Patent Publication Number: US-2017350479-A1

Title: Electric Actuator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/JP2016/052941, filed Feb. 1, 2016, which claims priority to Japanese Application No. 2015-018666, filed Feb. 2, 2015. The disclosures of the above applications are incorporating herein by reference. 
    
    
     FIELD 
     The present disclosure relates to electric actuators used in motors in general industries and driving sections of automobiles etc. to convert a rotary motion from an electric motor to linear motion of a driving shaft, via the ball screw mechanism. 
     BACKGROUND 
     Generally, gear mechanisms, such as a trapezoidal thread worm gear mechanisms or rack and pinion gear mechanisms, are used as the mechanism to convert a rotary motion, of an electric motor, to axial linear motion in an electric linear actuator, used in various kinds of driving sections. These motion converting mechanisms involve sliding contact portions. Thus, power loss is increased and, simultaneously, size of the electric motor and power consumption are also increased. Thus, the ball screw mechanisms have been widely used as more efficient actuators. 
     For example, in a prior art electric actuator, an output member connected to a nut can be axially moved by rotating a ball screw shaft, forming a ball screw mechanism, using an electric motor mounted on a housing. Since friction of the ball screw mechanism is very low and the ball screw shaft tends to be easily rotated by thrust loads acting on the output member, it is necessary to hold the position of the output member when the electric motor is stopped. 
     For example, the electric motor is provided with a brake or a power transmitting element, of low mechanical efficiency such as a worm gear, to solve such a problem. A representative electric actuator  50  is shown in  FIG. 8 . The electric actuator  50  has a ball screw mechanism  53  including a ball screw shaft  51 , rotationally driven by an electric motor (not shown), and a ball screw nut  52 , mated with the ball screw shaft  51  via balls (not shown). When a motor shaft (not shown) of the electric motor is rotated, the ball screw shaft  51 , connected to the motor shaft, is rotated and linearly moves the ball screw nut  52  in left and right directions. 
     The ball screw shaft  51  is rotationally supported on cylindrical housings  54 ,  55 , via two rolling bearings  56 ,  57 . These rolling bearings  56 ,  57  are secured by a rotation locking member  59 . The locking member  59  prevents loosening via a securing cover  58 . 
     The ball screw shaft  51  is formed, on its outer circumference, with a helical screw groove  51   a.  The helical screw groove  51   a  mates with the ball screw nut  52 , via balls. The ball screw nut  52  is also formed, on its inner circumference, with a helical screw groove  52   a.  The nut has, on its end, a larger diameter portion  60 . 
     Flat portions  61  are formed on the outer circumference of the larger diameter portion  60 . A cam follower (rotation locking mechanism)  62  projects radially outward from the flat portion  61  substantially at its center. 
     Since the cam follower  62  is fit into a notched portion, entrapment of the ball screw nut  52  accompanied with rotation of the ball screw shaft  51  are prevented. In addition, since the cam follower  62  can rotationally slide relative to the notched portion, sliding friction and sliding wear can be reduced (e.g., see JP2007-333046 A). 
     While the above prior art electric actuator  50  has these advantages and is able to achieve the lower driving torque, a problem exists in that it increases the manufacturing cost due to the use of rolling bearings in the cam follower. In addition, an anti-wear member is required when the housing  54  uses aluminum material. 
     Further, a simple structure electric actuator  63 , shown in  FIG. 9 , has been proposed in order to reduce the sliding friction, wear and its manufacturing cost. The electric actuator  63  includes a cylindrical housing  64 , an electric motor  65  mounted on the housing  64 , a speed reduction mechanism  68  and a ball screw mechanism  70 . The speed reduction mechanism  68  includes a pair of spur gears  66 ,  67  to transmit the rotational power of the electric motor  65 , via a motor shaft  65   a.  The ball screw mechanism  70  converts the rotational motion of the electric motor  65  into the axial linear motion of a driving shaft  69 . 
     The housing  64  is formed from aluminum alloy such as A 6063 TE, ADC 12 etc. It has a first housing  64   a  and a second housing  64   b  abutted and bolted to an end face of the first housing  64   a,  by securing bolts (not shown). The electric motor  65  is mounted on the first housing  64   a.  The first housing  64   a  and the second housing  64   b  form a blind bore  71  and a through bore  72 , respectively, to accommodate a screw shaft  74 . 
