Electromechanical actuator

Abstract

The invention relates to an electromechanical actuator (1), comprising a screw drive (2, 8) in the form of a planetary rolling screw drive (2, 8), wherein the planetary rolling screw drive (2, 8) provided is a true-pitch screw drive having a driven cage (5), guiding a plurality of planets (4), and having a threaded spindle (2), wherein the threaded spindle (2) and / or the planets (4) is / are formed from a steel of the following composition: --- C: 0.4 to 1.5% by weight, --- Mn: 12.0 to 22.0% by weight, --- Cr: up to 4.0% by weight, --- Ni: up to 0.5% by weight, --- Cu: up to 0.3% by weight, --- V: up to 0.3% by weight, --- S: up to 0.3% by weight, --- P: up to 0.1% by weight, --- Si: up to 4.0% by weight, --- Al: up to 0.05% by weight, --- the remainder: iron and smelting-induced impurities, on the surface of which there is, at least in the region of a thread (3) produced by a forming process on the threaded spindle (2) and / or the planets (4), a finish produced by martensite precipitation and strain hardening.

Description

[0001] Electromechanical actuator

[0002] The invention relates to an electromechanical actuator with a screw drive and a threaded spindle.

[0003] WO 2012 / 048917 A1 discloses a heat-treatable steel and its use as bar stock, particularly for the production of a threaded spindle. WO 2012 / 048917 A1 assumes that the material Cf53, as a widely used steel, is suitable for surface hardening or surface hardening and can be used, among other things, for the production of bolts, worm gears, gear wheels, shafts, or spindles. The composition of the steel Cf53 is suitable, in the sense of rolling bearing technology, for producing a fully load-bearing, martensitic hardened surface layer.

[0004] Based on this, WO 2012 / 048917 A1 describes an advanced tempering steel Cf53B, which achieves a hardening depth (600 HV) between 4 mm and 6 mm, measured from the end face of a quenched end specimen. The core structure typically consists of a mixture of pearlite and ferrite. The tempering steel according to WO 2012 / 048917 A1 contains, among other ingredients, 0.0031% to 0.005% boron and at least 3.5 parts titanium to one part nitrogen. The tempering steel Cf53B is suitable, among other things, for the production of a threaded spindle for a ball screw drive.

[0005] A ball screw drive known from DE 10 2017 121 942 A1, which is intended for use in a brake booster, has a threaded nut made of martensitic-hardened steel that is resistant to brake fluid rust. The steel contains at least 12 wt.% chromium. In addition, the steel can contain, among other things, 0.4% to 1.3% carbon, up to 2% silicon, up to 2% manganese, and up to 2% molybdenum. In particular, this can be steel with the material number 1.4108. DE 10 2017 121 942 A1 specifies a hardness of 55 HRC as the achievable hardness of the threaded nut.

[0006] EP 2 832 876 A1 describes a high-strength stainless steel wire that is said to exhibit excellent heat deformation resistance. The steel wire is said to be particularly suitable for the production of a high-strength spring. The spring is manufactured from cast steel, with a deformation-induced martensite formation index that must be within a specified range. NiAl-based composite particles with particle sizes of 50 nm or less can be contained in a matrix of the steel wire from which the high-strength spring is formed.

[0007] EP 2 465 964 A1 describes a Hadfield steel composition comprising 0.9 to 1.35 wt.% carbon, 11 to 14 wt.% manganese, a maximum of 0.8 wt.% silicon, a maximum of 0.07 wt.% phosphorus, a maximum of 0.05 wt.% sulfur, at least 0.01 wt.% hafnium, the remainder being iron and impurities. Such steels are non-magnetic, have low conductivity, and exhibit improved impact toughness through cold working.

[0008] EP 0 142 873 A1 discloses an austenitic manganese steel with 0.8 to 1.8 wt.% carbon, 6 to 18 wt.% manganese, 0 to 3 wt.% chromium, 0 to 2 wt.% nickel, 0 to 2.5 wt.% molybdenum, 0 to 1 wt.% silicon, at least 0.01 wt.% titanium, at least 0.01 wt.% vanadium, a total of 0.05 to 0.08 wt.% titanium and vanadium, and the remainder iron, wherein a ratio of carbon to manganese is in the range of 1:8 to 1:14. Such a steel has a hardening capacity during cold working.

