Power steering apparatus and preload adjustment method thereof

A power steering apparatus includes: a worm driven by a motor for assistance; a worm wheel engaged with the worm and transmitting an assist force to a steering shaft; a housing which houses the worm and the worm wheel; a plug having a male screw on an outer peripheral surface of the plug, the male screw being screwed with a female screw formed at the housing; an elastic member, one end of which in a biasing direction of the elastic member is supported by the worm and other end of which in the biasing direction is supported by the plug, giving a preload between the worm and the worm wheel; and a plug fixing member for fixing the plug, a position of which is adjusted to a screwing position at which the preload is a predetermined value, so that the plug is unrotatable with respect to the housing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2014-260205 filed on Dec. 24, 2014, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power steering apparatus and a preload adjustment method thereof.

2. Description of Related Art

In a motor-driven power steering apparatus, a rotation force of a motor for assistance generating an assist force is generally transmitted through a speed reduction mechanism to a pinion shaft and so on connected to a steering shaft through a torsion bar. The speed reduction mechanism is formed by, for example, a worm gear mechanism. In the case where the motor-driven power steering apparatus is a pinion-assist type apparatus, the speed reduction mechanism is formed by including a worm connected to an output shaft of a motor and a worm wheel fixed to the pinion shaft and engaged with the worm.

In the above motor-driven power steering apparatus, it is necessary to set a center distance between the worm and the worm wheel easily without being affected by dimension errors of parts such as the worm at the time of assembly as well as to maintain the center distance suitably to thereby eliminate backlash even when the engagement between the worm and the worm wheel is changed over time after the assembly.

In response to the above problems, there is disclosed in JP-A-2013-208932 (Patent Document 1), a power steering apparatus having a preload means for biasing a bearing supporting a distal shaft portion of the worm to a given preload direction so as to apply the preload to an engaged portion of the worm and the worm wheel. The preload means is formed by a compression coil spring. The center distance between the worm and the worm wheel is suitably maintained by an elastic force of the preload means to prevent a backlash.

The preload means described in Patent Document 1 includes a bearing case which is a C-shaped annular body in which part in a circumferential direction is cut out. An inner circumferential surface of the bearing case is formed as a bearing housing hole in which a guide surface for guiding the bearing to the preload direction is formed. One end of a compression coil spring abuts on the bearing through a spring insertion hole formed in the bearing case and the other thereof abuts on a lid fixed on a lid attachment surface of a housing. The lid is fixed to the lid attachment surface of the housing by a bolt.

A biasing force of the compression coil spring depends on a compression amount of the spring. The compression amount depends on a distance between a position of the worm and the lid attachment surface, a free length of the spring and the like. However, variation may occur in the distance between the position of the worm and the lid attachment surface, the free length of the spring and the like due to manufacturing errors, assembly errors and so on. Therefore, variation may occur in the biasing force of the compression coil spring and variation may occur in the preload between the worm and the worm wheel due to the above variation.

SUMMARY OF THE INVENTION

The present invention has been made for solving the above problems, and an object thereof is to provide a power steering apparatus and a load adjustment method thereof capable of adjusting the preload between the worm and the worm wheel to a desired value regardless of manufacturing errors, assembly errors and so on.

According to an embodiment of the present invention, there is provided a power steering apparatus having: a worm driven by a motor for assistance; a worm wheel engaged with the worm and transmitting an assist force to a steering shaft; a housing which houses the worm and the worm wheel; a plug having a male screw on an outer peripheral surface of the plug, the male screw being screwed with a female screw formed at the housing; an elastic member, one end of which in a biasing direction of the elastic member is supported by the worm and other end of which in the biasing direction is supported by the plug, giving a preload between the worm and the worm wheel; and a plug fixing member for fixing the plug, a position of which is adjusted to a screwing position at which the preload is a predetermined value, so that the plug is unrotatable with respect to the housing.

Thereby, even when manufacturing errors of the elastic member, assembly errors around the worm and the worm wheel and so on exist, a desired preload can be obtained between the worm and the worm wheel by adjusting the position of the plug to the screwing position. The desired preload can be maintained by providing the plug fixing member.

In the present invention, the plug fixing member may be a lock nut screwed to the male screw of the plug.

Thereby, the plug fixing member can be realized by a simple structure, and a fixing work of the plug can be also performed easily.

In the present invention, the plug fixing member may be caulking between the plug and the housing.

Thereby, the plug fixing member can be realized by a simple structure, and the fixing work of the plug can be also performed easily.

The power steering apparatus according to the embodiment may further include a gauge-head insertion hole formed at the housing, which is for inserting, from an outside of the housing, a gauge head for measuring a load of the worm with regard to the preload and a closing member which closes the gauge-head insertion hole.

Thereby, the preload between the worm and the worm wheel can be adjusted easily by a simple structure in which the hole is formed at the housing and the closing member which closes the hole is provided.

