JP2013063741A - Electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique that can more improve the steering feeling of an electric power steering device that has a worm gear mechanism.SOLUTION: The worm gear mechanism 44 comprises a worm 70 provided in a worm shaft 46 and a wheel that engages with this worm 70, one end part 46a of the worm shaft 46 is united to a motor shaft 43a of the electric motor, while the end faces are facing mutually and is rotatably supported to the housing 51 by a first bearing 47, and other end part 46b of the worm shaft 46 is rotatably supported to the housing 51 by the second bearing 48. Moreover, either of the first bearing 47 and the second bearing 48 is offset to an axis WL of the motor shaft 43a. While both of the first bearing 47 and the second bearing 48 are composed by the rolling bearing, the entire outer peripheral surface of the outer race is supported by the housing 51.

Description

  The present invention relates to an electric power steering apparatus, and more particularly to an improved technique of a worm gear mechanism for transmitting auxiliary torque generated by an electric motor to a load.

  The worm gear mechanism mounted on the electric power steering apparatus includes a worm connected to an electric motor and a worm wheel for torque transmission connected to a load. The auxiliary torque generated by the electric motor based on the steering torque of the steering wheel is transmitted from the worm to the load via the worm wheel. In such a worm gear mechanism, a backlash (gap between tooth surfaces) is provided between the worm and the worm wheel so as to achieve smooth meshing.

  However, if there is a backlash, for example, a sound that hits teeth when the vehicle is traveling on a rough road, a so-called rattling sound is generated. Such rattling noise can be a cause of noise. Furthermore, in the electric power steering apparatus, a minute delay time (time lag) may occur in the control response by the amount of backlash. In order to improve the steering feeling (steering feeling) of the electric power steering apparatus, it is required to have a more stable control performance. For this purpose, it is preferable to suppress backlash. In recent years, techniques for suppressing backlash have been developed (see, for example, Patent Document 1).

  A worm gear mechanism of an electric power steering device known from Patent Document 1 includes a worm provided on a worm shaft and a worm wheel meshing with the worm. One end portion of the worm shaft is coupled to the motor shaft of the electric motor by a shaft coupling while the end surfaces thereof face each other, and is rotatably supported by the housing by a ball bearing. The other end of the worm shaft is rotatably supported by the housing by a worm shaft support member.

  The worm shaft support member is a resin-made hollow disk-like member, and is fitted into a perfect circular hole formed in the housing. The worm shaft support member is formed with a support hole for supporting the worm shaft and a slit located on the outer side in the radial direction of the support hole.

  The center of the support hole is offset with respect to the entire center of the worm shaft support member. The center position of the support hole can be adjusted by rotating the worm shaft support member with respect to the hole of the housing. By adjusting the position of the center of the support hole, it is possible to adjust the backlash between the worm and the worm wheel.

  The slit is formed in a concentric arc shape with respect to the entire center of the worm shaft support member. By providing the worm shaft support member with the slit, it is possible to absorb the change in the distance between the centers of the worm and the worm wheel due to the influence of the temperature change.

  However, when the auxiliary torque is transmitted from the worm to the worm wheel, a reaction force is generated from the worm wheel to the worm. This reaction force is transmitted from the worm to the worm shaft support member via the worm shaft. Since this worm shaft support member has a slit, it is elastically displaced in a direction away from the worm meshing portion with the worm wheel. When such elastic displacement is repeated over a long period of time, the resin-made worm shaft support member can be plastically deformed due to aging. This can be a factor of lowering the function of suppressing the initial backlash.

  The control unit of the electric power steering apparatus controls the electric motor to generate auxiliary torque according to the steering torque applied to the steering wheel. There is a minute time lag between when the control unit issues a control signal and when the electric motor issues auxiliary torque according to the control signal. For this reason, disturbance such as overshoot occurs in the control signal, and it takes a minute time to stabilize. Meanwhile, the auxiliary torque generated by the electric motor fluctuates slightly.

  Furthermore, in order to enhance the steering feeling, it is preferable that the so-called robustness is high so that the torque control is not disturbed even if there is a noise entering the control system or a change in the control parameter. When the torque control is disturbed, the auxiliary torque generated by the electric motor fluctuates slightly during that time. Such torque fluctuation is preferably suppressed as much as possible in order to enhance the steering feeling of the electric power steering apparatus.

