Worm reduction gear

Abstract

[Problem] To provide a worm reduction gear capable of preventing generation of unpleasant abnormal noise which is due to metal-to-metal contact and generated at a part other than a meshing portion between wheel teeth and worm teeth. [Solution] A worm 14 has an annular protrusion 40 at a part located radially inside an inner ring 28. The annular protrusion 40 has two contact ends 41 at both ends in the axial direction of the outer peripheral surface, the contact ends 41 restricting the worm 14 from inclining further with respect to the inner ring 28 by coming into contact with the inner peripheral surface of the inner ring 28 when the worm 14 is inclined to a maximum inclination angle θmax with respect to the inner ring 28, and each of the two contact ends 41 being constituted by a convex curved surface.

Description

worm reducer

[0001] The present disclosure relates to a worm reducer that is incorporated into various mechanical devices such as an electric power steering device.

[0002] A worm reducer is used to reduce the rotational speed of an electric motor to a speed appropriate for the mechanical device that uses the high-speed rotation, thereby obtaining a large torque from a small power source.

[0003] For example, in the field of steering devices, electric power steering devices that use an electric motor as an auxiliary power source to reduce the force required by the driver to operate the steering wheel are widespread, and these electric power steering devices are equipped with a worm reducer to increase the torque of the electric motor.

[0004] The worm reducer includes a housing, a worm wheel, and a worm.

[0005] The worm wheel has wheel teeth on its outer circumferential surface and is rotatably supported inside the housing.

[0006] The worm has worm teeth on its outer circumferential surface that mesh with the wheel teeth, and is rotatably supported inside the housing.

[0007] The output shaft of the electric motor is connected to one axial end of the worm so as to transmit torque. The torque of the output shaft of the electric motor is amplified by being transmitted to the worm wheel via the worm, and then applied as auxiliary power to a steering force transmission member such as a steering shaft or a pinion shaft or rack shaft of a steering gear unit. This reduces the force required by the driver to operate the steering wheel.

[0008] In a worm reducer, unavoidable backlash exists at the meshing portion between the wheel teeth and the worm teeth due to dimensional errors, assembly errors, etc., of the components that make up the worm reducer. Due to the existence of this backlash, when changing the direction of rotation of the steering wheel, i.e., when changing the direction of rotation of the worm, an unpleasant rattle noise may occur at the meshing portion.

[0009] International Publication No. 2004 / 052712 describes an electric power steering device equipped with a worm reducer having a structure for suppressing the generation of rattle noise at the meshing portion between the wheel teeth and the worm teeth.

[0010] In the worm reducer disclosed in WO 2004 / 052712, a support bearing for rotatably supporting one axial end of the worm relative to the housing is configured as a deep groove ball bearing. A minute gap is formed between the inner peripheral surface of the inner ring constituting the support bearing and the outer peripheral surface of one axial end of the worm, allowing the worm to tilt relative to the inner ring of the support bearing. In other words, the existence of this minute gap and an internal gap in the support bearing enables the worm to oscillate relative to the housing.

[0011] Additionally, the other axial end of the worm is elastically biased toward the worm wheel by a biasing means provided between the housing and the other axial end of the worm, thereby reducing backlash at the meshing portion between the wheel teeth and the worm teeth and reducing the occurrence of teeth rattle noise at the meshing portion when the direction of rotation of the steering wheel is changed.

[0012] Furthermore, two worm dampers are provided, arranged on either axial side of the inner ring of the support bearing, to elastically support the worm in the axial direction relative to the inner ring. The worm can be displaced in the axial direction relative to the inner ring by elastic deformation of the worm dampers. This reduces the force of collision between the tooth surfaces of the wheel teeth and worm teeth when the direction of rotation of the steering wheel is changed, thereby suppressing the generation of rattle noise at the meshing portion between the wheel teeth and worm teeth.

[0013] International Publication No. 2004 / 052712

[0014] However, even the worm reducer described in WO 2004 / 052712 still has room for improvement in terms of further suppressing the generation of abnormal noise caused by contact between metals.

[0015] An object of the present disclosure is to provide a worm reducer that can suppress the generation of abnormal noise caused by metal-to-metal contact at locations other than the meshing portion between the wheel teeth and the worm teeth.

[0016] The inventors of the present disclosure have conducted extensive research into the cause of abnormal noise caused by metal-to-metal contact occurring at locations other than the meshing portion between the wheel teeth and the worm teeth.

[0017] In a worm reducer equipped with a biasing means, a portion of the worm located radially inward of the inner ring is provided with an annular protrusion that protrudes radially outward further than the adjacent portions on both axial sides, and the outer surface of the annular protrusion is positioned closely opposite the inner surface of the inner ring, thereby restricting the maximum inclination angle of the worm relative to the inner ring of the support bearing within an appropriate range.

[0018] When the worm is inclined to the maximum inclination angle relative to the inner ring, a corner at the connection between the outer circumferential surface of the annular convex portion and the axial side surface comes into contact with the inner circumferential surface of the inner ring. It was discovered that when the worm is displaced in the axial direction in this state, a so-called stick-slip phenomenon occurs, in which slippage and sticking alternate repeatedly in a short period of time, resulting in an abnormal noise known as stick-slip noise. The present disclosure was completed based on this discovery.

