Patent Publication Number: US-2016223066-A1

Title: Rack guide mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-016911 filed on Jan. 30, 2015, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a rack guide mechanism. 
     2. Related Art 
     There is a rack and pinion type steering apparatus which includes a housing, a pinion shaft provided with a pinion gear, to which an operation force of a steering wheel is transmitted, a rack shaft provided with a rack gear engaged with the pinion gear and a rack guide mechanism guiding a moving direction of the rack shaft and biasing the rack shaft to the pinion shaft by an elastic restoring force (for example, refer to JP-A-2002-370654 (Patent Literature 1)). 
     The rack guide mechanism includes a rack guide provided to the housing so as to slide, a screw (lid member) screwed to the housing, and a compression coil spring provided between the rack guide and the screw, biasing the rack guide so that the rack shaft is constantly pressed onto the pinion shaft. As the rack shaft is pressed and biased to the pinion shaft, backlash between the pinion gear and the rack gear can be reduced and the operation force of the steering wheel can be positively transmitted to the rack shaft. 
     When a large external force is added to the rack shaft, the rack shaft may be largely separated from the pinion shaft. As a method of restoring from the separation state smoothly, a method of increasing an elastic force (spring rate) of a compression coil spring can be considered. However, as the force of pressing and biasing the rack guide to the rack shaft is constantly strong in this method, there is a problem that the sliding resistance of the rack shaft is increased and the steering feeling is liable to be deteriorated. 
     In response to the above, a technique in which a disc spring is interposed between the rack guide and the screw in addition to the compression coil spring is known.  FIG. 4  is an explanatory view of the above, and a numeral  51  denotes a pinion shaft, a numeral  52  denotes a rack shaft, a numeral  53  denotes a rack guide, a numeral  54  is a screw, a numeral  55  denotes a compression coil spring, a numeral  56  denotes a disc spring and a numeral  57  denotes a housing. A female screw  58  is formed in the housing  57 , and a male screw  59  of the screw  54  is screwed into the female screw  58 . In the structure of  FIG. 4 , the rack guide  53  is biased only by the elastic restoring force of the compression coil spring  55  in a normal state or in a range in which a relatively small external force acts on the rack shaft  52 . Then, when the pinion shaft  51  is largely separated from the rack shaft  52  as the large external force acts on the rack shaft  52 , the elastic restoring force of the disc spring  56  is allowed to act on the rack guide  53  in addition to the elastic restoring force of the compression coil spring  55 . Accordingly, both the securement of good steering feeling in the normal state and the smooth restoration of engagement between the pinion shaft  51  and the rack shaft  52  in an emergency are realized. 
     However, the number of parts is increased in the structure of using the disc spring  56  in addition to the compression coil spring  55 , which arises a problem that an assembly work of the rack guide mechanism is complicated. 
     Moreover, as a height dimension of the disc spring  56  is small, a stroke of a compression amount of the disc spring  56 , namely, an adjustment stroke of a screwing amount of the screw  54  becomes small, which arises a problem that it is difficult to perform the initial setting of the compression amount of the disc spring  56 . 
     Furthermore, there is a danger that an outer edge of the disc spring  56  is caught by the female screw  58  at the time of assembly and the disc screw  56  is not sandwiched by the rack guide  53  and the screw  54  correctly. 
     SUMMARY OF INVENTION 
     An illustrative aspect of the present invention is to provide a rack guide mechanism capable of realizing both the securement of good steering feeling in the normal state and the smooth restoration of engagement between the pinion shaft and the rack shaft in an emergency by using a single elastic member. 
     According to an embodiment of the present invention, there is provided a rack guide mechanism including: a housing; a rack guide housed in the housing so as to move freely, supporting a rack shaft; a lid member attached to the housing; and an elastic member interposed between the rack guide and the lid member, biasing the rack guide to the rack shaft, in which the elastic member includes a first elastic portion constantly sandwiched between the rack guide and the lid member, and a second elastic portion sandwiched between the rack guide and the lid member when the rack guide moves to the lid member side by a given distance, and the first elastic portion and the second elastic portion are integrally formed. 
     With the configuration of the rack guide mechanism, only an elastic restoring force of the first elastic portion acts on the rack guide in the normal state or when a small external force acts on the rack shaft. Accordingly, the rack shaft slides smoothly on a sliding surface of the rack guide without a large load and good steering feeling can be obtained. In an emergency when a large external force acts on the rack shaft, both the elastic restoring force of the first elastic portion and the elastic restoring force of the second elastic portion act on the rack guide. Accordingly, the rack shaft largely separated from the pinion shaft can be returned to the pinion shaft side immediately. 
