JP3062852B2 - Electric power steering system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering device for an automobile or the like, and more particularly, to a power steering device for a motor vehicle. The present invention relates to a power steering device that moves a rack shaft by a driver's steering force and an auxiliary steering force of an electric motor corresponding to the steering force.

[0002]

2. Description of the Related Art Conventionally, as an electric power steering apparatus of this kind, a screw portion is formed on an outer peripheral surface of a rack shaft at a position shifted in an axial direction from a rack, and this screw portion can rotate in a housing. It is known that a ball nut mounted so as not to move in the axial direction is screwed through a number of circulating balls, and the ball nut is rotated by an electric motor via a reduction gear mechanism. For example, see Japanese Patent Publication No. 3-15591).

[0003]

However, the conventional electric power steering apparatus has the following problems. That is,
When the motor locks due to a failure, the axial movement of the rack shaft by turning the steering wheel is hindered, and as a result, it becomes impossible to transmit the input from the steering wheel to the steered wheels. It is necessary to provide a large connecting / disconnecting mechanism in order to cut off. Further, after the driver turns the steering wheel by a predetermined angle by moving the rack shaft by the driver's steering force by turning the steering wheel and the auxiliary steering force of the electric motor according to the steering force, the driver turns the steering wheel. When the wheel is released, the steered wheels naturally return to the neutral position. At this time, a force is applied to the ball nut to rotate the ball nut from the rack shaft, which is transmitted to the reduction gear mechanism and the electric motor. Therefore, the return performance of the rack shaft and the steering wheel deteriorates.

An object of the present invention is to provide an electric power steering apparatus which solves the above problem.

[0005]

SUMMARY OF THE INVENTION An electric power steering apparatus according to the present invention comprises: a rack shaft formed with a rack supported by a housing and engaged with a pinion of a shaft connected to a steering shaft; A motor for applying a steering force, wherein the ball nut is mounted on the housing so that the ball nut can rotate with respect to the housing but does not move in the axial direction, and the rotation of the motor is provided between the motor and the ball nut. Is transmitted to a ball nut, and a power transmitting means for rotating the ball nut is provided, and at a position shifted in the axial direction from the rack,
Around the rack shaft, a cylindrical ball screw shaft is fitted so that it can rotate with respect to the rack shaft but does not move in the axial direction, and the ball screw shaft is screwed into a ball nut via a circulating ball. Between the shaft and the ball screw shaft, lock the two when the inertia force of the ball screw shaft rotating around the rack shaft is equal to or greater than a predetermined value, and release the lock when the inertial force is less than the predetermined value. A clutch for freely rotating with each other is provided.

[0006]

When the electric motor is locked due to a failure, the rack shaft and the cylindrical ball screw shaft are moved in the axial direction by slowly turning the steering wheel, and a force for rotating the ball nut acts on the ball nut. Due to the reaction force of this force, an inertial force rotating around the rack axis is applied to the cylindrical ball screw axis. However, since this inertial force is less than a predetermined value, the ball screw shaft and the rack shaft rotate freely with each other by the action of the clutch. Therefore, the movement of the rack shaft in the axial direction is not hindered, and the input by turning the steering wheel can be transmitted to the steered wheels.

When the rotation of the electric motor is transmitted to the ball nut by the transmission means to apply the auxiliary steering force and the ball nut is rotated, the inertia force for rotating the ball screw shaft around the rack shaft is equal to or more than a predetermined value. Therefore, the cylindrical ball screw shaft and the rack shaft are locked by the operation of the clutch, and the mutual free rotation of both is prevented. Therefore, the rotation of the cylindrical ball screw shaft causes the rack shaft to move in the axial direction together with the ball screw shaft, and as a result, an auxiliary steering force is applied to the rack shaft.