[0019] A worm reducer according to one embodiment of the present disclosure comprises: a housing; a worm wheel having wheel teeth on its outer peripheral surface and rotatably supported inside the housing; a worm having worm teeth on its outer peripheral surface that mesh with the wheel teeth and rotatably supported inside the housing; a support bearing including an outer ring fixed to the housing and an inner ring loosely fitted onto one axial end of the worm, supporting the one axial end of the worm relative to the housing to allow rotation, swinging displacement, and axial displacement; a biasing means that elastically biases the other axial end of the worm toward the worm wheel; and two worm dampers that are arranged on both axial sides of the inner ring and elastically support the worm in the axial direction relative to the inner ring.

[0020] The worm has an annular protrusion at a portion located radially inside the inner ring, the protrusion protruding radially outward beyond adjacent portions on both axial sides.

[0021] The annular convex portion has two contact ends at both axial ends of the outer peripheral surface which come into contact with the inner peripheral surface of the inner ring when the worm is inclined to the maximum inclination angle relative to the inner ring, thereby preventing the worm from inclining further relative to the inner ring.

[0022] Each of the two contact ends is formed by a convex curved surface.

[0023] In the worm reducer according to one aspect of the present disclosure, the radius of curvature of the convex curved surface may be equal to or greater than 0.2 mm and equal to or less than 3.0 mm.

[0024] In a worm reducer according to one aspect of the present disclosure, the worm may have an annular recess that opens to the axially middle portion of the outer peripheral surface of the annular protrusion, and an O-ring may be provided that is elastically compressed between the bottom surface of the annular recess and the inner peripheral surface of the inner ring.

[0025] In the worm reducer according to one aspect of the present disclosure, the two contact ends may have an arithmetic mean roughness Ra of 1.6 μm or less.

[0026] According to the worm reducer of one aspect of the present disclosure, it is possible to effectively suppress the generation of harsh noise between the inner ring of the support bearing and the worm.

[0027] FIG. 1 is a diagram illustrating an electric power steering device incorporating a worm reducer according to a first embodiment of the present disclosure. FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1. FIG. 3 is an enlarged view of portion B in FIG. 2. FIG. 4 is an enlarged view corresponding to portion C in FIG. 3. FIG. 5(A) is a cross-sectional view of a main portion of the worm reducer according to the first embodiment, illustrating the contact state between the inner peripheral surface of the inner ring and the annular protrusion of the worm when the worm oscillates relative to the inner ring of the support bearing. FIG. 5(B) is a diagram similar to FIG. 5(A) but illustrating a worm reducer according to a comparative example of the first embodiment. FIG. 6 is an exploded perspective view of the upper left portion of FIG. 2. FIG. 7 is a view viewed from the left side of FIG. 2 with a cover and a retaining ring removed from the left side of FIG. 2. FIG. 8 is a view corresponding to FIG. 4 but illustrating a worm reducer according to a second embodiment of the present disclosure. FIG. 9 is a view corresponding to FIG. 4 but illustrating a worm reducer according to a third embodiment of the present disclosure. FIG. 10 is a diagram corresponding to FIG. 4 and illustrating a worm reducer according to a fourth example of an embodiment of the present disclosure.

[0028] First Example A first example of an embodiment of the present disclosure will be described with reference to FIGS. 1 to 7. FIG.

[0029] The worm reducer of this example is widely applicable to worm reducers incorporated in electric power steering devices as well as worm reducers incorporated in various mechanical devices. Furthermore, the worm reducer of this example is applicable to any type of electric power steering device, such as a column-assist type that applies auxiliary power to a steering shaft rotatably supported inside a steering column, a pinion-assist type that applies auxiliary power to a pinion shaft of a steering gear unit, or a rack-assist type that applies auxiliary power to a rack shaft of a steering gear unit. Hereinafter, the worm reducer of this example will be described using as an example a case where the worm reducer is applied to a column-assist type electric power steering device.

[0030] As shown in FIG. 1, the electric power steering device 1 includes a steering wheel 2, a steering shaft 3, a steering column 4, a pair of universal joints 5a, 5b, an intermediate shaft 6, a steering gear unit 7, and an electric assist device 8.

[0031] The steering wheel 2 is fixedly supported at the rear end of a steering shaft 3. The steering shaft 3 is rotatably supported inside a steering column 4 that is supported on the vehicle body. The front end of the steering shaft 3 is connected to a pinion shaft 9 of a steering gear unit 7 via a rear universal joint 5a, an intermediate shaft 6, and a front universal joint 5b.

[0032] Therefore, when the driver turns the steering wheel 2, the rotation of the steering wheel 2 is transmitted to the pinion shaft 9 via the steering shaft 3, the pair of universal joints 5a, 5b, and the intermediate shaft 6. The rotation of the pinion shaft 9 is converted into linear motion of a rack shaft (not shown) of the steering gear unit 7 that meshes with the pinion shaft 9. As a result, a steering angle corresponding to the amount of rotation of the steering wheel 2 is applied to the pair of steered road wheels.

