Electric actuator

Abstract: An electric actuator has a body (12) with a hole (14) defined therein and an electric motor (16) fixed to an end of the body (12). The electric motor (16) has a rotatable shaft to which a feed screw (20) is coaxially connected, and a feed nut (22) is threaded over the feed screw (20). The hole (14) receives an elongate cylindrical tubular member (24) slidably fitted therein which surrounds the feed screw (20). The feed nut (22) is secured to an end of the tubular member (24), and a support (26) for feeding a workpiece supported thereon is fixed to the other end of the tubular member (24). The feed screw (20), which serves as a drive force transmitting shaft, is protected by the surrounding tubular member (24) from the deposition thereon of dust, dirt, or other foreign matter and hence from unwanted shutdown or breakage.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to an electric actuator for displacing a support to feed a workpiece with linear motion which is converted from rotational drive forces of an electric motor by a drive force transmitting means mounted in an actuator body. Description of the Related Art: Electric actuators have heretofore been used as means for conveying workpieces. A conventional electric actuator basically comprises an electric motor disposed in an actuator body and a feed screw serving as a drive force transmitting shaft coupled to a rotatable shaft of the electric motor. The feed screw and the rotatable shaft are coaxially coupled to each other by a coupling. A feed nut is threaded over the feed screw, the feed nut having an end fixed to a support that is positioned outside of the actuator body for feeding a workpiece. When the electric motor is energized, the feed screw is rotated by the rotatable shaft thereof through the coupling, moving the feed nut in an axial direction of the feed screw thereby to linearly displace the support that is fixed to the feed nut. Instead of the feed nut US-A 4 403 387 shows a tubular shuttle with an internal thread residing at an intermedicate location in an axial bore thereof. DE-A 34 13 182 discloses a similar actuator with a tube on one side of the support surrounding a part of the feed screw and being supported moveable in a through hole of the actuator body. Another conventional electric actuator comprises an electric motor disposed in an actuator body and having a hollow cylindrical rotor with an internally threaded inner wall threaded by a feed screw serving as a drive force transmitting shaft. The feed screw extends through a hole defined in the actuator body and is fixed to a support that is positioned outside of the actuator body for feeding a workpiece. When the electric motor is energized, the cylindrical rotor is rotated to move the feed screw axially into or out of the hole defined in the actuator body for thereby linearly displacing the support. The document US-A 4,288,182 discloses a machining apparatus for machining a workpiece with a work-performing tool such as a drill. The machining apparatus includes a frame to which guides or ways are secured for guideably supporting a tool carrier for reciprocal movement relative to the workpiece. The tool carrier is secured to a slide that is movably mounted on the guide ways. The slide and the tool carrier are reciprocally movable back and forth in a generally linear path relative to the workpiece. The feed and retract system moving the tool includes a drive transmitting assembly with a hollow longitudinal tubular member disposed in a bore in the frame. The tubular member is reciprocally movable into and out of the bore. A hollow drive shaft is telescoped inside the tubular member for longitudinal movement relative thereto, wherein the drive shaft comprises an internal thread threaded over a screw member rotation of which extending the drive shaft out of the tubular member. The drive shaft and the tubular member form extensible and contractible means for moving the tool carrier between its partially retracted position and its fully retracted position, wherein the hollow shaft protects the screw member from the deposition thereon of dust, dirt or other foreign matter. The slide to which the tool carrier is secured and the guide ways to which the slide is movably mounted, however, are disposed in front of the feed and retract system that houses the screw member. This enlarges the overall size of the apparatus and provides problems in correctly aligning the guide ways to the feed and retract system. SUMMARY OF THE INVENTION It is the object of the present invention to provide an improved electric actuator providing for a precise and reliable linear movement of a support in alignment with the drive transmitting means, wherein the actuator is of a compact size. This object is solved with an electric actuator comprising the features of claims 1 or 5. Advantageous embodiments of the invention are evident from the