  However, in the worm gear mechanism of Patent Document 1, since the worm shaft support member has a slit, elastic displacement can occur according to torque fluctuation. This is disadvantageous in increasing the steering feeling.

JP 2002-173036 A

  An object of the present invention is to provide a technique capable of further improving the steering feeling of an electric power steering apparatus having a worm gear mechanism.

  In the invention according to claim 1, the control unit controls the electric motor based on the steering torque applied to the steering wheel, and transmits the auxiliary torque generated by the electric motor to the steering wheel via the worm gear mechanism. In the electric power steering apparatus in which the steering wheel is steered by a combined torque obtained by adding the auxiliary torque to the steering torque, the worm gear mechanism includes a worm provided on a worm shaft housed in a housing; A worm wheel that meshes with the worm, and one end portion of the worm shaft is coupled to the motor shaft of the electric motor with the end surfaces facing each other, and is rotatably supported by the housing by a first bearing. And the other end of the worm shaft is connected to the housing by a second bearing. The first bearing and the second bearing are offset with respect to the shaft center of the motor shaft, and both the first bearing and the second bearing are rolling bearings. And the entire outer peripheral surface of the outer ring is supported by the housing.

  The invention according to claim 2 is characterized in that the bearing that is offset with respect to the axis of the motor shaft is the second bearing.

  The invention according to claim 3 is characterized in that the first bearing is constituted by a bearing capable of self-regulating deformation in the worm shaft direction.

  The invention according to claim 4 is characterized in that the offset direction of the bearing that is offset with respect to the axis of the motor shaft is a direction in which the worm shaft approaches the outer peripheral surface of the worm wheel.

  In the invention which concerns on Claim 1, any one of the 1st bearing and the 2nd bearing for supporting the both ends of a worm shaft is offset with respect to the shaft center of a motor shaft. Moreover, the entire outer peripheral surface of the outer ring is supported by the housing in the first and second bearings. Since the first and second bearings are constituted by rolling bearings, the first and second bearings include an inner ring, an outer ring, and a plurality of moving bodies (such as spheres) arranged between the inner and outer rings. Since the worm shaft is inclined and fitted to the offset inner ring of the bearing, the load direction acting on the bearing from the worm shaft is slightly inclined rather than the direction perpendicular to the axis of the worm shaft. Therefore, in the bearing which is offset, the rolling friction resistance between the inner ring and the plurality of spheres and the rolling friction resistance between the outer ring and the plurality of spheres are increased as compared to the case where the offset is not offset. By increasing this rolling frictional resistance, it is possible to add a rotational resistance (rotational resistance) to the worm shaft. This rotational resistance can suppress wobbling (fluctuation) of the auxiliary torque generated by the electric motor. That is, stable torque control can be performed. Therefore, the steering feeling of the electric power steering device can be further enhanced.

  Furthermore, in the invention according to claim 1, since the entire outer peripheral surface of the outer ring of each bearing is supported by the housing, either the first bearing or the second bearing is offset with respect to the axis of the motor shaft. In spite of this, it is possible to prevent an excessive surface pressure from acting on each bearing. Accordingly, sufficient rigidity can be secured for the first and second bearings, so that rotational resistance can be added to the worm shaft over a long period of time.

  In the invention according to claim 2, the bearing that is offset with respect to the axis of the motor shaft is the second bearing that is located away from the motor shaft. That is, the first bearing closer to the motor shaft is not offset with respect to the axis of the motor shaft. The first bearing supports one end of the worm shaft. Since the first bearing is not offset, the one end of the worm shaft can be positioned so that the end surfaces of the worm shaft are coupled to each other while facing each other. For this reason, the freedom degree of selection of the shaft coupling for connecting a worm shaft to a motor shaft increases. Since the shafts can be directly connected to each other, a small cylindrical joint can be employed with a simple configuration. Examples of the cylindrical joint include a serration shaft joint in which serration teeth are formed in the cylinder and a spline shaft joint in which spline teeth are formed in the cylinder.