     A smaller spur gear  66  is press-fit onto the end of the motor shaft  65   a  of the electric motor  65 . A larger spur gear  67  is integrally formed with a nut  73 . The nut  73  forms part of the ball screw mechanism  70 . The larger spur gear  67  meshes with the smaller spur gear  66 . The drive shaft  69  is integrated with the screw shaft  74  to form part of the ball screw mechanism  70 . 
     The ball screw mechanism  70  includes the screw shaft  74  and the nut  73  inserted onto the screw shaft  74 , via balls  75 . The screw shaft  74  is formed, on its outer circumference, with a helical screw groove  74   a.  The screw shaft  74  is axially movably supported, but not rotationally. The nut  73  is formed, on its inner circumference, with screw groove  73   a,  that corresponds to the screw groove  74   a  of the screw shaft  74 . A large number of balls  75  are rollably contained between the screw grooves  73   a  and  74   a.  The nut  73  is rotationally supported by two supporting bearings  76 ,  77 , but is axially immovably supported relative to the housing  64 . 
     A cylindrical sleeve  78  is fit into the blind bore  71  of the first housing  64   a.  The sleeve  78  is formed from sintered alloy formed by an injection molding machine with plastically prepared metallic powder. During the injection molding, metallic powder and binder, with plastics and wax, are firstly mixed and kneaded by a mixing and kneading machine. This forms pellets from the mixed and kneaded material. The pellets are fed into a hopper of the injection molding machine. The molten material is pushed into dies under a heated and melted state and finally forms the sleeve by a so-called MIM (Metal Injection Molding) method. 
     The sleeve  78  is formed with a pair of diametrically opposite positioned axially extending recessed grooves  78   a.  A guide pin  80  is fit into a radially through aperture  79  formed at the end of the screw shaft  74 . An annular groove  81  is formed on the open end of the blind bore  71  of the housing  64   a.  The sleeve  78  is axially immovably secured by an annular stopper ring  82  fit into the annular groove  81 . The side surfaces of the stopper ring  82  are not flat similar to a usual C-shaped stopper ring. It has a bent cross-section with a vertex at a central position symmetric about a notch of the stopper ring  82 . 
     The rotation locking mechanism is constituted by the guide pin  80  and the sleeve  78 , of sintered metal fit, into the first housing  64   a  of aluminum light alloy. Thus, it is possible to reduce the sliding friction and wear of the aluminum housing  64   a  as well as the manufacturing cost due to its simple structure. 
     In addition, axial movement of the sleeve  78  is prevented by the urging spring force of the stopper ring  82  against the sleeve  78 . Thus, the generation of vibration and noise can be prevented (see e.g. JP 2013-167334 A). 
     However, in the prior art electric actuator  63 , a problem exists in that once the stopper ring  82  has been attached to the housing  64 , the stopper ring  82  cannot be easily removed from the first housing  64   a  without damaging the housing  64   a.  Accordingly, the removal operability and thus the maintenance workability of the electric actuator  63  would be impaired. 
     SUMMARY 
     It is, therefore, an object of the present disclosure to provide an electric actuator that can improve the maintenance workability and reduce the manufacturing cost due to its simple structure. 
     In order to achieve the object of the present disclosure, an electric actuator comprises a housing formed of aluminum light alloy with an electric motor mounted on the housing. A speed reduction mechanism transmits rotational driving power of the motor to a ball screw mechanism. The ball screw mechanism converts the rotational motion of the electric motor to axial linear motion of a driving shaft. The ball screw mechanism has a nut formed with a helical screw groove on its inner circumference. The nut is rotationally supported by supporting bearings mounted on the housing. The nut is axially immovable relative to the housing. A screw shaft, coaxially integrated with the driving shaft, has a helical screw groove on its outer circumference that corresponds to the helical screw groove of the nut. The screw shaft is inserted into the nut via a plurality of balls. A cylindrical sleeve is fit into a blind bore formed in the housing to accommodate the screw shaft. The sleeve&#39;s inner circumference has a pair of diametrically oppositely positioned axially extending recessed grooves to receive a guide pin mounted on one end of the screw shaft. The pin guides the screw shaft so that it is able to move axially, but not rotationally, relative to the housing. An annular groove is formed on the open end of the blind bore of the housing. The sleeve is axially immovably secured by an annular stopper ring fit into the annular groove. The stopper ring is formed with one notch. A recess is formed on the inner circumference of the stopper ring near each of the ends of the stopper ring to enable engagement of a detaching tool. The contour of each recess includes a circular arc portion and a flat portion. The circular arc portion has a predetermined radius of curvature. The flat portion extends tangentially from the circular arc portion. The width of the opening of the recess is smaller than the diameter of the circular arc portion. 