     As the first elastic portion and the second elastic portion can be integrally formed, an assembly work of the rack guide mechanism is facilitated without the increase in the number of parts. 
     The rack guide mechanism may have a configuration in which the first elastic portion is formed of a tubular cylindrical portion formed so that both ends of the tubular cylindrical portion open, the first elastic portion is housed in a housing hole of the rack guide, an axial direction of the tubular cylindrical portion is parallel to a moving direction of the rack guide, the second elastic portion is formed of a flange portion extended in an outward radial direction at one of the ends of the first elastic portion, and a gap is formed between the rack guide and the flange portion in a normal state. 
     With this configuration of the rack guide mechanism, the shape of the elastic member and the layout structure can be simplified. As it is not necessary to use a disc spring as in the related art, a disadvantage in which an outer edge of the disc spring is caught by a female screw of the housing does not occur. 
     The rack guide mechanism may have a configuration in which a liquid body may be interposed between an inner peripheral surface of the housing hole and an outer peripheral surface of the tubular cylindrical portion. 
     With this configuration of the rack guide mechanism, the liquid body is interposed between the inner peripheral surface of the housing hole and the outer peripheral surface of the cylindrical portion, therefore, the sliding resistance between them can be reduced and it is possible to prevent the cylindrical portion from being twisted when housed in the housing hole. 
     According to the rack guide mechanism discussed above, both the securement of good steering feeling in the normal state and the smooth restoration of engagement between the pinion shaft and the rack shaft in an emergency can be realized by the single elastic member, therefore, the assembly property of the rack guide mechanism can be improved without the increase in the number of parts. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic structural view of a motor-driven power steering apparatus; 
         FIG. 2  is a cross-sectional view showing a structure of a rack guide mechanism according to the present invention; 
         FIG. 3  is a cross-sectional view showing the structure of the rack guide mechanism according to the present invention, in which a second elastic portion is sandwiched between a rack guide and a screw; and 
         FIG. 4  is a cross-sectional view showing a structure of a related-art rack guide mechanism. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be explained with reference to the drawings. The embodiment will be explained by citing an example in which a rack guide mechanism of the present invention is applied to a motor-driven power steering apparatus, however, the present invention is not limited to this and may be applied to a hydraulic power steering apparatus and a manual steering apparatus. 
     As shown in  FIG. 1 , a motor-driven power steering apparatus  1  is a rack and assist type apparatus including a steering mechanism  2  having a rack shaft  6  in which two rack gears which are a rack gear (steering wheel side)  5   b  and a rack gear (assist side)  5   c  are formed along a shaft center O 1  and an auxiliary torque mechanism  3  giving an auxiliary steering force to the rack shaft  6 . 
     The steering mechanism  2  includes a steering wheel  4   a  operated by a driver, a steering shaft  4   b  rotating by the operation of the steering wheel  4   a,  a pinion shaft  4   c  provided on a lower side of the steering shaft  4   b  through a not-shown torsion bar and a rack shaft  6  to which right and left steered wheels  8 ,  8  are connected on both ends through tie rods  7 ,  7 . A pinion gear (steering wheel side)  5   a  of the pinion shaft  4   c  is engaged with the rack gear (steering wheel side)  5   b  of the rack shaft  6 . When the driver rotates the steering wheel  4   a,  the rack shaft  6  moves in right and left directions to steer right and left steered wheels  8 ,  8 . 
     The auxiliary torque mechanism  3  includes a motor for assistance  30 , a worm gear mechanism  31  and an assist shaft  32  provided with a pinion gear (assist side)  32   a,  in which the pinion gear (assist side)  32   a  of the assist shaft  32  is engaged with the rack gear (assist side)  5   c  of the rack shaft  6 . The worm gear mechanism  31  includes a worm  33  rotatably attached to the motor for assistance  30  and a worm wheel  34  engaged with the worm  33 . The worm wheel  34  is rotatably attached to the assist shaft  32 . In the auxiliary torque mechanism  3 , a torque added to the steering wheel  4   a  is detected by a not-shown torque sensor, and the motor for assistance  30  is driven and controlled by a not-shown control device in accordance with the detected torque. Accordingly, the generated torque of the motor for assistance  30  is transmitted to the rack shaft  6  as an auxiliary steering force through the worm gear mechanism  31  and the assist shaft  32 . 
     “Rack Guide Mechanism  10 ” 
     The motor-driven power steering apparatus  1  is provided with a rack guide mechanism  10  as shown in  FIG. 2  for reducing backlash between the pinion gear  5   a  and the rack gear  5   b.  The rack guide mechanism  10  according to the present invention can be also applied to the pinion gear  32   a  and the rack gear  5   c  in the auxiliary torque mechanism  3 . 
