Brake booster for a vehicle's braking system, as well as transmission and drive assembly for an electromechanical brake booster

Abstract

A gearbox for an electromechanical brake booster comprises a worm shaft made of a metal material with external teeth, which in a first axial section has a first pitch and in a second axial section adjoining the first axial section has a second pitch which is greater than the first pitch, and a worm wheel made of a plastic material with external teeth which engage with the external teeth of the worm shaft such that at least one tooth of the external teeth of the worm wheel engages in the first axial section of the worm shaft and another tooth engages at least partially in the second axial section of the worm shaft.

Description

Technical FieldThe present invention relates to an electromechanical brake booster for a brake system of a vehicle, a transmission and a drive assembly for an electromechanical brake booster.Prior ArtIn electromechanical brake boosters, a torque is generated by means of an electric motor, which is converted via a transmission into a movement of a piston of a master brake cylinder in order to increase or decrease hydraulic pressure by means of a brake fluid. Typically, the electric motor drives a worm shaft which is in engagement with a worm wheel. In order to achieve cost advantages, it is desirable to make the worm wheel from a plastic material. In particular at high temperatures and / or high torques, however, relatively strong deformations of the worm wheel can occur if it is manufactured from a plastic material. In this case, there is the risk that so-called graduation errors occur in the transition between the loaded and unloaded plastic teeth of the worm wheel. That is, when a tooth of the worm wheel engages with a gear of the worm shaft, high pressures may occur between the tooth flank of the resin tooth and the tooth of the worm shaft defining the gear. This can lead to damage to the plastic tooth.DE 10 2006 027 039 A1 describes an electromechanical brake booster having a transmission which has a variable transmission ratio.Disclosure of the InventionAccording to the invention, a transmission having the features of claim 1, a drive assembly having the features of claim 8 and a brake booster having the features of claim 9 are provided.According to a first aspect of the invention, a gear for an electromechanical brake booster comprises a worm shaft formed from a metal material and having an external toothing which has a first pitch in a first axial section and a second pitch in a second axial section which adjoins the first axial section, which second pitch is greater than the first pitch, and a worm wheel formed from a plastic material and having an external toothing which is in engagement with the external toothing of the worm shaft such that at least one tooth of the external toothing of the worm wheel is in engagement in the first axial section of the worm shaft and a further tooth is in engagement at least partially in the second axial section of the worm shaft.According to a second aspect of the invention, a drive assembly for an electromechanical brake booster comprises a gear according to the first aspect of the invention and an electric motor which is connected to the worm shaft.According to a third aspect of the invention, a brake booster for a brake system of a vehicle comprises a drive assembly according to the second aspect of the invention and a master brake cylinder having a piston which is kinematically coupled to the transmission.An idea on which the invention is based consists in modifying the pitch or pitch of the external toothing of the worm shaft along the length of the worm shaft, wherein the external toothing has a first pitch in a first axial section of the worm shaft and a larger, second pitch in a second axial section adjoining the first axial section. The worm wheel is engaged with the worm shaft in such a way that, when the worm shaft rotates in a first rotational direction, a respective tooth of the external toothing of the worm wheel first engages with the external toothing of the worm shaft in the second axial section, in which the external toothing of the worm shaft has the greater pitch, and then engages with the external toothing of the worm shaft in the first axial section.Due to the greater pitch of the external toothing of the worm shaft in the second axial section, the pitch of the external toothing of the worm shaft is increased there in the region of the external toothing of the worm shaft with which a respective tooth of the worm wheel first engages when rotated in a predetermined direction. Thus, more axial clearance is available for the respective tooth of the worm wheel at the beginning of engagement. Thus, even in the case of a deformation of the worm wheel, which can occur, for example, at high loads and / or temperature due to the elastic properties of the plastic material of the worm wheel, a secure engagement of the external toothing of the worm wheel and the external toothing of the worm shaft can be facilitated and the risk of damage to the worm wheel is reduced.A further advantage of the worm shaft according to the invention with different pitches of the external toothing in the first and second axial sections is that the load on the meshing teeth of the worm wheel is distributed more uniformly over