Abstract:
An electric power steering device having a speed reduction gear using a toothed belt and capable of preventing the speed reduction gear from being locked and steering from being disabled even if the toothed belt is broken. A clearance Δ between the inner surface ( 21   x ) of the second housing ( 21   b ) of the speed reduction gear ( 40 ) and the rear surface ( 29   a ) of the toothed belt ( 29 ) applied to a toothed pulley ( 27 ) is formed larger than the tooth depth (H) of the toothed belt ( 29 ) (Δ&gt;II). Thus, even if the toothed belt ( 29 ) is broken, the toothed belt ( 29 ) is not caught between the inner surface ( 21   x ) of the second housing ( 21   b ) and the outer peripheral surface ( 27   a ) of the second toothed pulley ( 27 ), and the toothed pulley ( 29 ) is not locked.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to an electric power steering device and particularly to the electric power steering device including a speed reduction gear using a toothed belt.  
       BACKGROUND ART  
       [0002]     In an electric power steering device adapted to generate auxiliary torque by an electric motor, the magnitude of steering torque generated on a steering shaft is detected, and the electric motor is driven by a control device for controlling a motor current depending on the detected steering torque to supply the auxiliary torque to the steering shaft.  
         [0003]     The device is, for example, so constructed that a pinion connected to the steering shaft meshes with a rack shaft supported to displace in the axial direction, thereby converting the rotation of the steering shaft to the axial movement of the rack shaft to operate the direction of a steered wheel through a wheel steering mechanism, and in the mechanism, a ball screw mechanism is disposed between the electric motor and the rack shaft.  
         [0004]     The ball screw mechanism is composed of a helical groove formed on the outer peripheral surface of the rack shaft, a nut member fitted to the outer periphery of the helical groove and provided with a helical groove formed on the inner peripheral surface, and a number of balls disposed between the two helical grooves. The electric motor is coupled to the nut member through a speed reduction gear, wherein the rotation of the electric motor rotates the nut member of the ball screw mechanism through the speed reduction gear to thereby move the rack shaft in the axial direction, thereby turning the steered wheel through the wheel steering mechanism.  
         [0005]     In this type of electric power steering device, when the rack shaft is displaced from the neutral position in the axial direction by the operation of the steering wheel, the axial load and the load in the shearing direction (in the radial direction) are transmitted to the rack shaft so that bending moment is generated in the rack shaft by the load in the shearing direction to affect the ball screw mechanism.  
         [0006]     That is, since the ball screw mechanism has very low acceptable capacity to the load in the shearing direction and high acceptable capacity to the axial load, when the bending moment is applied to the rack shaft to be deformed, the pitch of the helical groove formed on the rack shaft changes, and when the meshing accuracy of the rack shaft and the pinion is low, the rack shaft is vibrated in the radial direction with the rotation of the pinion. This results in that when excessive load is applied to the balls of the ball screw mechanism to disable the balls from smoothly rotating, the transmission efficiency of turning torque from the electric motor is lowered, and also the durability is lowered to inhibit the improvement in steering feeling.  
         [0007]     As a measure against this, the constitution has been proposed in Japanese Utility Model Publication (B) No. H6-49489 (49489/1994), in which a toothed pulley is coaxially disposed on a rack shaft provided with a helical groove of a ball screw mechanism, the toothed pulley is coupled to a nut of the above ball screw mechanism to be integrally rotated and be displaced in the radial direction, and on the other hand, another toothed pulley is mounted on an electric motor, and a toothed belt is wrapped between the toothed pulley coaxially disposed on the rack shaft and the toothed pulley mounted on the electric motor to thereby transmit the turning force of the electric motor through a speed reducing mechanism and the ball screw mechanism to the rack shaft.  
         [0008]     In the above constitution, excessive load is not applied to the ball screw mechanism so that the ball screw mechanism is smoothly rotated not to lower the transmission efficiency of turning torque from the electric motor and also the disadvantages such as lowering of durability and inhibition of the improvement in steering feeling can be nearly overcome.  