[0033] The electric assist device 8 includes an electric motor 10, which is an auxiliary power source, and a worm reduction gear 11. The electric assist device 8 increases the auxiliary power of the electric motor 10 by the worm reduction gear 11 and then applies it to the front end of the steering shaft 3. This reduces the force required by the driver to rotate the steering wheel 2.

[0034] The worm reducer 11 includes a housing 12 , a worm wheel 13 , a worm 14 , a support bearing 15 , two worm dampers 16 , and a biasing means 17 .

[0035] The housing 12 includes a wheel accommodating portion 18 and a worm accommodating portion 19 having a central axis that is twisted relative to the central axis of the wheel accommodating portion 18 and an axially intermediate portion that opens into the wheel accommodating portion 18. The housing 12 is fixedly coupled to the front end of the steering column 4.

[0036] The wheel accommodating portion 18 is cylindrical. In Fig. 2, the central axis of the wheel accommodating portion 18 extends in the front-to-rear direction. The wheel accommodating portion 18 is disposed coaxially with the steering column 4.

[0037] The worm housing 19 is cylindrical and has openings at both axial ends. In Fig. 2, the central axis of the worm housing 19 extends in the left-right direction.

[0038] In the following description, one axial side of the worm housing portion 19 and each member housed in the worm housing portion 19 is the right side in FIG. 2, and the other axial side is the left side in FIG.

[0039] An opening on one axial side of the worm accommodating portion 19 is closed by the electric motor 10 fixedly coupled to the housing 12. An opening on the other axial side of the worm accommodating portion 19 is closed by a lid 21 attached to the opening using a retaining ring 20.

[0040] The worm wheel 13 has wheel teeth 22 on its outer circumferential surface, and is rotatably supported inside the housing 12. Specifically, the worm wheel 13 is housed inside the wheel accommodating portion 18, and is fitted and fixed to the front end of the steering shaft 3, which is rotatably supported inside the wheel accommodating portion 18.

[0041] The worm 14 has worm teeth 23 that mesh with the wheel teeth 22 at an axially intermediate portion of its outer circumferential surface, and is rotatably supported inside the housing 12. Specifically, the worm 14 is housed inside the worm accommodating portion 19, and is rotatably supported relative to the worm accommodating portion 19 by a support bearing 15 and another support bearing 43.

[0042] One axial end of the worm 14 is connected to the tip of the output shaft 25 of the electric motor 10 by spline engagement or via a coupling, allowing torque transmission, swinging displacement, and axial displacement. In this example, the worm 14 has a female spline portion 24 on the inner peripheral surface of the one axial end. The female spline portion 24 is spline-engaged with a male spline portion 26 provided on the outer peripheral surface of the tip of the output shaft 25 of the electric motor 10. In this way, the one axial end of the worm 14 is connected to the tip of the output shaft 25, allowing torque transmission, swinging displacement, and axial displacement.

[0043] The support bearing 15 includes an outer ring 27 fixed to the housing 12 and an inner ring 28 fitted loosely onto one axial end of the worm 14, and supports the one axial end of the worm 14 relative to the housing 12, allowing rotation, swinging displacement, and axial displacement.

[0044] In this example, the support bearing 15 is configured as a four-point contact ball bearing. That is, the support bearing 15 includes an outer ring 27, an inner ring 28, and a plurality of rolling elements 29, each formed of a ball, arranged between a gothic arch grooved outer ring raceway provided on the inner peripheral surface of the outer ring 27 and a gothic arch grooved inner ring raceway provided on the outer peripheral surface of the inner ring 28.

[0045] In this example, the outer ring 27 is fitted and fixed to one axial end of the worm accommodating portion 19. Specifically, the outer ring 27 is clearance-fitted to a cylindrical fitting surface portion 30 provided on the inner peripheral surface of one axial end of the worm accommodating portion 19. Furthermore, the outer ring 27 is sandwiched from both axial sides by a stepped surface 31 facing one axial side and provided in a portion of the worm accommodating portion 19 adjacent to the other axial side of the fitting surface portion 30, and a retaining ring 32 engaged with a portion of the worm accommodating portion 19 adjacent to one axial side of the fitting surface portion 30.

[0046] In this example, the inner ring 28 is loosely fitted onto the outer peripheral surface of one axial end of the worm 14 .

[0047] In this example, the outer peripheral surface of one axial end of the worm 14 is configured as a stepped cylindrical surface whose outer diameter increases stepwise from one axial end to the other axial end, as shown in Fig. 3. Specifically, the outer peripheral surface of one axial end of the worm 14 has, in order from one axial end, a small diameter portion 33, a medium diameter portion 34, a large diameter portion 35, and a flange portion 36.

[0048] The small diameter portion 33 is configured to have a substantially cylindrical surface. The worm 14 has an engagement groove 37 formed around the entire circumference at one axial end of the small diameter portion 33.

[0049] The medium diameter portion 34 is configured in a substantially cylindrical surface shape and has an outer diameter larger than the outer diameter of the small diameter portion 33. The small diameter portion 33 and the medium diameter portion 34 are connected by a small diameter side step surface 38 facing one axial side.