When the steered wheel returns to the neutral position,
The rack shaft and the cylindrical ball screw shaft are moved in the axial direction. As a result, a force to rotate the ball nut acts on the ball nut, and the reaction force of this force causes the cylindrical ball screw shaft to rotate around the rack shaft. The rotating inertial force is applied to. However, since this inertial force is less than a predetermined value, the operation of the clutch causes the ball screw shaft and the rack shaft to rotate freely with each other, and the ball screw shaft also rotates freely with respect to the ball nut. Therefore, the ball nut does not rotate, and the force is not transmitted to the transmission means and the electric motor.

In the same manner as described above, when a large input is generated from the steered wheel side to the rack shaft when the steered wheels return, such as when the vehicle rides on a curb, the ball screw shaft rotates about the rack shaft. However, since the inertial force is equal to or greater than a predetermined value, the rack shaft and the ball screw shaft are locked to prevent free rotation of the rack shaft and the ball screw shaft. Therefore, the input is transmitted to the transmission means and the electric motor via the ball nut.

[0010]

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

FIG. 1 shows a part of an electric power steering apparatus for an automobile, and FIGS. 2 to 4 show details of a main part thereof. In addition,
In the following description, the left and right in FIGS . 1 and 2 are referred to as left and right.

A housing which is long in the left-right direction on the body of an automobile
(1) is fixed, and a rack shaft (2) extending left and right in the housing (1) is supported so as not to rotate but to be movable in the left and right direction (axial direction). housing
An input shaft (4) and an output shaft (5) connected by a torsion bar (not shown) are rotatably supported in a gear box (3) formed near the left end of (1). The output shaft (5) is integrally formed with a pinion (6) that meshes with a rack (not shown) of the rack shaft (2), and the input shaft (4) is connected to a steering wheel (not shown). The gearbox (3) is provided with a torque sensor (7) for detecting a steering torque from a steering wheel by detecting a torsion of a torsion bar between the input shaft (4) and the output shaft (5). Tie rods (10) (11) are attached to both left and right ends of the rack shaft (2) protruding from the housing (1) via ball joints (8) (9).
Are connected.

An intermediate part of the housing (1) and a rack shaft
A portion swelling upward is formed on the right side of the rack of (2), and the motor is provided on the left side surface of the swelling portion (1a).
(12) is fixed to the right so that it is parallel to the rack axis (2). The motor shaft penetrates the wall of the bulging portion (1a) and protrudes into the bulging portion (1a), and the small gear (13) is fixed to this portion. The small gear (13) meshes with a large gear (14) rotatably supported in the bulging portion (1a).

The electric motor (12) has a ball screw (1) as follows.
It is connected to the rack shaft (2) via 5). That is,
Around the rack shaft (2), a cylindrical ball screw shaft (16) having a thread formed on the outer peripheral surface is fitted so that it can rotate with respect to the rack shaft (2) but does not move in the axial direction. I have. The ball screw shaft (16) fits in the middle part of the housing (1).
A number of circulating balls (19) are mounted on a ball nut (18) that is rotatably supported so as not to move in the axial direction via (17).
Is screwed through. Teeth (18a) meshing with the teeth (14a) of the large gear (14) are formed on the outer peripheral surface of the ball nut (18). Then, by the teeth (18a) formed on the small gear (13), the large gear (14) and the ball nut (18), the electric motor is
The rotation of (12) is reduced and transmitted to the ball nut (18),
It is transmitted to the rack shaft (2) via the ball screw (15).