Also according to the present invention, there is provided a preload adjustment method of a power steering apparatus having a worm driven by a motor for assistance, a worm wheel engaged with the worm and transmitting an assist force to a steering shaft, a housing which houses the worm and the worm wheel, a plug having a male screw on an outer peripheral surface of the plug, the male screw being screwed with a female screw formed at the housing, and an elastic member, one end of which in a biasing direction of the elastic member is supported by the worm and other end of which in the biasing direction is supported by the plug, giving a preload between the worm and the worm wheel, which includes the steps of adjusting a position of the plug to a screwing position at which the preload is a predetermined value and fixing the plug, the position of which is adjusted in the adjusting of a position of the plug, so that the plug is unrotatable with respect to the housing.

Thereby, even when manufacturing errors of the elastic member, assembly errors around the worm and the worm wheel and so on exist, a desired preload can be obtained between the worm and the worm wheel by adjusting the position of the plug to the screwing position. The desired preload can be maintained by providing the plug fixing member.

In the present invention, a gauge-head insertion hole may be formed at the housing, and in the plug adjustment process, it is preferable that a load of the worm with regard to the preload is measured in a state where the gauge head of a load meter is inserted into the housing through the gauge-head insertion hole and the worm is driven by the motor for assistance to decide a screwing position of the plug from the measured load and that the gauge-head insertion hole is closed after the measuring.

Thereby, the preload between the worm and the worm wheel can be adjusted easily by a simple structure in which the hole is formed at the housing and the closing member which closes the hole is provided.

By applying the present invention, it is possible to adjust the preload between the worm and the worm wheel to a desired value regardless of manufacturing errors, assembly errors and so on.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

Structure of Motor-Driven Power Steering Apparatus

InFIG. 1, a motor-driven power steering apparatus100(steering apparatus) is a pinion assist type apparatus in which an assist force is inputted to a pinion shaft114. However, the present invention can be also applied to a column-assist type and a rack-assist type motor-driven power steering apparatuses.

A motor-driven power steering apparatus100includes a steering wheel111operated by a driver, a steering shaft112rotating together with the steering wheel111, a torsion bar113connected to a lower end of the steering shaft112, a pinion shaft114connected to a lower end of the torsion bar113and a rack shaft115extending in a vehicle width direction (right and left direction).

Pinion teeth114aof the pinion shaft114are meshed with rack teeth115aof the rack shaft115. Then, when the pinion shaft114rotates around a rotation axis O1(seeFIG. 2), the rack shaft115moves in the vehicle width direction and steered wheels117(vehicle wheels) connected through tie rods116are steered. The rack shaft115is housed in a cylindrical housing118through a bush and the like.

The motor-driven power steering apparatus100also includes a worm wheel120coaxially fixed to the pinion shaft114, a worm130engaged with the worm wheel120, an electric motor for assistance (hereinafter referred to as merely a motor)140generating an assist force and rotating the worm130, a joint150connecting an output shaft141of the motor140to the worm130and a housing20housing the worm130and so on (seeFIG. 2)

The worm wheel120is serration-connected with the pinion shaft114. That is, the worm wheel120is rotatably attached to the pinion axis114in a coaxial manner. The worm wheel120is engaged with the worm130to transmit the assist force of the motor140to the steered shaft (rack shaft115).

As shown inFIG. 2, the worm wheel130is a part having an approximately columnar shape rotating around a rotation axis O2, which has a worm body131in which gear teeth131aare formed on a peripheral surface, a first shaft portion132formed on one end side (motor140side) of the worm body131and a second shaft portion133formed on the other end side (opposite side of the motor140) of the worm body131.

The first shaft portion132is rotatably supported by the housing20through a first bearing11. The second shaft portion133is rotatably supported by the housing20through a second bearing12and a holder30.

When the motor140is driven and the worm130is rotated, the gear teeth131aof the worm130are going to climb over wheel teeth formed in an outer peripheral surface of the worm wheel120. That is, a separating force to separate from the worm wheel120is generated at the worm130.

The motor140is an electric motor driven in accordance with an instruction by a not-shown ECU (Electronic Control Unit) and generating the assist force. The output shaft141of the motor140is connected to the first shaft portion132of the worm130through the joint150. The ECU detects a torsional torque generated in the torsion bar113(FIG. 1) through a torque sensor (not shown), and drives the motor140so as to correspond to the magnitude of the torsional torque to thereby generate the assist force.

The joint150connects between the output shaft141and the first shaft portion132to transmit the power of the motor140to the worm130.

The first bearing11is provided between the first shaft portion132of the worm130and the housing20, rotatably supporting the first shaft portion132with respect to the housing20. The first bearing11is formed by, for example, a radial ball bearing, supporting a load in a radial direction. The first bearing11is held by the housing20by a cylindrical stopper member13screwed to the housing20.

