Reduction gear of an electric power steering system, and electric power steering system

Abstract

The reduction gear (13) comprises a support (15), an endless screw (19), and a gear (18) that meshes with a helical spline (33) on the endless screw, the endless screw having a rotatable first end (28) and a second end (32), the first end (28) and the second end (32) receiving a first rotation guide means (20) and a second rotation guide means (21), respectively, that allows the endless screw to turn relative to the support (15), the second rotation guide means (21) having a damping sleeve (41) and a rotation guide device (42), the damping sleeve surrounding the rotation guide device, the second rotation guide means comprising a centering means (43) including a press-fit portion (44) inserted into the rotation guide device (42), and a connecting portion that is rigidly connected to the support (15).

Description

Technical Field

[0001] The present invention relates to the field of electric power assist steering systems for vehicles. In the following description, the "electric power steering system" refers to an electric power assist steering system for vehicles.

[0002] More specifically, the present invention relates to a reduction gear of an electric power steering system, an electric power steering system, and an automobile equipped with the electric power steering system.

Background Art

[0003] Typically, as shown in FIG. 1, an electric power steering system 1 is composed of a steering column assembly 2 including an upper steering column 3 connected to a lower steering column 4 via a universal joint 5, for example, and an electric power steering device 6.

[0004] The upper steering column 3 supports a steering wheel 8 at an end 7, and the lower steering column 4 supports a steering pinion (not shown) intended to be engaged with a rack 9 at an end. The input of torque to the steering pinion by the steering column assembly 2 enables the translation of the rack 9, causing the rotation of the wheels (not shown) of the vehicle by a tie rod 10 hinge-connected to an end of the rack 9.

[0005] The electric assist device 6 includes, for example, a motor 11 capable of rotationally driving a rotary shaft 12 connected to a reduction gear 13. The motor 11 is controlled by a control unit 14 (for example, an electronic board) based on information given by a signal from a sensor (not shown) arranged in the steering column assembly 2. Such a motor 11 supplies an assist torque according to the information received by the sensor. This assist torque is transmitted to the steering column assembly 2 via the reduction gear 13 and added to the torque exerted by the user on the steering wheel 8. In this way, the force exerted by the user required to turn the wheels is reduced, and the handling of the steering wheel 8 becomes easier.

[0006] To transmit the assist torque to the steering column, the motor is connected to a reduction gear 13 coupled to a worm screw having a helical spline with which a gear (also called a reduction wheel) meshes. The gear is integral with a pinion or a shaft carrying the pinion. The gear and the worm screw are generally surrounded by a protective casing made of a metal alloy.

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, such a reduction gear has drawbacks. That is, for example, it has been observed that there is play between the gear and the helical spline, which occurs due to wear of the gear and temperature fluctuations. However, such play generates noise and harmful vibrations in the steering column assembly.

[0008] European Patent Application No. 2797014 (JTEKT) provides a reduction gear for an electric power steering comprising a worm shaft rotatably attached to a housing using ball bearings provided at both ends respectively. One end of the worm screw is driven by an electric motor, and the other free end is connected to a pressing mechanism for reducing the play between the worm screw and the gear. Such a pressing mechanism is provided in a hole provided in the housing. This mechanism consists of a compression spring sandwiched between a contact portion integral with the housing and a pressing piece that slides in the hole and contacts the worm screw.

[0009] This can reduce the play between the worm screw and the gear, but such a mechanism requires the manufacture of a hole with a very good surface condition for the sliding of the pressing piece. In addition, the installation of the said mechanism requires the installation of a sliding pressing piece, a prestress spring, and the fixing of the contact portion. Finally, this mechanism requires space and affects the compactness of the housing.

[0010] Japanese Patent Application No. 20017204147 (NSK) discloses a reduction system for a power steering system. The worm shaft is rotatably attached to a casing using two ball bearings respectively arranged at a first proximal end connected to a motor and a second distal end. The worm screw is prestressed against a gear using an elastic portion arranged between the distal end bearing and the casing.

[0011] However, the attachment of the worm screw and the adjustment of the prestress force cannot be easily achieved. The casing also requires the manufacture of a complex shape to enable the positioning of the distal end.

[0012] The present invention particularly aims to solve the above-mentioned drawbacks.

[0013] The first object is to provide a speed reduction device for a power steering that can reduce the play between a gear and a worm screw.

[0014] The second object is to provide a speed reduction device for a power steering that has a housing without a complicated shape for manufacturing and has the smallest possible installation area.