  In the invention according to claim 3, the first bearing receives a load (thrust) in the worm shaft direction accompanying the transmission of the auxiliary torque from the worm to the steering wheel. Since the second bearing is offset with respect to the axis of the motor shaft, it is not optimal for receiving the thrust. In general, when a rolling bearing receives a load, a minute elastic deformation occurs at a contact portion between a rolling element such as a ball and a raceway surface (a surface on which a sphere rolls). On the other hand, the first bearing is constituted by a rolling bearing capable of self-regulating deformation in the worm axis direction, for example, a four-point contact ball bearing. For this reason, the first bearing can prevent excessive elastic deformation by self-regulating deformation in the worm axis direction. As a result, the durability of the first bearing can be increased.

  In the invention according to claim 4, the offset direction of the bearing that is offset with respect to the axis of the motor shaft is a direction in which the worm shaft approaches the outer peripheral surface of the worm wheel. For this reason, since the distance between the centers of the worm and the worm wheel is reduced, backlash between the worm and the worm wheel can be suppressed. Therefore, the occurrence of rattling noise can be suppressed, and the time lag of control response can be suppressed.

  Furthermore, depending on the materials of the worm, worm wheel, and housing, the difference in thermal expansion increases as the temperature increases. As a result, the backlash can be increased by increasing the distance between the centers of the worm and the worm wheel. On the other hand, in the invention according to claim 4, the offset direction of the bearing that is offset with respect to the axis of the motor shaft is a direction in which the distance between the centers is reduced. For this reason, the expansion of the backlash due to the heat effect can be suppressed.

1 is a schematic diagram of an electric power steering apparatus according to the present invention. It is a whole block diagram of the electric power steering apparatus shown by FIG. FIG. 3 is a sectional view taken along line 3-3 in FIG. 2. FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. It is a principal part enlarged view of FIG. It is an enlarged view of a 2nd bearing.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  An electric power steering apparatus according to an embodiment will be described with reference to FIGS.

  As shown in FIG. 1, the electric power steering apparatus 10 includes a steering system 20 that extends from a steering wheel 21 of a vehicle to steering wheels 29 and 29 (for example, front wheels) of the vehicle, and an auxiliary torque that applies an auxiliary torque to the steering system 20. Mechanism 40.

  In the steering system 20, a pinion shaft 24 (rotary shaft 24) is connected to a steering wheel 21 via a steering shaft 22 and universal shaft joints 23, 23, and a rack shaft 26 is connected to the pinion shaft 24 via a rack and pinion mechanism 25. The left and right steering wheels 29, 29 are connected to both ends of the rack shaft 26 via left and right tie rods 27, 27 and knuckle 28, 28.

  The rack and pinion mechanism 25 includes a pinion 31 formed on the pinion shaft 24 and a rack 32 formed on the rack shaft 26.

  According to the steering system 20, when the driver steers the steering wheel 21, the left and right steering wheels 29 and 29 are steered by the steering torque via the rack and pinion mechanism 25 and the left and right tie rods 27 and 27. Can do.

  The auxiliary torque mechanism 40 detects the steering torque of the steering system 20 applied to the steering wheel 21 with the steering torque sensor 41, and generates a control signal with the control unit 42 based on the torque detection signal of the steering torque sensor 41. An auxiliary torque corresponding to the steering torque is generated by the electric motor 43 based on the signal, this auxiliary torque is transmitted to the pinion shaft 24 through the worm gear mechanism 44, and the auxiliary torque is further transmitted from the pinion shaft 24 to the rack and This is a mechanism for transmitting to the pinion mechanism 25.

  The steering torque sensor 41 detects the torque applied to the pinion shaft 24 and outputs it as a torque detection signal, and is constituted by, for example, a magnetostrictive torque sensor or a torsion bar torque sensor.

  According to the electric power steering apparatus 10, the steering wheels 29 and 29 can be steered by the rack shaft 26 by a combined torque obtained by adding the auxiliary torque of the electric motor 43 to the steering torque of the driver.

