Feed screw mechanism and electric actuator

Abstract

To provide a feed screw mechanism which can suppress deterioration of operation efficiency of the feed screw mechanism to suppress intrusion of a foreign matter from the outside and leakage of lubricant to the outside.SOLUTION: A feed screw mechanism 30 comprises: a rotary member 31 which has a female screw part 31a on an inner peripheral surface; a linear motion member 32 which has a male screw part 32a directly or indirectly screwed to the female screw part 31a on an outer peripheral surface to linearly move accompanying rotation of the rotary member 31; lubricant which is housed between the female screw part 31a and the male screw part 32a; a housing 5 which houses the rotary member 31 and the linear motion member 32; and a wall part 36 which radially projects from the outer peripheral surface of the linear motion member 32. The wall part 36 linearly moves with the linear motion member 32, and an outer radial end of the wall part 36 is arranged so as not to contact the inner peripheral surface of the housing 5 and a member provided on the inner peripheral surface of the housing 5, or to be slidable to the inner peripheral surface of the housing 5 or the member provided on the inner peripheral surface of the housing 5.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a feed screw mechanism and an electric actuator. [Background technology]

[0002] 2. Description of the Related Art Known electric actuators used in automatic transmissions, braking mechanisms, steering mechanisms, and the like of automobiles use feed screw mechanisms that convert the rotational motion of an electric motor into linear motion.

[0003] In such a feed screw mechanism, grease is generally filled between the screw shaft and the nut as a lubricant to improve operability and durability. However, when the screw shaft moves linearly, the grease inside the nut gradually leaks out. If the amount of grease between the nut and the screw shaft decreases as a result of the grease leaking out, operability and durability will decrease.

[0004] In order to solve this problem, a conventional configuration has been proposed in which a seal member is provided between the nut and the screw shaft to prevent grease from leaking out (see, for example, Patent Documents 1 to 4). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2013 / 168432 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-2566 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-25321 [Patent Document 4] Patent No. 5192074 Summary of the Invention [Problem to be solved by the invention]

[0006] In the feed screw mechanism described above, the screw shaft moves forward and backward in the axial direction as the nut rotates, so if foreign matter adheres to the outer peripheral surface of the screw shaft exposed from the nut, the foreign matter may enter the nut as the screw shaft moves backward.If the foreign matter enters the nut and mixes with the grease, the properties of the grease may change, and the operability and durability of the feed screw mechanism may be reduced.

[0007] One method for effectively preventing foreign matter from entering the nut is to increase the contact area of ​​the seal member with the screw shaft or the contact pressure. However, increasing the contact area of ​​the seal member with the screw shaft or the contact pressure increases the sliding resistance between the screw shaft and the seal member, which may reduce the operating efficiency of the feed screw mechanism.

[0008] Therefore, an object of the present invention is to provide a feed screw mechanism that can suppress a decrease in the operating efficiency of the feed screw mechanism and suppress the intrusion of foreign matter from the outside and the leakage of lubricant to the outside, and an electric actuator equipped with such a feed screw mechanism. [Means for solving the problem]

[0009] In order to solve the above problems, the present invention provides a feed screw mechanism that converts the rotational motion of an electric motor into linear motion and transmits it to an object to be operated, and is characterized in that it comprises a rotating member having a female thread portion on its inner surface, a linear-acting member having a male thread portion on its outer surface that screws directly or indirectly into the female thread portion and that moves linearly as the rotating member rotates, a lubricant contained between the female thread portion and the male thread portion, a housing that contains the rotating member and the linear-acting member, and a wall portion that protrudes radially from the outer surface of the linear-acting member, the wall portion moving linearly together with the linear-acting member, and the outer radial end portion of the wall portion is arranged so as not to come into contact with the inner surface of the housing or a member provided on the inner surface of the housing, or so as to be slidable relative to the inner surface of the housing or a member provided on the inner surface of the housing.