     The electric actuator housing is formed from aluminum light alloy with an electric motor mounted on the housing. A speed reduction mechanism transmits rotational driving power of the motor to a ball screw mechanism, via a motor shaft. The ball screw mechanism converts the rotational motion of the electric motor to axial linear motion of a driving shaft. The ball screw mechanism includes a nut and a screw shaft. The nut has a helical screw groove on its inner circumference. The nut is rotationally, but axially immovably, supported relative to the housing by supporting bearings mounted on the housing. The screw shaft is coaxially integrated with the driving shaft. A helical screw groove is on the screw shaft outer circumference and corresponds to the helical screw groove of the nut. The screw shaft is inserted in the nut, via a plurality of balls. A cylindrical sleeve is fit into a blind bore formed in the housing to accommodate the screw shaft. The sleeve inner circumference has a pair of diametrically oppositely positioned axially extending recessed grooves to receive a guide pin mounted on one end of the screw shaft. This guides the screw shaft so that it is able to move axially, but not rotationally, relative to the housing. An annular groove is formed on the open end of the blind bore of the housing. The sleeve is axially immovably secured by a annular stopper ring fit into the annular groove. The stopper ring is formed with one notch. A recess is formed on the inner circumference of the stopper ring near each of the two ends of the stopper ring to enable engagement of a detaching tool. The contour of each recess has a circular arc portion and a flat portion. The circular arc portion has a predetermined radius of curvature. The flat portion extends tangentially from the circular arc portion. The width of the opening of the recess is smaller than the diameter of the circular arc portion. Thus, it is possible to provide an electric actuator of simple design and low manufacturing cost. Also, the maintenance operability is improved by enabling a removing tool for the stopper ring to easily engage the recess. Thus, this prevents disengagement from the stopper ring. 
     The stopper ring has a curved portion formed with at least one vertex at positions symmetric about the notch. The sleeve is securely held under a pressed state by the stopper ring. This makes it possible to generate the axial spring force of the stopper ring against the sleeve. This applies a predetermined pre-pressure to the sleeve and prevents the generation of noise or vibration of the housing. 
     The recess of the stopper ring has a contour with a circular arc portion formed by a circular arc larger than a semicircle. The flat portion extends tangentially from the circular arc portion. An angle is formed between a horizontal line passing through the center of the circular arc portion and a line passing through the center of the circular arc portion. The edge of the opening opposite to the flat portion is within a range of 20°˜40°. 
     The recess of the stopper ring has a contour with a semicircular arc portion. The flat portion extends tangentially from the semicircular arc portion. Each of the portions of the opening between a horizontal line passing through the center of the semicircular arc portion and the edge of the opening is formed by a flat surface. 
     An angle of the radially outer side corner of the end of the stopper ring is formed by an obtuse angle. This makes it possible to prevent the angled portion of the radially outer side corner of the end of the stopper ring from being caught on the groove surface of the annular groove of the housing. Thus, this prevents the housing from being damaged as well as to smoothly perform the removing operation of the stopper ring. 
     The sleeve is formed of sintered alloy formed by MIM (Metal Injection Molding). This makes it possible to easily form the sleeve with a desirable configuration, dimension and high accuracy even if the sleeve has a complicated configuration. 
