Abstract:
To provide the electric motor assist type power steering apparatus, which can prevent the aggravation of the feeling of the steering, and which can transmit the assist steering force brought by the electric motor to the steering shaft completely, by preventing the flexure of the worm shaft of the electric motor assist type power steering apparatus, preferably the pinion assist type electric power steering apparatus.  
     To attain these object, there is provided an electric power steering apparatus comprising; an electric motor for adding the assist steering force to the steering system, a controller which drives said electric motor, a rack shaft which steers a steered wheel by displacing along the axial direction thereof, a pinion shaft which is engaged with said rack shaft through the rack-and-pinion mechanism, a worm shaft which is rotated by the electric motor, and a worm wheel which is engaged with said worm shaft and is integrally arranged with the pinion shaft, one end part of the worm shaft is joined with the electric motor, and holding means, which holds one end part and another end part of the worm shaft on allowing the rotation thereof, is further arranged.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an electric motor assist type power steering apparatus, which assists the steering effort of a driver by making the electric motor power act on a steering system directly.  
           [0003]    2. Prior Art  
           [0004]    An electric motor assist type power steering apparatus (hereinafter defined as an electric power steering apparatus) assists the steering force of a driver by making use of the driving force of an electric motor directly. A vehicle, on which an electric power steering apparatus is mounted, is widely spreads, and according to the electric poser steering apparatus, since a movement of the steering becomes light, a driver operates the steering without strong force.  
           [0005]    As an example of the electric power steering apparatus, a pinion assist type electric power steering apparatus, in which a worm shaft and a worm wheel are used as the means for transmitting the driving force of an electric motor, is known. The electric power steering apparatus, disclosed in the Japanese patent unexamined patent publication H11-43062, is one example thereof.  
           [0006]    In this electric power steering apparatus, as shown in FIG. 8, an electric motor  51  for adding an assist steering force to a steering shaft, which is rotated by the operation of the steering, is provided. The worm shaft  52  is connected to the electric motor  51 . A pinion shaft  53  is joined with the steering shaft. The worm wheel  54 , which is engaged with a worm gear  52 A of the worm shaft  52 , is fixed to a pinion shaft  53  so that it will be in the coaxial location with respect to the pinion shaft  53 .  
           [0007]    Both end parts  52 B and  52 C of the worm shaft  52  are held by ball bearings  55  and  56 , respectively. The assist steering force brought by the electric motor  51  is add to the pinion shaft  53  through the worm gear  52 A and the worm wheel  54  which are engaged together. By adding this assist steering force, the steering force required for steering operation of a driver is decreased.  
           [0008]    In the electric power steering apparatus  50  disclosed in the above described Japanese unexamined patent application, however, both end parts of the worm shaft  52  are rotatably held only by the ball bearing  55  and  56  with backlash.  
           [0009]    Since the flexural rigidity (flexural resistance) of the worm shaft  52  is not so high, the flexure of the worm shaft  52  tends to be arisen. When the steering is turned back in the other way while turning in the one direction, since the steering torque sensed by the torque sensor (not shown) decreases, the electric motor  51  is controlled so that the assist steering force will be decreased.  
           [0010]    In this case, the steering must be turned back with smoothly by rotating the electric motor  51  through the worm wheel  54  and worm shaft  52 . If the flexure is arising on the worm shaft  52 , however, since the bending moment affected to the rotation shaft of the worm gear  52 A is decreased by the decreasing of the assist steering force brought by the electric motor  51 , the restorative deformation is arisen for correcting the flexure.  
           [0011]    Since this restorative deformation acts as an undesirable force and inhibits the rotation of the worm gear  52 A, the rotation force from the worm wheel  54  is not smoothly transmitted to the electric motor  51 . Thus, the mismatch on the suitable relationship among the torque sensor (not shown), the control unit, and the electric motor  51  is arisen. When the moment of the inertia is add thereto, mismatch is further promoted. Then, the turning back of the steering is disturbed and thus the response of the steering becomes worse.  
           [0012]    In the electric power steering apparatus  50 , moreover, the clearance which exceeds a predetermined clearance value may be arose between the worm gear  52 A and the worm wheel  54  as the occurrence of the flexure of the worm shaft  52 . When such clearance arises, the dispensable backlash between the worm gear  52 A and the worm wheel  54  becomes larger. Thus, the assist steering force brought by the electric motor  51  cannot be transmitted to the steering shaft completely.  