the entire meshing region. As a result, contact pressures on the tooth flanks of the teeth of the worm wheel and stresses on the tooth root fillets of the teeth of the worm wheel are advantageously reduced. Furthermore, by the configuration of the worm shaft according to the invention, a smaller toothing on the worm wheel can be realized and thus a more compact construction can be achieved.Advantageous embodiments and refinements emerge from the further dependent claims and from the description with reference to the figures of the drawing.According to some embodiments, it can be provided that the pitch of the external toothing of the worm shaft is constant within the second axial section. Alternatively, it can be provided that the pitch of the external toothing of the worm shaft changes within the second axial section, e.g. increases with increasing distance from the first axial section.According to some embodiments, it can be provided that the second axial section extends as far as one end of the worm shaft. This simplifies the structural design and thus the production of the worm shaft.According to some embodiments, it may be provided that the second axial section has a length which lies in a range between 15 percent and 50 percent of an overall length of the worm shaft.According to some embodiments, it can be provided that the worm wheel is formed from a thermoplastic material, in particular from polyoxymethylene, POM for short, or polyetheretherketone, PEEK for short. By realizing the worm shaft with a greater pitch in the second axial section, tribologically well suited plastics, such as POM or PEEK, can be used, although these have a relatively low mechanical strength compared to metal materials.According to some embodiments, it can be provided that the gear has a rotationally fixed threaded spindle with an external thread, which is in engagement with an internal thread of the worm wheel. The worm wheel can have, for example, a central recess which is provided with an internal thread, wherein the threaded spindle extends through the central recess and is in engagement with the internal thread. The threaded spindle can be connected in a rotationally fixed manner to a plate which is mounted in an axial guide, for example on a housing, in an axially displaceable manner. The rotation of the worm wheel thus results in an axial displacement of the threaded spindle. Optionally, the piston of the master brake cylinder can be coupled to the threaded spindle.The invention is explained below with reference to the figures of the drawings. Of the figures, FIG. 1 shows a schematic sectional view of a drive assembly according to an exemplary embodiment of the invention; FIG. 2 is a partial sectional view of a transmission according to an embodiment of the invention; and FIG. 3 shows a schematic sectional view of a brake booster according to an exemplary embodiment of the invention.In the figures, the same reference numerals designate identical or functionally identical components, unless indicated to the contrary.FIG. 1 schematically shows a drive assembly 200 for an electromechanical brake booster 300 which can be used, for example, in a hydraulic brake system of a vehicle. The drive assembly 200 includes a transmission 100 and an electric motor 210.As shown purely schematically and greatly simplified in FIG. 1, the gear 100 has a worm shaft 1, a worm wheel 2 and an optional spindle 3.The worm shaft 1 is rotatably supported about a first rotation axis A 1 and extends between a first end 1A and a second end 1B located opposite the first end 1A along the rotation axis A 1. The worm shaft 1 is kinematically coupled to the electric motor 210 and is rotatable or drivable by the latter about the axis of rotation A 1. For example, as schematically shown in FIG. 1, the first end 1A of the worm shaft 1 may be connected or coupled to a motor shaft 211 of the electric motor 210.As is indicated purely schematically in FIG. 1 by the dashed lines and is shown in more detail in FIG. 2, the worm shaft 1 has an external toothing 10. For example, the worm shaft 1 may have a tooth which extends helically or spirally on the outer periphery of the worm shaft 1. The external toothing 10 of the worm shaft 1 has a predetermined pitch. The pitch can also be expressed as pitch s11, s12. As is merely indicated symbolically in FIG. 1, the external toothing 10 has a first pitch in a first axial section 11 and a second pitch in a second axial section 12 which adjoins the first axial section 11 along the axis of rotation A 1, which second pitch is greater than the first pitch. The first axial portion 11 may extend from the first end 1A of the worm shaft 1, for example, and have a predetermined length L 11. The second axial section 12 extends from the end of the first axial section 11 with a predetermined length L 12 and can optionally extend as far as the end, here the second end 1B of the worm shaft 1, as shown in FIG. 1 purely by way of example. Optionally, it can be provided that the second axial section 12 has a length L 12 which lies in a range between 15 percent and 50 percent of an overall length L 1 of the worm shaft 1, measured from the first to the second end 1A, 1B.FIG. 2 shows the external toothing 10 of the worm shaft 1 in section. In the first axial region, the external toothing 10 has a first pitch s 11. This can be constant within the first