         [0009]     On the other hand, in the electric power steering device including this type of speed reduction gear using the toothed belt, it is expected that a long-time use or application of excessive load will cause breakage of the toothed belt. When the toothed belt is broken, the turning force of the electric motor is not transmitted to the rack shaft to enter the manual steering state. Even in this case, steering for emergency refuge can be performed, so it has been considered that the steering mechanism is probably provided with a failsafe function.  
         [0010]     However, when the broken toothed belt slips out of the toothed pulley to be caught in a space up to a housing, the speed reduction gear is locked. Especially, when the broken toothed belt is caught between the toothed pulley on the driven side, that is, on the rack shaft side and the housing so that the toothed pulley on the driven side is locked, the steering shaft is locked and disabled from steering to cause a very dangerous condition where the failsafe function of the steering mechanism will not work because the toothed pulley on the driven side and the rack shaft interlock with each other through the ball screw mechanism.  
         [0011]     The invention has been made to solve the above problems and provide an electric power steering device including a speed reduction gear using a toothed belt, in which the speed reduction gear is so constructed that at least a clearance gap Δ between the inner peripheral surface of a housing of a driven side rotary member opposite to the toothed belt and the back face side of the toothed belt wrapped round a toothed pulley is larger than the tooth depth H of the toothed belt (Δ&gt;H), whereby even if the toothed belt is broken, the toothed belt is not gripped between the inside surface of the housing and the outer peripheral surface of the toothed pulley to lock the toothed belt and there is no fear of causing disability of steering.  
       DISCLOSURE OF THE INVENTION  
       [0012]     According to the invention, an electric power steering device adapted to supply steering assisting force from a motor to a steering mechanism includes a speed reduction gear in which a toothed belt is stretched between a drive side rotary member mounted on a motor shaft and a driven side rotary member mounted on a steering mechanism, and the speed reduction gear is provided with a housing in which at least the clearance gap between the inner peripheral surface of a housing part for storing the driven side rotary member opposite to the toothed belt and the back face side of the toothed belt is set equal to or larger than the tooth depth of the toothed belt.  
         [0013]     Further, according to the invention, an electric power steering device adapted to supply steering assisting force from a motor to a steering mechanism includes a drive side toothed pulley mounted on a motor shaft, a driven side toothed pulley mounted on a rotary member on the steering mechanism side, and a speed reduction gear formed of a toothed belt stretched between both toothed pulleys, and the speed reduction gear includes a housing in which at least the clearance gap between the inner peripheral surface opposite to the toothed belt wrapped round the driven side toothed pulley and the back face side of the toothed belt wrapped round the toothed pulley is set equal to or larger than the tooth depth of the toothed belt. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  is a cross section for explaining the constitution of the principal part of a rack shaft assembly provided with a belt speed reduction gear in an electric power steering device.  
         [0015]      FIG. 2  is a front section for explaining the principal part of the conventional speed reduction gear using a toothed belt.  
         [0016]      FIG. 3  is a front section for explaining the condition where the toothed belt is broken in the conventional speed reduction gear using the toothed belt shown in  FIG. 2 .  
         [0017]      FIG. 4  is a front section for explaining a speed reduction gear using a toothed belt according to an embodiment of the invention.  
         [0018]      FIG. 5  is a front section for explaining the condition where the toothed belt is broken in the speed reduction gear using the toothed belt according to the embodiment shown in  FIG. 4 .  
         [0019]      FIG. 6  is a diagram for explaining the relationship between the clearance gap Δ between the inside surface of a housing A and the outer peripheral surface of a toothed pulley B and the tooth depth H of a toothed belt C.  