[0010] As described above, in the present invention, a wall portion protruding radially from the outer peripheral surface of the linearly moving member is provided, thereby preventing the intrusion of foreign matter from the outside and the leakage of lubricant to the outside. Furthermore, the outer radial end of the wall portion is arranged so as not to contact the inner peripheral surface of the housing or a member provided on the inner peripheral surface of the housing, or so as to be slidable relative to the inner peripheral surface of the housing or a member provided on the inner peripheral surface of the housing. Therefore, when the linearly moving member moves linearly, the wall portion moves linearly together with the linearly moving member. In this case, the sliding resistance generated by the wall portion can be reduced compared to the sliding resistance generated by a sealing member disposed between a rotating member that performs rotational motion and a linearly moving linear member, as in the conventional case. This prevents a decrease in the operating efficiency of the feed screw mechanism and prevents the intrusion of foreign matter from the outside and the leakage of lubricant to the outside.

[0011] The object to be operated by the linear motion member may be, for example, a hydraulic device. In this case, even if oil splashed from the hydraulic device adheres to the outer circumferential surface of the linear motion member, the wall portion disposed between the male thread portion and the hydraulic device can prevent the oil from moving to the male thread portion. This prevents the oil from mixing with the lubricant, thereby maintaining the functionality of the lubricant. Furthermore, the oil splashed from the hydraulic device can be prevented from splashing directly into the feed screw mechanism, preventing the oil from mixing with the lubricant and maintaining the functionality of the lubricant.

[0012] It is preferable that at least a portion of the outer diameter of the wall portion is larger than the inner diameter of the female thread portion of the rotating member. By making at least a portion of the outer diameter of the wall portion larger than the inner diameter of the female thread portion, it is possible to effectively prevent foreign matter from entering from the outside and lubricant from leaking to the outside.

[0013] Furthermore, the wall portion only needs to be disposed on at least a portion of the outer circumferential surface of the linear motion member. In particular, when the wall portion is disposed continuously around the entire outer circumferential surface of the linear motion member, it is possible to effectively prevent foreign matter from entering from the outside and lubricant from leaking to the outside. Furthermore, the wall portion may be disposed on only a portion of the outer circumferential surface of the linear motion member.

[0014] A protrusion protruding radially from the inner peripheral surface of the housing may be provided between the wall and the hydraulic device. In this case, the protrusion provided on the inner peripheral surface of the housing can prevent oil from migrating from the hydraulic device side to the wall side. The protrusion can also prevent leaked lubricant from migrating to the hydraulic device side.

[0015] The feed screw mechanism according to the present invention can be applied to, for example, an electric actuator including an electric motor and a feed screw mechanism that converts the rotational motion of the electric motor into linear motion. [Effects of the Invention]

[0016] According to the present invention, it is possible to suppress a decrease in the operational efficiency of the feed screw mechanism, and to suppress the intrusion of foreign matter from the outside and the leakage of lubricant to the outside. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a vertical cross-sectional view of an electric actuator according to an embodiment of the present invention. [Figure 2] 1 is an enlarged longitudinal cross-sectional view showing a portion of an electric actuator according to an embodiment of the present invention. [Figure 3] FIG. 4 is a front view of the wall portion as seen from the axial direction of the screw shaft. [Figure 4] FIG. 10 is a front view showing a modified example of the wall portion. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

[0019] The electric actuator according to this embodiment is a so-called linear motion type electric actuator in which the output member moves back and forth in the axial direction (linear motion). In the following, an example will be described in which the electric actuator according to this embodiment is used in an electric brake system of an automobile.

[0020] As shown in Figure 1, the electric actuator 1 according to this embodiment includes an electric motor 2 that generates a rotational driving force, a motion conversion mechanism 3 that converts the rotational motion of the electric motor 2 into linear motion parallel to its rotation axis 2a and outputs the linear motion, a transmission gear mechanism 4 that transmits the rotational driving force from the electric motor 2 to the motion conversion mechanism 3, and a housing 5 that holds these components.

[0021] In this embodiment, for ease of assembly, the housing 5 is divided into two parts. The housing 5 is made up of a first housing 5A to which the electric motor 2 is attached, and a second housing 5B connected to the first housing 5A.