[0009] DE 28 53 582 A1 describes a non-magnetic steel alloy with not more than 1.5 wt.% carbon, 0.1 to 1.5 wt.% silicon, 5 to 30 wt.% manganese, 0.005 to 0.5 wt.% nitrogen and at least one element from the group comprising 0.05 to 1 wt.% sulfur, 0.05 to 1 wt.% lead, 0.05 to 1 wt.% selenium, 0.01 to 0.5 wt.% tellurium, 0.001 to 0.05 wt.% calcium and the remainder iron. GB 276 048 A describes another Hadfield manganese steel with at least 11 wt.% manganese and a maximum of 1.6 wt.% carbon.

[0010] The “Material Data Sheet 1.3401 / X120Mn12”, Team Edelstahl, 2020, discloses a composition of a manganese steel with 1.1 to 1.3 wt% carbon, 12 to 13 wt% manganese, 0.3 to 0.5 wt% silicon, a maximum of 0.1 wt% phosphorus, a maximum of 0.04 wt% sulfur and a maximum of 1.5 wt% chromium.

[0011] The dissertation “High-strength stainless austenitic CrMn steels”, Sascha Riedner, Ruhr University Bochum, 2010, describes, among other things, an investigation on austenitic steel of the type X120Mn12, which achieves a surface hardness of up to 700 HV through local work hardening under impact or shock loading.

[0012] Furthermore, reference is made to the following dissertation, which focuses in particular on sheet metal forming for automotive applications:

[0013] "Deformation-induced martensite formation during multi-stage forming and its use for optimizing the HCF and VHCF properties of austenitic stainless steel sheet," Dipl.-Wirt.-Ing. Carsten Müller-Bollenhagen, Department of Mechanical Engineering at Faculty IV of the University of Siegen, April 2011

[0014] The dissertation deals, among other things, with phase transformations of metastable austenite.

[0015] With regard to known compositions and properties of manganese steel, reference is made to documents DE 28 46 930 A1, EP 2 803 736 A1, DE 866 893 B, WO 2017 / 021459 A1, and EP 0 205 869 A1 as examples. In general, manganese steel is characterized by high wear resistance, particularly under shock or impact loads.

[0016] The invention is based on the object of achieving material-technical advances for an electromechanical actuator compared to the cited prior art, with particular consideration being given to the aspect of wear occurring in screw drives, for example in electric actuators.

[0017] This object is achieved according to the invention by an electromechanical actuator which operates with a screw drive and has the features of claim 1. The electromechanical actuator comprises a screw drive in the form of a planetary roller screw drive, wherein the planetary roller screw drive is a pitch-true screw drive with a driven cage which guides a plurality of planets, and with a threaded spindle, wherein the threaded spindle and / or the planets is / are formed from a steel of the following composition: o C: 0.4 to 1.5 wt.% o Mn: 12.0 to 22.0 wt.% o Cr: up to 4.0 wt.% o Ni: up to 0.5 wt.% o Cu: up to 0.3 wt.% o V: up to 0.3 wt.% o S: up to 0.3 wt.% o P: up to 0.1 wt.% o Si: up to 4.0 wt.% o Al: up to 0.05 wt.-% o Rest: Iron and impurities resulting from the melting process, on the surface of which there is martensite precipitation and work hardening at least in the area of ​​a thread produced by forming on the threaded spindle and / or the planets.

[0018] A steel with the following composition is selected as the starting material for the production of the threaded spindle and / or the planets: o C: 0.4 to 1.5 wt.% o Mn: 12.0 to 22.0 wt.% o Cr: up to 4.0 wt.% o Ni: up to 0.5 wt.% o Cu: up to 0.3 wt.% o V: up to 0.3 wt.% o S: up to 0.3 wt.% o P: up to 0.1 wt.% o Si: up to 4.0 wt.% o Al: up to 0.05 wt.% o Rest: iron and impurities from the melting process

[0019] In particular, the manganese content is in the range of 12.0 to 14.0 wt.% and the chromium content is a maximum of 1.8 wt.%.

