Patent Publication Number: US-7717225-B2

Title: Motor-driven steering assist apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a motor-driven steering assist apparatus. 
   2. Description of the Related Art 
   In a motor-driven steering assist apparatus of a buggy vehicle or the like, as described in Japanese Patent Application Laid-open No. 62-46790 (patent document 1), there is a structure which is interposed between a steering wheel and a tire wheel side steering member, and which assists steering force applied to the steering wheel by a driver on the basis of a torque generated by an electric motor. 
   In the motor-driven steering assist apparatus described in the patent document 1, a piping system applied to a hydraulic motor becomes complicated so as to deteriorate mounting characteristics, and a hydraulic pump and a tank are required so as to make the structure large in size and heavy in weight. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to improve the mounting characteristics and achieve compact size and low weight in a motor-driven steering assist apparatus. 
   In accordance with the present invention, there is provided a motor-driven steering assist apparatus interposed between a steering wheel and a tire wheel side steering member which assists steering force applied to the steering wheel by a driver on the basis of torque generated by an electric motor. The new structure includes an input shaft to which the steering wheel is connected and an output shaft to which the tire wheel side steering member is connected. A torque sensor is provided between the input shaft and the output shaft. An electric motor is driven in correspondence to a detected torque of a torque sensor. A worm gear is coupled to a rotating shaft of the electric motor. A worm wheel which is coupled to the output shaft is engaged with the worm gear. 
   The input shaft, the output shaft, the torque sensor, the electric motor, the worm gear and the worm wheel is installed within a single unit covered by first to third housings. 
   An upper end portion of the input shaft and the torque sensor are supported to the first housing. An upper end portion of the output shaft, the electric motor, the worm gear and the worm wheel are supported to the second housing. A lower end portion of the output shaft is supported to the third housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood from the detailed description given below and from the accompanying drawings which should not be taken to be a limitation on the invention, but are for explanation and understanding only. The drawings: 
       FIG. 1  is an entire schematic view showing a motor-driven steering assist apparatus; 
       FIG. 2  is a front elevational view showing the motor-driven steering assist apparatus; 
       FIG. 3  is a cross sectional view along a line III-III in  FIG. 2 ; and 
       FIG. 4  is a cross sectional view along a line IV-IV in  FIG. 2 . 
       FIG. 5  is a figure illustrating the claimed buggy vehicle. 
       FIG. 6  is a figure illustrating the claimed snowmobile. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As shown in  FIG. 1 , a motor-driven steering assist apparatus  10  is applied to a vehicle, for example, a buggy vehicle, a snow mobile or the like. The apparatus is interposed between a steering wheel side steering member  1  and a tire wheel side steering member  2 , and assists steering force applied to the steering wheel by a driver on the basis of torque generated by an electric motor  24 . 
   The motor-driven steering assist apparatus  10  includes a single unit body  10 A covered by first to third housings  11  to  13 , as shown in  FIGS. 1 to 4 . The unit body  10 A has an input shaft  21 , an output shaft  22 , a torque sensor  23 , an electric motor  24 , a worm gear  25  and a worm wheel  26  built-in. 
   The motor-driven steering assist apparatus  10  includes an upper end portion of the input shaft  21  to which the steering wheel side steering member  1  is connected by a connecting device  1 A which is supported to the first housing  11  by a bearing  31  ( FIG. 3 ). Upper and lower end portions of the output shaft  22  to which the tire wheel side steering member  2  is connected by a connecting device  2 A are supported to the second housing and the third housing  13  by upper and lower bearings  32 A and  32 B ( FIG. 3 ). The input shaft  21  is provided with a serration  21 A for connection to the connecting device  1 A in an upper end outer peripheral portion, and the output shaft  22  is provided with a serration  22 A for connection to the connecting device  2 A in a lower end outer peripheral portion. A torsion bar  27  is inserted in to a hollow portion of the input shaft  21 . One end of the torsion bar  27  is coupled to the input shaft  21  by a coupling pin  27 A, and the other end of the torsion bar  27  is inserted to a hollow portion of the output shaft  22  so as to be connected by serration. 
   A torque sensor  23  is provided with two detecting coils  23 A and  23 B surrounding a cylindrical core  23 C engaged with the input shaft  21  and the output shaft  22 , in the first housing  11 , as shown in  FIG. 3 . The core  23 C is provided with a vertical groove  23 E engaging with a guide pin  23 D of the output shaft  22  so as to be movable only in an axial direction, and is provided with a spiral groove  23 G engaging with a slider pin  23 F of the input shaft  21 . 
