Ball-screw assembly

Abstract

The present disclosure relates to a ball-screw assembly including: a spindle having a first spiral groove formed on its outer surface; a ball nut having a second spiral groove formed on its inner surface, the spindle being configured to move inside the ball nut; a plurality of first rolling elements inserted into a space formed between the first spiral groove of the spindle and the second spiral groove of the ball nut; and a ball bearing configured to support the ball nut. The ball bearing includes an inner race having a raceway surface on its outer surface and fixed around the ball nut, a ring shaped outer race spaced radially apart from the raceway surface of the inner race and having a raceway surface on its inner surface, and a plurality of second rolling elements disposed between the inner race and the outer race. The ball bearing is an angular contact ball bearing. A flinger having an axial position fixed with respect to the outer race of the ball bearing is disposed at a front end side of the ball bearing, and a snap ring is provided on the inner race of the ball bearing between the flinger and the second rolling elements in the axial direction of the ball bearing. When an axial force directed toward a front side of the spindle is applied to the ball nut, separation between the inner race and the outer race of the ball bearing is prevented by contact between the flinger and the snap ring.

Description

BALL-SCREW ASSEMBLY

[0001] This application claims priority to and the benefit of Korean Patent Application No. 10-2024-0180898, filed on December 6, 2024, and Korean Patent Application No. 10- 2025-0172898, filed on November 14, 2025, each filed with the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference in their entirety.

[0002] The present disclosure relates to a ball-screw assembly, and more particularly, to a ball-screw assembly that may be used as an actuator of an electro-mechanical brake apparatus for a vehicle.

[0003] A ball screw is a mechanical device that converts rotational motion generated by, for example, a motor, into linear motion. A ball screw generally includes a rotation shaft, a ball nut, and rolling elements inserted therebetween. When rotational torque generated by, for example, a motor, is applied to one of the rotation shaft and the ball nut, the rolling elements support rotational motion internally, and the other of the rotation shaft and the ball nut performs linear motion. Ball screws are widely utilized in machine tools and industrial machinery that require high precision and efficiency, and are known as essential devices for high-precision machines such as CNC lathes.

[0004] In a general ball screw, rolling elements are disposed in a space defined by threads between the rotation shaft and the ball nut, and when rotation occurs, friction is minimized by rolling motion of the rolling elements. Due to the rolling motion of the rolling elements, the ball screw may maintain a relatively low friction coefficient and allow precise position control. In addition, depending on a circulation method of the rolling elements, ball screws are classified into an external circulation type and an internal circulation type, and driving characteristics appear differently according to each method.

[0005] Recently, in the field of vehicle brake systems, many technologies have been proposed to improve, for example, characteristics and performance of an electro-mechanical brake (EMB) system, which generates braking force through electrical signals. Among these, there is a technique that uses a ball-screw actuator as a measure to achieve miniaturization of an EMB system. The ball-screw actuator generates brake pressure by moving linearly by receiving rotational torque generated by a drive unit such as a motor within the EMB system. During this process, large axial and radial loads of about 65 kN or more are generated, and when the ball-screw actuator is miniaturized, common wheel bearings or ball bearings cannot sufficiently support such loads.

[0006] To solve this problem, a technique has been proposed in which an angular contact ball bearing capable of simultaneously supporting an axial load and a radial load is applied as a bearing that supports the ball-screw actuator on a vehicle chassis. However, the angular contact ball bearing, which has no separate step for preventing rolling elements inserted therein from escaping, has a problem in that the inner race and the outer race may be easily separated during transportation or assembly, which significantly reduces productivity.

[0007] [Prior Art Documents]

[0008] [Patent Documents]

[0009] (Patent Document 1) Korean Laid-open Patent Publication No. 2020-0013850

[0010] According to an embodiment of the present disclosure, there is provided a ball-screw assembly comprising: a spindle having a first spiral groove formed on its outer surface; a ball nut having a second spiral groove formed on its inner surface, the spindle being configured to move inside the ball nut; a plurality of first rolling elements inserted into a space formed between the first spiral groove of the spindle and the second spiral groove of the ball nut; and an angular contact ball bearing configured to support the ball nut. The angular contact ball bearing comprises an inner race having a raceway surface on its outer surface, a ring-shaped outer race spaced apart radially from the raceway surface of the inner race and having a raceway surface on its inner surface, a plurality of second rolling elements provided between the inner race and the outer race, and a cage configured to maintain a circumferential spacing of the plurality of second rolling elements. The cage comprises: a first ring, a second ring having a larger diameter than the first ring, a plurality of bridges each having one end connected to the first ring and the other end connected to the second ring and arranged at uniform intervals in a circumferential direction, and a plurality of holes each formed by the first ring, the second ring, and two adjacent bridges, each hole configured to accommodate one of the second rolling elements.

[0011] The first ring of the cage may comprise, on an inner surface thereof, at least one protrusion that obliquely protrudes backward and radially inward from a body portion of the first ring, and the inner race may comprise at least one undercut at a radial position corresponding to the protrusion.

[0012] The protrusion may have a free end positioned within the undercut.

[0013] The first ring of the cage may comprise a hinge portion having a reduced thickness between the protrusion and a remaining portion of the first ring, and the hinge portion may elastically deform during assembly of the angular contact ball bearing.

[0014] The protrusion may be formed at a position on an inner surface of the first ring corresponding to each of the plurality of holes, and the undercut may be formed over an entire circumferential direction of an outer surface of the inner race.