[0050] The large diameter portion 35 is configured in a substantially cylindrical surface shape and has an outer diameter larger than the outer diameter of the medium diameter portion 34. The medium diameter portion 34 and the large diameter portion 35 are connected by a large diameter side step surface 39 facing one axial side.

[0051] The flange portion 36 is formed in a circular ring shape and protrudes radially outward from a portion adjacent to the other axial side of the large diameter portion 35 .

[0052] In this example, the inner ring 28 is loosely fitted onto the medium diameter portion 34. Therefore, a radial gap exists between the inner circumferential surface of the inner ring 28 and the medium diameter portion 34. The support bearing 15 supports one axial end of the worm 14 so as to be able to oscillate relative to the worm accommodating portion 19, based on the radial gap between the inner circumferential surface of the inner ring 28 and the medium diameter portion 34 and the existence of an internal gap in the support bearing 15. Furthermore, the worm 14 is able to move axially relative to the inner ring 28, based on the radial gap between the inner circumferential surface of the inner ring 28 and the medium diameter portion 34.

[0053] The worm 14 has an annular protrusion 40 at a portion located radially inside the inner ring 28 that protrudes radially outward beyond adjacent portions on both axial sides.

[0054] In this example, the annular protrusion 40 is provided around the entire periphery of the axially intermediate portion of the medium diameter portion 34. The outer diameter (maximum outer diameter) of the annular protrusion 40 is smaller than the inner diameter of the inner ring 28, and the axial dimension of the annular protrusion 40 is smaller than the axial dimension of the inner peripheral surface of the inner ring 28. The outer diameter of the annular protrusion 40 is not limited to this, but can be preferably smaller than the inner diameter of the inner ring 28 by approximately 0.005 mm to 0.08 mm, and more preferably by approximately 0.01 mm to 0.05 mm. Furthermore, the axial dimension of the annular protrusion 40 is not limited to this, but can be preferably smaller than the axial dimension of the inner ring 28 by approximately 3.0 mm to 8.0 mm, and more preferably by approximately 4.0 mm to 7.0 mm.

[0055] The annular protrusion 40 has two contact ends 41 at both axial ends of its outer peripheral surface. Specifically, the two contact ends 41 are provided at the connection between an axially intermediate portion of the outer peripheral surface of the annular protrusion 40 and an axial end face 60. In this example, the axially intermediate portion of the outer peripheral surface of the annular protrusion 40 is formed by a cylindrical surface portion 42 whose outer diameter does not change in the axial direction, and the axial end face 60 is formed by a flat surface perpendicular to the central axis of the worm 14.

[0056] As shown in FIG. 5A, the two contact end portions 41 are inclined at a maximum inclination angle θ max When the worm 14 is inclined to the maximum inclination angle θ , the two contact ends 41 contact the inner peripheral surface of the inner ring 28, thereby restricting the worm 14 from inclining further relative to the inner ring 28. max 5A ), when the worm 14 is tilted in a direction away from the worm wheel 13 as it moves toward the other axial side, the end of the contact end 41 on one axial side that is closer to the worm wheel 13 comes into contact with the inner circumferential surface of the inner ring 28 at one axial side, and the end of the contact end 41 on the other axial side that is farther from the worm wheel 13 comes into contact with the inner circumferential surface of the inner ring 28 at the other axial side.

[0057] Each of the two contact end portions 41 is formed by a convex curved surface. That is, the two contact end portions 41 are arranged on either side of the cylindrical surface portion 42 and are formed by two convex curved surfaces facing in opposite directions.

[0058] Specifically, the two contact end portions 41 are configured with two convex arc-shaped cross sections that are inclined radially inward as they move away from each other in the axial direction. The two contact end portions 41 and the cylindrical surface portion 42 are smoothly connected so that their axial ends have a common tangent.

[0059] According to the worm reducer 11 of this example, by configuring each of the two contact end portions 41 with a convex curved surface, it is possible to effectively suppress the generation of abnormal noise known as stick-slip noise between the inner ring 28 of the support bearing 15 and the worm 14.

[0060] 5(B), when the contact end portions 41z, which are the connection portions between the cylindrical surface portion 42z constituting the outer peripheral surface of the annular protrusion 40z of the worm 14z and the axial end surface 60z, are configured with pointed edges or corners, each contact end portion 41z is likely to come into edge contact with and get caught on the inner peripheral surface of the inner ring 28. For this reason, if the worm 14z is displaced in the axial direction while each contact end portion 41z is in contact with the inner peripheral surface of the inner ring 28, a so-called stick-slip phenomenon occurs in which slippage and sticking alternate repeatedly in a short period of time, and an abnormal noise known as stick-slip noise is likely to occur.

[0061] In contrast, in this example, as shown in Figure 5(A), the two contact end portions 41 provided at the connection between the cylindrical surface portion 42 of the annular convex portion 40 of the worm 14 and the axial end face 60 are each configured with a convex curved surface. Therefore, each contact end portion 41 is less likely to come into edge contact with or get caught on the inner circumferential surface of the inner ring 28. Therefore, even if the worm 14 is displaced in the axial direction with each contact end portion 41 in contact with the inner circumferential surface of the inner ring 28, the stick-slip phenomenon is less likely to occur, and the generation of abnormal noise is effectively suppressed.