           [0013]    These disadvantages caused by the flexure of the worm shaft  52  will be remarkable when the assist steering force brought by the electric motor  51  becomes large.  
         SUMMARY OF THE INVENTION  
         [0014]    This is therefore, the present invention aim at providing the electric motor assist type power steering apparatus, preferably the pinion assist type electric power steering apparatus, which can prevent the aggravation of the feeling of the steering by preventing the flexure of the worm shaft, and which can transmit the assist steering force of the electric motor to the steering shaft completely.  
           [0015]    For attaining these problems, there is provided an electric power steering apparatus comprising; an electric motor for adding the assist steering force to the steering system, a controller which drives said electric motor, a rack shaft which steers a steered wheel by displacing along the axial direction thereof, a pinion shaft which is engaged with said rack shaft through the rack-and-pinion mechanism, a worm shaft which is rotated by the electric motor, and a worm wheel which is engaged with said worm shaft and is integrally arranged with the pinion shaft, one end part of the worm shaft is joined with the electric motor, and holding means, which holds one end part and another end part of the worm shaft on allowing the rotation thereof, is further arranged.  
           [0016]    In the electric power steering apparatus according to the present invention, one end part and another end part of the worm shaft are held without play. The worm shaft, in the conventional manner, is held with play, thus the flexural rigidity is not so high and the flexure of the worm shaft is easily arisen. On the other hand, the worm shaft, in the present invention, is held without play, thus the worm shaft can be held with high flexural rigidity.  
           [0017]    Accordingly, by preventing the occurrence of the flexure of the worm shaft, not only the occurrence of the mismatch caused by the flexure of the worm shaft at the time of turning back of the steering but also the occurrence of the aggravation of the steering feeling are prevented. Thus, the certain transmission of the steering force brought by the electric motor to the pinion shaft can be achieved.  
           [0018]    In the electric power steering apparatus, preferably, said holding means is composed of plural ball bearings which hold one end part of the worm shaft, and plural ball bearings which hold the another end part of the worm shaft.  
           [0019]    According to this electric power steering apparatus, both end parts of the worm shaft are held without play by plural of ball bearings. Thus, the holding of the worm shaft can be carried out on keeping the rotating condition at the both ends of the worm shaft into the smooth condition. And also the holding of the worm shaft can be carried out using ball bearings, which are general-purpose parts without using the specific parts. Therefore, the contribution to a cost reduction may be attained.  
           [0020]    In the electric power steering apparatus, preferably, said holding means is composed of plural ball bearings which hold the one end part of the worm shaft, and a needle bearing which holds another end par of the worm shaft.  
           [0021]    According to this electric power steering apparatus, one end part side of the worm shaft joined with the electric motor is held by the plural of ball bearings. Since one end part of the worm shaft is joined with the electric motor, flexural rigidity in the one end part of the worm shaft is comparatively high, and that of in the another end part becomes low a little.  
           [0022]    In the present electric power steering apparatus, therefore, another end part of the worm shaft is held by a needle bearing without play. When the worm shaft is held by the needle bearing without play, since the flexural resistance of the worm shaft in the diameter direction is higher than the worm shaft held by the plural of ball bearings, the flexural rigidity along the worm gear whole may be higher. Thus, the occurrence of the flexure of the worm shaft is thus prevented, and also the occurrence of the feeling gap between in the case the steering is turned back in the anticlockwise direction and in the case the steering is turned back in the clockwise direction can be decreased. Thus, not only the prevention of the aggravation of steering feeling but also the certain transmission of the steering force brought by the electric motor  7  to the pinion shaft  3  are achieved.  
           [0023]    In the present invention, furthermore, there is provided an electric power steering apparatus comprising; an electric motor for adding the assist steering force to the steering system, a controller which drives said electric motor, a rack shaft which steers a steered wheel by displacing along the axial direction thereof, a pinion shaft which is engaged with said rack shaft through the rack-and-pinion mechanism, a worm shaft which is rotated by the electric motor, a worm wheel which is engaged with said worm shaft and is integrally arranged with the pinion shaft, end part holding means which holds the one end part of the worm shaft on allowing the rotation of the worm shaft, and center supporting means which holds the center part in the longitudinal direction of the worm shaft on allowing the rotation of the worm shaft, wherein one end part of said worm shaft is joined with the electric motor.  