     The rack guide mechanism  10  includes a housing  11  in which a housing chamber  12  opening toward the rack shaft  6  is formed, a rack guide  13  housed in the housing chamber  12  of the housing  11  so as to move freely and supporting the rack shaft  6 , a screw (lid member)  14  screwed to the housing  11  and forming a bottom wall of the housing chamber  12  and an elastic member  15  interposed between the rack guide  13  and the screw  14 , biasing the rack guide  13  to the rack shaft  6 . 
     “Housing  11 ” 
     The housing  11  is a casing member housing various components such as the pinion shaft  4   c  and the rack shaft  6 . The housing chamber  12  is formed as an approximately columnar through hole linearly extending from the housing position of the rack shaft  6  toward the opposite side of the pinion shaft  4   c  along a shaft center O 2 . When a portion where the rack shaft  6  is housed is referred to as one end side of the housing chamber  12  and a portion opening toward the outside of the housing  11  is referred to as the other end side of the housing chamber  12 , a female screw  11   b  into which the screw  14  is screwed is formed on an inner peripheral surface  11   a  of the housing  11  on the other end side of the housing chamber  12 . 
     “Screw  14 ” 
     The screw  14  is an approximately short columnar member, which closes the other end side of the housing chamber  12  by screwing a male screw  14   a  formed on an outer peripheral surface into the female screw  11   b.  In an outer surface  14   c  of the screw  14 , a tool hole  14   b  having a polygonal shape for inserting a rotating tool of the screw  14  is formed. Also in the screw  14 , a through hole  14   e  piercing in the shaft center O 2  direction is formed. The through hole  14   e  is a hole for allowing a gauge head for measuring a pressing force of the rack shaft  6  to pass through after screwing the screw  14 . The through hole  14   e  is closed by a bush  16  afterward. After the screw  14  is screwed to the housing  11 , a lock nut  17  is screwed to the male screw  14   a.    
     “Rack Guide  13 ” 
     A rack guide  13  is a member having an approximately columnar shape. The rack guide  13  is housed in the housing chamber  12  so as to move freely along the shaft center O 2  direction in a state where an outer peripheral surface thereof faces the inner peripheral surface  11  a of the housing  11 . A pair of annular grooves  18  extending in a circumferential direction is formed on the outer peripheral surface of the rack guide  13 , and O-rings  19  sliding on the inner peripheral surface  11  a are fitted to respective grooves  18 . 
     An end surface of the rack guide  13  is formed as a sliding surface  13   a  on which the rack shaft  6  slides in the shaft center O 2  direction, which is formed to have an approximately arc shape along the outer peripheral surface of the rack shaft  6 . On the other end surface  13   b  of the rack guide  13 , a housing hole (hereinafter referred to as a cylindrical portion housing hole)  20  for housing a later-described cylindrical portion  25  and a flange portion housing hole  21  formed to have a larger diameter than an inner diameter of the cylindrical portion housing hole  20  and for housing a later-described flange portion  26  are formed in order from the rack shaft  6  side around the shaft center O 2 . An annular stepped surface  22  is formed between the cylindrical portion housing hole  20  and the flange portion housing hole  21 . 
     “Elastic Member  15 ” 
     The elastic member  15  includes a first elastic portion  23  constantly sandwiched by the rack guide  13  and the screw  14  and a second elastic portion  24  sandwiched by the rack guide  13  and the screw  14  when the rack guide  13  moves to the screw  14  side by a given distance, which are integrally formed. In the embodiment, the first elastic portion  23  is formed as a tubular cylindrical portion  25  formed so that both ends of the tubular cylindrical portion  25  open, and the first elastic portion  23  is housed in the cylindrical portion housing hole  20  of the rack guide  13 . Here, an axial direction of the tubular cylindrical portion  25  is parallel to the moving direction of the rack guide  13 . In the embodiment, the axial direction is the same as the moving direction (shaft center O 2  direction) of the rack guide  13 . The second elastic portion  24  is formed as the flange portion  26  extending in the whole circumference in an outward radial direction at the end (on the other end side) of the first elastic portion  23 . A material of the elastic member  15  is, for example, a rubber material, a resin material and so on. 
     The cylindrical portion  25  abuts on a hole bottom surface  27  of the cylindrical portion housing hole  20  at one end surface thereof and abuts on an inner surface  14   d  of the screw  14  at the other end surface thereof, which is housed in the cylindrical portion housing hole  20  in a state of being constantly compressed in the shaft center O 2  direction. Accordingly, the elastic restoring force of the cylindrical portion  25  constantly acts on the rack guide  13 , and the rack guide  13  is in a state of being constantly pressed onto the rack shaft  6  side. 
