Method for machining a rack and rack machined according to said method

Abstract

The invention relates to a method for machining a rack and to a rack (1) machined according to said method, for example a steering rack. In said method, the stress pattern that is present after hardening and/or straightening the rack and that has a chaotic internal stress distribution of tensile and compressive stresses is converted into a stress pattern that optimizes the strength and the use of the material and also the diameter of the rack, such that, without altering the structure, at least the region of the gear teeth (2) is pre-stressed, in a functionally combined series of steps of a machining pass, with a deliberately introduced internal compressive stress without tensile stress and with a predominantly uniform stress distribution or stress plane.

Claims

1. A method for machining a rack made of a metallic material (1), comprising the steps of: after hardening, or after hardening and straightening, at least in a region of a gear teeth (2) initially a chaotic inherent stress profile (σ) of tensile stresses (σ+) and compressive stresses (σ−) is present, converting the chaotic inherent stress profile (σ) into a more uniform inherent stress profile (σ.sub.perm) without change in microstructure in a functionally combined series of steps of a machining operation, and at least in the region of the gear teeth (2) receiving a pre-stress with defined introduced compressive inherent stresses (σ−.sub.E) without the tensile stresses (σ+), so that a physical system with stress values of the defined introduced compressive inherent stresses (σ−.sub.E) and the more uniform inherent stress profile (σ.sub.perm) of a substantially uniform stress distribution or stress plane is present in at least the region of the gear teeth (2), stress peaks (σ+.sub.peak) or amplitudes of the present tensile stresses (σ+) are intentionally eliminated, smoothed or reduced by introducing near-surface inherent compressive stresses (σ−.sub.E) by shot peening or by inductive heating, after the shot peening or the inductive heating and elimination, smoothing or reduction of the stress peaks (σ+.sub.peak) or amplitudes, the region of the gear teeth (2) attains the pre-stress with the defined introduced compressive inherent stresses (σ−.sub.E) without the tensile stresses (σ+) by way of subsequent steel wire shot blasting or steel shot blasting as well as steel shot peening, to attain the more uniform inherent stress profile (σ.sub.perm); and wherein after hardening, or after hardening and straightening, the tensile stresses (σ+) and the compressive stresses (σ−) in the range of ±<2.0 MPa×10.sup.3 are measured in selectable sections (2.2, 2.3, 2.4), of which at least ±0.5 MPa×10.sup.3 are defined as the stress peaks (σ+.sub.peak) or amplitudes.

2. The method of claim 1, wherein after hardening, or after hardening and straightening, in selectable sections (2.2, 2.3, 2.4) in at least one rack (1) of the gear teeth (2), the tensile stresses (σ+) in an axial direction (x) and the compressive stresses (σ−) in a transverse direction (y) in form of measured values of axial stresses (σ+x) and transverse stresses (σ−.sub.y) are used to define the chaotic inherent stress profile (σ) as a reference stress profile or a theoretically assumed stress profile (σ) for lots, series or sets of the racks (1) to be machined with identical parameters.

3. The method of claim 2, wherein the chaotic inherent stress profile (σ) is measured by X-ray diffraction measurements in at least one near-surface layer of the sections (2.2, 2.3, 2.4) of the gear teeth (2) in the axial direction (x) and the transverse direction (y).

4. The method of claim 2, wherein the axial stresses (σ+.sub.x) and the transverse stresses (σ−.sub.y) are measured up to a defined depth of <100 μm.

5. The method of claim 2, wherein the measured values of the axial stresses (σ+.sub.x) and the transverse stresses (σ−.sub.y) for the defined chaotic inherent stress profile (σ) as a sample with the reference stress profile or the theoretically assumed stress profile (σ) for the lots or the series of the racks (1) to be machined with the identical parameters.

6. The method of claim 1, wherein the shot peening is performed briefly for <10 s.

7. The method of claim 1, wherein the inductive heating takes place in a range of 120° C.-160° C., and wherein, before the inductive heating and after the inductive heating, values according to respective stress states are measured.