           [0024]    According to this electric power steering apparatus, about the center part in the longitudinal direction of the worm shaft is supported on allowing the rotation of the worm shaft. Thus, the worm shaft can be made into the rotatable condition, and the occurrence of the flexure of the worm shaft can be prevented. This is therefore, the aggravation of the feeling of the steering is prevented, and the steering force brought by the electric motor can be transmitted to the pinion shaft certainly. According to the center supporting means, the worm shaft is held without arranging the holding means at one end part of the worm shaft, the assembling efficiency of the apparatus as a whole is improved.  
           [0025]    In the electric power steering apparatus, preferably, said center supporting means has urging means which gives the urging force towards the engaging part between the worm shaft and the worm wheel from the opposite direction with respect to the engaging part.  
           [0026]    According to the present invention, the urging means which urges the worm shaft to the engaging part side with respect to the worm wheel is arranged. Since the flexure of the worm shaft is prevented, and the worm shaft is pressed to the worm wheel with sufficient force, the clearance between the worm shaft and the worm wheel is maintained within the predetermined range. Thus, the occurrence of the unpleasant backlash between the worm gear and the worm wheel can be prevented, and the certain transmission of the rotation of the worm shaft to the worm wheel can be achieved.  
           [0027]    In the electric power steering apparatus, preferably, said center supporting means has a first roller and a second roller which are touched with the worm shaft and press the worm shaft toward the engaging part between the worm shaft and the worm wheel.  
           [0028]    According to the present invention, the first roller and the second roller, which are fitted with the worn shaft, is applied to the engaging part between the worm shaft and the worm wheel. Thus, the displacement in the ups-and-downs directions of the worm shaft is restricted, and the worm shaft is applied to the engaging part between the worm shaft and the worm wheel. Since the roller is used, the supporting mechanism with slightest fiction can be obtained. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIG. 1 is a whole block diagram of the electric power steering apparatus according to the first preferred embodiment of the present invention.  
         [0030]    [0030]FIG. 2 is a plan view of the substantial part of the electric power steering apparatus according to the first preferred embodiment of the present invention.  
         [0031]    [0031]FIG. 3 is a sectional view along the line X-X in FIG. 2.  
         [0032]    [0032]FIG. 4A is an explaining view of the model of the beam both ends part of which are held without play, and the bending moment applied to the beam.  
         [0033]    [0033]FIG. 4B is an explaining view of the model of the beam both ends part of which are held with play, and the bending moment applied to the beam.  
         [0034]    [0034]FIG. 2 is a plan view of the substantial part of the electric power steering apparatus according to the second preferred embodiment of the present invention.  
         [0035]    [0035]FIG. 6 is a plan view of the substantial part of the electric power steering apparatus according to the third preferred embodiment of the present invention.  
         [0036]    [0036]FIG. 6 is a sectional view along the line Y-Y in FIG. 2.  
         [0037]    [0037]FIG. 8 is a plan exploded view of the substantial part of the conventional electric power steering.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0038]    The preferred embodiments of the present invention will now be described by referring to the attached drawings. FIG. 1 is a block diagram of the electric power steering apparatus according to the present invention. FIG. 2 is a plan view of the principal part of the electric power steering apparatus. FIG. 3 is a sectional view along the line X-X in FIG. 2.  
         [0039]    As shown in FIG. 1, an electric power steering apparatus  1  according to the present invention has a steering  2 . The steering  2  is connected to a pinion shaft  4  through a steering shaft  3 . A torque sensor  5  and a torque transmitter  6  are attached to the pinion shaft  4 . The torque sensor  5  detects the steering torque add to the steering system. The torque transmitter  6  is an assistor, and connected to an electric motor  7 , which adds the assists steering force to the steering system.  
         [0040]    A pinion  4 A, arranged at the bottom part of the pinion shaft  4 , is engaged with a gear rack  8 A provided on a rack shaft  8 . In this construction, the rotation of the pinion shaft  4  is converted into the displacement in the longitudinal direction with respect to the rack shaft  8 , and then a steered wheel  9  and  9  are steered. The torque sensor  5  is connected to a control unit  10 , and outputs the toque signal to the control unit  10 . The control unit  10  computes the assist steering force using at least the torque signal T outputted form the torque sensor  5 , and outputs the electric motor control signal VO to the electric motor  7 , and thus the electric motor  7  is controlled.  