[0015] The outer race may comprise an annular retaining step that protrudes from an inner surface of the outer race at an axial position between the second ring and the second rolling elements.

[0016] A diameter of an imaginary circle formed by the peak of the retaining step may be determined by adding a diameter of the second rolling element to a pitch circle diameter, which is a diameter of an imaginary circle formed by centers of the second rolling elements, and then subtracting 3% to 10% of the diameter of the second rolling element therefrom.

[0017] The retaining step may be asymmetric with respect to a plane perpendicular to an axial direction and including the peak of the retaining step.

[0018] The inner race may be formed integrally with the ball nut.

[0019] The outer race may further comprise a concave portion on at least a portion of the inner surface located forward of the raceway surface of the outer race.

[0020] The ball-screw assembly may further comprise a spindle head coupled to a front end of the spindle.

[0021] According to another embodiment of the present disclosure, there is provided a ball screw assembly comprising: a spindle having a first spiral groove formed on its outer surface; a ball nut having a second spiral groove formed on its inner surface, the spindle being configured to move inside the ball nut; a plurality of first rolling elements inserted into a space formed between the first spiral groove of the spindle and the second spiral groove of the ball nut; and a ball bearing configured to support the ball nut. The ball bearing comprises an inner race having a raceway surface on its outer surface and fixed around the ball nut, a ring shaped outer race spaced radially apart from the raceway surface of the inner race and having a raceway surface on its inner surface, and a plurality of second rolling elements disposed between the inner race and the outer race. The ball bearing is an angular contact ball bearing. A flinger having an axial position fixed with respect to the outer race of the ball bearing is disposed at a front end side of the ball bearing, and a snap ring is provided on the inner race of the ball bearing between the flinger and the second rolling elements in the axial direction of the ball bearing. When an axial force directed toward a front side of the spindle is applied to the ball nut, separation between the inner race and the outer race of the ball bearing is prevented by contact between the flinger and the snap ring.

[0022] A recess may be formed on the outer surface of the inner race of the ball bearing, and the snap ring may be seated in the recess.

[0023] The flinger may comprise a vertical extension portion extending perpendicular to the axial direction so as to separate an internal space from an external space of the ball bearing.

[0024] The flinger may be configured to be mounted on an outer surface of the outer race of the ball bearing.

[0025] The flinger may be configured to be mounted on the inner surface of the outer race of the ball bearing such that the flinger is inserted between the outer race and the inner race of the ball bearing.

[0026] The ball bearing may further comprise a cage configured to maintain a circumferential spacing of the plurality of second rolling elements.

[0027] The cage may comprise at least one protrusion extending toward the inner race of the ball bearing, the inner race of the ball bearing may comprise at least one undercut on its outer surface at a position corresponding to that of the protrusion, and the protrusion may have a free end positioned within the undercut.

[0028] The inner race of the ball bearing may be formed integrally with the ball nut.

[0029] The inner race of the ball bearing may be formed separately from the ball nut.

[0030] According to an embodiment of the present disclosure, there is provided a ball-screw assembly that can achieve miniaturization while securing sufficient load-supporting capability. The ball-screw assembly may comprise an angular contact ball bearing having high assembly compatibility to improve productivity and efficiency of transportation and assembly operations. Furthermore, the angular contact ball bearing of the ball-screw assembly can be economically provided while exhibiting improved coupling performance and reliability in preparation for abnormal driving conditions that may intermittently occur.

[0031] The angular contact ball bearing may be provided as a support bearing to enable miniaturization of an actuator in an electric-mechanical brake device for a vehicle and implement a compact electric-mechanical brake system.

[0032] In addition, the ball-screw assembly may comprise an accidental separation prevention structure inside the angular contact ball bearing as the support bearing to prevent separation of the inner race, the ball set, and the outer race constituting the angular contact ball bearing during transportation or assembly, thereby substantially improving productivity and efficiency of transportation and assembly operations of the ball-screw assembly.

[0033] Furthermore, even when a reverse load exceeding a design load is intermittently applied, separation of the inner race and the outer race of the angular contact ball bearing can be prevented, thereby improving reliability of the apparatus. In addition, an economical and high mass-productivity-friendly solution may be provided for abnormal driving conditions that do not occur continuously.

[0034] The problems to be solved by the present disclosure are not limited to those described above, and other problems not mentioned may be clearly understood by those ordinarily skilled in the art from the description below.

[0035] The effects obtainable from embodiments of the present disclosure are not limited to those described above, and other effects not mentioned herein will be clearly understood by those of ordinary skill in the art from the descriptions below.

[0036] Features and advantages of embodiments of the present disclosure will become more apparent from the following description, which is to be read together with the accompanying drawings.

[0037] FIGS. 1A and 1B are cross-sectional views schematically illustrating a ball-screw assembly according to an embodiment of the present disclosure.

[0038] FIG. 2 is an exploded view of the ball-screw assembly according to an embodiment of the present disclosure.

[0039] FIG. 3 is an enlarged cross-sectional view schematically illustrating a cross-section of an angular contact ball bearing, which is a support bearing of the ball-screw assembly according to an embodiment of the present disclosure.

[0040] FIGS. 4A and 4B are views schematically illustrating a cage provided in the angular contact ball bearing included in the ball-screw assembly according to an embodiment of the present disclosure. FIG. 4A is a perspective view illustrating the entire cage, and FIG. 4B is a cross-sectional view illustrating a cross-section of the cage.