8. The method of claim 1, wherein the values of the defined introduced compressive inherent stresses (σ−.sub.E) are introduced by a beam pressure p1 in a range of ±p1.

9. The method of claim 1, wherein the values of the defined introduced compressive inherent stresses (σ−.sub.E) are in a range of >−0.6 MPa×10.sup.3 to <2.0 MPa×10.sup.3 present in selectable sections (2.2, 2.3, 2.4).

10. The method of claim 1, wherein the defined introduced compressive inherent stresses (σ−.sub.E) are controlled and detected as a measured intensity by way of an Almen intensity measurement.

11. The method of claim 10, wherein the chaotic inherent stress profile (σ) according to the Almen intensity measurement is used as a reference stress profile or a theoretically assumed stress profile (σ) for lots or series of racks (1) to be machined with identical parameters.

12. The method of claim 1, wherein the chaotic inherent stress profile (σ) of the tensile stresses (σ+) and the compressive stresses (σ−) is converted into the more uniform inherent stress profile (σ.sub.perm) depending on a type of gear teeth (2) or a characteristic of the metallic material of the rack (1).

13. The method according to claim 12, wherein the type of the gear teeth (2) is a uniform or variable gear teeth.

14. The method of claim 1, wherein dimensioning of a diameter (D) of the rack (1) is optimized.

15. The method of claim 14, wherein the rack (1) is a steering rack of a vehicle.

16. The method of claim 1, wherein values of the defined introduced compressive inherent stresses (σ−.sub.E) are in the range of >−0.6 MPa×10.sup.3 to <2.0 MPa×10.sup.3 present in the gear teeth (2).

17. The method of claim 1, wherein use of the rack (1) with the measured values of the defined introduced compressive inherent stresses (σ−.sub.E) in a range of >−0.6 MPa×10.sup.3 to <2.0 MPa×10.sup.3 as a sample for lots or series of the racks (1) to be machined with identical parameters.

18. The method according to claim 1, wherein the shot peening is glass bead blasting.

19. The method according to claim 1, wherein the inductive heating is stress relieving.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The drawings show in

(2) FIG. 1 a plan view of a rack 1 to be machined according to the invention, shown as a steering rack with a measuring range that is split in a Section 2.2 of the gear teeth 2, defined up to the 5th tooth starting from a pin 1.1, Section 2.3 of the gear teeth 2, defined as the measuring range center, and Section 2.4 of the gear teeth 2, defined up to the 5th tooth starting from a shaft 1.2,
and a detail A from these sections 2.2, 2.3, 2.3 of the gear teeth 2 emphasized for the purpose of illustration, in which the tensile stresses σ+ are measured in an axial direction x and the compressive stresses σ− are measured in a transverse direction y;

(3) FIG. 2.1 the diagram of a measured stress profile σ of tensile stresses σ+ and compressive stresses σ− in the gear teeth 2 according to FIG. 1 after hardening and straightening with identification of stress peaks σ+.sub.peak of the tensile stresses σ+ and stress peaks σ−.sub.peak of the compressive stresses σ− in the rack 1 of FIG. 1, which is used as a basis as designated “σ-profile” of a selected sample of the rack 1 for lots or series of racks 1 to be machined according to the invention with identical parameters as a reference stress profile σ or as a theoretically assumed stress profile σ of the lots or series of racks 1 having the same parameter;

(4) FIG. 2.2 the diagram “σ-profile” of tensile stresses σ+ and compressive stresses σ− in the gear teeth 2 of the rack 1 according to FIG. 1 after inventive elimination or smoothing of stress peaks σ+.sub.peak of the tensile stresses σ+, here referred to as “stress peak smoothing”;