         [0041]    As shown in FIG. 2 and FIG. 3, the torque transmitter  6  has a worm wheel  11 , which is fixed to the pinion shaft  4  so that it might be in the coaxial location with respect to the pinion shaft  4 . A worm shaft  12  is joined to the rotation shaft of the electric motor  7  through the coupling and the like. A worm gears  12 A provided on the worm shaft  12  is engaged with the worm wheel  11 .  
         [0042]    When the worm shaft  12  is rotated by the actuation of the electric motor  7 , the worm wheel  11  engaged with the worm gears  12 A is rotated, and thus the pinion shaft  4  is rotated along with the rotation of the worm wheel  11  through the mechanism such as a planetary gear and the like. The assist steering force (steering force) brought by the electric motor  7  is transmitted to the pinion shaft  4 , and then transmitted to the steering shaft  3  through the pinion shaft  4 .  
         [0043]    One end part  12 B of the worm shaft  12  is held by a first ball bearing  14 A and a second ball bearing  14 B, which are arranged in the adjoining location along the longitudinal direction of the worm shaft  12 , on allowing the rotation of the worm shaft  12 .  
         [0044]    The ball bearings  14 A and  14 B are a holding means, which holds the one end part  12 B of the worm shaft  12  with sufficient flexural rigidity. In other words, the holding means restricts the flexure of the worm shaft  12 .  
         [0045]    Another end part  12 C of the worm shaft  12 , furthermore, is held by a first ball bearing  15 A and a second ball bearing  15 B, which are arranged in the adjoining location along the longitudinal direction of the worm shaft  12 , on allowing the rotation of the worm shaft  12 .  
         [0046]    The ball bearing  15 A and  15 B are also a holding means, which holds the one end part  12 C of the worm shaft  12  with sufficient flexural rigidity. In other words, the holding means restricts the flexure of the worm shaft  12 .  
         [0047]    The location interval L 1  between the ball bearing  14 A and  14 B and the location interval L 2  between the ball bearing  15 A and  15 B are same. The location interval L 1  and the location interval L 2  are established as long as possible so that it can restrict the flexure of the worm shaft  12  efficiently.  
         [0048]    The explanation about the function and the operation of the electric power steering apparatus having the above-described construction will be carried out.  
         [0049]    When the driver operates the steering  2  shown in FIG. 1, the torque sensor  5  detects the steering torque. The steering torque detected by the torque sensor  5  is output to the control unit  10  as a torque signal T.  
         [0050]    In the control unit  10 , the assist steering force, which is brought by the electric motor  7  and add to the steering system, is computed using at least the steering torque represented by the torque signal T on considering the factors such as traveling speed and the steering angle.  
         [0051]    Based on this computed assist steering force, the electric motor control signal Vo is output to the electric motor  7  from the control unit  10 .  
         [0052]    The electric motor  7  is driven by the electric motor control signal Vo, and rotates the worm shaft  12  of shown in FIG. 2. One end part  12 B of the worm shaft  12  is held by the ball bearing  14 A and  14 B while the worm shaft  12  is rotating. Another end part  12 C of the worm shaft  12  is also held by the ball bearing  15 A and  15 B. Thus, both end parts of the worm shaft  12  is in the held condition by holding both end part  12 B and  12 C of the worm shaft  12  on allowing the rotation of the worm shaft.  
         [0053]    The explanation about the flexural resistance of the worm shaft  12  will be carried out on considering the worm shaft  12  as a beam.  
         [0054]    [0054]FIG. 4A is an explaining view of the bending moment in the case that the force P is applied to the center in the longitudinal direction of a beam from the upper direction, wherein both end parts of the beam are rigidly held without play (play means it has a loose fit).  
         [0055]    [0055]FIG. 4A is an explaining view of the bending moment in case that the force P is applied to the center in the longitudinal direction of a beam from the upper direction, wherein both end parts of the beam are held with play (play means it has a loose fit).  