[0041] FIG. 5 is a cross-sectional view schematically illustrating a ball-screw assembly according to another embodiment of the present disclosure.

[0042] FIG. 6 is a cross-sectional view schematically illustrating a ball bearing in the ball-screw assembly according to still another embodiment of the present disclosure.

[0043] FIGS. 7A and 7B are cross-sectional views schematically illustrating states in which a forward force and a reverse force are applied to the ball-screw assembly according to the embodiment illustrated in FIG. 5, respectively.

[0044] Embodiments of the present disclosure are exemplified for describing the technical spirit of the present disclosure. The scope of the present disclosure is not limited to the embodiments presented below or the detailed descriptions of such embodiments.

[0045] In describing embodiments of the present disclosure, detailed descriptions of well-known technologies related to the present disclosure will be omitted when it is determined that such descriptions may unnecessarily obscure the gist of the present disclosure. Unless otherwise defined, all technical and scientific terms used in the present disclosure have meanings generally understood by those ordinarily skilled in the technical field to which the present disclosure pertains. All of the terms used in the present disclosure are selected for the purpose of describing the present disclosure more clearly, and are not selected to limit the scope of rights according to the present disclosure.

[0046] As used herein, expressions such as "including," "comprising," and "having" are to be understood as open-ended terms having the possibility of encompassing other embodiments, unless otherwise mentioned in the phrase or sentence including such expressions.

[0047] Herein, when a portion such as a layer, a film, a region, or a plate is described as being located "on" or "over" another portion, this includes not only cases in which the portion is directly on the other portion but also cases in which another portion is interposed therebetween. Conversely, when a portion is described as being "directly above" another portion, this means there is no other portion therebetween. In addition, describing that a portion is located "on" or "over" a reference portion means that the portion is positioned above or below the reference portion, and does not necessarily mean that the portion is positioned in a direction opposite to the direction of gravity. This applies similarly to the expressions "beneath" or "under."

[0048] As used herein, the expression "in a plan view" refers to when a subject of the present disclosure is viewed from above, and the expression "in a cross-sectional view" refers to when a cross-section obtained by cutting the subject vertically with respect to the ground or installation surface is viewed from the side.

[0049] As used herein, the expressions "the same" and "identical" not only indicate a strictly identical state but also encompass states in which variations such as tolerances or differences that allow substantially the same function to be obtained are present.

[0050] As used herein, expressions indicating relative or absolute arrangements, such as "in a certain direction," "along a certain direction," "in parallel," "perpendicular," "toward a center," "concentric," or "coaxial," not only indicate strictly such arrangements, but also encompass states in which variations in angles or distances, such as tolerances or differences that allow substantially the same function to be obtained, are present.

[0051] As used herein, the term "front" refers to a direction in which the spindle advances such that when the ball-screw assembly operates, a spindle head moves away from the ball nut, and the term "rear" refers to the opposite direction. For example, in FIGS. 1A and 1B, "front" refers to the left side, and "rear" refers to the right side.

[0052] Singular expressions described herein may include the meaning of the plural unless otherwise specified, and this similarly applies to singular expressions described in the claims.

[0053] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In this process, the thickness of lines or the size of components illustrated in the drawings may be exaggerated for clarity and convenience of description. In the following descriptions of embodiments, descriptions of components that are identical or correspond to each other may be omitted to avoid redundancy. However, even if the description of a component is omitted, it is not intended that such a component is excluded from any embodiment.

[0054] Furthermore, the embodiments described below are merely illustrative examples of the components presented in the claims of the present disclosure and are not intended to limit the scope of rights of the present disclosure. Embodiments that fall within the technical spirit described throughout the specification of the present disclosure and include components that may be substituted as equivalents for the components recited in the claims may be included in the scope of rights of the present disclosure.

[0055] FIGS. 1A and 1B are cross-sectional views schematically illustrating a configuration of a ball-screw assembly 10 according to an embodiment of the present disclosure, and FIG. 2 is an exploded view of the ball-screw assembly 10 according to an embodiment of the present disclosure. Referring to FIGS. 1 and 2, the ball-screw assembly 10 according to an embodiment of the present disclosure includes a spindle 11 having a first spiral groove on its outer surface, a ball nut 12 having a second spiral groove on its inner surface, the spindle 11 being configured to move inside the ball nut, a plurality of first rolling elements 13 inserted into a space defined between the first spiral groove of the spindle 11 and the second spiral groove of the ball nut 12, and an angular contact ball bearing 20 mounted on a cylinder body and configured to support the ball nut 12. The ball-screw assembly 10 according to an embodiment of the present disclosure may further include a spindle head 14 coupled to a front end of the spindle 11.

[0056] The spindle 11 may be provided as a member having a generally rod-like shape with the first spiral groove formed on its outer surface. In this case, the first spiral groove may be continuously formed along a length direction of the spindle 11. The first spiral groove may be formed over an entire length of the spindle 11 or may be formed only on at least a portion of the outer surface of the spindle 11. As the ball nut 12 rotates about a central axis, the spindle 11 may be moved forward toward the front or rearward toward the rear inside the ball nut 12 through the rolling of the first rolling elements 13. In addition, the spindle head 14, which provides a connection between the piston member and the spindle 11, may be coupled to a front end of the spindle 11, and a rear end of the spindle 11 may remain as a free end.

