 21 is connected by a second end 211 to the ring 5 of the braking system 1. The rod 21 therefore passes through the second surface 250 of the hollow body 25 via a hole 251. This hole 251 is fitted with a seal 212 which ensures a seal around the rod 21 between the pressurized hydraulic fluid 4 and the outside at atmospheric pressure. When the piston 4 moves, the seal 212 slides on the rod 4, which can lead to degradation of the seal 212 if the surface of the rod 21 does not have sufficient surface hardness and roughness.

[0042] Figure 3 shows two examples of wear on the seal 212 where scratches R are visible. The surface finish required to meet dynamic sealing requirements is therefore crucial. Current requirements for chrome-plated rods call for an average roughness Ra < 0.2 pm. In reality, the roughness is significantly lower (Ra < 0.1 pm) due to the grinding process used to close the crazing of the chrome plating, which would otherwise constitute a potential source of hydraulic leakage. Furthermore, full-scale tests with surface finishes exceeding Ra 0.1 pm show wear on the rod seals 212, which could indicate leakage problems during operation.

[0043] The roughness of a surface is characterized by irregularities and includes several parameters: the average roughness Ra, the average maximum height of the peaks Rp and the maximum amplitude of the profile, Rz, over the measured length.

[0044] A specific study was therefore carried out with rods 21 which were manufactured with different levels of average roughness Ra: 0.04, 0.08, and 0.2. Tribological tests were carried out on these rods and gave the results illustrated in figures 4 and 5.

[0045] It can be seen on [Fig.4] that a chrome rod Te with an average roughness Ra of about 0.05 pm causes an average wear of less than 0.4%, while a first rod Til in nickel alloy with a roughness Ra of less than 0.1 pm causes an average wear of less than 0.6% and a second rod Ti2 in nickel alloy with a roughness close to Ra = 0.2 pm causes an average wear of more than 1.2%.

[0046] It can also be seen in [Fig.5] that the maximum height Rp has an influence on the average wear of the seal.

[0047] The surface of the different rods is visible in [Fig. 6], with, from left to right, the surface of the chrome-plated Te rod, then the two Ti1 and Ti2 rods before and after friction on the seal. The chrome-plated Te rod shows wear striations due to repeated friction with the rod, whereas the nickel alloy rods, such as Inconel 718, do not. They do not show, and are therefore able to withstand wear from the friction of the seal.

[0048] These tests made it possible to define the minimum requirements to be specified in order to ensure performance: Ra max 0.2 pm.

Claims

Demands

1. A surface treatment method for a rod (21) of a piston (2), the rod (21) being made of a high mechanical strength alloy with a minimum hardness greater than 45 HRC, characterized in that it comprises the following steps: - a grinding step and, - a finishing step until an average roughness Ra < 0.2pm is obtained.

2. Surface treatment process according to the preceding claim, characterized in that the alloy is a nickel-based superalloy.

3. Surface treatment process according to the preceding claim, characterized in that the alloy has a tensile strength Rm > 1100MPa.

4. A surface treatment process according to any one of the preceding claims, characterized in that the finishing step is a tribofinishing.

5. Piston rod (21) obtained by the surface treatment process according to any one of the preceding claims.

6. Piston (2) comprising a piston rod (21) according to the preceding claim.

7. Piston (2) according to the preceding claim, characterized in that the piston rod (21) comprises a sealing joint (212) between a pressurized medium and a non-pressurized medium.

8. Piston (2) according to the preceding claim, characterized in that the rod (21) is disposed in a deformation tube (22) and comprises a sphere (23) at a first end (210) cooperating with the deformation tube (22).

9. Braking system (1) characterized in that it comprises at least one piston (2) according to any one of claims 7 to 8.

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