  As shown in FIG. 2, the housing 51 extends in the vehicle width direction (left-right direction in the figure), and accommodates the rack shaft 26 so as to be slidable in the axial direction. The rack shaft 26 is connected to tie rods 27, 27 via ball joints 52, 52 at both longitudinal ends protruding from the housing 51.

  As shown in FIG. 3, the electric power steering apparatus 10 houses the pinion shaft 24, the rack and pinion mechanism 25, the steering torque sensor 41, and the worm gear mechanism 44 in the housing 51, and the upper opening of the housing 51 is formed in the upper cover portion 53. It was closed with. The steering torque sensor 41 is attached to the upper cover portion 53.

  The housing 51 has an upper portion 24u, a longitudinal center portion 24m, and a lower end portion 24d of the pinion shaft 24 extending vertically through three bearings (a first bearing 55, a second bearing 56, and a third bearing 57 in order from top to bottom). Further, the electric motor 43 is attached and a rack guide 60 is provided. The three bearings 55 to 57 are ball bearings. The outer ring of the third bearing 57 is fixed to the fitting hole of the housing 51 by press fitting.

  The rack guide 60 is rack pressing means that includes a guide portion 61 that contacts the rack shaft 26 from the side opposite to the rack 32 and an adjustment bolt 63 that presses the guide portion 61 via a compression spring 62.

  As shown in FIG. 4, the electric motor 43 is attached to the side surface of the housing 51 and includes a lateral motor shaft 43 a. The motor shaft 43 a extends into the housing 51 and is connected to the worm shaft 46 by a coupling 45. The housing 51 supports both end portions 46a and 46b of the worm shaft 46 extending horizontally through bearings 47 and 48 while restricting movement in the axial direction. The two bearings 47 and 48 are both rolling bearings.

  Hereinafter, the bearing 47 that supports one end portion 46a of the worm shaft 46, that is, the end portion 46a on the motor shaft 43a side, is referred to as "first bearing 47", and the bearing 48 that supports the other end portion 46b is referred to as "first bearing". It will be referred to as “two bearings 48”.

  As shown in FIGS. 3 and 4, the worm gear mechanism 44 is an auxiliary torque transmission mechanism that transmits auxiliary torque generated by the electric motor 43 to the pinion shaft 24, that is, a booster mechanism. More specifically, the worm gear mechanism 44 includes a worm 70 and a worm wheel 80 that meshes with the worm 70. The worm wheel 80 is hereinafter abbreviated as “wheel 80”. The center line CL of the wheel 80 is disposed substantially perpendicular to the center line WL of the worm 70. The center line CL of the wheel 80 is also the center line CL of the pinion shaft 24.

  The worm 70 is a metal product integrally formed with the worm shaft 46, for example, a steel product such as a carbon steel material for machine structure (JIS-G-4051). The wheel 80 is a resin product such as a nylon resin as a whole or at least a portion of the teeth 81. Since the resin product wheel 80 is meshed with the metal product worm 70, meshing can be made relatively smooth and noise can be further reduced.

The thread 71 (that is, the teeth 71) of the worm 70 is set to one. On the outer peripheral surface of the wheel 80, a plurality of teeth 81 having an equal pitch are formed over the entire circumference. The wheel 80 is attached such that relative movement in the axial direction is restricted with respect to the pinion shaft 24 and relative rotation is restricted. For example, the wheel 80 is connected to the pinion shaft 24 by a serration or a spline in the rotational direction, and is attached by a retaining ring in the axial direction. By engaging the load-side wheel 80 with the drive-side worm 70, torque can be transmitted from the worm 70 to the load via the wheel 80.
The details of the worm 70 and the wheel 80 will be described in the following figures.

  As shown in FIGS. 5 and 6, the first bearing 47 and the second bearing 48 are supported by the housing 51 on the entire outer peripheral surfaces of the outer rings 47 a and 48 a, respectively. The first bearing 47 is positioned by a retaining ring 82 and a nut 83. Although the details will be described later, the worm shaft 46 is restricted from being deformed in the axial direction by the first bearing 47 positioned by the retaining ring 82 and the nut 83. On the other hand, the second bearing 48 is offset in a direction in which the axis BC approaches the outer peripheral surface of the wheel 80 with respect to the axis WL of the motor shaft 43.