     The electric actuator of the present disclosure comprises a housing formed of aluminum light alloy with an electric motor mounted on the housing. A speed reduction mechanism transmits rotational driving power of the motor to a ball screw mechanism, via a motor shaft. The ball screw mechanism converts the rotational motion of the electric motor to the axial linear motion of a drive shaft. The ball screw mechanism includes a nut and a screw shaft. The nut has a helical screw groove on its inner circumference. The nut is rotationally supported by supporting bearings mounted on the housing. The nut is axially immovable relative to the housing. The screw shaft is coaxially integrated with the drive shaft. The screw shaft has a helical screw groove on its outer circumference that corresponds to the helical screw groove of the nut. The screw shaft is inserted into the nut, via a plurality of balls. A cylindrical sleeve is fit into a blind bore formed in the housing to accommodate the screw shaft. The sleeve inner circumference has a pair of diametrically oppositely positioned axially extending recessed grooves to receive a guide pin mounted on one end of the screw shaft. This guides the screw shaft so that it is able to move axially, but not rotationally, relative to the housing. An annular groove is formed on the open end of the blind bore of the housing. The sleeve is axially immovably secured by a annular stopper ring fit into the annular groove. The stopper ring has one notch and a recess is formed on the inner circumference of the stopper ring near each of the two ends of the stopper ring to enable engagement with a detaching tool. The contour of each recess includes a circular arc portion and a flat portion. The circular arc portion has a predetermined radius of curvature. The flat portion extends tangentially from the circular arc portion. The width of the opening of the recess is smaller than the diameter of the circular arc portion. Thus, it is possible to provide an electric actuator of a simple structure and low manufacturing cost. Also, the maintenance operability is improved by engaging a removing tool for the stopper ring to easily engage the recess. Thus, this prevents disengagement from the stopper ring. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a longitudinal sectional view of one preferable embodiment of an electric actuator. 
         FIG. 2  is an enlarged longitudinal sectional view of the ball screw mechanism of  FIG. 1 . 
         FIG. 3  is a cross-sectional view taken along a line III-III of  FIG. 1 . 
         FIG. 4( a )  is a front elevation view of a stopper ring. 
         FIG. 4( b )  is a cross-sectional view of the stopper ring of  FIG. 4( a ) . 
         FIG. 4( c )  is an enlarged view of a IV part of  FIG. 4( b ) . 
         FIG. 5  is an enlarged view of a V part of  FIG. 4( a ) . 
         FIG. 6  is an explanatory view of a method for measuring the axial spring load applied to the stopper ring. 
         FIG. 7  is a partially enlarged view of a modification of the stopper ring of  FIG. 5 . 
         FIG. 8  is a longitudinal sectional view of a prior art electric actuator. 
         FIG. 9  is a longitudinal sectional view of another prior art electric actuator. 
     
    
    
     DETAILED DESCRIPTION 
     An electric actuator has a cylindrical housing formed of aluminum light alloy with an electric motor mounted on the housing. A speed reduction mechanism transmits rotational driving power of the motor to a ball screw mechanism, via a motor shaft. The ball screw mechanism converts the rotational motion of the electric motor to the axial linear motion of a driving shaft. The ball screw mechanism includes a nut and a screw shaft. The nut has a helical screw groove on its inner circumference. The nut is rotationally supported by supporting bearings mounted on the housing. The nut is axially immovably supported relative to the housing. The screw shaft is coaxially integrated with the driving shaft. The screw shaft has a helical screw groove on its outer circumference that corresponds to the helical screw groove of the nut. The screw shaft is inserted into the nut, via a plurality of balls. A cylindrical sleeve is fit into a blind bore formed in the housing to accommodate the screw shaft. The sleeve inner circumference has a pair of diametrically oppositely positioned axially extending recessed grooves to receive a guide pin mounted on one end of the screw shaft. This guides the screw shaft so that it is able to move axially, but not rotationally, relative to the housing. An annular groove is formed on the open end of the blind bore of the housing. The sleeve is axially immovably secured by an annular stopper ring. The stopper ring has a curved portion formed with a vertex at a position symmetric about the notch. The stopper ring is formed with one notch. A recess is formed on the inner circumference of the stopper ring near each end of the stopper ring. This enables engagement of a detaching tool. The contour of each recess includes a circular arc portion and a flat portion. The circular arc portion has a predetermined radius of curvature. The flat portion extends tangentially from the circular arc portion. The width of the opening of the recess is smaller than the diameter of the circular arc portion. 
     One preferred embodiment of the present disclosure will be hereinafter described with reference to the drawings. 