[0057] The ball nut 12, which has the second spiral groove formed on its inner surface, is provided as a cylindrical member having a through-hole into which the spindle 11 is inserted. The ball nut 12 rotates by receiving rotational torque from an external drive device. When the ball nut 12 rotates, the spindle 11 may be moved forward or rearward by the rolling motion of the plurality of first rolling elements 13 caused by the second spiral groove.

[0058] The spindle 11 and the ball nut 12 may be a constituent of a general ball screw that converts rotational motion into linear motion through the rolling of the plurality of first rolling elements 13 inserted into the space formed between the first spiral groove and the second spiral groove.

[0059] The plurality of first rolling elements 13 are inserted into the space formed between the first spiral groove of the spindle 11 and the second spiral groove of the ball nut 12. An inner circumference of each first rolling element 13 is in rolling contact with the first spiral groove of the spindle 11, and an outer circumference of each first rolling element 13 is in rolling contact with the second spiral groove of the ball nut 12. Each first rolling element 13 may have a generally spherical shape. The first rolling elements 13 roll along the first and second spiral grooves during the rotation of the ball nut 12 and convert the rotational motion of the ball nut 12 into linear reciprocating motion of the spindle 11.

[0060] A first spring 231 may be disposed between the plurality of first rolling elements 13. In addition, a second spring 232 may be disposed at one or more of opposite ends of the plurality of first rolling elements 13. In this case, the first spring 231 may have a shorter length than the second spring 232.

[0061] The first spring 231, which is a short spring disposed between the plurality of first rolling elements 13, may be additionally provided so as to attenuate friction between the first rolling elements 13 during rotation of the ball nut 12. Specifically, as the ball nut 12 rotates and each first rolling element 13 rolls along the first spiral groove and the second spiral groove, two adjacent first rolling elements 13 rotate in opposite directions. As a result, significant friction loss occurs between the two adjacent first rolling elements 13. By inserting the first spring 231 between the two adjacent first rolling elements 13, the friction loss may be significantly reduced.

[0062] The first spring 231 may be disposed between every n first rolling elements 13 (where n is an integer of 1 or greater). As the number of the first springs 231 increases, the friction-resistance attenuation effect also increases, and therefore it is desirable that the first springs 231 be disposed between every adjacent ones of the plurality of first rolling elements 13.

[0063] The second spring 232 is provided in the form of a long spring disposed at one or more of opposite ends of the plurality of first rolling elements 13. Unlike the first spring 231, the role of the second spring 232 is distinguished depending on the position where it is disposed.

[0064] The second spring 232 disposed at a front end among the opposite ends of the plurality of first rolling elements 13 is compressed when the spindle 11 moves toward the front and is restored and pushes the plurality of first rolling elements 13 rearward when the spindle 11 moves rearward. Through this, energy required for the rearward movement of the spindle 11 may be significantly saved, while a fast response time may be secured. In addition, in a non-limiting embodiment, the return of the first rolling elements 13 may be enabled only by the restoring force of the second spring disposed at the front end, without the reverse rotation of the ball nut 12.

[0065] The second spring 232 disposed at a rear end among the opposite ends of the plurality of first rolling elements 13 serves to push the first rolling elements 13 forward with a predetermined force in a state in which the first spring 231 is not compressed, so as to align the first rolling elements 13 at correct positions.

[0066] The ball-screw assembly 10 according to an embodiment of the present disclosure may include an angular contact ball bearing 20 as a support bearing for supporting the assembly on a separate support member.

[0067] The angular contact ball bearing 20 is a bearing that supports loads between rotating components and has a structure capable of simultaneously withstanding an axial load and a radial load. This bearing is designed such that raceway surfaces of an inner race 21 and an outer race 22 are inclined, allowing for stable rotation when a load is applied in the axial direction. In addition, because the angular contact ball bearing 20 is open on both front and rear sides of the rolling elements without steps for fixing positions of the rolling elements, a large space is secured, allowing the rolling elements to be maximized in size. Accordingly, a large supporting load may be secured while miniaturizing the support bearing. However, conversely, a structure in which no step for supporting the rolling elements is present as a drawback in that the angular contact ball bearing may be easily disassembled during transport or assembly and may hence reduce productivity and efficiency of a ball-screw assembly process. The angular contact ball bearing 20 included in the ball-screw assembly 10 according to an embodiment of the present disclosure includes an accidental separation prevention structure so as to solve such problems.

[0068] In the present disclosure, the angular contact ball bearing 20 includes an inner race 21 that has a raceway surface on its outer surface, a ring-shaped outer race 22 that is radially spaced apart from the raceway surface of the inner race 21 and has a raceway surface on its inner surface, a plurality of second rolling elements 23 disposed between the inner race 21 and the outer race 22, and a cage 24 configured to maintain circumferential spacing between the plurality of second rolling elements 23. As illustrated in FIGS. 1A and 1B, the angular contact ball bearing 20 functions as a support bearing supporting the ball-screw assembly 10 with respect to a separate support member by including a structure in which the inner race 21 is coupled to the ball nut 12 and the outer race 22 is coupled to the separate support member.

[0069] FIG. 3 is an enlarged cross-sectional view schematically illustrating a cross-sectional structure of the angular contact ball bearing 20 of the present disclosure. Referring to FIG. 2, the angular contact ball bearing 20 generally includes the inner race 21, the outer race 22, the plurality of second rolling elements 23, and the cage 24. In an assembled state, the inner race 21, the outer race 22, and the cage 24 are arranged concentrically, and their central axes may coincide with a rotational axis of the angular contact ball bearing 20 and a rotational axis of the ball nut 12.