axial section 11, in particular. In the second axial section 12, the external toothing 10 has a second pitch s 12 which is greater than the first pitch s 11. In FIG. 2, the line x 12 in the second axial section 12 represents the actual course of the toothing 10, while the line x 1 is only represented as a reference and indicates a pitch corresponding to the first pitch s 11. The second pitch of the external toothing 10 of the worm shaft 1 can be constant, for example, within the second axial section 12. Alternatively, the pitch of the external toothing 10 of the worm shaft 1 can also change within the second axial section 12. It is thus shown purely by way of example in FIG. 2 that the second gradient in the second axial section 12 increases with increasing distance from the first axial section 11, e.g. increases linearly. Also, a progressive increase or other change along the rotational axis A 1 may be provided within the second axial portion 12.The worm shaft 1 is formed of a metal material.The worm wheel 2 is rotatably mounted about a second axis of rotation A 3 which extends transversely to the first axis of rotation A 1, as is schematically illustrated in FIG. 1. The worm wheel 2 has an external toothing 20, which is illustrated only symbolically as a dashed line in FIG. 1. As shown in detail in FIG. 2, the external toothing 20 of the worm wheel 2 comprises a plurality of teeth 21 arranged one after the other along the circumference of the worm wheel 2. The worm wheel 2 is formed from a plastic material, in particular from a thermoplastic plastic material such as, for example, polyoxymethylene or polyetheretherketone.As shown in FIGS. 1 and 2, the external teeth 20 of the worm wheel 2 are engaged with the external teeth 10 of the worm shaft 1. A rotation of the worm shaft 1 about the first axis of rotation A 1 thus causes a rotation of the worm wheel 2 about the second axis of rotation A 3. When worm shaft 1 rotates about first axis of rotation A 1 in a first direction of rotation D 1, this causes worm wheel 2 to rotate about second axis of rotation A 3 in a direction of rotation D 2 (clockwise in the illustration of FIG. 2 ), as is symbolically illustrated in FIG. 2.As can also be seen in FIG. 2, the external toothing 20 of the worm wheel 2 is in engagement with the external toothing 10 of the worm shaft 1 in such a way that at least one tooth 21, 21A of the external toothing 20 of the worm wheel 2 is in engagement in the first axial section 11 of the worm shaft 1 and a further tooth 21, 21B is in engagement at least partially in the second axial section 12 of the worm shaft 1. During the rotation of the worm shaft 1 in the first rotational direction D 1, a respective tooth 21 of the external toothing 20 of the worm wheel 2, e.g. the tooth 21C, first engages with the external toothing 10 thereof in the second axial section 12 of the worm shaft 1. Upon further rotation of the worm shaft 1 in the first direction of rotation, the tooth 21 in question only comes into engagement with the external toothing 10 of the worm shaft 1 subsequently, i.e. after it has been "tracked" in the second axial section 12, in the first axial section 11.As can be seen in FIG. 2 at the tooth 21B, the greater pitch s 12 of the external toothing 10 of the worm shaft 1 in the second axial section 12 creates a certain free space or a certain tolerance t 12 between the external toothing 10 of the worm shaft 1 and the respective tooth 21 ( 21B) of the external toothing 20 of the worm wheel 2. Thus, an undesirably high load of the tooth 21 ( 21B) of the external toothing 20 of the worm wheel 2 is effectively counteracted even if the worm wheel 2 deforms, e.g. due to a high torque acting in the opposite direction to the rotational direction D 2.The optional threaded spindle 3 has an external thread 30, which is in engagement with an internal thread 25 of the worm wheel 2. As schematically shown in FIG. 2, the worm wheel 2 may have a bore that is positioned coaxially with the second axis of rotation A 3 and is provided with the internal thread 25. The threaded spindle 3 is mounted in a rotationally fixed manner and is thus linearly displaceable along the second axis of rotation A 3 by rotation of the worm wheel 2.FIG. 3 shows, purely schematically and greatly simplified, an electromechanical brake booster 300. This comprises the drive unit 200 and a master brake cylinder 310, in which a piston 311 is mounted so as to be linearly displaceable in order to displace brake fluid for pressure build-up or pressure reduction. As is schematically shown in FIG. 3, the spindle 3 can be connected, for example, in a rotationally fixed manner to a plate 4 which is in turn mounted on a linear guide 5, for example on a housing, such that it can be displaced parallel to the second axis of rotation A 3. The piston 311 is kinematically coupled to the spindle 3, e.g. via a punch 6 or the like. A torque generated by the electric motor 210 can thus be converted via the transmission 100 into a linear displacement of the piston 311.Although the present invention has been explained above by way of example with reference to exemplary embodiments, it is not restricted thereto, but can be modified in a variety of ways. In particular, combinations of the above exemplary embodiments are also conceivable.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 10 2006 027 039 A1