         [0020]      FIG. 7  is a diagram for explaining the condition where the clearance gap Δ between the inside surface of the housing A and the outer peripheral surface of the toothed pulley B is set smaller than the tooth depth H of the toothed belt C (Δ&lt;H) and the toothed belt C is caught there.  
         [0021]      FIG. 8  is a diagram for explaining the constitution in which the clearance gap Δ between the inside surface of the housing A and the outer peripheral surface of the toothed pulley B is set larger than the tooth depth H of the toothed belt C (Δ&gt;H). 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     An embodiment of the invention will now be described.  
         [0023]      FIG. 1  is a cross section for explaining the constitution of the principal part of a rack shaft assembly  10  including a belt speed reduction gear of an electric power steering device according to an embodiment of the invention.  
         [0024]     In  FIG. 1 , the reference numeral  11  is a steering shaft,  12  is a rack shaft, a pinion not shown, which is mounted at the end part of the steering shaft  11 , and the rack shaft  12  mesh with each other to transmit the rotation of the steering shaft  11  to the rack shaft  12 .  
         [0025]     A rack shaft housing  21  is composed of a first housing  21   a  on which a motor flange  24  is mounted, a second housing  21   b  storing first and second toothed pulleys mentioned later, and a third housing  21   c  storing a ball screw mechanism also mentioned later.  
         [0026]     A motor  22  includes a stator and a rotor in the interior not shown, a rotating shaft  22   a  extended from the rotor is rotatably supported by bearings  25   a  and  25   b  disposed in the motor flange  24 , and a first toothed pulley  26  is fitted to the rotating shaft  22   a.    
         [0027]     A second toothed pulley  27 , which is a driven side pulley, is cylindrical, the rack shaft  12  is passed through the interior thereof, and the second toothed pulley  27  is rotatably supported at both end parts in the axial direction of the cylinder by a bearing  28   a  disposed in the first housing  21   a  and a bearing  28   b  disposed in the second housing  21   b.    
         [0028]     A spline groove is formed on the inner peripheral surface of the second toothed pulley  27 , and the spline groove and a spline projection formed on a nut  32  of the ball screw mechanism mentioned later are spline-coupled SP to be movable in the axial direction and relatively unrotatable in the rotating direction.  
         [0029]     The ball screw mechanism  30  is composed of a helical groove  31  formed on the rack shaft  12 , the nut  32  and a number of balls  33  disposed between the helical groove  31  and the nut  32 , and the nut  32  is rotatably held by a bearing  34  disposed in the third housing  21   c.    
         [0030]     The assembling procedure of the rack shaft assembly including the above belt speed reduction gear will be described simply. The bearings  25   a  and  25   b  are disposed in the motor housing  24 , the first toothed pulley  26  is fitted to the motor rotating shaft  22   a  supported by the bearings  25   a  and  25   b , and in this state, the motor housing  24  is mounted on the first housing  21   a . The bearing  28   a  is disposed in the first housing  21   a , thereby rotatably supporting the second toothed pulley  27 , and a toothed belt  29  is wrapped between the first toothed pulley  26  and the second toothed pulley  27 .  
         [0031]     The second housing  21   b  where the bearing  28   a  is mounted is mounted on the first housing  21   a , and the other end of the second toothed pulley  27  supported by the bearing  28   a  is rotatably supported by the bearing  28   b.    
         [0032]     Further, the third housing  21   c  where the ball screw mechanism  30  is assembled is fitted to the second housing  21   b , the rack shaft  12  is passed through the second toothed pulley  27 , and the spline groove of the inside surface of the second toothed pulley  27  and the spline projection of the nut  32  of the ball screw mechanism  30  are spline-coupled SP to complete the rack shaft assembly  10  including a belt speed reduction gear.  
         [0033]     In the above rack shaft assembly  10 , the first toothed pulley  26 , the second toothed pulley  27  and the toothed belt  29  constitute a belt speed reduction gear  40 .  