[0015] The third object is to provide a speed reduction device that is easy to install.

[0016] The fourth object is to provide a power steering system including the speed reduction device as described above.

[0017] The fifth object of the present invention is to provide a power-driven vehicle such as an automobile including such a power steering system.

Means for Solving the Problems

[0018] For this purpose, firstly, the speed reduction device for an electric power steering includes a support, a worm screw, and a gear. The gear meshes with a helical spline disposed on the worm screw. The worm screw has a first end for rotational drive and a free second end. The first end and the second end respectively receive a first rotation guiding means and a second rotation guiding means so that the worm screw can pivot with respect to the support. The second rotation guiding means includes a damping sleeve and a rotation guiding device. The sleeve surrounds the rotation guiding device. The second rotation guiding means includes centering means in which a press-fitting portion inserted into the rotation guiding device is incorporated, and a fixing portion integral with the support.

[0019] With this speed reduction device for a power steering, it is possible to limit the play between the worm screw and the gear caused by wear of the meshing mechanism and dimensional changes of parts due to thermal stress. On the other hand, this device is compact and can be easily obtained.

[0020] According to one embodiment, the second end portion includes a cavity having a shape complementary to that of the damping sleeve, and the damping sleeve is arranged to be inserted into the cavity.

[0021] According to one embodiment, the support includes an orifice for receiving a fixing portion.

[0022] According to one embodiment, the orifice is cylindrical and defines a central axis, the first rotation guiding means pivots around a rotation axis, the orifice is arranged such that an offset occurs between the central axis and the rotation axis, and the worm screw applies a pressing force to the gear.

[0023] According to one embodiment, the centering means is a cylindrical pin.

[0024] According to one embodiment, the rotation guiding device is a needle bearing cage or a draw cup needle bearing.

[0025] According to one embodiment, the support is a housing that forms a housing surrounding the worm screw, the gear, and the rotation guiding means.

[0026] Second, there is provided an electric power steering system including the above-described speed reduction device and an electric motor having a rotating shaft integrated with the first end portion.

[0027] Third, there is provided an automobile including the electric power steering system as described above.

[0028] Other features and advantages of the present invention will become clearer and more specific by reading the following description of the embodiments made with reference to the accompanying drawings.

Brief Description of the Drawings

[0029]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying out the Invention

[0030] An example of the electric power steering system 1 is shown in FIG. 1.

[0031] An automobile (not shown) incorporates, for example, such an electric power steering system 1. The automobile refers to any power-driven vehicle equipped with four wheels and intended to run on road facilities, such as a passenger car, a truck, or a bus.

[0032] The purpose of the electric power steering system 1 is to facilitate the operation of the steering wheel 8 by applying the torque generated by the rotating shaft 12 of the motor 11 to the steering column assembly 2 via the reduction gear 13.

[0033] The reduction gear 13 will be described in more detail with reference to FIGS. 2 to 5.

[0034] The reduction gear 13 includes a support 15 and a gear 16 including a worm assembly 17 and a gear 18.

[0035] As will be described later, the worm assembly 17 includes a worm screw 19 and rotation guiding means 20, 21 that enable the worm screw 19 to turn relative to the support, for example, along axis A. The gear 18 is rotatably attached along axis B orthogonal to the axis A using a known method, which will not be described in detail herein.

[0036] According to the illustrated embodiment, the gear 18 is provided with an opening 22 so as to be able to introduce the lower steering column 4. The opening 22 is preferably covered with a projection 23 complementary to the recess of the lower steering column 4, integrating the rotational movement of the gear 18 with the lower steering column 4.

[0037] Advantageously, the support 15 is a housing or casing that encloses the gear 16. Thereby, the gear 16 is protected from external impurities and dirt that may risk stopping its operation.

[0038] As shown in FIG. 3, the support 15 preferably includes a first section 24 that enables the installation of the worm screw 19 and a second section 25 that enables the placement of the gear 18. The sections 24 and 25 are arranged side by side and communicate via an opening 26 so as to be able to form an engagement between the gear 18 and the worm screw 19.

[0039] In the illustrated embodiment, the support 15 is a single member, obtained, for example, by molding.

[0040] In other embodiments not shown, the support 15 is manufactured by assembling several parts.