       FIG. 1  is a longitudinal sectional view of one preferable embodiment of an electric actuator.  FIG. 2  is an enlarged longitudinal sectional view of the ball screw mechanism of  FIG. 1 .  FIG. 3  is a cross-sectional view taken along a line III-III of  FIG. 1 .  FIG. 4( a )  is a front elevation view of a stopper ring.  FIG. 4( b )  is a cross-sectional view of the stopper ring of  FIG. 4( a ) .  FIG. 4( c )  is an enlarged view of a IV part of  FIG. 4( b ) .  FIG. 5  is an enlarged view of a V part of  FIG. 4( a ) .  FIG. 6  is an explanatory view of a method for measuring the axial spring load applied to the stopper ring.  FIG. 7  is a partially enlarged view of a modification of the stopper ring of  FIG. 5 . 
     As shown in  FIG. 1 , the electric actuator  1  has a cylindrical housing  2 , an electric motor  3  mounted on the housing  2 , a speed reduction mechanism  6  and a ball screw mechanism  8 . The speed reduction mechanism  6  has a pair of spur gears  4 ,  5  to transmit the rotational driving power of the motor  3 , via its motor shaft  3   a.  The ball screw mechanism  8  converts rotational motion of the electric motor  3  to axial linear motion of a drive shaft  7 , via the speed reduction mechanism  6 . 
     The housing  2  is formed of aluminum alloy such as A 6063 TE, ADC 12 etc. The housing  2  has a first housing  2   a  and a second housing  2   b  abutted against and bolted to an end face of the first housing  2   a.  The electric motor  3  is mounted on the first housing  2   a.  The first housing  2   a  and the second housing  2   b  are formed with a blind bore  9  and a through bore  10 , respectively, to accommodate the screw shaft  12 . 
     The smaller spur gear  4  is press-fit onto the end of the motor shaft  3   a  of the electric motor  3 . It is non-rotatably on the motor shaft  3   a.  The motor shaft is rotationally supported by a rolling bearing  11  mounted on the second housing  2   b.  The larger spur gear  5  mates with the smaller spur gear  4 . The larger spur gear  5  is integrally formed with a nut  14  that forms part of the ball screw mechanism  8 . The driving shaft  7  is coaxially integrated with the screw shaft  12  forming another part of the ball screw mechanism  8 . 
     As shown in the enlarged view of  FIG. 2 , the ball screw mechanism  8  includes the screw shaft  12  and the nut  14 . The nut  14  mates with the screw shaft  12 , via balls  13 . The screw shaft  12  outer circumference has a helical screw groove  12   a.  The screw shaft  12  is axially movably, but not rotationally supported on the housing  7 . The nut  14  inner circumference has a helical screw groove  14   a.  The screw groove  14   a  corresponds to the screw groove  12   a  of the screw shaft  12 . A large number of balls  13  are accommodated between the screw grooves  12   a  and  14   a.  The nut  14  is supported by two supporting bearings  15 ,  16 . The nut is rotationally, but axially immovably, supported relative to the housings  2 . The numeral  17  denotes a bridge member that connects opposite ends of the nut screw groove  14   a  to achieve an endless circulating passage of balls  13 . 
     The cross-sectional configuration of each screw groove  12   a,    14   a  may be either a circular-arc or a Gothic-arc configuration. However, the Gothic-arc configuration is adopted in this embodiment. It provides a large contacting angle with the ball  13  and sets a small axial gap. This provides a large rigidity against the axial load and thus suppresses the generation of vibration. 
     The nut  14  is formed of case hardened steel such as SCM 415 or SCM 420. Its surface is hardened to HRC 55˜62 by vacuum carburizing hardening. This enables the omission of operations such as buffing, for scale removal, after heat treatment and thus reduces the manufacturing cost. The screw shaft  12  is formed of medium carbon steel such as S55C or case hardened steel such as SCM 415 or SCM 420. Its surface is hardened to HRC 55˜62 by induction hardening or carburizing hardening. 
