Worm reduction gear

Abstract

To provide a worm reduction gear capable of reducing noise to occur in an engagement part between a wheel tooth part and a worm tooth part.SOLUTION: The worm reduction gear includes: a worm wheel 2 having a wheel tooth part 9 on an outer peripheral face; and a worm 3 having a worm tooth part 12 on the outer peripheral face for engaging with the wheel tooth part 9. When a lead angle of the wheel tooth part 9 is α1 and a lead angle of the worm tooth part 12 is α2, a relation of α1<α2 is satisfied. When a pressure angle of the wheel tooth part 9 is β1 and a pressure angle of the worm tooth part 12 is β2, a relation of β1>β2 is satisfied.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to a worm reduction gear that can be incorporated into an electric power steering device or the like. [Background technology]

[0002] 2. Description of the Related Art In recent years, electric power steering devices that use an electric motor as an auxiliary power source to reduce the force required for a driver to operate a steering wheel have become widespread.

[0003] The electric power steering device includes a worm reduction gear for increasing the torque of an electric motor. The worm reduction gear includes a worm wheel having wheel teeth on its outer circumferential surface, and a worm having worm teeth on its outer circumferential surface meshing with the wheel teeth. The torque of the electric motor is increased by being transmitted to the worm wheel via the worm, and is then applied as auxiliary power to a steering force transmission member such as a steering shaft or a pinion shaft or a rack shaft of a steering gear unit. This reduces the force required for the driver to operate the steering wheel.

[0004] In a worm reducer, the angle of intersection between the central axis of the worm wheel and the central axis of the worm is usually set to 90°. In this case, taking into consideration the fit of the meshing between the wheel teeth and the worm teeth, the pressure angles and lead angles of the wheel teeth and the worm teeth are set to be equal to each other.

[0005] When a worm reduction gear is in operation, noise such as teeth rattling occurs at the meshing portion between the wheel teeth and the worm teeth. In order to reduce such noise, it has been proposed to make the portion of the worm wheel where the wheel teeth are present out of synthetic resin instead of metal (see, for example, JP 2020-128803 A).

[0006] Japanese Patent Application Laid-Open No. 2016-217469 proposes making the pressure angle of the worm tooth portion smaller than the pressure angle of the wheel tooth portion in order to reduce torque fluctuations between the worm and the worm wheel. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2020-128803 A [Patent Document 2] JP 2016-217469 A Summary of the Invention [Problem to be solved by the invention]

[0008] As proposed in JP 2020-128803 A and the like, if the portion of the worm wheel where the wheel teeth are present is made of synthetic resin, the noise generated at the meshing portion between the wheel teeth and the worm teeth can be reduced. However, there are cases where the strength level required for the wheel teeth is high and the portion where the wheel teeth are present must be made of metal. In this case, it is necessary to reduce the noise generated at the meshing portion between the wheel teeth and the worm teeth by other means. Even if the portion where the wheel teeth are present can be made of synthetic resin, it is necessary to further reduce the noise generated at the meshing portion between the wheel teeth and the worm teeth.

[0009] On the other hand, JP 2016-217469 A describes that torque fluctuation between the worm and the worm wheel can be reduced by making the pressure angle of the worm tooth portion smaller than the pressure angle of the wheel tooth portion, but does not describe whether noise generated at the meshing portion between the wheel tooth portion and the worm tooth portion can be reduced at the same time. In other words, JP 2016-217469 A does not describe specific means for reducing the noise.

[0010] An object of the present disclosure is to provide a worm reducer capable of reducing noise generated at the meshing portion between the wheel tooth portion and the worm tooth portion. [Means for solving the problem]

[0011] A worm reducer according to one aspect of the present disclosure includes a worm wheel having wheel teeth on its outer circumferential surface, and a worm having worm teeth on its outer circumferential surface that mesh with the wheel teeth.

[0012] When the lead angle of the wheel tooth portion is α1 and the lead angle of the worm tooth portion is α2, the relationship α1<α2 is satisfied.

[0013] When the pressure angle of the wheel tooth portion is β1 and the pressure angle of the worm tooth portion is β2, the relationship β1>β2 is satisfied.