[0070] The inner race 21 is a component that rotates inside the angular contact ball bearing 20. As illustrated in FIG. 1A, the inner race 21 may be manufactured as a component separate from the ball nut 12 and then coupled to the ball nut 12 through, for example, press-fitting, or, as illustrated in FIG. 1B, the inner race 21 may be formed integrally with at least a portion of the outer surface of the ball nut 12. Referring to FIG. 3, the inner race 21 may include at least one raceway surface configured to form at least one row of raceways together with the outer race 22 on its radially outer surface. The raceway surface of the inner race 21 is in rolling contact with the plurality of second rolling elements 23. Through this, the raceway surface of the inner race 21 may be supported axially and radially by the plurality of second rolling elements 23. In a non-limiting embodiment, a plurality of inner races 21 may be provided, in which case each inner race 21 may include a raceway surface forming a separate row of raceways.

[0071] A front-side inner surface of the inner race 21 may be formed in a shape that is flat and parallel to the central axis. At least a portion of a rear-side inner surface of the inner race 21 may include an undercut 211 configured to receive a protrusion 245, which is formed on an inner surface of the first ring 241 of the cage 24 described later, at a radial position corresponding to the protrusion 245. Since the undercut 211 may be variously modified in design within the technical concept of limiting movement of the protrusion 245, the present disclosure does not particularly limit, for example, the size or shape of the undercut 211.

[0072] A remaining portion of the rear-side inner surface of the inner race 21, excluding the undercut 211, may be formed in a shape that is flat and parallel to the central axis. This flat portion may cooperate with a flat portion of a rear-side inner surface of the outer race 22, described later, to form a mounting portion for assembling a shield member or a sealing member 25 that is provided as a separate component. The shield member and the sealing member 25 used in the present disclosure may employ components that are ordinarily used in the technical field to which the present disclosure pertains, and therefore, the present disclosure does not particularly limit these components.

[0073] The outer race 22 may be provided as a ring-shaped component that is radially spaced apart from the raceway surface of the inner race 21 and has a raceway surface on its radially inner surface. At least a portion of the outer race 22 is coupled to the cylinder body to provide supporting force. The outer race 22 may include at least one raceway surface configured to form at least one row of raceways together with the inner race 21 on its radially inner surface. The raceway surface of the outer race 22 is in rolling contact with the plurality of second rolling elements 23. Through this, the raceway surface of the outer race 22 may support the plurality of second rolling elements 23 axially and radially.

[0074] In a non-limiting embodiment, when a plurality of inner races 21 are provided, the outer race 22 may include a number of raceway surfaces corresponding to the number of the inner races 21.

[0075] A front-side inner surface of the outer race 22 may define an insertion opening into which the second rolling elements 23 are inserted. The outer race 22 may include a retaining step 221 having a predetermined height on a portion of its front-side inner surface adjacent to the second rolling elements 23. As illustrated in FIG. 3, the retaining step 221 may protrude radially inward at an axial position between the second ring 242 of the cage 24 and the centers of the second rolling elements 23.

[0076] The height of the retaining step 221 may be dimensioned to allow the second rolling elements 23 to easily ride over the retaining step during assembly, and to prevent the second rolling elements from easily escaping after assembly. In a non-limiting embodiment, a diameter of an imaginary circle formed by the peak of the retaining step 221 may be determined as a value obtained by adding a diameter of the second rolling elements 23 to a pitch circle diameter (PCD), which is a diameter of an imaginary circle formed by centers of the second rolling elements 23, and then subtracting 3% to 10% of the diameter of the second rolling elements 23. In non-limiting embodiment, the retaining step 221 may be formed in an asymmetrical shape with respect to a plane perpendicular to an axial direction and including the peak of the retaining step 221.

[0077] A remaining portion of the front-side inner surface of the outer race 22, excluding the retaining step 221, may be formed in a flat shape parallel to the central axis. The flat portion of the front-side inner surface of the outer race 22 may cooperate with the front-side inner surface of the opposing inner race 21 to form a mounting portion for assembling a shield member or a sealing member 25 provided as a separate component.

[0078] In a non-limiting embodiment, the outer race 22 may further include a recess 222 on its front-side inner surface, in which the thickness of the outer race 22 is locally reduced. A front edge of the recess 222 may be connected to the flat portion of the front-side inner surface, and a rear edge thereof may be continuously connected to the retaining step 221 without a step difference. By forming the recess 222, a clearance may be provided between a radially outer edge of the second ring 242 of the cage 24 and the outer race 22, thereby preventing the second ring 242 of the cage 24 and the outer race 22 from contacting each other during rotation. In addition, since additional machining such as polishing of the inner surfaces of the inner race 21 and the outer race 22 is required for mounting the shield member or the sealing member 25, the recess 222 may allow such additional machining to be omitted, thereby achieving an incidental effect of reducing the area to be polished.

[0079] The rear-side inner surface of the outer race 22, unlike the front-side inner surface of the outer race 22, may be formed in a flat shape parallel to the central axis so that no separate step is formed. The flat portion of the rear-side inner surface of the outer race 22 may cooperate with the flat portion of the rear-side inner surface of the inner race 21 to form a mounting portion for assembling the shield member or the sealing member 25 that is provided as a separate component.