[0062] There are no sharp edges or corners extending in the circumferential direction at the two contact ends 41 or at the connection between the two contact ends 41 and the cylindrical surface portion 42. As long as there are no such edges or corners, the shape of the convex curved surfaces that make up the two contact ends 41 is arbitrary. There are no particular limitations on the radius of curvature R of the convex curved surfaces that make up the two contact ends 41, but from the viewpoint of more effectively suppressing the occurrence of the stick-slip phenomenon and the generation of abnormal noise, it can be preferably about 0.2 mm or more and 3.0 mm or less, more preferably about 0.5 mm or more and 1.5 mm or less.

[0063] In this example, the arithmetic mean roughness Ra of the two contact end portions 41 is preferably 1.6 μm or less, and more preferably 0.8 μm or less.

[0064] Setting the arithmetic mean roughness Ra of the two contact ends 41 to 1.6 μm or less makes it possible to further reduce the frictional engagement force of each contact end 41 with the inner circumferential surface of the inner ring 28. Therefore, it is possible to more effectively prevent the stick-slip phenomenon from occurring between each contact end 41 and the inner circumferential surface of the inner ring 28, and to more effectively prevent the occurrence of abnormal noise.

[0065] In this example, grease, which is a lubricant, is interposed between the inner circumferential surface of the inner ring 28 and the outer circumferential surface of the annular convex portion 40. That is, in this example, the two contact ends 41 contact the inner circumferential surface of the inner ring 28 via the grease. Preferably, grease containing an extreme pressure additive, such as molybdenum grease, can be used as the grease.

[0066] By providing grease between the two contact ends 41, the two contact ends 41 come into contact with the inner peripheral surface of the inner ring 28 via the grease, which effectively prevents the stick-slip phenomenon from occurring between the two contact ends 41 and the inner peripheral surface of the inner ring 28. This also effectively suppresses the generation of abnormal noise.

[0067] The worm reducer 11 of this example further includes an annular spacer 50 fitted and fixed to the outside of the small diameter portion 33 at one axial end of the worm 14. The spacer 50 has a cylindrical portion 51, a circular flange portion 52 that protrudes radially outward from the one axial end of the cylindrical portion 51, and a crimped portion 53 that extends toward one axial side from the radially inner end of the one axial end of the cylindrical portion 51. The outer diameter of the portion of the cylindrical portion 51 that is located on the other axial side of the flange portion 52 is equal to the outer diameter of the large diameter portion 35.

[0068] The spacer 50 is externally fitted and fixed to the small diameter portion 33 by press-fitting the cylindrical portion 51 onto the small diameter portion 33, and by engaging the tip half of the crimped portion 53, which is the half on one axial side, with the engagement groove 37, with the radially inner portion of the side surface on the other axial side of the cylindrical portion 51 abutting against the small diameter side step surface 38. Before the spacer 50 is externally fitted and fixed to the small diameter portion 33, the crimped portion 53 of the spacer 50 is configured in a simple cylindrical shape, and when the spacer 50 is externally fitted and fixed to the small diameter portion 33, the half on one axial side is plastically deformed radially inward to engage with the engagement groove 37.

[0069] In this example, the inner ring 28, which is loosely fitted onto the medium diameter portion 34, is disposed axially between the radially outer portion of the side surface on the other axial side of the cylindrical portion 51 and the large diameter side stepped surface 39. The axial width between the radially outer portion of the side surface on the other axial side of the cylindrical portion 51 and the large diameter side stepped surface 39 is 2L larger than the axial dimension of the inner ring 28. The worm 14 can be displaced axially by 2L relative to the inner ring 28.

[0070] In this example, the other axial end of the worm 14 is supported by another support bearing 43 relative to the worm accommodating portion 19 so as to be rotatable and displaceable in the axial direction.

[0071] In this example, support bearing 43 is configured as a deep groove ball bearing, that is, support bearing 43 includes an outer ring 44, an inner ring 45, and a plurality of rolling elements, each of which is configured as a ball, arranged between a deep groove outer ring raceway provided on the inner peripheral surface of outer ring 44 and a deep groove inner ring raceway provided on the outer peripheral surface of inner ring 45.

[0072] The inner ring 45 is press-fitted onto the other axial end of the worm 14. The outer ring 44 is disposed inside a retaining portion 47 provided on the inner peripheral surface of the other axial end of the worm accommodating portion 19 so as to be movable in the radial and axial directions.

[0073] The biasing means 17 elastically biases the other axial end of the worm 14 toward the worm wheel 13. As a result, backlash at the meshing portion suppresses backlash between the wheel teeth 22 and the worm teeth 23, thereby suppressing the generation of teeth rattle noise at the meshing portion when the rotation direction of the steering wheel 2 is changed.

[0074] The specific configuration of the biasing means 17 is not particularly limited, and various conventionally known biasing means configurations can be used. In this example, the biasing means 17 is configured by a biasing leaf spring 46. The biasing leaf spring 46 biases the support bearing 43 toward the worm wheel 13.