[0014] The worm reduction gear according to one aspect of the present disclosure satisfies the relationship α2-α1≦1.5°.

[0015] The worm reduction gear according to one aspect of the present disclosure satisfies the relationship β1-β2≦1.5°.

[0016] In the worm reducer according to one aspect of the present disclosure, the wheel tooth portion has a R-chamfered portion at a connection portion between a tooth surface and a tooth tip surface, the R-chamfered portion coming into contact with the tooth surface of the worm tooth portion.

[0017] In this case, the radius of curvature of the R-chamfered portion can be set to 2.0 mm or less.

[0018] In the worm reducer according to one embodiment of the present disclosure, the worm tooth portion has two teeth.

[0019] The worm reducer of the present disclosure can be implemented by appropriately combining the configurations of the above-described aspects to the extent that no contradiction occurs. Effect of the Invention

[0020] According to the worm reducer of one aspect of the present disclosure, it is possible to reduce noise generated at the meshing portion between the wheel teeth portion and the worm teeth portion. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a diagram illustrating an electric power steering device incorporating a worm reduction gear according to a first embodiment of the present disclosure. [Diagram 2] FIG. 2 is a cross-sectional view taken along line AA in FIG. [Diagram 3] FIG. 3 is a diagram showing a part of a worm and a worm wheel that constitute the worm reduction gear of the first example. [Figure 4] FIG. 4(a) is a plan view of a worm constituting the worm reduction gear of the first example, and FIG. 4(b) is a view of a worm wheel constituting the worm reduction gear as viewed from the radial outside. [Diagram 5] FIG. 5(a) is a partial cross-sectional view of a worm constituting the worm reduction gear of the first example, and FIG. 5(b) is a partial cross-sectional view of a worm wheel constituting the worm reduction gear. [Figure 6] FIG. 6 is a diagram for explaining locations where the worm teeth portion comes into contact with the wheel teeth portion in the worm reduction gear of the first example. [Figure 7] FIG. 7(a) is a cross-sectional view showing a schematic view of an engagement portion between worm teeth and wheel teeth in the first example, and FIG. 7(b) is a view corresponding to FIG. 7(a) for a comparative example. [Figure 8] FIG. 8(a) is a schematic diagram of the meshing portion between the worm teeth and the wheel teeth in the first example as viewed from the radial outside, and FIG. 7(b) is a view corresponding to FIG. 8(a) for the comparative example. [Figure 9] FIG. 9(a) is a conceptual diagram showing the change over time in the axial load acting on the worm during operation of the first example worm reducer, and FIG. 9(b) is a conceptual diagram equivalent to FIG. 9(a) for the comparative example. [Figure 10] FIG. 10 is a partial cross-sectional view of a wheel tooth portion of a worm reducer according to a second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] [Example 1] A worm reduction gear according to a first example of an embodiment of the present disclosure will be described with reference to Figs. 1 to 9.

[0023] (1) Worm reducer The worm reducer of the present disclosure is applicable to worm reducers incorporated into parts of various mechanical devices, but in this example, a case will be described in which the worm reducer of the present disclosure is applied to a worm reducer incorporated into part of an electric power steering device for an automobile.

[0024] The worm reduction gear 1 of this embodiment includes a worm wheel 2 and a worm 3, as shown in FIGS.

[0025] In this example, the worm wheel 2 and the worm 3 are housed in a housing 4 .

[0026] The housing 4 includes a wheel accommodating portion 5 and a worm accommodating portion 6 having a central axis that is twisted relative to the central axis of the wheel accommodating portion 5 and an axially intermediate portion that opens into the wheel accommodating portion 5.

[0027] The wheel accommodating portion 5 is configured in a cylindrical shape. In Fig. 2, the central axis of the wheel accommodating portion 5 extends in the front-rear direction.