(5) FIG. 2.3 the diagram of a permissible or predetermined stress profile σ.sub.perm of compressive stresses σ− in the gear teeth 2 of the rack 1 according to FIG. 1 according to the invention following a machining operation according to the invention, whereby the “σ-profile” in accordance with FIG. 2.1 was converted into a permissible, not designated stress profile σ.sub.perm and the region of the gear teeth 2 has a pre-stress with intentionally introduced, not designated inherent compressive stresses σ−.sub.E without tensile stresses σ+, here symbolically indicated with arrows, according to which in a first stage, designated as “Process I”, the functionally combined series of steps of a machining operation intentionally smoothed, reduced or eliminated the not designated stress peaks σ+.sub.peak of the tensile stresses σ+ of FIG. 1 by introducing near-surface, not designated inherent compressive stresses σ−.sub.E by way of shot peening, and in a second stage, designated as “Process II”, as a conclusion in the functionally combined series of steps of the machining operation according to the invention following the shot peening or inductive heating and eliminating, smoothing or reducing these stress peaks σ+.sub.peak the allowable inherent stress σ.sub.perm is achieved by a subsequent steel wire shot blasting according to a designated “σ-specification”;

(6) FIG. 3 three bar graphs with the measured values of inherent compressive stresses σ−.sub.E obtained in the gear teeth 2 of FIG. 1, as they were introduced according to the invention by means of shot peening or stress relieving and steel shot blasting with different beam pressures in a machining operation, namely in block diagrams designated with I, II, III;

(7) FIG. 4 an overview, depicted as diagrams, of the detailed stress values achieved in the sections 2.2, 2.3 and 2.4 of the gear teeth 2 of the toothed rack 1 according to FIG. 1;

(8) FIG. 5 a schematic diagram of the disadvantageous “failure-to-reach” the tooth base which is to be avoided when blasting with glass spheres.

(9) Exemplary embodiments of the invention will be explained in more detail below.

DESCRIPTION OF THE INVENTION

(10) For a better understanding of the reproducibility of a rack 1 to be machined according to the invention according to FIG. 1, technological conditions according to FIGS. 2.1, 2.2, 2.3, results according to FIG. 3, and data such as possibly mandatory data according to FIG. 4 are shown for the examples.

(11) In FIG. 1, a rack 1 is used as a basis, having gear teeth 2 with teeth 1 introduced between pin 1.1 and shaft 1.2 by non-cutting shaping, by end faces 1.3 of limited length and a diameter 2.5, which is to be machined according to the invention after hardening and straightening.

(12) In view of the problem initially analyzed, namely that after hardening and straightening of the rack 1, disadvantageous, undefinable or uneven stress states, here referred to as “chaotic”, are present in the gear teeth 2, with a stress profile σ, as shown in FIG. 2.1, of tensile stresses σ+ and compressive stresses σ− in the gear teeth 2 with particularly disadvantageous tensile stresses σ+ and their stress peaks σ−.sub.peak, and externally specified mandatory data, optimizations of the mass and material use while avoiding later cracks or breaks of the rack 1 are required,
the rack 1 must be processed so as to meet the requirements for weight saving by minimizing the rack diameter and thus also the envelope diameter of the gear teeth with a largely uniform stress distribution and without change in the microstructure, avoid the disadvantage of a decreasing fatigue strength of the rack and the formation of cracks or fractures, and offer at all a potential for diameter minimization.

(13) The following symbols designate consistently: σ+ a tensile stress, σ− a compressive stress, σ−.sub.E a compressive inherent stress, σ a stress profile, also “chaotic” as a (inherent) stress profile, and σ.sub.perm a permissible, desired, given or achieved (inherent) stress profile.

(14) According to the invention, in a functionally combined sequence of steps of a machining operation, the (inherent) stress profile σ which is disadvantageous for satisfying the required parameters is to be converted into an allowable (inherent) stress profile σ.sub.perm without changing the microstructure, wherein (at least) the region of the gear teeth 2 receives a pre-stress with defined introduced inherent compressive stresses σ−.sub.E without tensile stresses σ+, so that as a result of the machining a physical system with stress values of inherent compressive stresses σ−.sub.E and the permissible (inherent) stress profile σ.sub.perm of a largely uniform stress distribution or stress plane is present in the region of the gear teeth 2.