[0080] The plurality of second rolling elements 23 are inserted between the inner race 21 and the outer race 22, more specifically, between the raceway surface of the inner race 21 and the raceway surface of the outer race 22. At least a portion of each second rolling element 23 is in rolling contact with the raceway surface of the inner race 21, and at least another portion of each second rolling element 23 is in rolling contact with the raceway surface of the outer race 22. Each second rolling element 23 may have a generally spherical shape.

[0081] The angular contact ball bearing 20 may include a cage 24 configured to maintain a uniform circumferential spacing between the second rolling elements 23 by being engaged with the second rolling elements 23 arranged in a row of raceways between the inner race 21 and the outer race 22. FIGS. 4A and 4B schematically illustrate the cage 24 of the angular contact ball bearing 20 included in the present disclosure. FIG. 4A is a perspective view of the cage 24, and FIG. 4B is a partially enlarged cross-sectional view of the cage 24. Referring to FIGS. 4A and 4B, the cage 24 may include a first ring 241, a second ring 242 formed to have a larger diameter than the first ring 241, a plurality of bridges 243 each having one end connected to the first ring 241 and the other end connected to the second ring 242 and arranged at regular intervals in a circumferential direction, and a plurality of holes 244 each formed by the first ring 241, the second ring 242, and two adjacent bridges 243, and configured to receive the second rolling elements 23. The cage 24 may be made of an elastic material, and in a non-limiting embodiment, the cage 24 may be made of a synthetic resin such as plastic.

[0082] Each second rolling element 23 is supported in the circumferential direction on both sides by two adjacent bridges 243, and may be supported axially and radially by at least one surface of the first ring 241 and at least one surface of the second ring 242. Each of circumferentially opposite side surfaces of each bridge 243 may include a pocket surface that matches an outer contour of a second rolling element 23, and at least one surface of each of the first ring 241 and the second ring 242 may also include pocket surfaces that match the outer contours of the second rolling elements 23.

[0083] As illustrated in FIG. 4A, each bridge 243 may be formed in a shape bent at an intermediate portion. The bridge 243 may include a first bridge portion 2431 extending in an axial direction from at least a portion of the first ring 241, and a second bridge portion 2432 bent radially outward from the first bridge portion 2431 at a predetermined inclination angle with respect to the axial direction and extending to the second ring 242. Since the inclination angle with respect to the axial direction is a design variable, the range of which is determined depending on a contact angle of the angular contact ball bearing 20, the present disclosure does not particularly limit this inclination angle.

[0084] At least a portion of the first ring 241 including a pocket surface, at least a portion of the second ring 242 including a pocket surface, and the two adjacent bridges 243 may form a hole 244 configured to receive a second rolling element 23. A plurality of holes 244 may be provided to correspond to the number of second rolling elements 23, and the plurality of holes 244 may be arranged at equal intervals in the circumferential direction around the central axis. Since the dimensions of individual holes 244 are design variables determined depending on the size of the second rolling elements 23, the present disclosure does not particularly limit the dimensions of the holes 244.

[0085] The first ring 241 of the cage 24 may include a main body portion and a protrusion 245 extending from the main body portion and protruding rearward while being inclined radially inward. A plurality of protrusions 245 may be provided to correspond to the number of holes 244, and may be formed at positions on an inner surface of the first ring 241 corresponding to the plurality of holes 244. However, the present disclosure is not limited thereto, and in a non-limiting embodiment, the protrusions 245 may be provided in a number greater or smaller than the number of holes 244.

[0086] The protrusion 245 may be configured to cooperate with an undercut 211 of the inner race 21. The protrusion 245 may include a coupling portion 2451 coupled to a body portion of the first ring 241, an extension portion 2452 extending from the coupling portion 2451 toward the inner race 21, and a terminal end 2453 forming a free end of the protrusion 245. The extension portion 2452 may extend in an inclined manner such that it becomes closer to the central axis as it becomes farther from the first ring 241. The terminal end 2453 may be positioned within the undercut 211 of the inner race 21. As a result, the terminal end 2453 may be engaged with a rear side wall of the undercut 211 so that a range of relative axial movement of the cage 24 may be limited. That is, through an engagement structure formed by the protrusion 245 and the undercut 211, accidental separation between the inner race 21 and a ball set formed of the cage 24 and the second rolling elements 23 may be prevented.

[0087] A hinge portion 246 having a reduced thickness may be formed between the coupling portion 2451 and the body portion of the first ring 241. During an assembly process of the ball set and the inner race 21, when the ball set is externally fitted to the inner race 21 along the axial direction from the rear side of the inner race 21, a rear-side inner surface of the inner race 21 presses the protrusion 245 radially outward, and at this time, force is concentrated on the hinge portion 246 having the reduced thickness, causing the hinge portion 246 to elastically deform. When the external fitting of the ball set is completed, the pressing by the rear-side inner surface of the inner race 21 is released, and as the elastic deformation of the hinge portion 246 is restored, the terminal end 2453 of the protrusion 245 is positioned within a space defined by the undercut 211.

[0088] In a non-limiting embodiment, the first ring 241, the second ring 242, and the plurality of bridges may be manufactured as an integral component.