  The first bearing 47 receives a load (thrust) in the direction of the worm shaft 46 due to the transmission of the auxiliary torque from the worm to the steering wheel (FIG. 1, reference numeral 21). Since the second bearing 48 is offset with respect to the axis WL of the motor shaft, it cannot be said to be optimal for receiving the thrust.

  In general, when a rolling bearing receives a load, a minute elastic deformation occurs at a contact portion between a rolling element such as a ball and a raceway surface (a surface on which a sphere rolls). In such a rolling bearing, when there is no restriction of elastic deformation in the axial direction, if the acting thrust load is excessive, the rolling surface of the sphere and the raceway surface of the raceway (inner ring and outer ring) are uneven. An indentation is produced and the smoothness of rotation is lost. As a result, the rolling bearing generates an operating sound such as a grooving sound and vibration.

  On the other hand, the first bearing 47 is constituted by a rolling bearing capable of self-regulating deformation in the worm shaft 46 direction. Specifically, by using a four-point contact ball bearing for the first bearing 47, deformation of the worm shaft 46 in the axial direction is self-regulated. For this reason, the first bearing 47 can prevent excessive elastic deformation by self-regulating deformation in the direction of the worm shaft 46. As a result, the durability of the first bearing 47 can be enhanced and the operating noise and vibration can be suppressed.

In addition, since one four-point contact ball bearing can receive axial thrust in both directions (axial load), the rigidity in the axial direction is large.
Another type of rolling bearing with high axial rigidity is a double-row angular contact ball bearing. However, the four-point contact ball bearing has a smaller bearing width. By employing a four-point contact ball bearing, the worm gear mechanism 44 can be reduced in size.

  The second bearing 48 is a single-row ball bearing and includes an outer ring 48a, an inner ring 48b, and a plurality of spheres 48c. The outer ring 48a is supported on the entire outer peripheral surface of the housing 51, and the worm shaft 46 is fitted into the inner ring 48b. The center of the outer ring 48a coincides with the center of the outer diameter D, that is, the axis BC. The center of the inner ring 48b coincides with the axis WL.

  By the way, as shown in FIG. 6, the single row ball bearing 48 (second bearing 48) has an inherent gap t between each of the outer ring 48a, the inner ring 48b and the sphere 48c in consideration of thermal expansion and the like. Designed. According to the present invention, the axis BC of the second bearing 48 is offset with respect to the axis WL of the worm shaft 46. For this reason, the inner ring 48b is pushed and displaced by the worm shaft 46 in a direction opposite to the offset direction of the axis BC. By being displaced, the outer ring 48a, the inner ring, and the sphere 48c are always in contact with each other (see the sphere 48c at the top of the drawing). On the other hand, in the offset direction of the axis BC, the inner ring 48b and the sphere 48c are not in contact with each other, and a space 48d is formed between the inner ring 48b and the sphere 48c.

  That is, since the worm shaft 46 is inclined and fitted to the offset inner ring 48b of the second bearing 48, the load direction acting on the second bearing 48 from the worm shaft 46 is the axis of the worm shaft 46. It is not right-angled but slightly tilted. Therefore, in the second bearing 48, the rolling friction resistance between the inner ring 48b and the plurality of spheres 48c and the rolling friction resistance between the outer ring 48a and the plurality of spheres 48c are compared with the case where they are not offset. Increase. By increasing this rolling frictional resistance, resistance in the rotational direction (rotational resistance) can be added to the worm shaft 46. This rotational resistance can suppress wobbling (fluctuation) of the auxiliary torque generated by the electric motor (FIG. 4, reference numeral 43). That is, stable torque control can be performed. Therefore, the steering feeling of the electric power steering device can be further enhanced.

  Further, since the entire outer peripheral surfaces of the outer rings 47a and 48a of the bearings 47 and 48 are supported by the housing 51, one of the first bearing 47 and the second bearing 48 is connected to the shaft center WL of the motor shaft. In spite of being offset, it is possible to prevent an excessive surface pressure from acting on the bearings 47 and 48. Accordingly, the first and second bearings 47 and 48 can be sufficiently rigid, and thus a rotational resistance can be added to the worm shaft 46 over a long period of time.