The second bearing12is provided between the second shaft portion133of the worm130and the housing20, rotatably supporting the second shaft portion133with respect to the housing20. The second bearing12is formed by, for example, a radial ball bearing, supporting a load in a radial direction. The second bearing12is housed in a later-described holder30.

The housing20houses the worm wheel120, the worm130and the like. In the housing20, a holder housing hole21for housing the holder30is formed. The holder housing hole21has a short columnar shape, and an axial direction thereof extends in an axial direction of the worm130. The holder housing hole21is communicated with the outside through a communication hole22extending in a later-described biasing direction (a radial direction of the worm130(separating direction)). The communication hole22is closed by a plug23. At the housing20, a female screw25to which a male screw24of the plug23is screwed is formed.

At the housing20, a gauge-head insertion hole62into which a gauge head61of a load meter60(FIG. 5) which measures a load concerning the preload of the worm130is inserted from the outside is formed as described later. The gauge-head insertion hole62is formed in a position facing an end portion of the second shaft portion133approximately around the rotation axis O2as a hole axis center. The gauge-head insertion hole62is closed by a closing member63after the measurement by the load meter60. The closing member63is formed by, for example, a metal ball such as a steel ball, which closes the gauge-head insertion hole62by press fitting. The closing member63may be formed by members other than the metal ball. The closing member63may be provided so as to be detachable.

The motor-driven power steering apparatus100includes a worm biasing structure1which biases the second bearing12(worm130) toward the worm wheel120to give a preload to the worm130. The worm biasing structure1is formed by including the second bearing12, the holder30, a cup40and an elastic member50as shown inFIG. 4. In this case, the biasing direction in which the second bearing12(worm130) is biased is a right and left direction (vertical direction of the page) inFIG. 4, namely, a radial direction of the worm130.

The holder30is a member having an approximately annular shape and covering the outside of the second bearing12to hold the second bearing12as shown inFIGS. 3A, 3BandFIG. 4. The holder30has a C-shape in which a portion facing the worm wheel120is cut out seen from the axial direction of the second bearing12. Then, the holder30is housed inside the holder housing hole21in a state of being slightly reduced in diameter.

In an inner peripheral surface of the holder30, a pair of flat guide surfaces31,31extending in the biasing direction (right and left direction) of the worm130are formed. That is, the second bearing12can slide in the biasing direction of the worm130while slidingly contacting the guide surfaces31,31.

A short-cylindrical shaped spring housing portion32is formed in the holder30on the opposite side of the worm wheel120. The spring holding portion32is a portion which houses a compression coil spring50and a hollow portion thereof extends in the biasing direction (right and left direction) of the worm130.

The cup40has a bottomed cylindrical shape in which the second bearing12side is closed, which is housed in the spring housing portion32so as to be slidable. The cup40includes a cylindrical peripheral wall portion41and a bottom wall portion42formed in the peripheral wall portion41side of the second bearing12. The peripheral wall portion41has a certain degree of length in the sliding direction of the cup40and an outer peripheral surface of the peripheral wall portion41slidingly contacts an inner peripheral surface of the spring housing portion32. An outer surface of the bottom wall portion42linearly contacts an outer peripheral surface of the second bearing12.

The elastic member50is an elastic body which biases the second bearing12(worm130) toward the worm wheel120. In the present embodiment, the elastic member50is formed by the compression coil spring51. The compression coil spring51is arranged in a compressed state so as to extend along the biasing direction inside the spring housing portion32, and one end51A thereof abuts on the bottom wall portion42of the cup40and the other end51B thereof abuts on the plug23. That is, in the compression coil spring51, one end51A is supported by the worm130through the cup40and the second bearing12in the biasing direction and the other end51B is supported by the plug23. Accordingly, a preload is given between the worm130and the worm wheel120due to an elastic restoring force of the compression coil spring51. The elastic member50may be a disc spring, a rubber material and so on.

The motor-driven power steering apparatus100includes the plug23having the male screw24on an outer peripheral surface, the male screw24being screwed with the female screw25of the housing20. The plug23is a short-columnar member formed in an approximately fixed diameter in the axial length. On an end surface (referred to as an outer end surface26) on one end side of the plug23in the axial direction, a hexagonal hole28into which a screw fastening tool71(FIG. 5) is inserted is formed. On an end surface (referred to as an inner end surface27) on the other end side of the plug23in the axial direction, a concave portion29for housing the other end51B of the compression coil spring51is formed.