[0028] The worm accommodating portion 6 is cylindrically configured and has openings at both axial ends. In FIG. 2, the central axis of the worm accommodating portion 6 extends in the left-right direction. That is, the central axis of the wheel accommodating portion 5 and the central axis of the worm accommodating portion 6 are perpendicular to each other. The opening on one axial side of the worm accommodating portion 6 (the right side in FIG. 2) is closed by a cover 7. The opening on the other axial side of the worm accommodating portion 6 (the left side in FIG. 2) is closed by an electric motor 8 fixedly coupled to the housing 4.

[0029] The worm wheel 2 has a helical gear-shaped wheel tooth portion 9 on its outer circumferential surface. In this example, the worm wheel 2 is formed by fixing a synthetic resin gear portion 11 having the wheel tooth portion 9 on its outer circumferential surface to the periphery of a disk-shaped core portion 10 made of a metal such as steel. When implementing the structure of the present disclosure, the type of synthetic resin material that is the material for the gear portion 11 may be any type as long as the durability required for the gear portion 11 is ensured. For example, polyamide 66 (PA66) mixed with glass fiber (GF) may be used. The amount of glass fiber mixed may be any type as long as the durability required for the gear portion 11 is ensured. For example, the amount of glass fiber mixed may be about 25 wt %. The entire worm wheel 2 may be made of a metal such as steel.

[0030] The worm wheel 2 is rotatably supported inside the wheel accommodating portion 5. In this example, the worm wheel 2 is fitted and fixed to an axial portion of a rotating shaft rotatably supported inside the wheel accommodating portion 5 (in this example, a front portion of the steering shaft 20 in the front-rear direction of the vehicle).

[0031] The worm 3 has, on the outer peripheral surface of an axially intermediate portion, a screw-like worm tooth portion 12 that meshes with the wheel tooth portion 9 of the worm wheel 2. The worm 3 is made of a metal such as steel.

[0032] The worm 3 is rotatably supported inside the worm receiving portion 6. That is, the intersection angle between the central axis of the worm wheel 2 and the central axis of the worm 3 is set to 90°.

[0033] In this example, the portion of the worm 3 near the base end (the portion near the left end in FIG. 2) is rotatably supported by a ball bearing 13 with respect to the worm housing portion 6. In this example, the outer ring of the ball bearing 13 is fitted inside the worm housing portion 6 with a radial gap therebetween, and the inner ring of the ball bearing 13 is fitted outside the portion of the worm 3 near the base end with a radial gap therebetween. In this way, the portion of the worm 3 near the base end is supported so as to be able to rotate and oscillate with respect to the worm housing portion 6. Note that the structure for supporting the portion of the worm 3 near the base end with respect to the worm housing portion 6 so as to be able to rotate and oscillate with respect to the worm housing portion 6 is not limited to the structure of this example, and various structures can be adopted.

[0034] In this example, the tip end of the worm 3 (the right end in FIG. 2) is rotatably supported by a ball bearing 14 relative to the worm housing 6. A biasing mechanism 15 including an elastic body such as a coil spring or a leaf spring is attached between the outer circumferential surface of the ball bearing 14 and the inner circumferential surface of the worm housing 6. The biasing mechanism 15 elastically biases the tip end of the worm 3 toward the worm wheel 2. This configuration suppresses backlash between the wheel tooth portion 9 and the worm tooth portion 12, thereby suppressing the generation of teeth rattle noise when the rotation direction of the worm 3 is reversed. Note that the mechanism for elastically biasing the tip end of the worm 3 toward the worm wheel 2 is not limited to the structure of this example, and various structures can be adopted.

[0035] When implementing the worm reducer of the present disclosure, the configuration for supporting the base end of the worm so that it can swing relative to the worm accommodating portion and the configuration for elastically biasing the tip end of the worm toward the worm wheel can be omitted.

[0036] In this example, the base end of the worm 3 is connected to the tip end of the output shaft 16 of the electric motor 8 by using a coupling 17 so as to enable torque transmission and swinging displacement. Note that the base end of the worm 3 can also be connected to the tip end of the output shaft 16 of the electric motor 8 by spline engagement or the like so as to enable torque transmission and swinging displacement.