(15) The machining method according to the invention is based on the fact that in the gear teeth 2 with teeth 1 necessarily, i.e. measured or not measured or detected or not found, a “chaotic” (inherent) stress profile σ of tensile stresses σ+ and compressive stresses σ− according to FIG. 2.1 is present. In the context of the invention, this state is to be examined and defined on a sample of the rack 1 as follows. As an important aspect of the invention and as a quasi-technological preparation of the rack 1 according to FIG. 1 to be machined with these integrated steps, the section of the gear teeth 2 with the teeth 2.1 (=length of the gear teeth 2) is each divided into a section 2.2 as a measuring range up to the 5th tooth 2.1 starting from the pin 1.1, a section 2.3 as a measuring range center, and a section 2.4 as a measuring range up to the 5th tooth starting from the shaft 1.2.

(16) To this end, the emphasized detail A of the gear teeth 2 shows symbolic vector arrows for the measurement of tensile stresses σ+ in an axial direction x, and for the measurement of compressive stresses σ− in a transverse direction y.

(17) On the basis of this technological system prepared for the inventive machining, the tensile stresses σ+ of the gear teeth 2 in the axial direction x and the compressive stresses σ− in the transverse direction y are detected in the selected (near-surface) areas by X-ray diffraction measurements and recorded as measured values.

(18) Although X-ray diffraction measurements in workpieces are generally known, it should be emphasized that in the case of the present rack 1 measures had already to be developed and tested for the first technological approach that went beyond the actions expected from a skilled artisan, in order to ultimately attain a largely uniform stress distribution and usable results without changes in the microstructure. Since the X-ray diffraction measurement are performed by determining the lattice expansion as a result of inherent stresses in a crystalline lattice, a measurement could be carried out in the defined coordinate system, whereby the stresses are determined from the multiaxial stress state.

(19) Since the gear teeth 2 requires a special adjustment of the X-ray apparatus, for example a diffractometer, on the rack 1, the following criteria must be observed: the choice of areas to be irradiated, and the determination of the measurements concerning the measuring positions, such as interference lines, angular ranges, tilting.

(20) Accordingly, for this technologically preparatory step, the (near-surface layer) sections 2.2, 2.3, 2.4 of the gear teeth 2 to be selected for the x-ray diffraction measurements of the inherent stresses must be determined. Hereby, the present stress profile σ of tensile stresses σ+ and compressive stresses σ− in the gear teeth 2 is determined in the axial direction x and the transverse direction y after hardening and straightening with identification of stress peaks σ+.sub.peak of the tensile stresses σ+ and stress peaks σ−.sub.peak of compressive stresses σ− and recorded as measured values.

(21) This approach according to the invention has the advantage that the “chaotic” stress profile σ of a selected sample of the rack 1 can thereafter be used in practice as a defined reference stress profile σ or as a theoretically assumed stress profile σ for the lots or series of racks 1 to be machined with identical parameters.

(22) FIG. 2.1 shows in form of a diagram a reference stress profile σ or a theoretically assumed stress profile σ for lots or series of racks 1 with identical parameters, from which the stress profile σ and values in the dimension [MPa]×10.sup.3 of tensile stresses σ+ and compressive stresses σ− the gear teeth 2 after hardening and straightening with indicated stress peaks σ+.sub.peak of the tensile stresses σ+ and stress peaks σ−.sub.peak of the compressive stress σ− for the comparison rack 1 shown in FIG. 1 can be collected. This stress system, which has been obtained from a selected sample of the rack 1, can then be used for lots or series of racks 1 to be produced in accordance with the invention and with the same parameters as a reference stress profile or as a theoretically assumed stress profile σ with the values of tensile stresses σ+ and compressive stresses σ−.having the dimension [MPa]×10.sup.3.