   Accordingly, when steering torque applied to the steering wheel is applied to the input shaft  21 , and a relative displacement in a rotation direction is generated between the input shaft  21  and the output shaft  22  on the basis of an elastic torsional deformation of the torsion bar  27 , the displacement in the rotation direction of the input shaft  21  and the output shaft  22  displaces the core  23 C in an axial direction, and an inductance of the detecting coils  23 A and  23 B caused by a magnetic change around the detecting coils  23 A and  23 B due to the displacement of the core  23 C is changed. When the core  23 C moves close to the input shaft  21 , the inductance of the detecting coil  23 A to which the core  23 C moves close is increased, and the inductance of the detecting coil  23 B from which the core  23 C moves apart is reduced, whereby it is possible to detect steering torque on the basis of the change of the inductance. 
   The electric motor  24  is attached and supported to the second housing  12  by a mounting bolt  28 , and is driven by a controller (not shown) in correspondence to the detected torque of the torque sensor  23 . A worm gear  25  is coupled to a rotation shaft  24 A of the electric motor  24  by a joint  24 B, and the worm wheel  26  engaging with the worm gear  25  is fixed to the output shaft  22 . The worm gear  25  is supported at both ends to the second housing  12  by right and left bearings  41  and  42 , as shown in  FIG. 4 . The worm wheel  26  is fixed to the output shaft  22  just below an upper bearing  32 A in the output shaft  22 , in an inner portion of the second housing  12 . 
   In this case, the joint  24 B coupling the rotation shaft  24 A of the electric motor  24  and the worm gear  25  is structured such that a torque limiter  24 C constituted by an elastic ring is interposed in a fitting gap between both of the elements ( FIG. 4 ). The torque limiter  24 C keeps coupling the rotation shaft  24 A and the joint  24 B under normal torque conditions of the motor-driven steering assist apparatus  10 , makes them slip under abnormal torque conditions, and does not transmit the torque of the electric motor  24  to a side of the joint  24 B. 
   Accordingly, in the motor-driven steering assist apparatus, an integral unit body  10 A is structured by supporting the upper end portion of the input shaft  21  and the torque sensor  23  to the first housing  11 , supporting the upper end portion of the output shaft  22 , the electric motor  24 , the worm gear  25  and the worm wheel  26  to the second housing  12 , supporting the lower end portion of the output shaft  22  to the third housing  13 , coupling the first housing  11  and the second housing  12  by the mounting bolt  14 , and coupling the second housing  12  and the third housing  13  by the mounting bolt  15  ( FIG. 3 ). An oil seal  33  is attached in a sealing manner to an upper opening portion of the bearing  31  in the first housing  11 , and an oil seal  34  is attached in a sealing manner to a lower opening portion of the bearing  32 B in the third housing  13  ( FIG. 3 ). 
   In accordance with the motor-driven steering assist apparatus  10 , the steering torque applied to the steering wheel is detected by the torque sensor  23 , the electric motor  24  is driven by the detected torque, and the torque generated by the electric motor  24  is transmitted to the output shaft  22  via the worm gear  25  and the worm wheel  26 . Accordingly, the generated toque of the electric motor  24  can be used as an assist force with respect to the steering force which the driver applies to the steering wheel. 
   In accordance with the present embodiment, the following operations and effects can be achieved.
         (a) The motor-driven steering assist apparatus  10  which includes the single unit body  10 A, can be easily attached to the applied vehicle or the like by connecting the input shaft  21  to the steering wheel side steering member  1  and connecting the output shaft  22  to the tire wheel side steering member  2 , and resultingly has a high general-purpose application to various vehicles or the like.   (b) Since the torque sensor  23  is provided in the first housing  11 , and the worm wheel  26  and the like are provided in the second housing  12 , it is easy to avoid grease which has been applied to the worm wheel  26  and the like from entering into the side of the torque sensor  23 .   (c) Since the upper end portion of the output shaft  22  is supported to the second housing  12 , and the lower end portion of the output shaft  22  is supported to the third housing  13 , it is possible to secure a distance between the bearing  32 A in the upper end portion of the output shaft  22  and the bearing  32 B in the lower end portion, and it is possible to stably support the output shaft  22 .       

   As heretofore explained, embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configurations of the present invention are not limited to the illustrated embodiments but those having a modification of the design within the range of the presently claimed invention are also included in the present invention. 
   Although the invention has been illustrated and described with respect to several exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made to the present invention without departing from the spirit and scope thereof. Therefore, the present invention should not be understood as limited to the specific embodiment set out above, but should be understood to include all possible embodiments which can be encompassed within a scope of equivalents thereof with respect to the features set out in the appended claims.