     The larger spur gear  5  is integrally formed with the outer circumference of the nut  14 . The supporting bearings  15 ,  16  are press-fit onto the nut  14  at both sides of the larger spur gear  5 , via a predetermined interference. This prevents an axial movement of the supporting bearings  15 ,  16  and the larger spur gear  5  even if a thrust load is applied to them. In addition, each supporting bearing  15 ,  16  include a deep groove ball bearing with mounted shield plates on both its sides. This prevents lubricating grease sealed within the bearing body from leaking outside. Also, it prevents abrasive debris from entering into the bearing body from the outside. The larger spur gear  5  may be formed separately from the nut  14  and secured on the nut  14  via a key. 
     In the illustrated embodiment, a cylindrical sleeve  18  is fit into the blind bore  9  of the first housing  2   a.  The sleeve  18  is formed from a sintered alloy by an injection molding machine for molding plastically prepared metallic powder. In this injection molding, metallic powder and binder, including plastics and wax, are firstly mixed and kneaded by a mixing and kneading machine to form pellets, from the mixed and kneaded material. The pellets are fed into a hopper of the injection molding machine. The pellets under a heated and melted state are pushed into dies and finally formed into the sleeve by a so-called MIM (Metal Injection Molding). The MIM method can easily mold sintered alloy material into article having desirable accurate configurations and dimensions even though the article require high manufacturing technology and has intricate configurations that are hard to form. 
     One example of a metallic powder for the sintering alloy able to be carburized is SCM415 including C of 0.13 wt %, Ni of 0.21 wt %, Cr of 1.1 wt %, Cu of 0.04 wt %, Mn of 0.76 wt %, Mo of 0.19 wt %, Si of 0.20 wt % and remainder Fe etc. The sleeve  18  is cementation quenched and tempered with controlling temperature. Other materials may be used for the sleeve  18 . Examples are materials superior in workability and corrosion resistance and include Ni of 3.0˜10.0 wt % (FEN 8 of Japanese powder metallurgy industry standard) or stainless steel SUS  630  of precipitation hardening comprising C of 0.07 wt %, Cr of 17 wt %, Ni of 4 wt %, Cu of 4 wt %, remainder Fe etc. This stainless steel SUS 630 is able to properly increase its surface hardness to 20˜33 HRC by solid-solution heat treatment to obtain both the high toughness and hardness. It is possible to increase the strength and wear resistance of the sleeve  18  and thus its durability higher than those of the first housing  2   a  that is formed from aluminum alloy by adopting materials described above to the sleeve  18 . 
     As shown in  FIGS. 1 and 3 , the inner circumference of the sleeve  18  is formed with a pair of axially extending recessed grooves  18   a,    18   a  arranged radially opposed each other. A radially extending through aperture  19  is formed on the end of the screw shaft  12 . A guide pin  20  is fit into the through aperture  19 . The guide pin  20  engages the recessed grooves  18   a,    18   a.  This axially guides and prevents rotation of the screw shaft  12 . An annular groove  21  is formed on an opening of the blind bore  9 . The sleeve is axially positioned and secured by a stopper ring  22  fit into the annular groove  21 . 
     The guide pin  20  may be constituted by a needle roller, used in needle bearings, that is easily available at low cost with superior wear resistance and shear strength. More particularly, since the outer circumferential surface of the needle roller is crowned, it is possible to prevent edge load contact against the recessed grooves  18   a,    18   a  of the sleeve  18 . Thus, this improves the durability for a long term. 
     As described above, the guide pin  20  engages the recessed grooves  18   a,    18   a  of the sleeve made of strong sintered alloy. Thus, it is possible to reduce the sliding friction and wear of the first housing  2   a  made of aluminum alloy. This further provides an electric actuator that has tough strength, a simple structure and can be manufactured at a low cost. 
     The stopper ring  22  is mounted in the annular groove  21  formed in the opening of the blind bore  9  of the first housing  2   a.  Axial movement of the sleeve  18  is prevented by abutment of the stopper ring  22  against the sleeve  18 . The stopper ring  22  is formed of hard steel wire such as SWRH67A (JIS G3506). The stopper ring  22  has a configuration of an ended ring. Thus, it is deformable both in circumferential and radial directions as shown in  FIG. 4( a ) . The stopper ring  22  inner circumference, near each of the two ends  23 ,  23  of the stopper ring  22 , has a recess  24  that enables engagement of a tool (e.g. circlip plier). If the stopper ring  22  is elastically deformed, so as to reduce the outer diameter of its outer peripheral portion, by this tool inserted into the annular groove of the blind bore, it is secured by an elastic return force. 