(23) FIG. 2.2 shows schematically for the method that and how stress peaks or amplitudes σ+.sub.peak of the tensile stresses σ+ are present in the functional combined series of steps of a machining operation. Accordingly, the stress peaks or amplitudes σ+.sub.peak of tensile stresses σ+ having values with the dimension [MPa]×10.sup.3 can be defined and eliminated according to a first stage referred to as stress peak smoothing, designated in FIG. 2.3 as Process I.

(24) The functionally combined series of steps of a machining operation includes that the stress peaks σ+.sub.peak of tensile stresses σ+ are smoothed, reduced or eliminated by introducing near-surface inherent compressive stresses σ−.sub.E by shot peening, such as blasting with glass beads, as symbolically indicated with arrows according to the first stage as well as in FIG. 2.3 which will be described below in more detail. Shot peening can be carried out for a brief time of <10 s. In blasting with glass beads, the spheres of the blasting material are to be sized so that a respective tooth root radius in the tooth root of the gear teeth 2 can be irradiated, in order to avoid a disadvantageous “failure-to-reach” the tooth root, as schematically illustrated in FIG. 5.

(25) Alternatively, the stress peaks σ+.sub.peak of tensile stresses σ+(and possibly stress peaks σ−.sub.peak of compressive stresses σ−) can be eliminated, smoothed or reduced by inductive heating such as stress relieving. Heating can take place in the range from 120 to 160° C., whereby stress values can be measured before and after heating.

(26) Accordingly, after the shot peening or stress relieving according to FIG. 2.2 (initially as a technological intermediate stage of the method according to the invention), a (inherent) stress profile σ without stress peaks σ+.sub.peak is attained.

(27) According to the invention, a pre-stress with defined inherent compression stresses σ−.sub.E without tensile stresses σ+ is to be introduced in the region of the gear teeth 2 so that by looking ahead, a physical system with stress values of (inherent) compression stresses σ−.sub.E and the permissible (inherent) stress profile σ.sub.perm with a largely uniform stress distribution or stress plane is present in the region of the gear teeth 2. To this end, after shot peening or inductive heating and elimination, smoothing or reduction of the stress peaks σ+.sub.peak or amplitudes, the allowable inherent stress profile σ.sub.per (possibly to be achieved according to the compulsory data) is achieved by a subsequent steel wire shot blasting with a beam pressure p1, in particular in a second stage II according to FIG. 2.3 and as a conclusion within the functionally combined series of steps of the machining operation according to the invention. Thus, the pre-stress with the intentionally introduced compressive inherent stresses σ−.sub.E without tensile stresses σ+ is present in the area of the gear teeth 2, as shown in the diagram in FIG. 2.3 according to a σ-specification.

(28) FIG. 3 shows values of desired compressive inherent stresses σ−.sub.E (and thus an inherent stress profile or σ.sub.perm that is permissible or obtained according to compulsory data) for processed racks 1, as were deliberately introduced by the machining operation according to the invention by shot peening or stress relieving and steel wire shot blasting at beam pressures of, for example, p1 in accordance with block diagram I, 0.5×p1 in accordance with block diagram II, or 1.4×p1 in accordance with block diagram Ill.

(29) The steel wire operating here as strengthening beams can be controlled with regard to the specific values of inherent compressive stresses σ−.sub.E to be introduced after the “Almen test” integrated in the machining operation (with preceding paint test, if applicable) to ensure the quality. While the beam is operated, the operating pressure is controlled, and the rack 1 is available after the “Almen test” quasi as a test specimen, on which (see FIG. 4) it can be demonstrated that as a result of the steps linked with the present invention with the values a1=axial stress beginning, a2=transverse stress beginning, b1=axial stress center, b2=transverse stress center, c1=axial stress end, c2=transverse stress end, a permissible stress plane or stress distribution exists in the sections 2.1, 2.2, 2.3 of the gear teeth 2.