         [0034]     On the basis of the steering torque of the steering shaft detected by a torque sensor not shown, the driving turning force of the motor  22  driven by a control device also not shown is transmitted through the first toothed pulley  26 , the toothed belt  29  and the second toothed pulley  27  to the nut  32  of the ball screw mechanism  30 . By the rotation of the nut  32 , the rack shaft  12  is moved in the axial direction to turn the wheels so that steering is performed.  
         [0035]     The relationship between the toothed belt and the housing will now be described.  FIG. 2  is a front view showing the constitution of a speed reduction gear  140  using the conventional toothed belt, in which although the relationship between the clearance gap Δ between an inside surface  141   a  of a housing  141  of the speed reduction gear  140  and a back face side  145   a  of a toothed belt  145  wrapped round a toothed pulley  143  and the tooth depth H of the toothed belt  145  will be described later with reference to  FIG. 6 , the above clearance gap Δ is set smaller than the tooth depth H of the toothed belt  145  (Δ&lt;H).  
         [0036]     Consequently, when the toothed belt  145  is broken, as shown in  FIG. 3 , the toothed belt  145  is gripped between the inside surface  141   a  of the housing  141  and the outer peripheral surface of the toothed pulley  143  so that the toothed pulley  143  is locked.  
         [0037]      FIG. 4  is a front section showing the constitution of the speed reduction gear  40  part, which shows a section taken along a line A-A of  FIG. 1 . The clearance gap Δ between the inside surface  21 X of the second housing  21   b  of the speed reduction gear  40  and the back face  29   a  of the toothed belt  29  wrapped round the toothed pulley  27  is larger than the tooth depth H of the toothed belt  29  (Δ&gt;H). Accordingly, when the toothed belt  29  is broken, as shown in  FIG. 5 , the toothed belt  29  is not gripped between the inside surface  21 X of the second housing  21   b  and the outer peripheral surface of the second toothed pulley  27  to prevent the toothed pulley  27  from being locked.  
         [0038]     FIGS.  6  to  8  are diagrams for explaining the relationship between the clearance gap Δ between the inside surface of the housing A and the back face of the toothed belt C wrapped round the toothed pulley B and the tooth depth H of the toothed belt C, and the gripping of the toothed belt C,  FIG. 6  is a diagram for explaining the relationship between the clearance gap Δ between the inside surface of the housing A and the back face of the toothed belt C wrapped round the toothed pulley B and the tooth depth H of the toothed belt C,  FIG. 7  shows the condition where the clearance gap Δ is smaller than the tooth depth H (Δ&lt;H) and the toothed belt C is gripped there, and  FIG. 8  shows the condition where the clearance gap Δ is larger than the tooth depth H (Δ&gt;H) so that the toothed belt C is not gripped.  
         [0039]     In the case where the clearance gap Δ is smaller than the tooth depth H (Δ&lt;H), as shown in  FIG. 7 , the toothed belt C is gripped between the inside surface of the housing A and the outer peripheral surface of the toothed pulley B so that the toothed pulley B is locked. On the other hand, in the case where the clearance gap Δ is larger than the tooth depth H (Δ&gt;H), as shown in  FIG. 8 , the toothed belt C is not gripped between the inside surface of the housing A and the outer peripheral surface of the toothed pulley B so that the toothed pulley B is not locked.  
       INDUSTRIAL APPLICABILITY  
       [0040]     This invention may provide an electric power steering device including the speed reduction gear using the toothed belt, in which since the speed reduction gear is so constructed that at least the clearance gap Δ between the inner peripheral surface of the driven side rotary member opposite to the toothed belt of the housing and the back face side of the toothed belt wrapped round the toothed pulley is larger than the tooth depth H of the toothed belt (Δ&gt;H), even if the toothed belt is broken, the toothed belt is not gripped between the inside surface of the housing and the outer peripheral surface of the toothed pulley so that the toothed pulley is not locked, and there is no fear of disabling steering.