[0075] In the following description, the direction in which the worm 14 moves toward or away from the worm wheel 13 (the up-and-down direction in Figures 2 and 7), which is the biasing direction of the biasing leaf spring 46, is referred to as the "first direction," the axial direction of the worm accommodating portion 19 (the left-right direction in Figure 2, the front-to-back direction in Figure 7) is referred to as the "second direction," and the direction perpendicular to both the first and second directions (the front-to-back direction in Figure 2, the left-to-right direction in Figure 7) is referred to as the "third direction."

[0076] As shown in Fig. 7 , the biasing leaf spring 46 is disposed at an end of the portion between the retaining portion 47 and the outer peripheral surface of the outer ring 44 that is farther from the worm wheel 13 in the first direction, so as to extend in the third direction. The base end (right end in Fig. 7 ) of the biasing leaf spring 46 is fixedly sandwiched between the retaining portion 47 and a support pin 48 that is fixed to the other axial end of the worm accommodating portion 19. The tip end (left end in Fig. 7 ) of the biasing leaf spring 46 is elastically pressed against the outer peripheral surface of the outer ring 44. As a result, the biasing leaf spring 46 elastically biases the other axial end of the worm 14 toward the worm wheel 13 via the support bearing 43.

[0077] When torque is transmitted from the output shaft 25 of the electric motor 10 to the worm 14 and then from the worm 14 to the worm wheel 13, and when a large torque is input in reverse from the worm wheel 13 to the worm 14 due to the wheels of the automobile running over a curb or the automobile traveling on a rough road, a meshing reaction force is applied to the worm 14 from the meshing portion between the worm teeth 23 and the wheel teeth 22.

[0078] 7, F1 and F2 represent vectors of components of the meshing reaction force in a virtual plane perpendicular to the second direction. Specifically, F1 represents the vector of the component when the worm 14 and the worm wheel 13 rotate in a predetermined direction, and F2 represents the vector of the component when the worm 14 and the worm wheel 13 rotate in the direction opposite to the predetermined direction.

[0079] The components F1 and F2 include not only a first direction component but also a third direction component. The direction of the first direction component is opposite to the worm wheel 13. The directions of the third direction components are opposite for the components F1 and F2. Furthermore, the ratio of the first direction component to the third direction component is different for the components F1 and F2. That is, the vectors of the components F1 and F2 are oriented in directions asymmetric with respect to each other with respect to the third direction.

[0080] The biasing leaf spring 46 also functions to support the first direction component of the meshing reaction force acting on the worm 14. In particular, in this example, the biasing leaf spring 46 exhibits nonlinear spring characteristics such that the spring constant increases as the amount of deflection in the first direction increases. Therefore, when the first direction component of the meshing reaction force increases, the biasing leaf spring 46 can efficiently support the first direction component.

[0081] The worm reducer 11 of this example includes, as an optional or additional element, a clamping leaf spring 49 that elastically clamps the support bearing 43 from both sides in the third direction.

[0082] The clamping leaf spring 49 is configured in a generally notched cylindrical shape with a discontinuous portion at one location in the circumferential direction. The clamping leaf spring 49 is disposed between the outer peripheral surface of the outer ring 44 and the retaining portion 47 with the discontinuous portion facing away from the worm wheel 13 in the first direction. The portions of the clamping leaf spring 49 located on both sides in the third direction are disposed between the outer peripheral surface of the outer ring 44 and the retaining portion 47 in a state that allows elastic deflection in the third direction.

[0083] In this example, the portions of the clamping leaf spring 49 located on both sides in the third direction prevent the support bearing 43 and the other axial end of the worm 14 from rattling in the third direction inside the retaining portion 47.

[0084] The portions of the clamping leaf spring 49 located on both sides in the third direction exhibit nonlinear spring characteristics such that the spring constant increases as the amount of deflection in the third direction increases. Therefore, when the third direction component of the meshing reaction force applied to the worm 14 from the meshing portion between the wheel teeth 22 and the worm teeth 23 increases, the third direction component can be efficiently supported by the portions of the clamping leaf spring 49 located on both sides in the third direction.

[0085] The clamping leaf spring 49 can be omitted. In this case, by minimizing the gap in the third direction between the outer peripheral surface of the outer ring 44 of the support bearing 43 and the retaining portion 47, it is possible to prevent the end portions of the support bearing 43 and the worm 14 on the other axial side from rattling in the third direction inside the retaining portion 47.

[0086] The two worm dampers 16 are arranged on both axial sides of the inner ring 28 of the support bearing 15 and elastically support the worm 14 relative to the inner ring 28 in the axial direction.

[0087] In this example, each of the two worm dampers 16 is configured to be cylindrical as a whole, and includes a damper body 54 and two side plates 55 .

[0088] The damper body 54 is cylindrical and made of rubber. The two side plates 55 are annular and made of metal plates, and sandwich the damper body 54 from both axial sides. The axial side surfaces of the damper body 54 are vulcanization-bonded to the opposing axial side surfaces of the two side plates 55.