The motor-driven power steering apparatus100includes a plug fixing member80which fixes the plug23the position of which is adjusted to an arbitrary screwing position so as not to rotate with respect to the housing20for obtaining a desired preload between the worm130and the worm wheel120. As a preferred example of the plug fixing member80, a lock nut (locking nut)81can be cited. In this case, in the plug23the position of which is adjusted at the position where the desired preload is obtained, part of the male screw24is exposed to the outside from the outer surface of the housing20. The rock nut81is screwed to the exposed male screw24so as to abut on the outer surface of the housing20. Accordingly, the plug23is fixed to the housing20so as not to rotate and the movement of the plug23in the axial direction is inhibited. As the lock nut81, a lock nut of a well-known structure can be used and explanation of the detailed structure is omitted.

In a state where all the respective parts inside the housing20are assembled, the gauge-head insertion hole62(FIG. 2) is left unclosed. Then, as shown inFIG. 5, the screw fastening tool71is inserted into the hexagonal hole28and the male screw24of the plug23is screwed with the female screw25of the housing20to compress the compression coil spring51. The elastic restoring force generated in the compressed coil spring51is acted as a force of pressing the second bearing12to the worm wheel120side inside the holder30. Accordingly, the preload is given between the worm130and the worm wheel120. After that, a plug adjustment process and a plug fixing process are performed described below.

In the plug adjustment process, the position of the plug23is adjusted to an arbitrary screwing position for obtaining a desired preload. An example of a method of determining whether the desired preload can be obtained or not is as follows. The gauge head61of the load meter60is inserted into the housing20through the gauge-head insertion hole62and a tip thereof is locked in a gauge-head locking hole64formed on an end surface of the second shaft portion133. The load concerning the preload of the worm130is measured in the state where the worm130is driven by the motor140, and the screwing position of the plug23is adjusted by the screw fastening tool71and the compression amount of the compression coil spring51is adjusted so that the measurement value reaches a “predetermined load”. A value of the “predetermined load” is a load value when the desired preload is obtained, which is an approximately value obtained by subtracting the separating force of the worm130separating from the worm wheel120which is generated when the worm130is rotated as described above from the preload obtained when the worm130is not rotated. The value of the “predetermined load” is a value calculated by simulations and the like in advance.

When the measured value of the load member60becomes the predetermined load, the plug23is in the state where part of the male screw24is exposed to the outside from the outer surface of the housing20. A gap L is formed between the inner end surface27of the plug23and a wall portion65of the housing20in which the communication hole22is formed. After that, the gauge-head insertion hole62is closed by the closing member63.

In the plug fixing process, the plug23the position of which is adjusted in the plug adjustment process is fixed by the lock nut81so as not to rotate with respect to the housing20. Accordingly, the movement of the plug23in the axial direction is inhibited, and the preload between the worm130and the worm wheel120is held in a suitable value.

As described above, when applying the structure of including the plug fixing member80which fixes the plug23the position of which is adjusted to the arbitrary screwing position for obtaining the desired preload between the worm130and the worm wheel120so as not to rotate with respect to the housing20, the desired preload can be obtained by adjusting the position of the plug23to the arbitrary screwing position even when there are variations in length of the compression coil spring51, assembly errors around the worm130and the worm wheel120and so on. As the plug fixing member80is provided, the predetermined preload is maintained.

As the plug fixing member80is formed by the lock nut81screwed to the male screw24of the plug23, the plug fixing member80can be realized by a simple structure and a fixing work of the plug23can be also performed easily.

Also, when applying a structure in which the gauge-head insertion hole62formed at the housing20, into which the gauge head61for measuring the load concerning the preload of the worm130is inserted from the outside and the closing member63closing the gauge-head insertion hole62, the preload adjustment between the worm130and the worm wheel120can be easily performed by a simple structure in which the hole is formed at the housing20and the closing member63for closing the hole is provided.

Second Embodiment

A second embodiment will be explained with reference toFIGS. 6A and 6B. The second embodiment is an embodiment in which the plug fixing member80is formed by caulking between the plug23and the housing20. As the other components are the same as those of the first embodiment, the explanation thereof is omitted.

On an outer surface of the housing20around the female screw25, protrusions82for caulking are formed so as to protrude as shown inFIG. 6A. Plural protrusions82may be provided at intervals in a circumferential direction of the female screw25as well as one protrusion82may be provided. In the plug23of the present embodiment in which the position is adjusted so as to obtain the desired preload between the worm130and the worm wheel120, the outer end surface26is approximately flush with the outer surface of the housing20. When the protrusions82are bent to the inner diameter side by using a not-shown jig, the protrusions82bite into the plug23by deforming the male screw24as shown inFIG. 6B. Accordingly, the plug23is fixed so as not to rotate with respect to the housing20, and the movement of the plug23in the axial direction is inhibited.

As the plug fixing member80is formed by caulking between the plug23and the housing20, the plug fixing member80can be realized by a simple structure and the fixing work of the plug23can be also performed easily.

The two preferred embodiments have been explained as the above. The plug fixing member80may fix the plug23to the housing20so as not to rotate by using an adhesive in addition to the lock nut and the caulking.