Abstract:
An electrically driven linear actuator ( 1 ) has an electric motor with a ball screw mechanism ( 3  and  4 ) connected to an output shaft ( 2   a ) of the electric motor ( 2 ), via a power transmitting mechanism ( 10  and  12 ) to convert a rotary motion of the electric motor ( 2 ) to a linear motion of an object member to be controlled by the actuator ( 1 ). Housing members ( 5, 6  and  8 ) contain the ball screw mechanism ( 3  and  4 ) and the power transmitting mechanism ( 10  and  12 ). Stroke end members ( 14  and  14 ), of a ball screw shaft ( 4 ) or a ball screw nut ( 3 ) forming the ball screw mechanism ( 3  and  4 ), are rotatably supported relative to the housing members ( 5  and  8 ) via a free rotation mechanism ( 20  and  20 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to Japanese Patent Application No. 2006-276373, filed Oct. 10, 2006, which application is herein expressly incorporated by reference.  
       FIELD  
       [0002]     The present disclosure relates to an electrically driven linear actuator used in a drive train of a vehicle, such as an automobile, and more particularly to an electrically driven linear actuator to convert rotary motion of an electric motor to linear motion of an output member of the actuator, via a ball screw mechanism.  
       BACKGROUND  
       [0003]     In electrically driven linear actuator used in many types of driving mechanisms of a vehicle, such as an automobile and the like, a trapezoidal screw thread or a gear mechanism, such as a rack and pinion, are used as the mechanism to convert the rotary motion of the electric motor to the linear motion in an axial direction. These converting mechanisms usually have sliding contact portions and thus a defect of power loss, which needs power up of the electric motor and thus is obliged to increase the power consumption. Accordingly, ball screw mechanisms have been increasingly used as a more efficient actuator.  
         [0004]     The ball screw is a mechanical element to convert a rotational movement to an axial translation motion, a linear motion. The ball screw includes a ball screw shaft with a helical ball screw groove formed on its outer circumferential surface. A ball screw nut has a helical ball screw groove formed on its inner circumferential surface. A plurality of balls are contained in a ball raceway formed between the helical ball screw grooves of the ball screw shaft and nut.  
         [0005]     The reciprocal stroke of the ball screw actuator is finite and is several millimeters. In order to prevent overrunning of the linear motion of the ball screw shaft or the ball screw nut, it is necessary to provide mechanical stoppers at both ends of the stroke. Although it is possible to electrically control the stroke of the actuator using a limit sensor, the mechanical stoppers are required to prevent an accidental overrunning of the actuator.  
         [0006]     The ball screw mechanism acts as a boosting mechanism to convert a rotational force to a thrust force. Accordingly, a large thrust force is generated between an abutting member and the ball screw shaft or the ball screw nut when either one of them strikes against the abutting member at the end of the stroke. Since the ball screw shaft or the ball screw nut strikes against the abutting member during its rotation, it bites the abutting member due to its wedging action against the abutting member. Once it has been rigidly bitten, it is difficult to release from the bitten condition if trying to drive the electric motor in a reverse direction. This is due to the usual design of the electric motor which leaves less torque margin.  
         [0007]     One method of releasing the bitten rigid engagement is proposed in Japanese Laid-open Patent Publication No. 186065/1995. Here a screw arranged within a driving screw shaft can be rotated to release the bitten engagement. Although detailed description of this method is not therein, it may be easily supposed that the bitten engagement is released by adding a large torque on the screw using a dedicated tool having a large arm ratio.  
         [0008]     However there are several problems in the prior art. When the ball screw shaft or the ball screw nut strikes against the abutting member at the end of the stroke, it bites the abutting member due to its wedging action against the abutting member. According to the Japanese Laid-open Patent Publication No. 186065/1995, there are also problems that the strength of the ball screw shaft is reduced due to forming of screw threads within the shaft. Also, use of a dedicated tool is required, in order to release the bitten condition.  
       SUMMARY  
       [0009]     It is an object of the present disclosure to provide an electrically driven linear actuator that prevents generation of the bitten engagement in the actuator. Thus, no reduction of the strength of the screw shaft is caused and dedicated tools are not required.  