[0037] 3 and 4(b), the wheel tooth portion 9 of the worm wheel 2 is configured in the shape of a helical gear. That is, the wheel tooth portion 9 has a plurality of teeth 9a arranged at equal pitch in the circumferential direction, and the tooth trace direction of each tooth 9a is inclined by a lead angle α1 with respect to the direction of the central axis O1 of the worm wheel 2.

[0038] A cross section of the tooth 9a cut by a virtual plane perpendicular to the central axis O1 of the worm wheel 2 (hereinafter, simply referred to as a cross section) has a substantially isosceles trapezoidal shape in which the circumferential width decreases toward the radially outer side, i.e., the tooth tip side (i.e., the tooth surfaces 9b) extend in a direction toward the circumferential center side of the tooth 9a as it moves toward the radially outer side. A pressure angle β1 is set in the wheel tooth portion 9. In the cross section of the tooth 9a, the intersection point between the pitch cylinder C1 of the wheel tooth portion 9 and the tooth surfaces 9b is defined as P1, the radial line of the worm wheel 2 passing through the intersection point P1 is defined as a reference straight line L1, and the tangent line of the tooth surface 9b at the intersection point P1 is defined as S1. The pressure angle β1 is expressed as the inclination angle of the tangent line S1 with respect to the reference straight line L1.

[0039] The worm tooth portion 12 of the worm 3 is configured in a screw shape, as shown in Fig. 3 and Fig. 4(a). Specifically, the worm tooth portion 12 has n (n is a positive integer) teeth 12a each formed in a spiral shape, and each tooth 12a is inclined by a lead angle α2 with respect to a virtual plane perpendicular to the central axis O2 of the worm 3 when viewed from the outside in the radial direction. Here, the number n of teeth 12a of the worm tooth portion 12 is referred to as the number of threads of the worm tooth portion 12. The number n of threads of the worm tooth portion 12 can be set arbitrarily, but is 2 in this example.

[0040] A cross section of the tooth 12a cut by a virtual plane including the central axis O2 of the worm 3 (hereinafter, simply referred to as a cross section) has a substantially isosceles trapezoidal shape in which the axial width decreases toward the radially outer side, i.e., the tooth tip side (see FIG. 5(a)). In other words, the tooth flanks 12b, which are the side surfaces on both axial sides of the tooth 12a, extend in a direction toward the center of the axial width of the tooth 12a as they move toward the radially outer side. A pressure angle β2 is set in the worm tooth portion 12. In the cross section of the tooth 12a, the intersection point between the pitch cylinder C2 of the worm tooth portion 12 and the tooth flanks 12b is defined as P2, the radial line of the worm 3 passing through the intersection point P2 is defined as a reference straight line L2, and the tangent to the tooth flank 12b at the intersection point P2 is defined as S2. The pressure angle β2 is expressed as the inclination angle of the tangent S2 with respect to the reference straight line L2.

[0041] In this example, the lead angle α1 of the wheel tooth portion 9 and the lead angle α2 of the worm tooth portion 12 are set so that α2 is larger than α1, that is, so that the relationship α1<α2 is satisfied. Also, the pressure angle β1 of the wheel tooth portion 9 and the pressure angle β2 of the worm tooth portion 12 are set so that β2 is smaller than β1, that is, so that the relationship β1>β2 is satisfied. Therefore, in the structure of this example, noise generated at the meshing portion between the wheel tooth portion 9 and the worm tooth portion 12 can be suppressed. The reason for this will be explained below.

[0042] In the worm reducer 1, when the number of threads of the worm tooth portion 12 is n (n is a positive integer), the meshing state between the worm tooth portion 12 and the wheel tooth portion 9 alternates between a meshing state of n teeth and a meshing state of (n+1) teeth as the worm 3 and the worm wheel 2 rotate.

[0043] In the present example, when the number of threads n of the worm tooth portion 12 is 2, the meshing state between the worm tooth portion 12 and the wheel tooth portion 9 alternates between a meshing state of two teeth and a meshing state of three teeth as the worm 3 and the worm wheel 2 rotate.