(30) Since values of inherent compressive stresses σ−.sub.E in the range of, for example, >−0.6 MPa×10.sup.3 to <−2.0 MPa×10.sup.3 are possible in the sections 2.2, 2.3, 2.4 of the gear teeth 2, the invention enables the following with respect to the constructive development of racks 1:

(31) In the conventional machining of a rack 1, the designer commonly investigated its cross-sectional behavior and optimization with respect to the rack load, a bending line for the given load case, and the dynamic fatigue tests to achieve the corresponding elastic modulus and the desired fatigue life. He determined, for example, a toothed rod diameter D of 28 mm in the region of the gear teeth 2. Although the inherent compressive stresses—σ−.sub.E or the magnitude and direction of near-surface stresses a could be disregarded, however, tensile stresses σ+ in the gear teeth 2, i.e. the negative “chaotic” stress profile σ of tensile stresses σ+ and compressive stresses σ− (see FIG. 2.1) disadvantageously remained.

(32) The skilled artisan who now accepts the inventive teaching can advantageously deepen the structural optimization of racks 1 by investigating factors important for fatigue strength and service life, such as cross-sectional area and section module of the teeth 2.1 at the tooth root (including tooth root radii and their corresponding notch effect) and tooth width, tooth root plane or surface, furthermore converting the inherent stress profile σ of tensile stresses σ+ and compressive stresses σ− as a function of a variable gear teeth 2 i.sub.var or constant gear teeth 2 i.sub.constant or as a function of the characteristic of a material of the rack 1 into the permissible inherent stress profile σ.sub.perm, eliminating the tensile stresses σ+ through intentional introduction of near-surface inherent compressive stresses σ−.sub.E (e.g. 0.02 mm below the surface) and minimizing the diameter D of the racks 1 to, for example, 26 mm, and even optimizing the dimensioning of the diameter D of the rack 1, such as the steering rack of a vehicle, with the same axle loads of a vehicle.

(33) This is possible because the method according invention provides a rack 1 which allows values of compressive inherent stresses σ−.sub.E in the range of >−0.6 MPa×10.sup.3 to <−2.0 MPa×10.sup.3.

(34) FIG. 4 shows how the stress ratios which originally act unfavorable in the physical stress system of the microstructure, were converted as a result of the steps associated with the invention with the above defined values a1, a2; b1, b2; c1, c2 into a permissible stress plane or stress distribution in the sections 2.2, as measured on the 5th tooth starting from the pin 1.1, 2.3, in the measuring range center, 2.4, as measured on the 5th tooth starting from the shaft 1.2
of the gear teeth 2. Thus, default values such as external compulsory data can be compared to the achieved actual values and confirmed to be satisfied, namely that an advantageous pre-stress from inherent compressive stresses—σ−.sub.E without detrimental tensile stresses σ+ is present in the gear teeth 2.

(35) The invention is technologically applicable with relatively low costs and makes it possible to meet the demand for saving weight of racks by minimizing the rack diameter. The durability of racks is satisfied, and cracks or breaks in the gear teeth are avoided. In particular, in automotive steering systems, the demand for higher axle loads of vehicles can be realized.

LIST OF REFERENCE NUMERALS AND SYMBOLS USED

(36) 1=rack 1.1=pin 1.2=shaft 1.3=front side 2=gear teeth 2.1=tooth 2.2=section of the gear teeth 2 as a measuring range starting from the pin 1.1 2.3=section of the gear teeth as measuring range center 2.4=section of the gear teeth 2 as a measuring range starting from the shaft 1.2 2.5=diameter of the rack 1 x=axial direction y=transverse direction σ=stress profile, also “chaotic” (inherent) stress profile σ.sub.perm=permissible, desired, defined (inherent) stress profile, also referred to as “σ-specification” σ+=tensile stress σ−=compressive stress σ−.sub.E=compressive inherent stress σ+.sub.peak=Stress peak or amplitude of tensile stresses σ+.sub.x=axial stress σ+y=transverse stress i.sub.var=variable gear teeth i.sub.constant=constant gear teeth ±p1=variable beam pressure a1=axial stress value beginning a2=transverse stress value beginning b1=axial stress value middle b2=transverse stress value center c1=axial stress value end c2=transverse stress value end