         [0010]     To achieve the above mentioned object, an electrically driven linear actuator comprises an electric motor with a ball screw mechanism connected to an output shaft of the electric motor, via a power transmitting mechanism to convert a rotary motion of the electric motor to a linear motion of an object member to be controlled by the actuator. Housing members contain the ball screw mechanism and the power transmitting mechanism. Stroke end members, of a ball screw shaft or a ball screw nut forming the ball screw mechanism, are rotatably supported relative to the housing members via a free rotation mechanism.  
         [0011]     The free rotation mechanism can be rotated relative to the housing of the actuator while being entrained by the stroke end members when the stroke end members are urged toward the housing. Thus, the biting action of the stroke end members against the housing members can be effectively prevented.  
         [0012]     The free rotation mechanism may be a thrust washer. This enables prevention of the biting action by a simple structure.  
         [0013]     The free rotation mechanism may be a rolling bearing, such as a thrust needle bearing, a ball bearing etc. This enables prevention of the biting action due to lower coefficient of low rolling friction.  
         [0014]     The free rotation mechanism may be arranged via an axial gap relative to the housing. This enables easy rotation of the free rotation mechanism when the stroke end mechanism. Thus, this prevents an increase in friction between mutually contacting members, the free rotation mechanism, the housing members and the stroke end members.  
         [0015]     An electrically driven linear actuator comprises an electric motor with a ball screw mechanism connected to an output shaft of the electric motor, via a power transmitting mechanism to convert a rotary motion of the electric motor to a linear motion of an object member to be controlled by the actuator. Housing members contain the ball screw mechanism and the power transmitting mechanism. Stroke end members, of a ball screw shaft or a ball screw nut that forms the ball screw mechanism, are rotatably supported relative to the housing members via a free rotation mechanism. This design provides an electrically driven linear actuator that prevents generation of the bitten engagement in the actuator. Thus, no reduction of strength of the screw shaft is caused and dedicated tools are not required.  
         [0016]     An electrically driven linear actuator comprises an electric motor with a ball screw mechanism connected to an output shaft of the electric motor, via a power transmitting mechanism, to convert rotary motion of the electric motor to a linear motion of an object member to be controlled by the actuator.  
         [0017]     Housing members contain the ball screw mechanism and the power transmitting mechanism. Stroke end members, of a ball screw shaft or a ball screw nut forming the ball screw mechanism, are rotatably supported relative to the housing members via a free rotation mechanism.  
         [0018]     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.  
     
    
     DRAWINGS  
       [0019]     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein:  
         [0020]      FIG. 1  is a longitudinal section view of an embodiment of the electrically driven linear actuator;  
         [0021]      FIG. 2  is a partially enlarged view of the electrically driven linear actuator of  FIG. 1 ; and  
         [0022]      FIG. 3  is a view like  FIG. 2  with a bearing thrust member. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Preferable embodiments of the present disclosure will be hereinafter described with reference to the drawings.  
         [0024]      FIG. 1  is a longitudinal section view of an embodiment of the electrically driven linear actuator.  FIG. 2  is a partially enlarged view of the electrically driven linear actuator of  FIG. 1 .  FIG. 3  is like  FIG. 2  with a bearing thrust member.  
         [0025]     The electrically driven linear actuator  1  includes a ball screw mechanism with a ball screw nut  3  rotationally driven by an electric motor  2 . A ball screw shaft  4  threadably engages with the ball screw nut  3 , via balls (not shown). When a motor shaft  2   a  of the electric motor  2  is rotated, the ball screw nut  3  is rotated to linearly move the ball screw shaft  4  in the left and right directions in  FIG. 1 . An object member (not shown) to be controlled is connected to one end of the ball screw shaft  4  and is moved to the left and right directions by rotation of the electric motor  2 .  
         [0026]     The ball screw nut  3  and the ball screw shaft  4  are contained within a housing formed by housing members  5 ,  6  and  8 . The housing members  5  and  6  are united by fastening bolts  7 . The electric motor  2  is mounted on the right side of the housing member  5 . Housing member  8  is mounted on the left side of the housing member  6  and is united with it by fastening bolts  9 .  
         [0027]     The motor shaft  2   a  of the electric motor  2  is fit into a shaft  10   a  of a gear  10 . A gear portion  10   b  is fitted on the shaft  10   a  at its nearly central portion. The shaft  10   a  has a stepped portion  10   c  and a smaller diameter portion  10   d . The smaller diameter portion  10   d  is mounted on the housing member  6  via a rolling bearing  11 .  