dependent claims. An electric actuator is provided which has a drive force transmitting shaft disposed in a hole defined in an actuator body and surrounded by a tubular member having an end fixed to a support, so that the drive force transmitting shaft can be protected from the deposition thereon of dust, dirt, of other foreign matter and hence from unwanted shutdown or breakage. A feed screw serves as a drive force transmitting shaft and a feed nut is threaded over the feed screw and fixed to a tubular member surrounding the feed screw, so that the feed screw can be protected from the deposition thereon of dust, dirt, of other foreign matter and hence from unwanted shutdown or breakage. In an electric actuator which has a support for feeding a workpiece, the body housing the drive force transmitting means comprises guide members extending parallel to a feed screw which serves as the drive force transmitting shaft, so that the support is prevented from rotating upon energization of an electric motor of the electric actuator. According to the present invention an electric actuator has an electric motor including a hollow cylindrical rotor, a feed screw serving as a drive force transmitting shaft coupled to the electric motor, and a feed nut threaded over the feed screw and fixed coaxially to the hollow cylindrical rotor, so that the electric actuator is relatively small in size. According to another embodiment of the present invention an electric actuator is provided with an electric motor including a hollow cylindrical rotor with an internally threaded inner wall, a support for feeding a workpiece, and a feed screw serving as a drive force transmitting shaft fixed to the support and threaded in the internally threaded inner wall of the hollow cylindrical rotor, so that the electric actuator is made up of a relatively small number of parts and is relatively small in size. The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial longitudinal cross-sectional view of an electric actuator according to a first embodiment of the present invention; FIG. 2 is a longitudinal cross-sectional view of an electric actuator according to a second embodiment of the present invention; FIG. 3 is a longitudinal cross-sectional view of an electric actuator according to a third embodiment of the present invention; FIG. 4 is a side elevational view, as viewed from the right, of the electric actuator shown in FIG. 3; FIG. 5 is a longitudinal cross-sectional view of an electric actuator according to a fourth embodiment of the present invention; and FIG. 6 is a longitudinal cross-sectional view of a modification of the electric actuator according to the fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Identical parts are denoted by identical reference numerals throughout views. An electric actuator according to a first embodiment of the present invention will be described below with reference to FIG. 1. As shown in FIG. 1, the electric actuator, generally designated by the reference numeral 82, comprises an elongate body 84 having a hole 86 defined longitudinally therein and an electric motor 88 mounted in an end of the hole 86. The electric motor 88 has a hollow cylindrical rotor 90 with a feed nut 92 coaxially fixed thereto. The feed nut 92 is threaded over a feed screw 94 which has an end fastened by a screw 98 to a slide table 96 that is integral with a support for feeding a workpiece supported thereon. An elongate cylindrical tubular member 100 is fixed to the slide table 96 and disposed slidably in the hole 86 in surrounding relationship to the feed screw 94. A guide rail 102 is mounted on an upper surface of the body 84 and extends in the longitudinal direction of the body 84. The guide rail 102 is held in slidable engagement with a guide member 104 which is mounted on a lower surface of the support of the slide table 96. The guide rail 102 and the guide member 104 jointly serve as a guide means for guiding the slide table 96 to move with respect to the body 84. The electric actuator 82 operates as follows: When the electric motor 88 is energized by a power supply (not shown), the rotor 90 is rotated about its own axis. Because of the threaded engagement between the feed nut 92 and the feed screw 94, the rotation of the rotor 90 is converted into linear motion, thereby linearly displacing the feed screw 94 in its axial direction. As a result, the slide table 96 moves in the longitudinal direction of the electric actuator 82 in response to the energization of the electric motor 88. Inasmuch as the feed screw 94 is surrounded by the tubular member 100, the feed screw 94 is not exposed to the exterior, and is protected from the deposition thereon of dust, dirt, or other foreign matter. Since the feed screw 94 axially moves within the hollow rotor 90 upon energization of the electric motor 88, the length of the electric actuator 82 may be relatively small while the slide table 96 