         [0056]    As shown in FIG. 4A, in the case of the beam H, which has a length l and both end parts HA and HB of which are rigidly held without play, the bending moment add thereto is Pl/8.  
         [0057]    As shown in FIG. 4B, in the case of the beam H, which has a length l and both end parts HA and HB of which are held with play, the bending moment add thereto is Pl/4.  
         [0058]    The bending moment applied to the beam H, both end parts HA and HB of the which are rigidly held without play, becomes one half in the case that both end parts HA and HB of the beam H are supported with play. Thus, when both end parts HA and HB of the beam H are rigidly held without play, the bending moment might be smaller than the case where both end parts HA and HB are supported with play.  
         [0059]    When the force P is applied to the center in the longitudinal direction of the beam H from the upper direction, wherein both end parts of which HA and HB are rigidly held without play, the maximum flexural amount (δ max) of the beam H is formulated as formula (1).  
         δ  max=Pl   3 /192 EI   z    (1)  
         [0060]    P: the force added to the beam H  
         [0061]    l: the length of the beam H  
         [0062]    E: Young&#39;s modulus  
         [0063]    I z : geometrical moment of inertia  
         [0064]    On the contrarily, when the force P is applied to the center in the longitudinal direction of the beam H from the upper direction, wherein both end parts of which HA and HB are held with play, the maximum flexural amount (δ max) of the beam H is formulated as formula (2).  
         δ  max=Pl   3 /48 EI   z    (2)  
         [0065]    P: the force applied to the beam H  
         [0066]    l: the length of the beam H  
         [0067]    E: Young&#39;s modulus  
         [0068]    I z : geometrical moment of inertia  
         [0069]    Therefore, the flexural amount of the beam H, wherein both end parts are rigidly held without play becomes to one fourth with respect to the case where both end parts are held with play. As described above, when both end parts HA and HB of the beam H are rigidly held without play, the flexural amount of the beam H can be smaller than that of the beam H, both end parts of which are held with play. The beam with sufficient flexural rigidity and with high flexure resistance can be supplied when both end parts of the beam are held without play.  
         [0070]    Therefor, when both end parts of the beam H are rigidly held, the holding of the beam member can be carried out with sufficient rigidity than the case where the both end parts are supported with play, and thus the maximum flexural amount of the beam H can be smaller.  
         [0071]    As for the electric power steering apparatus  1  according to the present preferred embodiment, the worm shaft  12  is rigidly held by the ball bearing  14 A,  14 B,  15 A and  15 B without play. Since the same reason as described in the case of beam H can be applicable, the worm shaft  12  according to the present invention can be held with sufficient flexural rigidity as compared to the conventional holding manner.  
         [0072]    When the force P is applied to the center in the longitudinal direction of the worm shaft  12  as a result of the actuation of the electric motor  7 , therefore, the bending moment becomes to one half and the maximum bending amount becomes one fourth. Thus, the flexure of the worm shaft  12  is efficiently prevented.  
         [0073]    Then, the occurrence of the mismatch, which is caused by the flexure of the worm shaft at the time of the steering wheel is turning back in the reverse direction, and the aggravation of the steering responsibility in the electric power steering apparatus  1  are sufficiently prevented. Thus, the transmission of the assist steering force brought by the electric motor  7  to the steering shaft  3  can be achieved.  
         [0074]    In the present embodiment, furthermore, due to the weight of the electric motor  7 , the difference between the flexural rigidity at one end part  12 B and at the another end part  12 C of the worm shaft  12  may be arisen.  
         [0075]    When the rotation axis of the electric motor  7  and the worm shaft  12  are joined firmly, for example, the flexural rigidity at the one end part  12 B of the worm shaft  12  is higher than another end part  12 C of the worm shaft  12  (flexural angle becomes smaller).  
         [0076]    If the rotation axis of the electric motor  7  and the worm shaft  12  are joined firmly, it is acceptable that the location interval L 2  between the ball bearing  15 A and  15 B, which support another end part  12   c  of the worm shaft  12 , is established wider than the location interval L 1  between ball bearing  14 A and  14 B.  
