Electric power steering apparatus

Since a nut 45 is provided with paths for connecting rolling paths 23b, 45f and circulating paths 45c, that is, return paths 45d at end caps 45b, 45b attached to both ends of a main body 45a, a receiving portion for receiving power transmitted from an electric motor 35, that is, a driven gear portions 45e is provided at a center of an outer peripheral face on outer sides in a radius direction of the rolling paths and therefore, when power is transmitted from the electric motor 35 to the driven gear portion 45e, smooth operation can be carried out by restraining the nut 45 from being twisted.

BACKGROUND OF THE INVENTION

The present invention relates to an electric power steering apparatus having a ball screw mechanism.

There is known an electric power steering apparatus for driving an electric motor in accordance with a steering torque to assist steering by transmitting a rotational force of the electric motor to a rack shaft. Here, there is a case of using a ball screw mechanism for converting the rotational force of the electric motor into a thrust force of the rack shaft (refer to Patent Reference 1).

Examined Japanese Patent Application Publication No. Hei6-504

Meanwhile, according to the electric power steering apparatus described in Patent Reference 1, a gear is integrally formed with an outer periphery of a nut of the ball screw mechanism and power of the electric motor is transmitted to the nut of the electric motor via the gear. Although when the nut and the screw shaft are rotated relative to each other, there is needed a circulating path for returning a ball from one end to other end of a rolling path, nothing is disclosed in Patent Reference 1 with regard to the circulating path. Therefore, it seems that a tube or a frame which is a general circulating path is used. However, when the gear is formed at the outer periphery of the nut, it is difficult to arrange the tube or the frame to ride over the gear. Therefore, it is to be conceived that the gear of the nut is arranged to displace from the rolling path to an outer side in an axis line direction according to the electric power steering apparatus disclosed in Patent Reference 1 regardless of drawings of Patent Reference 1.

However, when it is conceived that the gear of the nut is arranged to displace from the rolling path to the outer side in the axis line direction, a torque transmitted from the electric motor is inputted to one end of the nut and therefore, the nut is liable to be twisted and power cannot be transmitted smoothly. Further, although when a large torque is transmitted, a tooth width of a gear is to be increased in order to reduce a face pressure of a tooth face, thereby, a length of the nut in the axis line direction is increased to pose also a problem that compact formation of the electric power steering apparatus cannot be achieved.

SUMMARY OF THE INVENTION

The invention is carried out in view of the problem of the background art and it is an object thereof to provide an electric power steering apparatus capable of ensuring to operate smoothly while achieving compact formation.

According to the first aspect of the present invention, there is provided an electric power steering apparatus including:

an electric motor;

a rack shaft connected to a steering mechanism and arranged in parallel with an axis line of the electric motor; and

a power transmitting mechanism for transmitting a power from the electric motor to the rack shaft, wherein

the power transmitting mechanism includes:

a screw shaft connected or integrated to the rack shaft and having a male screw groove;

a nut arranged at a surrounding of the screw shaft and having a female screw groove; and

a plurality of rolling members rotatable in a rolling path formed between the male screw groove and the female screw groove, and

the nut includes:

a main body provided with a receiving portion for receiving the power transmitted from the electric motor on an outer side in a radius direction of the rolling path and provided with a circulating path of the rolling member extended in an axis line direction; and

end caps provided at both ends of the main body and provided with paths for connecting the rolling path and the circulating path.

According to the second aspect of the present invention, there is provided the electric power steering apparatus according to the first aspect, wherein

the receiving portion is a gear portion formed at an outer peripheral face of the nut.

According to the third aspect of the present invention, there is provided the electric power steering apparatus according to the first aspect, wherein

the receiving portion is a gear portion formed at a driven member fitted to an outer peripheral face of the nut.

According to the forth aspect of the present invention, there is provided the electric power steering apparatus according to the third aspect, wherein

the nut and the driven member are coupled via a serration.

According to the fifth aspect of the present invention, there is provided the electric power steering apparatus according to the third aspect, wherein

a portion of the main body of the nut surrounding the circulating path is projected from the outer peripheral face in a cylindrical shape in the radius direction, and

an inner peripheral face of the driven member is recessed to be opposed to the portion surrounding the circulating path.