[0044] At this time, in the meshing state of three teeth, among the three teeth 9a(1), 9a(2), 9a(3) continuous in the circumferential direction of the wheel tooth part 9, the worm tooth part 12 contacts the three locations K1, K2, K3 shown in Fig. 6. The first location K1 counted from the front side in the rotation direction A of the worm wheel 2 is located on the root side of the first tooth 9a(1). The second location K2 counted from the front side in the rotation direction A is located between the root side and the tip side of the second tooth 9a(2). The third location K3 counted from the front side in the rotation direction A is located on the tip side of the third tooth 9a(3).

[0045] Loads F1, F2, F3 directed in the axial direction of the worm 3 are applied to the respective locations K1, K2, K3 from the worm tooth part 12. In Fig. 6, for the sake of convenience, all the loads F1, F2, F3 are drawn with the same magnitude, that is, all the load vectors of the loads F1, F2, F3 have the same length. However, in reality, due to the fact that the rotation radii R1, R2, R3, which are the distances from the central axis O1 of the worm wheel 2 to the respective locations K1, K2, K3, are different (R1 < R2 < R3), the magnitudes of the loads F1, F2, F3 tend to be different. Specifically, the load F3 at the location K3 with a relatively large rotation radius tends to be larger than the loads F1, F2 at the locations K1, K2 with relatively small rotation radii (see, for example, Figs. 7(a) and 7(b)).

[0046] Note that the comparative examples shown in Figs. 7(b), 8(b), and 9(b) are different from this example only in that they satisfy the relationship of α1 = α2 and the relationship of β1 = β2.

[0047] On the other hand, when the worm 3 and the worm wheel 2 rotate, the axial load Fx acting on the worm 3 changes periodically as shown in Figs. 9(a) and 9(b), for example. Specifically, the axial load Fx rapidly increases at the timing when the meshing state changes from the meshing state of two teeth to the meshing state of three teeth, that is, at the timing when the load F3 is applied to the third location K3 counted from the front side in the rotation direction A, and reaches the peak indicated by the arrow in Figs. 9(a) and 9(b).

[0048] From this, it can be seen that one of the causes of noise occurring at the meshing portion between the wheel teeth portion 9 and the worm teeth portion 12 is the fluctuation of the load F3 applied to the third point K3 counting from the front side in the rotation direction A. Therefore, if the magnitude of the load F3 applied to the point K3 can be reduced, the noise occurring at the meshing portion between the wheel teeth portion 9 and the worm teeth portion 12 can be reduced.

[0049] In this regard, in the structure of this example, the lead angle α2 of the worm tooth portion 12 is set to be greater than the lead angle α1 of the wheel tooth portion 9 (α1<α2), and the pressure angle β2 of the worm tooth portion 12 is set to be smaller than the pressure angle β1 of the wheel tooth portion 9 (β1>β2).

[0050] In this example, in which the lead angle α1 of the wheel tooth portion 9 and the lead angle α2 of the worm tooth portion 12 are set to satisfy the relationship of α1<α2, the contact area at the moment when the worm tooth portion 12 contacts the point K3 can be made smaller than in the comparative example in which the lead angle α1 of the wheel tooth portion 9 and the lead angle α2 of the worm tooth portion 12 are set to satisfy the relationship of α1=α2 (see Figs. 8(a) and 8(b)). That is, in this example, the worm tooth portion 12 starts to contact the point K3 from a small area, so that the rise amount of the load F3 at the moment of contact can be made smaller than in the comparative example (see Figs. 7(a) and 7(b)), and the rise amount of the peak of the axial load Fx can be made smaller than in the comparative example (see Figs. 9(a) and 9(b)).

[0051] In this example, in which the pressure angle β1 of the wheel tooth portion 9 and the pressure angle β2 of the worm tooth portion 12 are set so as to satisfy the relationship β1>β2, the contact area at the moment when the worm tooth portion 12 contacts the point K3 can be made smaller than in the comparative example in which the pressure angle β1 of the wheel tooth portion 9 and the pressure angle β2 of the worm tooth portion 12 are set so as to satisfy the relationship β1=β2. That is, in this example, by starting to bring the worm tooth portion 12 into contact with the point K3 from a small area, the rise amount of the load F3 at the moment of contact can be made smaller than in the comparative example (see Figures 7(a) and 7(b)), and the rise amount of the peak of the axial load Fx can be made smaller than in the comparative example (see Figures 9(a) and 9(b)).