         [0077]    To be more precise, the flexural rigidity of another end part  12   c  side can be stronger than that of one end part  12 B side, by establishing the interval between the ball bearing  15 A and  15 B wider than that of between the ball bearing  14 A and  14 B. Therefore, since one end part  12 B and another end part  12 C are rigidly held with the uniform flexural rigidity depending on the strength of the connecting part between the rotation axis of the electric motor  7  and the worm shaft  12 , the prevention of the flexure of the worm shaft  12  can be achieved more certainly.  
         [0078]    The second preferred embodiment according to the present invention will be described. FIG. 5 is a plan sectional view showing a substantial part of the electric power steering apparatus according to the present invention.  
         [0079]    In an electric power steering apparatus  20  according to the present preferred embodiment, only the construction of the torque transmitter is differing from the electric power steering apparatus  1 . Thus, in the following explanation, the explanation is mainly carried out about the different components, and the same components as that of explained in the first embodiment is emitted and indicates as the same symbol.  
         [0080]    As shown in FIG. 5, a torque transmitter  21  according to the present second preferred embodiment of the electric power steering apparatus  20 , has a worm wheel  11  fixed to the pinion shaft  4  so that it might be in the coaxial location with respect to the pinion shaft  4 .  
         [0081]    A worm gear  12  is joined to the electric motor  7 . The worm gears  12 A provided to the worm shaft  12  is engaged with the worm wheel  11 .  
         [0082]    When the worm shaft  12  is rotated by the actuation of the electric motor  7 , the worm wheel  11  engaged with the worm gears  12 A is rotated, and then the pinion shaft  4  is rotated along with the rotation of the worm wheel  11 . The assist steering force brought by the electric motor  7  is transmitted to the pinion shaft  4 . These compositions are same as that of disclosed in the first preferred embodiment.  
         [0083]    One end part  12 B of the worm shaft  12  is held by the ball bearing  14 A and  14 B which are arranged in the adjoining location along the longitudinal direction of the worm shaft  12 .  
         [0084]    On the other hand, another end part  12 C of the worm shaft  12  is held by a needle bearing  22  on allowing the rotation around the axis. The electric power steering apparatus  20  according to the present preferred embodiment differs in that the another end part  12 C of the worm shaft  12  is supported without play on allowing the rotation.  
         [0085]    As for the worm shaft  12  in the electric power steering apparatus  20  according to the present preferred embodiment, one end part  12 B of the worm shaft  12  is held without play by the two of ball bearings  14 A and  14 B, and another end part  12 C of the worm shaft  12  is held without play by the needle bearing  22 .  
         [0086]    As for the needle bearing  22 , since needle rollers (located at inner-side and outer-side in figure) are contacting with the worm shaft  12  along the longitudinal direction of the worm shaft  12 , wide range of the worm shaft  12  is held by the needle roller.  
         [0087]    Thus, flexural resistance in the diameter direction at the another end part  12 C of the worm shaft  12  where is supported by the needle bearing  22  is higher than one end part. Therefore, since another end part  12 C is held by the needle bearing  22 , the flexural rigidity at the another end part  12 C of the worm gear  12  is also higher than the one end part  12 B where the worm gear  12  is held by the ball bearing  14 A and  14 B.  
         [0088]    The flexural rigidity at one end part  12 B and another end part  12 C of worm shaft  12  can be uniform by holding the another ends part  12 C using the needle bearing  22 , which gives the superior flexural resistance, even if the rigidity at the one ends part  12 B becomes high as a result of the firmly connection between the rotating shaft of the electric motor  7  and the worm shaft  12 .  
         [0089]    The occurrence of the flexure of the worm shaft is thus prevented, and also the occurrence of the feeling gap between in the case the steering is turned back in the anticlockwise direction and in the case the steering is turned back in the clockwise direction can be decreased. Thus, not only the prevention of the aggravation of steering feeling but also the certain transmission of the steering force brought by the electric motor  7  to the pinion shaft  3  are achieved.  
         [0090]    The third preferred embodiment of the present invention will be explained. FIG. 6 is a plan sectional view of the substantial part of the electric power assist type steering apparatus according to the present invention. FIG. 7 is sectional view along the line Y-Y in FIG. 6.  
         [0091]    In the electric power steering apparatus  30  according to the present preferred invention, only the construction of the torque transmitter is differing from the electric power steering apparatus  1 . Thus, in the following explanation, the explanation is mainly carried out about the different components, and the same components as that of explained in the first embodiment is emitted and indicates as the same symbol.  