According to the sixth aspect of the present invention, there is provided the electric power steering apparatus according to any one of the third aspect through the fifth aspect, wherein

the nut and the driven member are coupled displaceably in the axis line direction via an elastic member.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be explained in reference to drawings as follows.FIG. 1is a sectional view of an essential portion of an electric power steering apparatus according to a first embodiment.FIG. 2is a view viewing the nut ofFIG. 1in an arrow mark direction by cutting the nut by a line II-II, in which a screw shaft and a ball are omitted. InFIG. 1, an electric power steering apparatus11includes a housing21fixed to a vehicle body, not illustrated. A rack shaft23is supported by the housing21movably in an axis line direction to penetrate the housing21horizontally. Although not illustrated, a pinion is formed at a lower end of an input shaft connected to a steering wheel and is brought in mesh with rack teeth of the rack shaft23and the rack teeth23is moved in a left and right direction in the drawing. Both ends of the rack shaft23is connected to a tie rod (not illustrated) of a steering mechanism.

An electric motor35is attached to the housing21such that an axis line thereof is in parallel with the rack shaft23. An output shaft35aof the electric motor35is fixed to a drive shaft37relatively displaceably in an axis line direction and integrally in a rotational direction by serration coupling. The drive shaft37is rotatably supported by the housing21by bearings20,22and includes a drive gear portion37aat a portion thereof interposed by the bearings20,22.

A middle shaft38is arranged between the drive shaft37and the rack shaft23. The middle shaft38is rotatably supported by the housing21by bearings24,25and includes a middle gear portion38abrought in mesh with the drive gear portion37a.

A nut45is arranged at a surrounding of the rack shaft23and is supported by the housing21rotatably by bearings26,27. The nut45includes a main body45ain a shape of a hollow cylinder at a center thereof and end caps45b,45bat both ends thereof. The main body45ais formed with two pieces of circulating paths45cpenetrated in an axis line direction (refer toFIG. 2) and formed with return paths (paths)45d,45dconnecting rolling paths thereof, mentioned later, to the circulating paths45c. Further, a driven gear portion (receiving portion)45ebrought in mesh with the middle gear portion38ais provided at a portion of the main body45ainterposed by the bearings26,27. A gear pair is constituted by the drive gear portion37a, the middle gear portion38aand the driven gear portion45e.

A male screw groove23bis formed at a portion of an outer peripheral face of the rack shaft23integral with the screw shaft (a separate parts may be connected thereto). The nut45is arranged at a surrounding of the male screw groove23b, and a female screw groove45fis formed at an inner peripheral face of the main body45aopposed to the male screw groove23b. A number of balls65are movably arranged at a space (rolling path) in a spiral shape formed by the male screw groove23band the female screw groove45f.

Operation of the embodiment will be explained. Although not illustrated, when a driver rotates the steering wheel, the rotational force is transmitted to the input shaft. When the input shaft is rotated, the rack teeth brought in mesh therewith by a pinion is pressed, the rack shaft23is moved in the axis line direction and the steering mechanism, not illustrated, is driven via the tie rod to thereby steer a wheel.

At this occasion, a torque sensor, not illustrated, detects a steering torque, CPU, not illustrated, supplies power to the electric motor35and therefore, the drive gear portion37ais rotated along with the output shaft, and the gear portion45ebrought in mesh therewith via the middle gear portion38ais rotated by a predetermined speed reducing ratio. Thereby, also the nut45is rotated, and rotational movement thereof is converted into an axis line movement of the rack shaft23via the ball65. By using the force in the axis line direction, the assisting steering force is made to be able to be outputted.

According to the electric power steering apparatus11of the embodiment, the nut45is provided with paths connecting the rolling paths (23b,45f) and the circulating paths45c, that is, return paths at the end caps45b,45bprovided at the both ends of the main body45aand therefore, the receiving portion for receiving power transmitted from the electric motor35, that is, the driven gear portion45ecan be provided at a center of an outer peripheral face on outer sides in a radius direction of the rolling paths and therefore, when power is transmitted from the electric motor35to the driven gear portion45e, smooth operation can be carried out by restraining the nut45from being twisted. Further, by providing the driven gear portion45efor receiving power transmitted from the electric motor35at the outer peripheral face on the outer sides in the radius direction of the rolling paths, a length in the axis line direction of the nut45can also be restrained.