[0020] This starting product, which is in rod-shaped form, is deformed during the manufacture of the threaded spindle and / or planets in such a way that martensite precipitation and work hardening occur on its surface, at least in the area of ​​a thread to be produced using forming processes. Austenitic manganese steel with the material number 1.3401 (X120Mn12) has proven to be particularly suitable for the manufacture of the threaded spindle and / or planets. This steel contains 1.1% to 1.3% carbon, 12 to 13% manganese, 0.3 to 4.0% silicon, up to 0.1% phosphorus, up to 0.04% sulfur and up to 1.5% chromium, the remainder being iron and melting-related impurities, each given as a percentage by weight, and it has a high level of wear resistance, particularly under shock or impact loads.The manganese steel mentioned, which is also known as manganese hard steel, has otherwise proven itself as a material for the production of excavator teeth or jaw crushers, for example, and is particularly suitable for hot forming in the temperature range from 850 °C to 1050 °C.

[0021] Surprisingly, it has been shown that a lower carbon content, compared to steel with the material number 1.3401, has positive effects. For example, a carbon content in the lower range of the specified interval, for example, a C content (in wt. %) in the range of 0.4% to 0.8%, in the range of 0.4% to 0.6%, or in the more narrow range of a minimum of 0.4% and a maximum of 0.5%, ensures that the hardening is less abrupt and thus allows for greater degrees of deformation. In these cases, the steel is also referred to as a weakened manganese steel.

[0022] The deformation of the rod-shaped starting material, which leads to martensite precipitation and work hardening, can initially involve a drawing process. During this drawing process, a rod that is available as a preliminary product and does not yet have a thread structure can be stretched. This has the advantage that intermediate products of uniform shape and quality created by the drawing process can be made available for further processing into different end products, i.e. in particular lead screws and / or planets with different thread profiles. The term "lead screw" is also used in cases in which a screw has a pitchless, i.e. groove-like, profile. In all cases, the initial drawing process benefits the mechanical load-bearing capacity of the end product, i.e. the lead screw and / or the planets.Within the actuator, the threaded spindle is subjected to considerable axial forces in interaction with the existing counterpart, in particular in the form of a nut, a roller or a worm, whereby steep force increases can occur.

[0023] Both in process variants with an initial drawing process and in variants without such plastic elongation of the rod-shaped starting material, thread forming by thread rolling is considered. Optionally, in addition to work hardening, heat treatment is also considered, whereby in each case, the thread formation plays a key role in martensite precipitation and work hardening. The heat treatment can be designed in several stages and, in particular, include a subsequent heat treatment, i.e., tempering. Deep freezing of the rod-shaped material in an intermediate step can also be provided.

[0024] Such steps, which follow heating the material to a temperature of more than 1,000 °C and quenching, can reduce stresses in the forming area and stabilize the microstructure. Regarding the forming of the rod-shaped starting material, forging, particularly of an end section of this material, can also be provided. At least a final machining of the thread can also be performed in a known manner by machining.

[0025] The plastic forming process described, in various variants, is particularly suitable for the production of threaded spindles with virtually undetectable, extremely low distortion, as well as planetary gears. A core strength of the workpiece of 800 MPa to 1080 MPa and a surface hardness of 650 HV and more can be achieved. This applies both to cases where the workpiece is a threaded spindle and to cases where components that interact directly or indirectly with a threaded spindle, such as nuts, bolts, rollers, or planetary gears, are machined as workpieces.

[0026] The threaded spindle is a spindle of a planetary roller screw drive, whereby its planets also represent profiled shafts that can be manufactured from rod-shaped starting material of the composition specified above.

[0027] The electromechanical actuator is preferably used as a steering actuator of a motor vehicle, i.e., as an actuator for a front-axle or rear-axle steering system. Examples in this context are documents DE 10 2019 103 385 A1 and DE 10 2011 082 514 A1. Alternatively, the actuator according to the application can be used, for example, in an actuating mechanism of a stationary industrial plant.