[0052] The effect of satisfying the relationship α1<α2 and the effect of satisfying the relationship β1>β2 are not mutually exclusive, but rather have an additive and synergistic effect of suppressing the rise in the peak of the axial load Fx. Therefore, in this example, the contact area at the moment when the worm tooth portion 12 contacts the point K3 can be made smaller by the amount that the relationship α1<α2 is satisfied, compared to the case where only the relationship β1>β2 is satisfied, and the effect of suppressing the rise in the load F3, i.e., the rise in the peak of the axial load Fx, is high.

[0053] In this example, the sums (F1+F2+F3) of the loads F1, F2, and F3 applied to the points K1, K2, and K3 are equal to each other. Therefore, in this example, the load F3 applied to the point K3 is smaller than that in the comparative example, and the loads F1 and F2 applied to the points K1 and K2 are larger than those in the comparative example (see FIG. 7(a) and FIG. 7(b)).

[0054] As described above, in this example, the magnitude of the load F3 applied to the point K3 can be reduced, thereby reducing noise generated at the meshing portion between the wheel tooth portion 9 and the worm tooth portion 12. When a material in which about 25 wt % glass fiber is mixed into polyamide 66 is used as the synthetic resin constituting the gear portion 11, the load F3 tends to be large because the material is a hard material for a synthetic resin. Therefore, the feature of this example that reduces the magnitude of the load F3 (α1<α2, β1>β2) is particularly effective in reducing noise generated at the meshing portion when using such a material.

[0055] When implementing the structure of this example, the lead angle α1 of the wheel tooth portion 9 can be set to any value, taking into consideration the efficiency and self-locking properties required for the worm reduction gear. Also, the pressure angle β1 of the wheel tooth portion 9 can be set to any value, taking into consideration the gear strength required for the worm reduction gear. In this example, the lead angle α1 is set to 19.36 degrees, and the pressure angle β1 is set to 14.50 degrees. Taking into consideration the gear efficiency, the lead angle α1 is most preferably 19.36 degrees.

[0056] When implementing the structure of this example, from the viewpoint of increasing the effect of reducing the load F3, the angle difference (α2-α1) between the lead angle α1 of the wheel tooth portion 9 and the lead angle α2 of the worm tooth portion 12, and the angle difference (β1-β2) between the pressure angle β1 of the wheel tooth portion 9 and the pressure angle β2 of the worm tooth portion 12 are basically better to be larger.

[0057] However, it is empirically estimated that when the angle differences (α2-α1) and (β1-β2) exceed 1.5°, the effect of reducing the load F3 is almost saturated. Also, it is empirically estimated that when the angle differences (α2-α1) and (β1-β2) exceed 1.5°, the tooth thickness of the worm tooth portion 12 becomes small and the amount of wear at the meshing portion between the wheel tooth portion 9 and the worm tooth portion 12 increases, which leads to a decrease in durability.

[0058] For this reason, when implementing the structure of this example, it is preferable that the angle difference (α2-α1) satisfies the relationship α2-α1≦1.5°, and the angle difference (β1-β2) satisfies the relationship β1-β2≦1.5°.

[0059] Incidentally, reducing the fluctuation range of the axial load Fx acting on the worm 3, i.e., the torque ripple, can alleviate the load applied to the tooth tip side of the wheel tooth portion 9, but on the other hand, the load applied to the tooth base side increases. If tooth surface contact is centered on the tooth base side, the durability of the wheel tooth portion 9 may decrease depending on the bias of the load and surface pressure. Therefore, as a countermeasure, the rate of decrease in the durability of the wheel tooth portion 9 can be estimated, and the angle differences (α2-α1) and (β1-β2) can be determined so that it falls within the safety factor.