     In addition as shown in  FIG. 4( b ) , the stopper ring  22  has a curved portion  22   a  formed with at least one vertices (one vertex at the center of the stopper ring  22  in the illustrated embodiment) at positions symmetric about the notch. The sleeve is securely held under a pressed state by the stopper ring  22 . Thus, the stopper ring  22  generates axial spring force against the sleeve  18  to apply a predetermined pre-pressure to the sleeve  18 . This prevents the generation of noise or vibration of the housing  2 . Furthermore, as shown in  FIG. 4( c ) , four corner edges, of a cross-section of the stopper ring  22 , are rounded to a radius R. This can be simply achieved without the necessity of after treatment if the stopper ring is press-formed from steel wires with corners that are previously rounded. Thus, mass productivity can be improved. The stopper ring  22  may be press-formed from austenitic stainless steel sheet (JIS SUS304 system) or preserved cold rolled steel sheet (JIS SPCC system) other than those described above. 
     As shown in the enlarged view of  FIG. 5 , an angle a of the radially outer side corner of the end  23  of the stopper ring  22  is formed with an obtuse angle. The obtuse angle may be between 110˜130°. This prevents the angled portion of the radially outer side corner of the end of the stopper ring  22  from being caught on the groove surface of the annular groove  21  of the housing  2   a.  Thus, this prevents the housing  2   a  from being damaged as well as it smoothly performs the removal operation of the stopper ring  22 . 
     The recess  24  of the stopper ring  22  has a contour with a circular arc portion  24   a.  The circular arc portion has a predetermined radius of curvature R 1 . A flat portion  24   b  tangentially extends from the circular arc portion  24   a.  The circular arc portion  24   a  is larger than a semicircle. More particularly, an angle β formed between a horizontal line passing through the center of the circular arc portion  24   a  and a line passing through the center of the circular arc portion  24   a  and the edge of the opening  24   c  opposite to the flat portion  24   b  is within a range between 20°˜40°. This enables a tool to more easily engage the recess  24  and prevent the tool from easily slipping off from the stopper ring  22 . Accordingly, this improves the workability. 
     The axial spring load of the stopper ring  22  is measured by sampling inspection as shown in  FIG. 6 . The axial spring load P is set at a value larger than 37 N at a height H (e.g. 2.2 mm) after compression to the thickness of the stopper ring  22  by measuring presser  25 . This makes it possible to prevent the permanent set-in fatigue of the stopper ring  22  caused by repeated applied axial loads. Thus, this improves the reliability of the stopper ring  22 . 
     A stopper ring  26  shown in  FIG. 7  is a modification of the stopper ring  22  described above. This stopper ring  26  is different from the stopper ring  22  only in configuration of the opening  24   c.  Accordingly, the same reference numerals are used to identify the same parts in this modification as those in the stopper ring  22 . 
     The stopper ring  26  is an ended ring deformable both in the circumferential and radial directions. It is press-formed from preserved cold rolled steel sheet. Its inner circumference, near each of both the ends  23 ,  23  of the stopper ring  26 , includes a recess  27 . The recess  27  has a contour with a circular arc portion  24   a  having a predetermined radius of curvature R 1 . A flat portion  24   b  tangentially extends from the circular arc portion  24   a.  The circular arc portion  24   a  is a semicircle. Each of portions of the opening  27   a  between a horizontal line passing through the center of the circular arc portion  24   a  and the edge of the opening  27   a  is formed by a flat surface. The width W of the opening  27   a  is set at a dimension equal to or smaller than 2R1 (W2R1). This enables a removing tool for the stopper ring to easily engage the recess  27  and prevent it from being disengaged from the stopper ring  26 . 
     The present electric actuator can be used in general industry driving portions of an automobile etc. The electric actuator is provided with a ball screw mechanism to convert a rotational input motion from an electric motor to a linear motion of a drive shaft, via a speed reduction mechanism. 
     The present disclosure has been described with reference to the preferred embodiments. 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 to include all such alternations and modifications insofar as they come within the scope of the appended claims or their equivalents.