[0022] The motion conversion mechanism 3 is composed of a cylindrical nut 31 as a rotating member, a screw shaft 32 as a linear motion member that moves linearly as the nut 31 rotates, and a ball screw mechanism 30 equipped with a plurality of balls 33. The nut 31 is rotatably supported by two rolling bearings 6 and 7 (here, ball bearings) provided in the first housing 5A and the second housing 5B, respectively.

[0023] An internal thread portion 31a consisting of a spiral groove is provided on the inner peripheral surface of the nut 31. Meanwhile, an external thread portion 32a consisting of a spiral groove is provided on the outer peripheral surface of the screw shaft 32 inserted into the inside of the nut 31. A plurality of balls 33 are housed between the opposing internal thread portion 31a and external thread portion 32a, and these balls 33 support the screw shaft 32 in parallel with the rotating shaft 2a of the electric motor 2. Furthermore, grease is filled between the internal thread portion 31a and the external thread portion 32a as a lubricant to improve the operability and durability of the ball screw mechanism 30.

[0024] A pair of anti-rotation members 34 are provided at the rear end (the right end in FIG. 1) of the screw shaft 32 to prevent the screw shaft 32 from rotating around its axis. The pair of anti-rotation members 34 are formed in a round bar (pin-like) shape, and a portion of each anti-rotation member 34 protrudes from the outer circumferential surface of the screw shaft 32. Each anti-rotation member 34 is disposed in a guide groove 35a of a cylindrical guide member 35 housed in the second housing 5B. Each guide groove 35a is formed to extend in the axial direction of the screw shaft 32, and each anti-rotation member 35 is configured to be movable along its corresponding guide groove 35a.

[0025] The transmission gear mechanism 4 is composed of a first transmission gear 41 provided on the rotary shaft 2a of the electric motor 2 and a second transmission gear 42 that meshes with the first transmission gear 41. The second transmission gear 42 is provided on the outer peripheral surface of the nut 31 and is configured to rotate together with the nut 31.

[0026] In the electric actuator 1 configured as described above, when the rotary shaft 2a of the electric motor 2 rotates, the rotational motion is transmitted to the nut 31 via the first transmission gear 41 and the second transmission gear 42. When the nut 31 rotates, a plurality of balls 33 circulate between the female thread portion 31a and the male thread portion 32a by a circulation member (not shown), causing the screw shaft 32 to advance or retreat in its axial direction. At this time, the screw shaft 32 attempts to rotate in the same direction as the rotational motion of the nut 31, but the rotation stop member 34 provided at the rear end of the screw shaft 32 comes into contact with the guide groove 35a of the guide member 35, thereby restricting the rotation of the screw shaft 32. As a result, the screw shaft 32 advances or retreats without rotating.

[0027] In this way, as the screw shaft 32 moves forward or backward in its axial direction, the tip (left end in FIG. 1) of the screw shaft 32 operates a device (here, a hydraulic cylinder that constitutes an electric brake system) that is not shown.

[0028] Here, when an electric actuator is used as a means for operating a hydraulic cylinder that constitutes an electric brake system, as in this embodiment, oil leaking from inside the hydraulic cylinder may splash and adhere to the outer circumferential surface of the screw shaft 32 or splash into the inside of the ball screw mechanism 30. In this case, if oil enters the nut 31 as the screw shaft 32 moves forward and backward, the oil may mix with the grease present between the nut 31 and the screw shaft 32, changing the properties of the grease and potentially reducing the operability and durability of the ball screw mechanism 30. Furthermore, if the grease inside the nut 31 leaks to the outside as the screw shaft 32 moves forward and backward, the amount of grease may decrease, potentially reducing the operability and durability.

[0029] Therefore, in the electric actuator according to this embodiment, in order to effectively prevent both the intrusion of foreign matter into the interior and the leakage of grease to the exterior, a wall portion 36 is provided on the outer circumferential surface of the screw shaft 32. The configuration of the wall portion 36 will be described in detail below.