[0028] The screw drive is designed as a planetary roller gear, providing a rotational drive to the cage that guides the planets of the screw drive. The planetary roller gear is designed as a pitch-stable screw drive, which allows for a less extreme transmission ratio compared to planetary roller gears with a driven threaded spindle or driven spindle nut.

[0029] An embodiment of the invention is explained in more detail below with reference to a drawing. In the drawings:

[0030] Fig. 1 shows a detail of a steering actuator with a planetary roller screw drive intended for use in a rear axle steering system, Fig. 2 shows a flow chart showing steps in the production of a threaded spindle of the steering actuator,

[0031] Fig. 3 shows in a diagram the dependence of the hardness of the workpiece on the distance from the workpiece surface in the threaded spindle according to the application and in a comparative example not claimed,

[0032] Fig. 4 shows data recorded in a further diagram during a tensile test on the threaded spindle.

[0033] An actuator identified overall by the reference numeral 1 is designed in the present case as an electromagnetic steering actuator for a rear axle steering system of a motor vehicle.

[0034] The actuator 1 comprises a threaded spindle 2, which is movable in its longitudinal direction to vary the steering angle of the rear wheels of a motor vehicle (not shown). The threaded spindle 2 is aligned in the transverse direction of the vehicle. Regarding the basic structure and function of the actuator 1, reference is made to the cited prior art.

[0035] Several planets 4, which are guided in a cage 5, roll on the thread of the threaded spindle 2, designated 3 in this case as a single-start thread. The cage 5 includes cage discs 6 on both end faces of the planets 4 and a cage sleeve 7, which surrounds the entire planetary gear 4 in a ring and is arranged concentrically to the central axis of the threaded spindle 2, designated MA, and thus of the entire actuator 1. The planets 4 and the cage 5 are components of a nut assembly, designated 8 overall. External teeth 9 are formed on the outer circumferential surface of the cage sleeve 7, enabling the entire cage 5 to be driven by means of a belt drive (not shown). The cage 5 is used as a rotating drive element, with the planetary roller screw drive of the actuator 1, formed from the threaded spindle 2 and the nut assembly 8, being designed as a pitch-stable planetary roller screw drive (SPWG).

[0036] Each planet 4 has a central section 10 and two adjoining, comparatively thin end sections 11. Each of the sections 10, 11 has a profile 12, 13 which, in contrast to the thread 3, is designed in the form of pitchless grooves. Only the central sections 10 of the planets 4 contact the threaded spindle 2. The end sections 11 of the planets 4, in contrast, are raised from the thread 3 and instead engage in profiles 17 formed by nut parts 14, 15. The nut parts 14, 15, which are part of the nut arrangement 8, are adjusted such that a preload is provided between the nut parts 14, 15, the planets 4, and the threaded spindle 2.

[0037] The relative positioning of the nut parts 14, 15 to one another is fixed by a lock nut 16. The inherently rigid assembly consisting of the screwed-together nut parts 14, 15 and the lock nut 16 is rotatably mounted in the cage 5 by means of two axial bearings 18. No drive power is fed into the nut parts 14, 15. The entire nut assembly 8 is mounted by means of two angular contact roller bearings 19 in a surrounding structure (not shown), i.e., an actuator housing. Attached to the actuator housing, among other components, is an electric motor, which drives the belt drive that rotates the cage 5. Alternatively, the electric motor can be built into the actuator housing.

[0038] During operation of the electromechanical actuator 1, i.e., the steering actuator, rapidly increasing, almost sudden loads acting in the longitudinal direction of the threaded spindle 2 can occur. The thread 3, in particular, must be able to withstand these loads. Reference is made below to the flowchart shown in Figure 2, in which steps S1 to S5 denote individual manufacturing steps in the production of the threaded spindle 2. Alternatively or additionally, the planetary gears 4 can be manufactured in a similar manner.

[0039] In step S1, round steel is provided as the starting product. This is manganese steel X120Mn12 (material number 1.3401). Before further processing, the round steel can be plastically stretched – still in step S1 – which already has a positive effect on the desired hardening of the steel.

[0040] The starting product prepared in step S1 is ground and rolled in steps S2 and S3. Rolling produces, in particular, thread 3. The martensite precipitation that occurs during forming is crucial for the hardening.