The inertial force for rotating the ball screw shaft (16) around the rack shaft (2) between the right end of the cylindrical ball screw shaft (16) and the rack shaft (2) is equal to or greater than a predetermined value. In this case, a clutch (21) is provided which locks the two in a case, and releases the lock of the two when they are less than a predetermined value to rotate freely with each other. Two recesses (22) extending in the circumferential direction are formed on the inner peripheral surface at the right end of the ball screw shaft (16) so as to be on the same diameter. Roll in the direction
(23) is arranged. The curvature of the bottom (22a) of each recess (22) is larger than the curvature of the outer peripheral surface of the rack shaft (2), and the bottom (22a) of the recess (22) and the outer peripheral surface of the rack shaft (2) are formed. Is larger than the diameter of the roller (23) at the center in the circumferential direction,
The diameter gradually decreases toward both ends in the circumferential direction, and at both ends, the diameter is smaller than the diameter of the roller (23). Therefore,
When the roller (23) is located at the center of the recess (22), the ball screw shaft (16) and the rack shaft (2) rotate freely with each other. 16) (2) is locked to prevent mutual free rotation. Further, on both sides of the roller (23) in the recess (22), a compression coil spring (24) for holding the roller (23) at a free position in the center of the recess (22) is arranged. . The biasing force of the spring (24) is equal to the rated torque of the motor (12), i.e., when the ball screw shaft (16) applies the auxiliary steering force to the rack shaft (2) by the motor (12), the ball screw shaft (16) The ball screw shaft (16) is smaller than the inertia force rotating around the rack shaft (2) when the steered wheels naturally return to the neutral position as described later. It has become.

In such a configuration, when the driver turns the steering wheel to rotate the steering shaft, the rotation is transmitted to the pinion (6) via the input shaft (4), the torsion bar and the output shaft (5). The rotation of the pinion (6) causes the rack shaft (2) to move in the left-right direction, and the wheels are steered. At this time, the torque sensor
The electric motor (12) is driven based on the direction and magnitude of the steering torque detected by (7), whereby the small gear (13),
Large gear (14), ball nut (18) and ball screw shaft (16)
To the rack shaft (2). The inertia force applied to the ball screw shaft (16) by the electric motor (12) about the rack shaft (2) is greater than the biasing force of the spring (24).
As shown in the figure, the ball screw shaft resists the urging force of the spring (24).
(16) rotates with respect to the rack shaft (2), the cylindrical ball screw shaft (16) and the rack shaft (2) are locked by the action of the clutch (21), and the mutual free rotation of both is locked. You. As a result, the rotation of the cylindrical ball screw shaft (16) causes the rack shaft (2) to move together with the ball screw shaft (16) in the same direction as that of the pinion (6).

When the motor (12) is locked due to a failure, the rack shaft (2) and the ball screw shaft (16) are moved in the axial direction by slowly turning the steering wheel, and as a result, the ball nut (18) ) Acts on the ball screw shaft (16) due to the reaction force of the force, and an inertia force rotating around the rack shaft (2) is applied to the ball screw shaft (16). Since this inertial force is smaller than the biasing force of the spring (24),
As shown in FIG. 3, the roller (23) is held at a free position in the center in the circumferential direction within the recess (22), and the rack shaft (2) and the ball screw shaft (16) rotate freely with each other. sell. Therefore, the ball nut (18) does not rotate, and the input by turning the steering wheel is prevented from being transmitted to the electric motor (12) via the large gear (14) and the small gear (13). As a result, the axial movement of the rack shaft (2) is not hindered, and the input can be transmitted to the steered wheels and steered.

Further, when the driver releases the steering wheel after turning the steered wheels by a predetermined angle, the steered wheels naturally try to return to the neutral position. At this time, the rack shaft (2) and the cylindrical ball screw shaft are used. (16) is moved in the axial direction.As a result, a force to rotate the ball nut (18) acts on the ball nut (18), and the reaction force of this force causes the ball screw shaft (16) to rotate the rack shaft (2). A rotating inertia force is applied. Since this inertial force is smaller than the biasing force of the spring (24), as shown in FIG. 3, the roller (23) is held at a free position at the center in the circumferential direction in the recess (22), and (2) and the ball screw shaft (16) can rotate freely with respect to each other. Therefore, ball nut (18)
Does not rotate, the input is prevented from being transmitted to the electric motor (12) via the large gear (14) and the small gear (13), and the rack shaft (2)
And the return performance of the steering wheel is improved.