FIG. 3is a sectional view of an essential portion of an electric power steering apparatus according to a second embodiment.FIG. 4is a view viewing a nut ofFIG. 3in an arrow mark direction by cutting the nut by a line IV-IV, in which a screw shaft and a ball are omitted. With regard to the embodiment, an explanation will be given only a point which differs from the embodiment shown byFIGS. 1,2, and an explanation will be omitted of a common constitution by attaching the same notation. Further, a nut145includes a main body145ahaving circulating paths145c,145cand end caps145b,145bhaving return paths145d,145dsimilar to the above-described embodiment.

In an electric power steering apparatus111shown inFIG. 3, a hollow driven member139is arranged at a surrounding of the rack shaft23. The driven member139is rotatably supported by the housing21by a bearing126, and a pair of angular contact ball bearings127,127outer rings of which are fixed by a screw member101screwed to the housing21. The driven member139includes a driven gear portion139aat a portion thereof interposed by the bearing126and the bearings127,127.

As shown byFIG. 4, the nut145is not constituted by a shape of a complete cylinder but is provided with raised portions (receiving portions)145g,145gto surround the circulating paths145c,145c. In correspondence with the raised portions145g,145g, recessed grooves139b,139bhaving substantially a rectangular sectional shape are provided at an inner peripheral face of the driven member139, and by engaging the raised portions145g,145gand the grooves139b,139b, the nut145is attached to the driven member139relatively displaceably in an axis direction and integrally in a rotational direction.

One end (left end inFIG. 3) of the driven member139is formed with a flange portion139cextended to an inner side in a radius direction, and an elastic member102made of resin or made of rubber is arranged between the flange portion139cand the end cap145b. On the other hand, a pressing member103in a shape of a circular disk is screwed to an inner periphery of other end (right end ofFIG. 3) of the driven member139and the elastic member102made of resin or made of rubber is arranged also between the pressing member103and the end cap145b. In a free state shown inFIG. 3, axis line direction clearances of Δ are present respectively between the end caps145b,145bof the nut145and an end face of the flange portion139cand an end face of the pressing member103.

According to the electric power steering apparatus111of the embodiment, power transmitted from the electric motor35is transmitted to the nut145by the driven member139via the grooves139b,139band the raised portions145g,145gengaged with each other. Here, the nut145is provided with the end caps145b,145battached with paths for connecting the rolling paths (23b,145f) and the circulating paths145c, that is, the return paths145dat both ends of the main body145aand therefore, receiving portions for receiving power transmitted from the electric motor35, that is, the raised portions145g,145gat an outer peripheral face on outer sides in the radius direction of the rolling paths and therefore, when power is transmitted from the electric motor35to the raised portions145g,145g, smooth operation can be carried out by restraining the nut145from being twisted. Further, by providing the raised portions145g,145gfor receiving power transmitted from the electric motor35at an outer peripheral face on outer sides in the radius direction of the rolling paths, a length in an axis line direction of the nut145can also be restrained.

Further, according to the embodiment, when the rack shaft23receives a force to abruptly move in the axis line direction by, for example, an impact force inputted from a road face via a wheel, not illustrated, the impact can be alleviated by deforming the elastic members102in a direction of reducing the axis line direction clearance Δ between the end cap145b,145bof the nut145and the end face of the flange portion139cor the end face of the pressing member103.

FIG. 5is a sectional view similar toFIG. 4according to a third embodiment. The embodiment applicable to the electric power steering apparatus ofFIG. 1or3is formed with a male serration245gat an outer peripheral face of a main body245aof a nut245in a cylindrical shape, and a female serration239bis formed at an inner peripheral face of a driven member239to be opposed thereto. By engaging the male serration245gand the female serration239b, the nut245is attached to the driven member239relatively displaceably in an axis line direction and integrally in a rotational direction. According to the modified example, the outer peripheral face of the main body245ais constituted by the cylindrical shape and therefore, it is easy to form the serration. Otherwise, a basic effect of the modified example is similar to those of the above-described embodiments.