[0041] Furthermore, a heat treatment takes place in step S3. In step S4, the threaded spindle 2 is machined by turning. The machining may also include other machining technologies, in particular milling. In the final step S5, the workpiece, i.e., the threaded spindle 2, is washed.

[0042] The mechanical properties of the threaded spindle 2 manufactured using the process shown in Figure 2 are shown in Figures 3 and 4. Figure 3 shows the hardness curve in the work-hardened state (bold line) as well as in the work-hardened and heat-treated state (top, thin line). For comparison, the hardness (300 HV) in the solution-annealed state is shown. As can be seen from Figure 3, a surface hardness of approximately 550 HV is achieved through work hardening alone. The dashed line refers to work hardening with an increased degree of deformation. The additional heat treatment increases the surface hardness to at least 650 HV. The simplified diagram shown in Figure 4 shows the yield strength ÖE (corresponding force: F) and tensile strength ÖB (corresponding force: Z), which can be determined using a tensile test.As can be seen from Figure 4, the force increases steadily with increasing elongation of the manganese steel from which the threaded spindle 2 is made. The core strength of the machined manganese steel used in the threaded spindle 2 ranges from 800 MPa to 1,080 MPa.

[0043] List of reference symbols

[0044] 1 actuator

[0045] 2 threaded spindles

[0046] 3 threads

[0047] 4 Planet

[0048] 5 cage

[0049] 6 cage disc

[0050] 7 Cage sleeve

[0051] 8 Nut arrangement

[0052] 9 External gearing

[0053] 10 middle section of a planet

[0054] 11 End section of a planet

[0055] 12 Profiling of the middle section

[0056] 13 Profiling of the end section

[0057] 14 Female part

[0058] 15 Female part

[0059] 16 Lock nut

[0060] 17 Profiling of a female part

[0061] 18 thrust bearings

[0062] 19 angular contact roller bearings

[0063] ÖE yield strength

[0064] ÖB tensile strength

[0065] F Force

[0066] MA central axis

[0067] S1 , ... S5 steps

[0068] Z Force

Claims

Patent claims 1. Electromechanical actuator (1) comprising a screw drive (2, 8) in the form of a planetary roller screw drive (2, 8), wherein a pitch-accurate screw drive with a driven cage (5) guiding a plurality of planets (4) is provided as the planetary roller screw drive (2, 8), and with a threaded spindle (2), wherein the threaded spindle (2) and / or the planets (4) is / are formed from a steel of the following composition: o C: 0.4 to 1.5 wt.% o Mn: 12.0 to 22.0 wt.% o Cr: up to 4.0 wt.% o Ni: up to 0.5 wt.% o Cu: up to 0.3 wt.% o V: up to 0.3 wt.% o S: up to 0.3 wt.% o P: up to 0.1 wt.% o Si: up to 4.0 wt.% o Al: up to 0.05 wt.% o Rest: iron and impurities caused by melting, on the surface of which there is martensite precipitation and work hardening at least in the area of ​​a thread (3) produced by forming on the threaded spindle (2) and / or the planets (4).

2. Electromechanical actuator (1) according to claim 1, wherein the threaded spindle (2) and / or the planets (4) is / are made of manganese steel (material number 1.3401).

3. Electromechanical actuator (1) according to claim 1 or 2, wherein the threaded spindle is formed from a rod-shaped starting material made of steel.

4. Electromechanical actuator (1) according to claim 3, wherein the rod-shaped starting material is formed in a drawing process.

5. Electromechanical actuator (1) according to claim 3 or claim 4, wherein at least one section, in particular end section, of the rod-shaped starting material is forged.

6. Electromechanical actuator (1) according to one of claims 1 to 5, wherein the work hardening of the threaded spindle (2) and / or the planets (4) is at least partially produced by thread rolling.

7. Electromechanical actuator (1) according to one of claims 1 to 6, wherein the threaded spindle and / or the planet (4) is / are further heat-treated.

8. Electromechanical actuator (1) according to one of claims 1 to 7, which is designed as a steering actuator.

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