[0030] As shown in FIG. 1, the wall portion 36 is provided on the distal side of the male thread portion 32a of the screw shaft 32, i.e., on the outer peripheral surface (a portion where the male thread portion 32a is not provided) between the male thread portion 32 and the hydraulic device (here, a hydraulic cylinder) to be operated. The wall portion 36 is fixed to the outer peripheral surface of the screw shaft 32 so as not to move axially or rotate circumferentially. Therefore, when the screw shaft 32 advances or retreats, the wall portion 36 advances or retreats together with the screw shaft 32. The wall portion 36 also protrudes radially (in a direction perpendicular to the axial direction) from the outer peripheral surface of the screw shaft 32, and its distal end in the protruding direction is disposed close to the inner peripheral surface of the housing 5 (first housing 5A). In other words, the distal end (outer radial end) of the wall portion 36 in the protruding direction is disposed so as not to come into contact with the inner peripheral surface of the housing 5 via a gap 10 (see FIG. 2).

[0031] FIG. 3 is a front view of the wall portion 36 as seen from the axial direction of the screw shaft 32. FIG.

[0032] As shown in FIG. 3 , the wall portion 36 is formed in the shape of a circular plate and is disposed continuously over the entire outer circumferential surface of the screw shaft 32. The shape of the wall portion 36 is not limited to a circular shape and may be changed as appropriate depending on the shape of the inner circumferential surface of the housing 5 (first housing 5A), etc. The wall portion 36 is provided over the entire outer circumferential surface of the screw shaft 32 so that no gap is formed between the wall portion 36 and the outer circumferential surface. In this embodiment, the wall portion 36 is configured as a separate body from the screw shaft 32, but the wall portion 36 may also be molded integrally with the screw shaft 32. The wall portion 36 can be formed from a heat-resistant metal material, for example.

[0033] As described above, in the electric actuator 1 according to this embodiment, the wall portion 36 is provided on the outer peripheral surface of the screw shaft 32. Therefore, even if oil scattered from a hydraulic device adheres to the outer peripheral surface of the screw shaft 32 or if oil scattered from the hydraulic device splashes directly into the ball screw mechanism 30, the oil can be effectively prevented from entering the nut 31 along the outer peripheral surface of the screw shaft 32. That is, even if oil moves along the outer peripheral surface of the screw shaft 32 toward the nut 31, the movement of the oil is blocked by the wall portion 36, and therefore the oil can be prevented from entering the nut 31. This effectively prevents oil from mixing with the grease in the nut 31, and prevents changes in the properties (lubricity) of the grease due to the mixing of oil. This makes it possible to maintain the operability and durability of the ball screw mechanism 30.

[0034] Furthermore, the wall portion 36 not only prevents oil from entering the nut 31, but also prevents grease from leaking from the nut 31. That is, even if grease inside the nut 31 moves along the outer circumferential surface of the screw shaft 32 toward the tip end (toward the hydraulic device) as the screw shaft 32 moves forward and backward, the wall portion 36 prevents the grease from moving. This prevents the amount of grease inside the nut 31 from decreasing, ensuring the function of maintaining the operability and durability of the ball screw mechanism 30 by the grease.

[0035] In a conventional configuration in which a seal member is provided between the nut and the screw shaft, when the screw shaft moves forward or backward, the seal member slides relative to the screw shaft in the linear and rotational directions, so if the contact area of ​​the seal member with the screw shaft or the contact pressure is increased, the sliding resistance of the seal member with respect to the screw shaft increases, resulting in a decrease in the operating efficiency of the screw shaft.In addition, instead of the conventional seal member, it is possible to provide an expandable bellows-shaped boot between the inner surface of the housing and the outer surface of the screw shaft, and use this boot to suppress the intrusion of oil, but this method generates resistance and fluctuations in the internal pressure of the housing due to the expansion and contraction of the boot.