         [0092]    As shown in FIG. 6, a torque transmitter  21  according to the present second preferred embodiment of the electric power steering apparatus  30 , has a worm wheel  11  fixed to the pinion shaft  4  so that it might be in the coaxial condition with the pinion shaft  4 .  
         [0093]    The worm gear  12  is joined to the electric motor  7 , and a worm gear  12 A provided thereto is engaged with the worm wheel  11 .  
         [0094]    When the worm shaft  12  is rotated by the actuation of the electric motor  7 , the worm wheel  11  engaged with the worm gear  12 A is rotated, and then the pinion shaft  4  is rotated along with the worm wheel  11 . The assist steering force brought by the electric motor  7  is transmitted to the pinion shaft  4 . These compositions are same as that of disclosed in the first and second preferred embodiment.  
         [0095]    One end part  12 B of the worm shaft  12  is held by a ball bearing  32  in the condition the rotation of the worm shaft  12  might be allowed. Another end part  12 C is also held by a ball bearing  33  in the condition where the rotation of the worm shaft  12  might be allowed.  
         [0096]    A central holding means  35  is arranged at the almost central part in the longitudinal direction of the worm shaft  12 . The central holding means  35  is positioned so that the worm shaft  12  might be sandwiched between the worm wheel  11  and the central holding means  35 , and has a urging means  34  which crowds the worm shaft  12  toward the direction of the worm wheel  11 .  
         [0097]    According to this urging means  34 , the worm shaft  12  is pressed toward the worm wheel  11  from the opposite direction side with respect to the engaging part where the worm gear  12 A and worm wheel  11  are engaged together. This urging means  34  is arranged for pressing the worm shaft  12  to the engaging part of the worm wheel  11 , and is composed of a spring  36 , an upper roll  37 , a lower roll  38 , and a roller holder  39  (see FIG. 7).  
         [0098]    As shown in FIG. 7, the urging means  34  has the upper roll  37  and the lower roll  38  which are arranged at the opposite side with respect to the engaging part where the worm wheel  11  and the worm gear  12 A of the worm shaft  12  are engaged together. The urging means  34  also has a roller holder  39 , which rotatably supports the upper roll  38  and the lower roll  39 , and the spring  36 , which press the roller holder  39  towards the worm shaft  12  urgently.  
         [0099]    The upper roll  37  and the lower roll  38  are fit with the worm gear  12 A, respectively, and transmit the urgent force brought by the spring  36  to the worm gear  12 A. The rotation shafts of the upper roll  37  and the lower roll  38  are supported by the roller folder  39  and thus the movement in the ups-and-downs directions thereof is restricted.  
         [0100]    In the electric power steering apparatus  30  according to the present preferred embodiment, about the center part in the longitudinal direction of the worm shaft  12  is supported by the center holding means  35 . The flexural degree of the worm shaft  12 , conventionally, tends to be maximum at the central part in the longitudinal direction of the worm shaft  12 . As for the present preferred embodiment, since the worm shaft  12  is supported by the center holding means  35 , the occurrence of the flexure of the worm shaft  12  is efficiently prevented. As a result of this prevention, the aggravation arose at the time of turning back of the steering is prevented, and thus the prevention of the aggravation of the steering feeling and the certain transmission of the steering force by the electric motor to the pinion shaft can be achieved.  
         [0101]    In the present invention, the central part between one end part and the another end part of the worm shaft  12  is supported, the scattering in the flexural resistance in the longitudinal direction of the worm shaft  12  can be smaller. Thus, the gaps in the steering feeling depending on the turning direction of the steering can be smaller.  
         [0102]    As for the present invention, furthermore, since the center holding means  35  has the urging means  34 , the urgent force brought by the spring  36  is transmitted to the worm shaft  12  through the upper roll  37  and the lower roll  38 . Accordingly, the clearance between the worm gear  12 A and the worm wheel  11  is maintained within the predetermined clearance even if the clearance, which exceeds a predetermined clearance value, come close to arising as a result of the occurrence of slight flexure of the worm gear  12 .  
         [0103]    The worm gear  12 A and the worm wheel  11  are certainly engaged together by the center holding means  35 , the unpleasant backlash between the worm gear  12 A and the worm wheel  11  thus can be prevented. Then, the transmission of the assist steering force brought by the electric motor  7  to the worm wheel  11  through the worm shaft  12  can be achieved certainly.  