[0089] FIG. 5 is a cross-sectional view schematically illustrating a configuration of a ball-screw assembly 100 according to another embodiment of the present disclosure. According to the embodiment illustrated in FIG. 5, the ball-screw assembly 100 includes a spindle 11 having a first spiral groove on its outer surface, a ball nut 12 having a second spiral groove on its inner surface, the spindle 11 being configured to move inside the ball nut, a plurality of first rolling elements 13 inserted into a space defined between the first spiral groove of the spindle 11 and the second spiral groove of the ball nut 12, and an angular contact ball bearing 20 mounted on a cylinder body and configured to support the ball nut 12. The ball-screw assembly 100 may further include a spindle head 14 coupled to a front end of the spindle 11.

[0090] In the embodiment illustrated in FIG. 5, the same reference numerals are used for the same components included in the ball-screw assembly 100 according to the embodiment of FIG. 5 and the ball-screw assembly 10 according to the embodiments illustrated in FIGS. 1 to 4. Hereinafter, differences of the embodiment of FIG. 5 compared with the embodiments of FIGS. 1 to 4 will be described. Except for the differences described below, the description of the embodiments of FIGS. 1 to 4 may be equally applied to the embodiment of FIG. 5.

[0091] The angular contact ball bearing 20 may include a flinger 26. In the embodiment illustrated in FIG. 5, the flinger 26 is mounted on an outer surface of an outer race of the angular contact ball bearing and includes a vertical extension portion extending perpendicular to an axial direction. The vertical extension portion separates an internal space and an external space of the angular contact ball bearing 20. The flinger 26 may prevent oil or lubricant inside the angular contact ball bearing 20 from being discharged to the outside and may prevent foreign substances outside the angular contact ball bearing 20 from entering the inside. The flinger 26 may be installed such that its axial position is fixed with respect to the outer race 22 of the angular contact ball bearing 20. For example, as illustrated in FIG. 5, the flinger 26 may be configured to be mounted on an outer surface of the outer race 22 of the angular contact ball bearing 20. According to a non-limiting embodiment, the flinger 26 may be fixedly press-fitted to the outer surface of the outer race 22.

[0092] In the embodiment illustrated in FIG. 5, the flinger 26 is provided on a front end side of the angular contact ball bearing 20, and second rolling elements 23 are disposed behind the flinger 26.

[0093] In FIG. 5, a snap ring 30 is provided on an inner race 21 of the angular contact ball bearing 20, and the snap ring 30 is disposed between the flinger 26 and the second rolling elements 23. According to a non-limiting embodiment, a recess 212 may be formed on an outer surface of the inner race 21 of the angular contact ball bearing 20, and the snap ring 30 may be seated in the recess 212.

[0094] The outer race 22 of the angular contact ball bearing 20 is supported at least in part by a housing (not illustrated) such that the axial position of the outer race 22 may be fixed. The inner race 21 of the angular contact ball bearing 20 is coupled to the ball nut 12 as described above, and when the ball nut 12 moves in the axial direction, the inner race 21 also moves in the axial direction. Accordingly, when the inner race 21 moves in the axial direction, a relative axial position between the flinger 26, whose axial position is fixed with respect to the outer race 22, and the snap ring 30 mounted on the inner race 21 may change. For example, as will be described in detail below, when an axial force directed toward a front side of the spindle 11 is applied to the ball nut 12, the flinger 26 and the snap ring 30 come into contact with each other, and accordingly, separation of the inner race 21 and the outer race 22 of the angular contact ball bearing 20 may be prevented.

[0095] The inner race 21 of the angular contact ball bearing 20 may be formed integrally with the ball nut 12. FIGS. 5 to 7 illustrate embodiments in which the inner race 21 of the angular contact ball bearing 20 is formed integrally with the ball nut 12. However, the present disclosure is not limited thereto, and the inner race 21 of the angular contact ball bearing 20 may be formed separately from the ball nut 12.

[0096] FIG. 6 is a cross-sectional view schematically illustrating a configuration of a ball bearing in the ball-screw assembly 100 according to still another embodiment of the present disclosure. The embodiment illustrated in FIG. 6 is basically identical to the embodiment illustrated in FIG. 5, except for the flinger 26. According to the embodiment of FIG. 6, the flinger 26 is mounted on the inner surface of the outer race 22 of the angular contact ball bearing 20 and is configured to be inserted between the outer race 22 and the inner race 21 of the angular contact ball bearing 20. The flinger 26 of FIG. 6 may also be installed such that the axial position of the flinger 26 is fixed with respect to the outer race 22 of the angular contact ball bearing 20, similar to the flinger 26 of FIG. 5.

[0097] FIGS. 7A and 7B are views schematically illustrating states in which a forward force (see reference numeral F in FIG. 7A) and a reverse force (see reference numeral F' in FIG. 7B) are applied to the ball-screw assembly 100 according to the embodiment illustrated in FIG. 5, respectively.

[0098] For reference, in the present disclosure, the term "forward force" refers to an axial force directed toward the rear side of the spindle 11, and the term "reverse force" refers to an axial force directed toward the front side of the spindle 11. In addition, the descriptions "a forward force is applied" and "a reverse force is applied" each indicate that a net force applied to an object is in the forward direction or in the reverse direction, respectively.