[0003]

Claims

A gear (100) for an electromechanical brake booster (300), comprising: a worm shaft (1) formed from a metal material and having an external toothing (10), which has a first pitch in a first axial section (11) and a second pitch in a second axial section (12), which adjoins the first axial section (11), which is greater than the first pitch; and a worm wheel (2) formed from a plastic material and having an external toothing (20), which is in engagement with the external toothing (10) of the worm shaft (1) such that at least one tooth (21) of the external toothing (20) of the worm wheel (2) is in engagement in the first axial section (11) of the worm shaft (1) and a further tooth (21) is in engagement at least partially in the second axial section (12) of the worm shaft (1).The gear (100) according to claim 1, wherein the pitch of the external toothing (10) of the worm shaft (1) is constant within the second axial portion (12).The transmission (100) according to claim 1, wherein the pitch of the external toothing (10) of the worm shaft (1) changes within the second axial section (12), in particular increases with increasing distance from the first axial section (11).The transmission (100) according to any one of the preceding claims, wherein the second axial portion (12) extends as far as an end (1B) of the worm shaft (1).The transmission (100) according to any one of the preceding claims, wherein the second axial portion (12) has a length (L12) which is in a range between 15 percent and 50 percent of an overall length (L1) of the worm shaft (1).Gear (100) according to one of the preceding claims, wherein the worm wheel (2) is formed from a thermoplastic material, in particular polyoxymethylene or polyetheretherketone.Gear (100) according to one of the preceding claims, additionally comprising: a threaded spindle (3) mounted in a rotationally fixed manner with an external thread (30) which is in engagement with an internal thread (25) of the worm wheel (2).A drive assembly (200) for an electromechanical brake booster (300), comprising: a gear (100) according to any one of the preceding claims; and an electric motor (210) connected to the worm shaft (1).Brake booster (300) for a brake system of a vehicle, comprising: a drive assembly (200) according to Claim 8; and a master brake cylinder (310) having a piston (311) which is kinematically coupled to the transmission (100).Brake booster (300) according to Claim 9, wherein the gear mechanism (100) of the drive assembly (200) is configured according to Claim 7, and wherein the piston (311) is coupled to the threaded spindle (3).

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