         [0104]    Since the urgent force brought by the spring  36  is transmitted to the worm shaft  12 A through the upper roll  37  and the lower roll  38  which are being rotatable, the rotation of the worm shaft  12  is carried out without any restriction.  
         [0105]    The displacement in the ups-and-downs directions of the upper roll  37  and the lower roll  38 , which are applied to the worm shaft  12 , are restricted, furthermore, the displacement in the ups-and-downs directions of the worm gear  12  is also prevented.  
         [0106]    According to this prevention, the flexure in the ups-and-downs directions of the worm gear  12  is also prevented. Thus, the aggravation of the steering feeling is prevented, and also the steering force, which is brought by the electric motor, is certainly transmitted to the pinion shaft. According to the supporting mechanism of the present invention, since the worm gear  12  supported by the upper roll  37  and lower roll  38 , the supporting mechanism with reduced unpleasant friction can be supplied.  
         [0107]    In the present preferred embodiment, both end parts of the worm gear  12  are held by the ball bearings  32  and  32 , but the holding manner of the worm gear  12  is not restricted to this case. The holding manner, for example, in which the worm gear  12  is held without the ball bearing arranged at another end part of the worm gear can be applicable as long as the flexural resistance brought by the center holding means is acceptable.  
         [0108]    In the present preferred embodiment, the explanation about the electric power steering apparatus, which assist the steering effort of the driver by adding the assist steering force brought by the electric motor in addition to the steering force brought by the driver&#39;s operation, is carried out.  
         [0109]    The application of the present invention is not restricted to the above-described case. The application to the another type of construction may be acceptable as long as the it has a construction that the diving force brought by the electric motor is add to the pinion shaft, which is joined to the rack shaft and steers the steering wheel. As an example of this, the steer-by-wire mechanism which steers the steered wheel only by the driving force, which is brought by the electric motor and is controlled by the electric signal, and the four-wheel-steering mechanism and the like are considerable.  
         [0110]    As described above, in the present invention, the worm shaft  12  is held with sufficient flexural rigidity by holding the worm shaft without play. According to this invention, since the flexure of the worm shaft  12  is prevented, the aggravation caused by the flexure of the worm shaft  12  at the time of turning back of the steering is prevented. As a result of this prevention, the aggravation of the steering feeling and the certain transmission of the steering force, which is brought by the electric motor, to the pinion shaft can be achieved.  
         [0111]    In the present invention, one end part and another end part of the worm shaft are held by plural of ball bearings, respectively. According to this invention, the worm shaft is held without play on maintaining the smooth rotation of the worm gear. Since the ball bearing which are general-purpose components is used, furthermore, it can contribute to a cost reduction.  
         [0112]    In the present invention, both end parts of the worm shaft are held with the uniform flexural rigidity by holding the another end part side, in which the flexural rigidity is low a little, by the needle bearing. According to this invention, the flexure of the worm shaft is certainly prevented and also the occurrence of the feeling gap between in the case the steering is turned back in the anticlockwise direction and in the case the steering is turned back in the clockwise direction can be decreased. the certain transmission of the steering force brought by the electric motor  7  to the pinion shaft  3  is achieved.  
         [0113]    In the present invention, the occurrence of the flexure of the worm shaft can be prevented while the worm shaft is made into the rotatable condition. According to this invention, the aggravation of the steering feeling and the certain transmission of the steering force brought by the electric motor to the pinion shaft can be achieved.  
         [0114]    In the present invention, since the worm shaft is pressed to the engaging part between the worm shaft and the worm wheel by the urgent force of the urging means with the suitable force, the clearance between the worm shaft and the worm wheel can be maintained within the predetermined ranges. According to the present invention, since the occurrence of the backlash between the worm gear and the worm wheel is prevented, the rotation of the worm shaft is certainly transmitted to the worm wheel.  
         [0115]    In the present invention, the displacement in the ups-and-downs directions of the worm shaft is restricted. The worm shaft is crowed toward the engaging part with worm wheel on allowing the rotation of the worm shaft. According to the invention, since the roller is used, the unpleasant friction with respect to the worm gear can be reduced.