Further, even when a large input is generated from the steered wheel side to the rack shaft (2) when the steered wheels are returned, such as when the steered wheels are returned, the ball screw shaft (16) is moved in the same manner as described above. Is applied to the rack shaft (2), but the inertia force is greater than the biasing force of the spring (24), which is opposite to the above.Therefore, the rack shaft (2) and the ball screw shaft (16) The input is locked via the ball nut (18).
4), transmitted to the small gear (13) and the electric motor (12). Therefore, sudden return of the rack shaft (2) and the steering wheel is prevented.

It should be noted that the clutch is not limited to the one having the above-mentioned structure, but can be changed as appropriate.

Further, in the above embodiment, the present invention is applied to a power steering device in which an electric motor is provided outside a housing. Instead, the present invention is applied to a power steering device having a built-in electric motor. Is also applicable. That is, the present invention can be applied to a power steering device in which a motor stator is mounted on a housing (1) supporting a rack shaft (2) and a motor rotor is mounted on a ball nut (18). In this case, the same effect as in the above embodiment can be obtained.

[0022]

According to the electric power steering apparatus of the present invention, the input by turning the steering wheel is transmitted to the steered wheels even when the motor is locked due to a failure, as described above. In addition, there is no need for a connection / disconnection mechanism for separating the motor from the reduction gear mechanism.

Further, when a small input from the steered wheels is generated on the rack shaft when the steered wheels return, the ball screw shaft and the rack shaft rotate freely with each other by the action of the clutch, and the ball screw shaft is rotated by the rack shaft and the ball. Since the ball nut rotates freely with respect to the nut, rotation of the ball nut is prevented. Therefore, the input from the steered wheels is prevented from being transmitted to the transmission means provided between the electric motor and the ball nut and the electric motor, and as a result, the return performance of the rack shaft and the steering wheel is improved.

Further, when a large input is applied to the rack shaft from the steered wheels when the steered wheels are returned, such as when the vehicle rides on a curb, the clutch shaft locks the rack shaft and the ball screw shaft. Therefore, the input is transmitted to the transmission means and the electric motor via the ball nut. Therefore, sudden return of the rack shaft and the steering wheel is prevented.

[Brief description of the drawings]

FIG. 1 is a partially cutaway rear view of an electric power steering apparatus according to an embodiment of the present invention.

FIG. 2 is an enlarged view showing a main part of FIG. 1;

FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 2, showing a state where the ball screw shaft and the rack shaft can freely rotate with each other.

FIG. 4 is a sectional view corresponding to FIG. 3, showing a state in which a ball screw shaft and a rack shaft are locked by a clutch.

[Explanation of symbols]

 Reference Signs List 1 housing 2 rack shaft 5 output shaft 6 pinion 12 electric motor 13 small gear 14 large gear 15 ball screw 16 ball screw shaft 18 ball nut 18a tooth 19 circulation ball 21 clutch

Claims (1)

(57) [Claims] A rack shaft formed with a rack supported by the housing and meshing with a pinion of a shaft connected to the steering shaft; and an electric motor for applying an auxiliary steering force to the rack shaft. The ball nut is rotatable with respect to the housing but is mounted so as not to move in the axial direction, and between the electric motor and the ball nut, a transmission means for transmitting the rotation of the electric motor to the ball nut and rotating the same is provided. An electric power steering device, comprising: a cylindrical ball screw shaft which is rotatable with respect to the rack shaft but is not moved in the axial direction around the rack shaft at a position shifted in the axial direction from the rack. As well as
When the ball screw shaft is screwed into the ball nut via the circulating ball, and the inertia force for rotating the ball screw shaft around the rack shaft between the rack shaft and the ball screw shaft is equal to or more than a predetermined value, the two are connected. An electric power steering apparatus provided with a clutch that locks and unlocks both when the value is less than a predetermined value and freely rotates with each other.