[0036] In contrast, in the embodiment of the present invention, even if the wall portion 36 moves forward or backward together with the screw shaft 32, a gap 10 (see FIG. 2) is provided between the outer radial end of the wall portion 36 and the inner circumferential surface of the housing 5, so the wall portion 36 does not slide against the inner circumferential surface of the housing 5. In this manner, in the present embodiment, the wall portion 36 is disposed so as not to come into contact with the inner circumferential surface of the housing 5, so no sliding resistance occurs between the wall portion 36 and the housing 5, and the wall portion 36 does not interfere with the forward and backward movement of the screw shaft 32. This makes it possible to ensure good operability of the ball screw mechanism 30.

[0037] As described above, in the embodiment of the present invention, the conventional sealing member and bellows-shaped boot are not used, and the wall portion 36 that does not come into contact with the housing 5 is provided on the outer peripheral surface of the screw shaft 32, thereby making it possible to effectively suppress the intrusion of oil from the outside and the leakage of grease to the outside, and also to ensure good operability of the screw shaft.

[0038] The outer diameter direction end of the wall portion 36 needs to be positioned so as not to come into contact with the inner circumferential surface of the housing 5, as well as with the inner circumferential surface including that of other members provided (fixed) on the inner circumferential surface of the housing 5. In the example shown in Fig. 1, nothing is provided on the inner circumferential surface of the housing 5 within the range of reciprocating movement of the wall portion 36, but if other members are provided on the inner circumferential surface of the housing 5, by positioning the wall portion 36 so as not to come into contact with the members provided on the inner circumferential surface, it is possible to avoid the occurrence of sliding resistance accompanying the movement of the wall portion 36.

[0039] Furthermore, when the ball screw mechanism 30 is used in a horizontally installed position, as in the example shown in Figure 1, oil scattered from the hydraulic equipment may accumulate at the bottom of the inner surface of the housing 5 due to gravity, and the oil may enter the nut 31 through the gap between the bottom of the inner surface and the wall 36.

[0040] In this regard, in this embodiment, an annular protrusion 50 (see FIG. 1 ) that protrudes inwardly along the entire inner circumferential surface of the housing 5 is provided between the wall 36 and the hydraulic device, and this protrusion 50 can prevent oil from moving toward the nut 31. Furthermore, even if grease inside the nut 31 moves along the lower part of the inner circumferential surface of the housing 5 toward the hydraulic device, the protrusion 50 can prevent the grease from moving toward the hydraulic device, and therefore the grease can be prevented from entering the hydraulic device. In this way, in this embodiment, the protrusion 50 of the housing 5 provided between the wall 36 and the hydraulic device can more reliably prevent the movement of oil and grease.

[0041] Although the wall portion 36 can suppress the intrusion of oil and the leakage of grease as long as it protrudes even slightly from the outer peripheral surface of the screw shaft 32, a larger protrusion of the wall portion 36 is preferable in terms of suppressing the intrusion of oil and the leakage of grease. Specifically, it is preferable that the outer diameter (maximum outer diameter) D of the wall portion 36 shown in FIG. 1 is larger than the inner diameter (maximum inner diameter) d of the female thread portion 31a of the nut 31. In this way, by making the outer diameter D of the wall portion 36 larger than the inner diameter d of the female thread portion 31a of the nut 31, it is possible to effectively suppress the leakage of grease from inside the nut 31 and the intrusion of oil into the nut 31. Furthermore, the entire outer diameter of the wall portion 36 is not necessarily larger than the inner diameter of the female thread portion 31a of the nut 31, and only a portion of the outer diameter of the wall portion 36 may be larger than the inner diameter of the female thread portion 31a of the nut 31. In other words, the outer diameter of the wall portion 35 may be formed to be larger than the inner diameter 31a of the female thread portion of the nut 31 in a part of the circumferential direction of the outer peripheral surface of the screw shaft 32.

[0042] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the invention.