[0060] When implementing the structure of this example, when changing the angular differences (α2-α1) and (β1-β2) between the wheel tooth portion 9 and the worm tooth portion 12, it is effective in terms of reducing labor and costs to change the specifications (α2, β2) of the worm tooth portion 12 without changing the specifications (α1, β1) of the wheel tooth portion 9. The reason for this is that in this example, changing the specifications (α1, β1) of the synthetic resin wheel tooth portion 9 requires changes to expensive and complex injection molding equipment, whereas the specifications (α2, β2) of the metal worm tooth portion 12 can be easily changed by machining.

[0061] In addition, by changing the specifications (α2, β2) of the worm tooth portion 12, the variation in the meshing position between the wheel tooth portion 9 and the worm tooth portion 12 increases, and as a result, there is a concern that the torque ripple in the no-load operation, which is the torque ripple in the range where the meshing reaction force between the wheel tooth portion 9 and the worm tooth portion 12 is small, may worsen. Therefore, as a countermeasure, the shapes of the wheel tooth portion 9 and the worm tooth portion 12 may be optimized by processing the wheel tooth portion 9 by lapping, that is, processing the wheel tooth portion 9 using a processing worm with electrolytically deposited abrasive grains. In addition, the range where the meshing reaction force between the wheel tooth portion 9 and the worm tooth portion 12 is small refers to a range where the worm 3 does not float up to the opposite side to the worm wheel 2 due to the meshing reaction force.

[0062] (2) Electric power steering device As shown in FIG. 1, the electric power steering device 18 of this example includes a steering wheel 19, a steering shaft 20, a steering column 21, a pair of universal joints 22a, 22b, an intermediate shaft 23, a steering gear unit 24, the worm reduction gear 1 of this example, and an electric motor 8.

[0063] The steering wheel 19 is supported and fixed to the rear end of the steering shaft 20. The steering shaft 20 is rotatably supported inside a steering column 21 supported on the vehicle body. The front end of the steering shaft 20 is connected to a pinion shaft 25 of a steering gear unit 24 via a rear universal joint 22a, an intermediate shaft 23, and a front universal joint 22b. Therefore, when the driver rotates the steering wheel 19, the rotation of the steering wheel 19 is transmitted to the pinion shaft 25 via the steering shaft 20, a pair of universal joints 22a and 22b, and the intermediate shaft 23. The rotation of the pinion shaft 25 is converted into a linear motion of a rack shaft (not shown) of the steering gear unit 24 that meshes with the pinion shaft 25. As a result, a pair of tie rods 26 are pushed and pulled, and a steering angle according to the amount of rotation of the steering wheel 19 is applied to the left and right steered wheels.

[0064] The electric power steering device 18 in this example is a column type in which a worm reduction gear 1 and an electric motor 8 are supported on the front end of a steering column 21, and is configured so that the force required by the driver to operate the steering wheel 19 can be reduced by increasing the auxiliary power of the electric motor 8 by the worm reduction gear 1 and then applying it to the front end of the steering shaft 20.

[0065] In the electric power steering device 18 of this example, the worm reduction gear 1 is located relatively close to the driver's seat, and noise generated at the meshing portion between the wheel tooth portion 9 and the worm tooth portion 12 can be suppressed, thereby improving the habitability of the driver's seat.

[0066] In addition, when implementing the worm reduction gear of the present disclosure, the number of threads n of the worm tooth portion 12 can be determined arbitrarily. In addition, the effect of suppressing noise generated at the meshing portion between the wheel tooth portion 9 and the worm tooth portion 12, that is, the effect of suppressing the magnitude of the load F(n+1) applied to the (n+1)th location of the wheel tooth portion 9 counting from the front side in the rotation direction of the worm wheel 2, increases as the number of threads n decreases, and is maximized when the number of threads n=1. However, in addition to the effect, when considering the torque ratio, reduction ratio, gear strength, etc. appropriate for a worm reduction gear incorporated in an electric power steering device, it is most preferable to set the number of threads=2 as in this example, rather than the number of threads=1. In other words, when the number of threads n=1, the reduction ratio can be increased compared to the case of the number of threads=2, which is advantageous in terms of ensuring auxiliary power, but is disadvantageous in terms of ensuring rolling tracking performance, and also, since twice the gear strength is required, it is disadvantageous in terms of manufacturing costs.