[0089] Of the two worm dampers 16, the worm damper 16 on one axial side is disposed around a portion of the cylindrical portion 51 of the spacer 50 that is located on the other axial side of the flange portion 52, and is axially sandwiched between a side surface on the other axial side of the flange portion 52 of the spacer 50 and a side surface on one axial side of the inner ring 28. Specifically, the side surface on one axial side of the side plate 55 on one axial side abuts against the side surface on the other axial side of the flange portion 52, and the side surface on the other axial side of the side plate 55 on the other axial side abuts against the side surface on one axial side of the inner ring 28.

[0090] In a neutral state in which torque is not being transmitted from the output shaft 25 of the electric motor 10 to the worm 14, the damper body 54 constituting the worm damper 16 on one axial side is elastically compressed in the axial direction between the two side plates 55. In this state, a gap of axial width L exists between the side surface on the other axial side of the cylindrical portion 51 of the spacer 50 and the side surface on one axial side of the inner ring 28.

[0091] Of the two worm dampers 16, the worm damper 16 on the other axial side is disposed around the large diameter portion 35 of the worm 14, and is axially sandwiched between the side surface on the other axial side of the inner ring 28 and the side surface on one axial side of the flange 36 of the worm 14. Specifically, the side surface on one axial side of the side plate 55 on the one axial side abuts against the side surface on the other axial side of the inner ring 28, and the side surface on the other axial side of the side plate 55 on the other axial side abuts against the side surface on one axial side of the flange 36.

[0092] In the neutral state, the damper body 54 constituting the worm damper 16 on the other axial side is elastically compressed in the axial direction between the two side plates 55. In this state, a gap of axial width L exists between the side surface on the other axial side of the inner ring 28 and the large-diameter-side stepped surface 39 of the worm 14.

[0093] The meshing reaction force applied to the worm 14 from the meshing portion between the worm teeth 23 and the wheel teeth 22 includes a second direction component, i.e., a component in the axial direction of the worm 14. The direction of this axial component is reversed depending on the rotational direction of the worm 14 and the worm wheel 13. That is, depending on the rotational direction of the worm 14 and the worm wheel 13, a component force fa directed toward one axial side may be applied to the worm 14 from the meshing portion, or a component force fb directed toward the other axial side may be applied to the worm 14.

[0094] When a component force fa directed toward one axial side is applied to the worm 14 from the neutral state, the worm 14 displaces toward one axial side while further compressing the damper body 54 of the worm damper 16 on the other axial side in the axial direction. This displacement stops when the large-diameter-side stepped surface 39 comes into contact with the side surface of the inner ring 28 on the other axial side. In other words, the stroke of this displacement is equal to the axial width L of the gap that existed between the large-diameter-side stepped surface 39 and the side surface of the inner ring 28 on the other axial side in the neutral state.

[0095] When a component force fb in a direction toward the other axial side is applied to the worm 14 from the neutral state, the worm 14 displaces toward the other axial side while further compressing the damper body 54 of the worm damper 16 on one axial side in the axial direction. This displacement stops when the side surface on the other axial side of the cylindrical portion 51 of the spacer 50 comes into contact with the side surface on one axial side of the inner ring 28. In other words, the stroke of this displacement is equal to the axial width L of the gap that existed between the side surface on the other axial side of the cylindrical portion 51 and the side surface on one axial side of the inner ring 28 in the neutral state.

[0096] As described above, when the component forces fa and fb in either direction in the axial direction are applied to the worm 14, the damper body 54 constituting one of the worm dampers 16 is further compressed in the axial direction. Therefore, based on this compression of the damper body 54, i.e., elastic deformation, the impact acting on the meshing portion can be alleviated, and the occurrence of rattle noise can be suppressed.

[0097] Second Example A second example of the embodiment of the present disclosure will be described with reference to FIG.

[0098] In this example, the worm 14a has an annular recess 56 that opens to the axially intermediate portion of the outer peripheral surface of the annular protrusion 40a.

[0099] The annular recess 56 is provided around the entire circumference of the cylindrical surface portion 42a of the annular protrusion 40a so as to open to the axially intermediate portion of the cylindrical surface portion 42a. The radial depth of the annular recess 56 is greater than the radial height of the annular protrusion 40a. In other words, the bottom surface of the annular recess 56 is located radially inward of the portions of the medium diameter portion 34 that are adjacent to both axial sides of the annular protrusion 40a.

[0100] In this example, the worm reducer includes an O-ring 57 held in the annular recess 56 of the worm 14a. The O-ring 57 is elastically compressed between the bottom surface of the annular recess 56 and the inner circumferential surface of the inner ring 28. This seals the gap between the inner circumferential surface of the inner ring 28 and the medium diameter portion 34, and also suppresses radial rattle between the inner ring 28 and one axial end of the worm 14a. The other configurations, functions, and effects of the second example are similar to those of the first example.

[0101] Third Example A third example of the embodiment of the present disclosure will be described with reference to FIG.