FIG. 6is a sectional view of a nut and a screw shaft showing other modified example of the embodiment, andFIG. 7is a view viewing a constitution ofFIG. 6in an arrow mark direction by cutting the constitution by a line VII-VII, in which a screw shaft and a ball are omitted. Although the modified example corresponds to the embodiment ofFIGS. 1,2, one streak groove (345f,323b) is adopted for a nut345and a screw shaft323and therefore, there are provided single ones of a circulating path345cprovided at a main body345aof the nut345and a return path345dof an end cap345b.

FIG. 8is a view similar toFIG. 7according to still other modified example. Although the modified example corresponds to the embodiment ofFIGS. 3,4, the nut445and a screw shaft (not illustrated) adopt one streak screw groove (445f), therefore, there are single ones of a circulating path445cand a raised portion445gprovided at the nut445and a return path (not illustrated) of an end cap. Further, it is sufficient to form one groove of a driven member, not illustrated, in accordance with the raised portion445gto be engaged therewith.

FIG. 9is a view similar toFIG. 7according to still other modified example. Although the modified example corresponds to the embodiment ofFIG. 5, a nut545having a male serration and a screw shaft (not illustrated) adopt one streak screw groove (545f) and therefore, there are single ones of a circulating path545cand a return path (not illustrated) of an end cap provided at the nut545.

FIG. 10is a view similar toFIG. 7according to still other modified example. Although the modified example corresponds to the embodiment ofFIGS. 1,2, a nut645and a screw shaft (not illustrated) adopt a three streaks screw groove (645f) and therefore, there are respective three of circulating paths645cprovided at the nut645and return paths (not illustrated) of the end cap.

FIG. 11is a views similar toFIG. 7according to still other modified example. Although the modified example corresponds to the embodiment ofFIGS. 3,4, a nut745and a screw shaft (not illustrated) adopt a three streaks screw groove (745f) and therefore, there are respective threes of circulating paths745c, raised portions745gand return paths (not illustrated) of an end cap provided at the nut745. Further, three of grooves of a driven member, not illustrated, are needed in accordance with raised portions745gto be engaged therewith.

FIG. 12is a view similar toFIG. 7according to still other modified example. Although the modified example corresponds to the embodiment ofFIG. 5, a nut845having a male serration and a screw shaft (not illustrated) adopt three streaks screw grooves (845f) and therefore, there are respective threes of circulating paths845cand return paths (not illustrated) of an end cap provided at the nut845.

Although a detailed explanation is given of the invention in reference to the embodiments as described above, the invention is not to be interpreted to be limited to the above-described embodiments but can naturally be modified or improved pertinently within the range of not deteriorating a gist thereof.

According to the electric power steering apparatus of the invention, the nut is provided with the paths for connecting the rolling paths and the circulating paths at the caps at both ends of the main body and therefore, the receiving portion for receiving the power transmitted from the electric motor can be provided at the outer peripheral face on the outer sides in the radius direction of the rolling paths or the like and therefore, when power is transmitted from the electric motor to the receiving portion, smooth operation can be carried out by restraining the nut from being twisted. Further, by providing the receiving portion for receiving the power transmitted from the electric motor at the outer peripheral face on the outer sides in the radius direction of the rolling paths or the like, a length in the axis line direction of the nut can also be restrained.

Although it is preferable that the receiving portion is a gear portion formed at the outer peripheral face of the nut and the power from the electric motor is transmitted via a gear pair, the power may also be transmitted via a toothed belt or a chain.

It is preferable that the receiving portion is a gear portion formed at a driven member fitted to an outer peripheral face of the nut.

Although it is preferable that the nut and the driven member are coupled via a serration, the nut and the driven member may be fitted by press-fitting or the like.

It is preferable that a portion of the main body of the nut surrounding the circulating path is formed to project from the outer peripheral face in a cylindrical shape in a radius direction and an inner peripheral face of the driven member is recessed to be opposed to the portion surrounding the circulating path.

It is preferable that the nut and the driven member is coupled displaceably in the axis line direction via an elastic member since a relative displacement in the axis line direction of the nut and the driven member can be absorbed by deforming the elastic member.