[0041] The worm screw 19 is connected, for example, to the rotating shaft 12 of the motor 11 via a connecting piece 27 and meshes with the gear 18 that enables the transmission of the rotational movement. The diameter of the gear is defined such that it enables the reduction of the rotational speed from the motor 11 and can obtain a torque large enough to contribute to the drive of the steering column assembly 2. Thereby, such a reduction device 13 can reduce the user's effort required for turning the wheels of a vehicle equipped with the electric power steering system 1.

[0042] Regarding the worm screw 19, a Cartesian coordinate system XYZ that constitutes a trihedron is defined, which includes three mutually perpendicular axes: The axis X defining the longitudinal direction in the horizontal direction substantially defines the overall direction of the extension of the worm screw 19. The axis Y defining the transverse direction in the horizontal direction, together with the axis X, defines the horizontal plane XY. The axis Z defining the vertical direction is perpendicular to the horizontal plane XY.

[0043] The worm screw 19 has a first end 28 that advantageously supports a connecting piece 27 integral with the motor 11 via a rotating shaft 12. Thereby, the rotational movement of the motor 11 is transmitted to the worm screw 19.

[0044] Advantageously, in order to make the connecting piece 27 integral with the worm screw 19, the first end 28 is provided with a positioning surface 29 formed on a cylinder 30 protruding from the first end 28. Such a configuration enables the positioning of the connecting piece 27 within the first end 28, and the position retention is ensured by, for example, a fixing part 31.

[0045] The worm screw 19 advantageously has a free second end 32.

[0046] In order to enable the engagement of the gear 18, the worm screw advantageously has a helical spline 33 disposed between the two ends 28, 32. The helical spline 33 is formed, for example, as a helix that engages with the teeth 34 of the gear 18.

[0047] The rotation guiding means will be described in particular specifically with reference to FIG. 5. Advantageously, the first rotation guiding means 20 and the second rotation guiding means 21 are respectively disposed at the first end 28 and the second end 32 of the worm screw 19.

[0048] The first rotation guiding means 20 incorporates, for example, a ball bearing 35 disposed between the first end 28 and a hole 36 provided in the support 15, providing a pivotal connection about the axis A' between the support 15 and the first end 28.

[0049] In order to enable the positioning and holding of the ball bearing 35 on the first end portion 28, the helical spline 33 has a shoulder 37 and the hole 36 has a bracket 38, whereby each ring of the ball bearing 35 can be stopped respectively. The holding in place is ensured by a first stop surface 39 arranged on the connecting piece 27 and a second stop surface 40 provided on the fixing part 31.

[0050] The second rotation guiding means 21 includes a damping sleeve 41, a rotation guiding device 42, and a centering means 43, and together they enable a pivoting connection about the central axis A'' between the support 15 and the second end portion 32.

[0051] Advantageously, the damping sleeve 41, the rotation guiding device 42, and the centering means are cylindrical and nested with each other. Thereby, the installation area of the second rotation guiding means 21 can be minimized.

[0052] The centering means includes a press-fitting part 44 inserted into the rotation guiding device 42. The press-fitting part 44 acts as a male part, for example, and the rotation guiding device 42 acts as a female part. In such a configuration, the press-fitting part 44 supports the rotation guiding device 42. The centering means includes, for example, a fixing part 45 integral with the support 15 and press-fitted into an orifice 46 formed in the support 15. Therefore, the position of such an orifice 46 makes it possible to define the position of the axis A'' with respect to the axis A'.

[0053] Advantageously, the second axis A'' is arranged offset from the first axis A' and positioned so that the worm screw 19 presses against the gear 18. In this way, the worm screw 19 can be attached in a state of being prestressed against the gear 18.

[0054] By prestressing the gear 16, the worm screw 19 is always in contact with the gear 18, thereby avoiding the appearance of any play caused by both wear of the gear 16 and deformation of the worm assembly 17 and the gear 18 caused by thermal fluctuations.

[0055] When the damping sleeve 41 is energized, it exerts a restoring force. Therefore, due to such a damping sleeve 41, the direction in which the shaft A'' extends can change following wear of the gear 16 or deformation of the helical spline 33 or the tooth 34 related to thermal fluctuations. Therefore, the damping sleeve 41 changes the extending direction of the worm screw 19 so as to press the worm screw 19 against the gear 18. As a result, the parasitic play in the radial direction is eliminated.

[0056] The damping sleeve 41 is made of, for example, a synthetic polymer material, such as an elastomer, or natural rubber. Such a damping sleeve 41 is compact, inexpensive, and easy to attach to the rotary guide device 42. In this way, the installation area and manufacturing complexity of the speed reduction device 13 are limited.