[0102] In this example, the annular protrusion 40b of the worm 14b includes a partially conical inclined surface 58 that slopes radially inward as it extends axially outward on the outer peripheral surface of the portion of each of the axially opposite ends of the annular protrusion 40b adjacent to the axially outer side (the opposite side of the cylindrical surface 42a in the axial direction) of each of the two contact ends 41. The axially outer ends of each inclined surface 58 are connected to portions of the medium diameter portion 34 located on both axial sides of the annular protrusion 40b. The inclined surface 58 does not contact the inner peripheral surface of the inner ring 28 even when the worm 14b oscillates relative to the inner ring 28. The inclined surface 58 provides the advantage of ensuring stable contact with the inner ring 28 even if the shape of the contact end 41 changes due to wear or other reasons. The remaining configuration and effects of the third example are similar to those of the second example.

[0103] Fourth Example A fourth example of the embodiment of the present disclosure will be described with reference to FIG.

[0104] In this example, the annular protrusion 40c of the worm 14c has small-diameter cylindrical surface portions 59 on the outer peripheral surface of portions of each of the axially opposite ends of the annular protrusion 40c adjacent to the axially outer side (the opposite side of the cylindrical surface portion 42a in the axial direction) of the two contact ends 41. The outer peripheral surface of each of the small-diameter cylindrical surface portions 59 has inclined surface portions 58a having a concave arc cross-sectional shape that slopes radially inward as it extends axially outward. The axially outer ends of each inclined surface portion 58a are connected to portions of the medium-diameter portion 34 located on both axial sides of the annular protrusion 40b. The small-diameter cylindrical surface portions 59 and inclined surface portions 58a do not contact the inner peripheral surface of the inner ring 28 even when the worm 14c oscillates relative to the inner ring 28. The small-diameter cylindrical surface portions 59 and inclined surface portions 58a provide the effect of ensuring stable contact with the inner ring 28 even if the contact ends 41 change shape due to wear or other reasons. The other configurations and effects of the fourth example are the same as those of the second example.

[0105] REFERENCE SIGNS LIST 1 electric power steering device 2 steering wheel 3 steering shaft 4 steering column 5a, 5b universal joint 6 intermediate shaft 7 steering gear unit 8 electric assist device 9 pinion shaft 10 electric motor 11 worm reducer 12 housing 13 worm wheel 14, 14a, 14b, 14c, 14z worm 15 support bearing 16 worm damper 17 biasing means 18 wheel accommodating portion 19 worm accommodating portion 20 retaining ring 21 cover body 22 wheel teeth 23 worm teeth 24 female spline portion 25 output shaft 26 male spline portion 27 outer ring 28 inner ring 29 rolling element 30 mating surface portion 31 stepped surface 32 retaining ring 33 small diameter portion 34 medium diameter portion 35 Large diameter portion 36 Flange portion 37 Engagement groove 38 Small diameter side stepped surface 39 Large diameter side stepped surface 40, 40a, 40b, 40c, 40z Annular convex portion 41, 41z Contact end portion 42, 42a, 42z Cylindrical surface portion 43 Support bearing 44 Outer ring 45 Inner ring 46 Biasing leaf spring 47 Retaining portion 48 Support pin 49 Clamping leaf spring 50 Spacer 51 Cylindrical portion 52 Flange portion 53 Caulking portion 54 Damper body 55 Side plate 56 Annular recess portion 57 O-ring 58, 58a Inclined surface portion 59 Small diameter cylindrical surface portion 60 Axial end face

Claims

1. Housing and A worm wheel having wheel teeth on its outer circumference and rotatably supported inside the housing, A worm having worm teeth on its outer circumferential surface that mesh with the wheel teeth, and rotatably supported inside the housing, A support bearing comprising an outer ring fixed to the housing and an inner ring fitted onto one axial end of the worm in a clearance fit, which supports the axial end of the worm relative to the housing, enabling rotation, oscillating displacement, and axial displacement, A biasing means that elastically biases the other axial end of the worm toward the worm wheel, Two worm dampers are positioned on both axial sides of the inner ring and elastically support the worm in the axial direction relative to the inner ring, Equipped with, The worm has an annular projection on the portion located radially inward of the inner ring that protrudes radially outward more than the portions adjacent to it on both axial sides. The annular projection has an outer surface that is in close proximity to and facing the inner surface of the inner ring, and has a cylindrical surface portion in the axial middle of the outer surface in which the outer diameter does not change in the axial direction, and has two contact ends on both ends of the outer surface in the axial direction that, when the worm is tilted to the maximum tilt angle with respect to the inner ring, contact the inner surface of the inner ring, thereby restricting the worm from tilting any further with respect to the inner ring. Each of the two contact ends is formed by a convex curved surface. Worm gear reducer.

2. The worm gear reducer according to claim 1, wherein the radius of curvature of the convex curved surface is 0.2 mm or more and 3.0 mm or less.

3. The worm has an annular recess that opens in the axial middle portion of the outer circumferential surface of the annular protrusion, An O-ring is provided, which is elastically compressed between the bottom surface of the annular recess and the inner circumferential surface of the inner ring. The worm gear reducer according to claim 1.

4. The worm gear reducer according to claim 1, wherein the arithmetic mean roughness Ra of the two contact ends is 1.6 μm or less.