[0099] In general, a ball-screw assembly may be intermittently exposed to abnormal driving conditions depending on various external factors or system conditions. Under such intermittent abnormal driving conditions, a reverse force (F') exceeding a design load may be applied to the ball nut 12. In a typical structure, when the reverse force F' applied to the ball nut 12 exceeds the design load, the inner race 21 and a ball set including the cage 24 and the second rolling elements 23 may be separated, and the inner race 21 and the outer race 22 may be separated. However, according to an embodiment of the present disclosure, the separation of the inner race 21 and the outer race 22 may be prevented even when the reverse force F' exceeding the design load is applied to the ball nut 12.

[0100] In the ball-screw assembly 100 according to an embodiment of the present disclosure, when the forward force F is applied to the ball nut 12, the flinger 26 and the snap ring 30 may be spaced apart from each other (see region X of FIG. 7A). However, when the reverse force F' applied to the ball nut 12, the flinger 26 comes into contact with the snap ring 30 (see region Y of FIG. 7B). By such contact between the flinger 26 and the snap ring 30, the reverse force F' may be supported. The flinger 26 and the snap ring 30 may be formed of a metal material. Accordingly, the flinger 26 and the snap ring 30 may have relatively high rigidity. Therefore, a contact portion between the flinger 26 and the snap ring 30 may reliably support even the reverse force F' exceeding the design load. As a result, even when the reverse force applied to the ball nut 12 exceeds the design load, the inner 21 and the ball set including the cage 24 and the second rolling elements 23 may not be separated, and the inner race 21 and the outer race 22 may be prevented from being separated.

[0101] In addition, according to an embodiment of the present disclosure, since the ball-screw assembly 100 additionally includes the snap ring 30 on a front end side of the angular contact ball bearing 20, as compared with a case in which only the flinger 26 is used, sealing and shielding functions of the angular contact ball bearing 20 (i.e., functions of prevent oil leakage from inside the angular contact ball bearing and preventing foreign substances from entering the inside) may be improved. In addition, since the snap ring 30 itself may perform sealing and shielding functions, an additional flinger for a structure to prevent separation of the inner race and the outer race may be omitted, thereby reducing costs.

[0102] Meanwhile, as described above, an abnormal driving condition such as the reverse force F' exceeding the design load being applied to the ball nut 12 occurs only intermittently. Accordingly, modifying the structure of the angular contact ball bearing 20 itself or separately designing and manufacturing special components to prepare for such intermittent abnormal driving conditions is uneconomical and degrades mass productivity. In contrast, according to an embodiment of the present disclosure, even when the reverse force F' exceeding the design load is applied, the separation of the inner 21 and the outer 22 may be prevented by using the snap ring 30, which is a commercially available product, thereby improving economic efficiency and mass productivity.

[0103] The above description is merely illustrative of the technical spirit of the present disclosure, and those ordinarily skilled in the art to which the present disclosure pertains will appreciate that various modifications and changes may be made without departing from the essential characteristics of the present disclosure. Therefore, the embodiments disclosed herein are provided for illustrative purposes rather than for limiting the technical spirit of the present disclosure, and the scope of the technical spirit of the present disclosure should not be construed as being limited by such embodiments. The scope of protection of the present disclosure should be interpreted based on the claims below, and all technical ideas falling within an equivalent scope should be interpreted as being included within the scope of rights of the present disclosure.

Claims

A ball-screw assembly comprising:a spindle having a first spiral groove formed on its outer surface;a ball nut having a second spiral groove formed on its inner surface, the spindle being configured to move inside the ball nut;a plurality of first rolling elements inserted into a space formed between the first spiral groove of the spindle and the second spiral groove of the ball nut; anda ball bearing configured to support the ball nut, the ball bearing comprising:an inner race having a raceway surface on its outer surface and fixed around the ball nut,a ring-shaped outer race spaced radially apart from the raceway surface of the inner race and having a raceway surface on its inner surface, anda plurality of second rolling elements disposed between the inner race and the outer race,wherein the ball bearing is an angular contact ball bearing,wherein a flinger having an axial position fixed with respect to the outer race of the ball bearing is disposed at a front end side of the ball bearing, and a snap ring is provided on the inner race of the ball bearing between the flinger and the second rolling elements in the axial direction of the ball bearing, andwherein, when an axial force directed toward a front side of the spindle is applied to the ball nut, separation between the inner race and the outer race of the ball bearing is prevented by contact between the flinger and the snap ring.The ball-screw assembly of claim 1, wherein a recess is formed on the outer surface of the inner race of the ball bearing, and the snap ring is seated in the recess.The ball-screw assembly of claim 1, wherein the flinger comprises a vertical extension portion extending perpendicular to the axial direction so as to separate an internal space from an external space of the ball bearing.The ball-screw assembly of claim 3, wherein the flinger is configured to be mounted on an outer surface of the outer race of the ball bearing.The ball-screw assembly of claim 3, wherein the flinger is configured to be mounted on the inner surface of the outer race of the ball bearing such that the flinger is inserted between the outer race and the inner race of the ball bearing.The ball-screw assembly of claim 1, wherein the ball bearing further comprises a cage configured to maintain a circumferential spacing of the plurality of second rolling elements.The ball-screw assembly of claim 6, wherein the cage comprises at least one protrusion extending toward the inner race of the ball bearing,wherein the inner race of the ball bearing comprises at least one undercut on its outer surface at a position corresponding to that of the protrusion, andwherein the protrusion has a free end positioned within the undercut.The ball-screw assembly of claim 1, wherein the inner race of the ball bearing is formed integrally with the ball nut.The ball-screw assembly of claim 1, wherein the inner race of the ball bearing is formed separately from the ball nut.

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