[0057] Advantageously, the rotary guide device 42 is a bearing, such as a roller bearing like a draw cup needle bearing or a needle bearing cage, which allows, for example, deviation of the rotation axis A' along the transverse axis caused by deviation resulting from prestressed attachment of the worm screw 19. In addition, such a rotary guide device 42 also offers the advantages of being compact, inexpensive, and easy to install on the rotary guide device 42. In this way, the installation area and manufacturing complexity of the speed reduction device 13 are limited.

[0058] The centering means 43 is preferably manufactured in a cylindrical shape that facilitates the pivotal connection between the second end portion 32 and the support 15. Such a configuration also allows offsetting the central axis A'' in the direction Y, Z alone or in combination, that is, in the radial direction.

[0059] In the illustrated embodiment, the centering means 43 includes a cylindrical pin. By using a cylindrical pin, centering means can be provided that serves both to set and hold a fixing portion at a position on the support 15. Thereby, the number of parts used in the manufacture of the centering means 43 is reduced. Further, the attachment of the second rotation guiding means 21 becomes easier.

[0060] Advantageously, the worm screw 19 has a cavity 47 for accommodating the second rotation guiding means 21. The cavity 47 is, for example, cylindrical and has a shape and dimensions complementary to those of the damping sleeve 41. In this way, the installation area of the damping sleeve 41, the rotation guiding device 42, and the press-fitting portion 44 is limited. Thus, apart from the manufacture of the holes 36, the openings 26, and the orifices 46, there is no need to create other specific additional geometric shapes within the first compartment 24, so the size of the reduction gear 13 is limited, which particularly facilitates the manufacture of the support 15.

[0061] The reduction gear 13 as described above has particularly many advantages.

[0062] Reduction of play between the gear 18 and the helical spline 33, Small installation area, Easy attachment, reduction in the number of operating times, Particularly because there is no particularly complex shape, the manufacturing cost of the support 15 is relatively limited, and particularly because the installation area of the second rotation guiding means 21 is limited, parts are saved.

Claims

1. A reduction gear (13) for an electric power steering, comprising: a support (15); a worm screw (19); a gear (18), wherein the gear (18) meshes with a helical spline (33) disposed on the worm screw (19), the worm screw (19) has a first end (28) for rotational drive and a free second end (32), the first end (28) and the second end (32) respectively receive a first rotation guide element (20) and a second rotation guide element (21) such that the worm screw (19) is rotatable with respect to the support (15), the second rotation guide element (21) comprises: a rotation guide device (42); a damping sleeve (41) surrounding the rotation guide device (42); and a centering member (43) having a first end (44) forming a press-fitting portion inserted into the rotation guide device (42) and a second end (45) integral with the support (15). The reduction gear is characterized by this.

2. In the reduction gear (13) according to Claim 1, the second end (32) has a cavity (47) having a shape complementary to that of the damping sleeve (41), and the damping sleeve (41) is arranged to be inserted into the cavity (47). The reduction gear is characterized by this.

3. In the reduction gear (13) according to Claim 1 or 2, the support (15) has an orifice (46) for receiving the second end (45) of the centering member (43). The reduction gear is characterized by this.

4. In the reduction gear (13) according to Claim 3, the orifice (46) is cylindrical and defines a central axis (A''), the first rotation guide element (20) rotates around a rotation axis (A'), the orifice (46) is arranged such that an offset occurs between the central axis (A'') and the rotation axis (A'), and the worm screw (19) applies a pressing force to the gear (18). The reduction gear is characterized by this.

5. In the reduction gear (13) according to Claim 1, the centering member (43) is a cylindrical pin. The reduction gear is characterized by this.

6. In the reduction gear (13) according to any one of Claims 1 to 5, the rotation guide device (42) is a needle bearing cage or a drawn cup needle bearing. The reduction gear is characterized by this.

7. In the speed reducer (13) according to any one of claims 1 to 5, the support (15) is a housing forming a housing that surrounds the worm screw (19), the gear (18), and the first rotary guide element (20) and the second rotary guide element (21), the speed reducer being characterized thereby.

8. An electric power steering system (1) comprising the speed reducer (13) according to any one of claims 1 to 7 and an electric motor (11) having a drive shaft (12) integral with the first end portion (28).

9. An automobile comprising the power steering system (1) according to claim 8.

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