     In the normal state (a large external force is not added to the rack shaft  6  and the pinion gear  5   a  and the rack gear  5   b  are normally engaged), a gap C is set between the stepped surface  22  of the rack guide  13  and the flange portion  26 . The gap C is set to be smaller than a gap D between the other end surface  13   b  of the rack guide  13  and the inner surface  14   d  of the screw  14 . An outer diameter of the flange portion  26  is set to be slightly smaller than an inner diameter of the flange portion housing hole  21  so that a gap E is formed between an outer peripheral surface of the flange portion  26  and an inner peripheral surface of the flange portion housing hole  21 . 
     A liquid body  28  is interposed between the inner peripheral surface of the cylindrical portion housing hole  20  and the outer peripheral surface of the cylindrical portion  25 . The liquid body  28  is applied to, for example, the outer peripheral surface of the cylindrical portion  25  in advance in a stage before the cylindrical portion  25  is housed. 
     “Operation” 
     As shown in  FIG. 2 , in the normal state, or when an external force is added to the rack shaft  6 , the flange portion  26  is not elastically deformed as there exists the gap C between the stepped surface  22  and the flange portion  26  in a state where the pinion gear  5   a  is slightly separated from the rack gear  5   b  within a range in which the stepped surface  22  of the rack guide  13  does not abut on the flange portion  26 . Accordingly, only the elastic restoring force of the cylindrical portion  25  acts on the rack guide  13 , which presses the rack guide  13  onto the rack shaft  6  side. 
     When a large external force is added to the rack shaft  6  and the pinion gear  5   a  is largely separated from the rack gear  5   b  against the biasing force of the cylindrical portion  25 , the stepped surface  22  of the rack guide  13  abuts on the flange portion  26 , and the flange portion  26  is sandwiched between the rack guide  13  and the screw  14  so as to be compressed in the shaft center O 2  direction as shown in  FIG. 3 . Accordingly, the elastic restoring force of the flange portion  26  acts on the rack guide  13  in addition to the elastic restoring force of the cylindrical portion  25 , which presses the rack guide  13  onto the rack shaft  6  side with the large elastic restoring force. An elastic deformation amount of the flange portion  26  in an outward radial direction can be effectively released to the gap E formed between the outer peripheral surface of the flange portion  26  and the inner peripheral surface of the flange portion housing hole  21 . 
     As described above, when the elastic member  15  is configured to have the first elastic portion  23  constantly sandwiched by the rack guide  13  and the screw  14  and the second elastic portion  24  sandwiched by the rack guide  13  and the screw  14  when the rack guide  13  moves to the screw  14  side by a given distance, the following advantages can be obtained. 
     In the normal state or when a small external force is added to the rack shaft  6 , only the elastic restoring force of the first elastic portion  23  acts on the rack guide  13 . Therefore, the rack shaft  6  smoothly slides on the sliding surface  13   a  of the rack guide  13  without a large load, and good steering feeling can be obtained. In an emergency when the large external force is added to the rack shaft  6 , the rack shaft  6  largely separated from the pinion shaft  4   c  can be returned to the pinion shaft  4   c  side smoothly by allowing both the elastic restoring force of the first elastic portion  23  and the elastic restoring force of the second elastic portion  24  to act on the rack guide  13 . 
     Then, the first elastic portion  23  and the second elastic portion  24  are integrally formed, thereby suppressing the increase in the number of parts as well as facilitating the assembly work of the rack guide mechanism  10 . 
     Moreover, the first elastic portion  23  is formed by the cylindrical portion  25 , the second elastic portion  24  is formed by the flange portion  26  and the gap C is set between the rack guide  13  and the flange portion  26  in the normal state, thereby simplifying the shape of the elastic member  15  and the layout structure. As it is not necessary to use the disc spring as in the related art, a disadvantage in which the outer edge of the disc spring is caught by the female screw  11   b  does not occur. 
     When the liquid body  28  (for example, oil and fat such as grease) is interposed between the inner peripheral surface of the cylindrical portion housing hole  20  and the outer peripheral surface of the cylindrical portion  25 , the following advantages can be obtained. When the screw  14  is screwed to the housing  11 , the elastic member  15  is co-rotated as the other end of the elastic member  15  is fixed by pressure to the inner surface  14   d  of the screw  14 . At that time, there is a danger that the cylindrical portion  25  is rubbed on the inner peripheral surface of the cylindrical portion housing hole  20  and is twisted. In response to this, when the liquid body  28  is interposed between the inner peripheral surface of the cylindrical portion housing hole  20  and the outer peripheral surface of the cylindrical portion  25 , the rotational resistance generated in the cylindrical portion  25  can be reduced and generation of twisting of the cylindrical portion  25  can be prevented.