  The first bearing 47 closer to the motor shaft 43 is not offset with respect to the axis WL of the motor shaft. The first bearing 47 supports one end 46 a of the worm shaft 46. Since the first bearing 47 is not offset, the one end portion 46 a of the worm shaft 46 is positioned so that the end surfaces are coupled to the motor shaft WL of the electric motor 43 while facing each other. And the eccentricity of the coupling surface of the motor shaft WL can be suppressed. As a result, in addition to joints (such as rubber couplings) that allow eccentricity and declination, it is also possible to select joints (such as serrations) that do not allow decentration and allow only declination. For this reason, the freedom degree of selection of the shaft coupling for connecting the worm shaft 46 to the motor shaft 43 increases. Since the shafts can be directly connected to each other, a small cylindrical joint can be employed with a simple configuration. Examples of the cylindrical joint include a serration shaft joint in which serration teeth are formed in the cylinder and a spline shaft joint in which spline teeth are formed in the cylinder.

  When the first bearing 47 is offset with respect to the axis of the motor shaft 43, for example, a rubber coupling can be used to allow eccentricity, so that the worm shaft 46 can be connected to the motor shaft 43.

  The second bearing 48 is offset in the direction in which the worm shaft 46 approaches the outer peripheral surface of the wheel 80 with respect to the shaft center WL of the motor shaft. For this reason, since the distance between the centers of the worm 70 and the wheel 80 is reduced, the backlash between the worm 70 and the wheel 80 can be suppressed. Therefore, the occurrence of rattling noise can be suppressed, and the time lag of control response can be suppressed.

  Further, depending on the materials of the worm 70, the wheel 80, and the housing 51, the difference in thermal expansion increases as the temperature increases. As a result, the backlash can be increased by increasing the distance between the centers of the worm 70 and the wheel 80. On the other hand, the offset direction of the second bearing 48 with respect to the axis WL of the motor shaft is a direction in which the distance between the centers becomes smaller. For this reason, the expansion of the backlash due to the heat effect can be suppressed.

  In the electric power steering apparatus according to the present invention, the case where a four-point contact ball bearing or a double-row angular ball bearing is used as the first bearing has been described as an example, but a single-row ball bearing may be used to reduce the cost. Any bearing can be applied.

  The electric power steering device of the present invention is suitable for use in passenger cars.

  DESCRIPTION OF SYMBOLS 10 ... Electric power steering apparatus, 21 ... Steering wheel, 29 ... Steering wheel, 42 ... Control part, 43 ... Electric motor, 44 ... Worm gear mechanism, 46 ... Worm shaft, 46a ... One end part, 46b ... Other end part, 47 ... First bearing 48, second bearing 51, housing 70, worm, 80 worm wheel, WL worm center (motor shaft axis), BC second bearing center line (offset bearing axis) ).

Claims (4)

The control unit controls the electric motor based on the steering torque applied to the steering wheel, and transmits the auxiliary torque generated by the electric motor to the steering wheel via the worm gear mechanism, whereby the auxiliary torque is added to the steering torque. In the electric power steering apparatus adapted to steer the steered wheel by the added composite torque,
The worm gear mechanism is composed of a worm provided on a worm shaft housed in a housing, and a worm wheel meshing with the worm,
One end portion of the worm shaft is coupled to the motor shaft of the electric motor with end surfaces facing each other, and is rotatably supported by the housing by a first bearing,
The other end of the worm shaft is rotatably supported by the housing by a second bearing,
Either one of the first bearing and the second bearing is offset with respect to the axis of the motor shaft,
Both the first bearing and the second bearing are constituted by rolling bearings, and the entire outer peripheral surface of the outer ring is supported by the housing.   2. The electric power steering apparatus according to claim 1, wherein the bearing that is offset with respect to the axis of the motor shaft is the second bearing.   The electric power steering apparatus according to claim 2, wherein the first bearing is a bearing capable of self-regulating deformation in the worm shaft direction.   The offset direction of the bearing that is offset with respect to the axis of the motor shaft is a direction in which the worm shaft approaches the outer peripheral surface of the worm wheel. The electric power steering device according to claim 1.

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