[0067] When implementing an electric power steering device equipped with the worm reduction gear of the present disclosure, the worm reduction gear 1 and the electric motor 8 can be disposed at a position where auxiliary power is applied to the pinion shaft or rack shaft of the steering gear unit.

[0068] [Example 2] A worm reduction gear according to a second example of an embodiment of the present disclosure will be described with reference to FIG.

[0069] In the structure of this example, the wheel tooth portion 9 has an R-chamfered portion 27 that comes into contact with the tooth surface of the worm tooth portion at the connection portion between the tooth surface 9b and the tooth tip surface 9c on both circumferential sides.

[0070] In the structure of this example, when the worm tooth portion comes into contact with the point K3 (see FIG. 6) of the wheel tooth portion 9, the worm tooth portion comes into contact with the R-chamfered portion 27, thereby making it possible to increase the contact area of ​​the contact portion, and as a result, to reduce the load F3 applied to the point K3 of the wheel tooth portion 9. Therefore, it is possible to further suppress the noise generated at the meshing portion between the wheel tooth portion 9 and the worm tooth portion.

[0071] The radius of curvature R27 of the R chamfered portion 27 can be set arbitrarily, but is preferably set to 2.0 mm or less. If the radius of curvature R27 exceeds 2.0 mm, the thickness of the wheel tooth portion 9 is reduced, making it difficult to ensure the durability of the wheel tooth portion 9. That is, if the radius of curvature R27 is 2.0 mm or less, it is easy to suppress the noise generated at the meshing portion and ensure the durability of the wheel tooth portion 9. More preferably, if the radius of curvature R27 is within the range of 1.0 ± 0.1 mm, it is possible to suppress the noise and ensure the durability of the wheel tooth portion 9 at a higher level. That is, if the radius of curvature R27 is smaller than 0.9 mm, it is difficult to obtain the effect of improving the suppression of noise, and if the radius of curvature R27 is larger than 1.1 mm, the thickness of the wheel tooth portion 9 is reduced, and the durability of the wheel tooth portion 9 is reduced accordingly. The other configurations and effects of this example are the same as those of the first example. [Explanation of symbols]

[0072] 1 Worm reducer 2 Worm Wheel 3. Warm 4. Housing 5 Wheel storage area 6 Worm housing 7 Lid 8 Electric motor 9 Wheel teeth 9a Teeth 9b Tooth surface 9c Tooth tip 10 core 11 Gear section 12 Worm teeth 12a Tooth 12b Tooth surface 13 Ball bearings 14 Ball bearing 15 Biasing mechanism 16 Output shaft 17 Coupling 18 Electric power steering device 19 Steering Wheel 20 Steering shaft 21 Steering column 22a, 22b Universal joint 23 Intermediate shaft 24 Steering gear unit 25 Pinion shaft 26 Tie rod 27 R chamfer

Claims

1. A worm wheel having wheel teeth on an outer circumferential surface thereof; a worm having worm teeth on an outer circumferential surface thereof that mesh with the wheel teeth; Equipped with When the lead angle of the wheel tooth portion is α1 and the lead angle of the worm tooth portion is α2, a relationship of α1<α2 is satisfied, When the pressure angle of the wheel tooth portion is β1 and the pressure angle of the worm tooth portion is β2, a relationship of β1>β2 is satisfied. Worm reducer.

2. 2. The worm reduction gear according to claim 1, wherein the relationship α2-α1≦1.5° is satisfied.

3. 2. The worm reduction gear according to claim 1, wherein the relationship β1-β2≦1.5° is satisfied.

4. The worm reduction gear according to claim 1 , wherein the wheel tooth portion has a R-chamfered portion at a connection between a tooth surface and a tooth tip surface, the R-chamfered portion coming into contact with the tooth surface of the worm tooth portion.

5. 5. The worm reduction gear according to claim 4, wherein the radius of curvature of the R-chamfered portion is 2.0 mm or less.

6. 2. The worm reducer according to claim 1, wherein the number of teeth of the worm tooth portion is two.

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