         [0028]     An annular flange  12  is fitted on the outer circumferential surface of the ball screw nut  3 , at its right side. Teeth are formed on the outer circumferential surface of the flange  12 . The teeth mate with the teeth of the gear portion  10   b . The inner circumferential surface at the right side of the ball screw nut  3  is formed with a countersunk portion  3   b . The countersunk portion  3   b  prevents a stroke end member  14  of the ball screw shaft  4  from being contacted with the ball screw nut  3  when the ball screw shaft  4  is axially moved. The outer circumferential surface of the ball screw nut  3  is rotatably supported on the housing member  6  via a rolling bearing  13 . The inner circumferential surface of the ball screw nut  3  is formed with a helical screw groove  3   a.    
         [0029]     The outer circumferential surface of the ball screw shaft  4 , at its nearly central portion, is formed with a helical screw groove  4   a . A plurality of balls (not shown) are arranged between the helical screw grooves  3   a  and  4   a . Stroke end members  14  and  14  each have a cylindrical configuration and are fitted on the ball screw shaft  4  at either end of the helical screw groove  4   a . The stroke end members  14  prevent mechanical overrunning of the ball screw shaft  4  when it is moved toward the left and right directions in  FIG. 1 .  
         [0030]     A cutout portion  4   b  is formed on the outer circumferential surface of the ball screw shaft  4 , at its right side in  FIG. 1 . A bolt  15  is screwed into the cutout portion  4   b  to prevent rotation of the ball screw shaft around its axis. The left and right side ends of the ball screw shaft  4  are slidably supported by the housing members  8  and  5  via bushes  17  and  16 , respectively. A male thread  4   c , formed on the rightmost end of the ball screw shaft  4 , secures an object member (not shown) to be controlled by the actuator  1 .  
         [0031]     Annular thrust washer  20 , positioned around the ball screw shaft  4 , is arranged within a cylindrical bore  8   a  formed in the housing member  8 . The thrust washer  20  is positioned adjacent to the bottom surface of the cylindrical bore  8   a . Thus, the thrust washer  20  opposes the stroke end member  14  on the left side. The thrust washer  20  is secured by a snap ring  21 , via a predetermined axial gap. The outer diameter of the thrust washer  20  is slightly smaller than the diameter of the bore  8   a  so that it can freely rotate within the bore  8   a . Accordingly, the thrust washer  20  can rotate when the stroke end member  14 , on the left side, abuts the thrust washer  20 . Thus, the biting action of the ball screw shaft  4  against the housing member  8  can be effectively prevented.  
         [0032]     Similarly annular thrust washer  20 , positioned around the ball screw shaft  4 , is arranged within a cylindrical bore  5   a  formed in the housing member  5 . The thrust washer  20  is positioned adjacent to the bottom surface of the cylindrical bore  5   a . Thus, the thrust washer  20  opposes the stroke end member  14  on the right side. The thrust washer  20  is secured by a snap ring  21 , via a predetermined axial gap. The outer diameter of the thrust washer  20  is slightly smaller than the diameter of the bore  5   a . Thus, the thrust washer  20  can freely rotate within the bore  5   a . Accordingly, the thrust washer  20  can rotate when the stroke end member  14 , on the right side, abuts the thrust washer  20 . Thus, the biting action of the ball screw shaft  4  against the housing member  5  can be effectively prevented.  
         [0033]     Although the illustrated embodiment uses thrust washers as the free rotation means, the present invention is not limited to thrust washers. Rolling bearings such as thrust needle bearings, ball bearings etc  20 ′ ( FIG. 3 ) may be used as the free rotation means. In addition, although it has been described where the electrically driven linear actuator rotates the ball screw nut and the ball screw shaft is axially moved, the present invention is not limited, and can be applied to an electrically driven linear actuator where the ball screw shaft is rotated and the ball screw nut is axially moved.  
         [0034]     The electrically driven linear actuator of the present disclosure can be applied to electrically driven linear actuators used in driving portions of a vehicle such as an automobile.  
         [0035]     The present disclosure has been described with reference to the preferred embodiment. Obviously, modifications and alternations will occur to those of ordinary skill in the art upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such alternations and modifications insofar as they come within the scope of the appended claims or their equivalents.