[0043] In the above-described embodiment, the protruding tip of the wall portion 36 is positioned so as not to contact the inner circumferential surface of the housing 5. However, to improve sealing, the protruding tip of the wall portion 36 may be in sliding contact with the inner circumferential surface of the housing 5 or the inner circumferential surface of another component provided on the inner circumferential surface of the housing 5. This configuration also reduces the decrease in operational efficiency of the ball screw mechanism and the intrusion of foreign matter from the outside and the leakage of lubricant, compared to a conventional configuration in which a sealing component is disposed between a rotating component that performs rotational motion and a linearly moving component that performs linear motion. In other words, in a conventional configuration, when the rotating component rotates and the linearly moving component linearly moves, the sealing component slides against the linearly moving component in both the linear and rotational directions. However, in the case of the wall portion 36 according to the present invention, the sealing component slides against the inner circumferential surface of the housing 5 or another component only in the linear direction, thereby reducing sliding resistance and preventing a decrease in operational efficiency of the ball screw mechanism. The wall portion 36 can be made of a slidable resin material, rubber material, or the like.

[0044] 4, the wall portion 36 may be provided on only a portion of the outer circumferential surface of the screw shaft 32. In the example shown in Fig. 4, the wall portion 36 is provided in a semicircular shape over the circumferential region of the lower half of the screw shaft 32, but the arrangement and shape of the wall portion 36 can be changed as appropriate. Also, a plurality of wall portions 36 may be arranged at intervals from one another over the circumferential direction of the screw shaft 32.

[0045] In the above embodiment, the electric actuator according to the present invention is used as an electric actuator for operating hydraulic equipment in an electric brake system, but the device or member to be operated may be something other than hydraulic equipment. Therefore, the electric actuator according to the present invention can also be used as an electric actuator for operating devices or members other than hydraulic equipment, and the wall portion 36 can also be used to prevent the intrusion of foreign matter other than oil.

[0046] Furthermore, the motion conversion mechanism that converts the rotational motion of the electric motor into linear motion is not limited to a ball screw mechanism in which a nut (female thread portion) and a screw shaft (male thread portion) are indirectly threaded together via balls, but may also be a sliding screw mechanism in which a nut (female thread portion) and a screw shaft (male thread portion) are directly threaded together without using balls. In other words, the present invention is not limited to a ball screw mechanism, which is an example of a feed screw mechanism, but can also be applied to another example, a sliding screw mechanism. [Explanation of symbols]

[0047] 1 Electric Actuator 2 electric motors 3. Motion conversion mechanism 4 Transmission gear mechanism 5. Housing 10 Gap 30 Ball screw mechanism 31 Nut (rotating part) 31a Female thread 32 Screw shaft (linear motion member) 32a male thread 33 Ball 36 Wall 50 Protrusion

Claims

1. A feed screw mechanism that converts the rotational motion of an electric motor into linear motion and transmits it to an object to be operated, a rotating member having a female thread portion on an inner circumferential surface; a linear motion member having a male thread portion on an outer circumferential surface that is directly or indirectly threaded with the female thread portion and that moves linearly in accordance with the rotation of the rotating member; a lubricant accommodated between the female thread portion and the male thread portion; a housing that accommodates the rotary member and the linearly moving member; a wall portion protruding radially from an outer circumferential surface of the linear motion member; the wall portion moves linearly together with the linear motion member, an outer radial end of the wall portion is arranged so as not to come into contact with an inner circumferential surface of the housing or a member provided on the inner circumferential surface of the housing, or so as to be slidable relative to the inner circumferential surface of the housing or a member provided on the inner circumferential surface of the housing, A feed screw mechanism, characterized in that only a portion of the outer diameter of the wall portion in the circumferential direction of the outer peripheral surface of the linear motion member is larger than the inner diameter of the female thread portion of the rotating member.

2. the operation target is a hydraulic device, The feed screw mechanism according to claim 1 , wherein the wall portion is disposed between the male thread portion of the linear motion member and a hydraulic device that is moved by the linear motion member.

3. The feed screw mechanism according to claim 1 or 2, wherein the wall portion is disposed on at least a portion of the outer circumferential surface of the linear motion member in the circumferential direction.

4. 3. The feed screw mechanism according to claim 2, wherein a protrusion protruding radially from an inner peripheral surface of the housing is provided between the wall portion and the hydraulic device.

5. An electric actuator comprising: an electric motor; and the feed screw mechanism according to any one of claims 1 to 4, which converts rotational motion of the electric motor into linear motion.

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