may be movable in a relatively large range. FIG. 2 illustrates an electric actuator according to a second embodiment of the present invention. As shown in FIG. 2, the electric actuator, generally designated by the reference numeral 106, differs from the electric actuator 82 according to the first embodiment in that a support 108 is employed in place of the slide table 96 of the electric actuator 82, and two rods 110a, 110b are employed as a guide means in place of the guide rail 102 and the guide member 104. Details of only those parts of the electric actuator 106 which differ from those of the electric actuator 82 according to the first embodiment will be described below. As shown in FIG. 2, the electric actuator 106 comprises an elongate body 112 having a first hole 114 and a pair of laterally spaced second holes 116a, 116b which are defined longitudinally therein. The first hole 114, which is positioned between the second holes 116a, 116b, houses an electric motor 88, a rotor 90, a feed nut 92, a feed screw 94, and a tubular member 100 in the same manner as with the electric actuator 82 according to the first embodiment. A support 108 is fixed to an end of the feed screw 94 remote from the electric motor 88. Bushings 118a, 118b are mounted in respective end portions of the second holes 116a, 116b close to the support 108. The rods 110a, 110b are slidably inserted in the respective bushings 118a, 118b. The rods 110a, 110b and the second holes 116a, 116b jointly serve as a guide means for guiding the support 108 to move with respect to the body 112. When the electric motor 88 is energized by a power supply (not shown), the feed screw 94 is linearly displaced in its axial direction. As a result, the support 108 moves in the longitudinal direction of the electric actuator 106 in response to the energization of the electric motor 88. Since the feed screw 94 is surrounded by the tubular member 100, it is not exposed to the exterior, and hence is protected from the deposition thereon of dust, dirt, or other foreign matter. Furthermore, because the feed screw 94 axially moves within the hollow rotor 90 upon energization of the electric motor 88, the length of the electric actuator 108 may be relatively small. FIGS. 3 and 4 show an electric actuator according to a third embodiment of the present invention. As shown in FIG. 3, the electric actuator, generally designated by the reference numeral 120, comprises an elongate body 122 having a hole 124 defined longitudinally therein which has an open end closed by a cover 126. An electric motor 128 is disposed in the body 122 coaxially with the hole 124. The electric motor 128 has a hollow cylindrical rotor 130 with an internally threaded inner wall surface 130a. The rotor 130 is rotatably supported by bearings 132a, 132b in the electric motor 128. A feed screw 134 extends axially in the hole 124 and is held in threaded engagement with the internally threaded inner wall surface 130a of the rotor 130. The feed screw 134 has an end to which a support 136 is fixed remotely from the cover 126. A tubular member 138 having an inside diameter greater than the outside diameter of the electric motor 128 is fixed at an end thereof to the support 136 coaxially with the feed screw 134, and slidably inserted in an annular hole 140 that is defined in the body 122 around the electric motor 128. Therefore, the tubular member 138 surrounds the feed screw 134. A slide table 142 is coupled to the support 136. Alternatively, the slide table 142 may be integral with the support 136. As shown in FIG. 4, a guide rail 144 is mounted on an upper surface of the body 122 and extends in the longitudinal direction of the body 122. The guide rail 144 has a pair of V-shaped grooves 146a, 146b defined in outer side surfaces thereof. Two guide members 148a, 148b that are laterally spaced from each other are fixed to a lower surface of the slide table 142. The guide members 148a, 148b, which extend in the longitudinal direction of the body 122, have respective V-shaped grooves 150a, 150b defined in respective inner side surfaces thereof. A plurality of cylindrical roller bearings 152 are rotatably fitted in spaces of square cross-section which are defined by the V-shaped grooves 146a, 146b and the V-shaped grooves 150a, 150b. The guide members 148a, 148b, the guide rail 144, and the roller bearings 152 jointly serve as a guide means for guiding the slide table 142 to move with respect to the body 122. An encoder 154 is mounted on an end of the electric motor 128. Operation of the electric actuator 120 is as follows: When the electric motor 128 is energized by a power supply (not shown), the rotor 130 is rotated about is own axis. The threaded engagement between the feed screw 134 and the internally threaded inner wall surface 130a of the rotor 130 converts the rotation of the rotor 130 into linear motion, thereby linearly displacing the feed screw 134 in its axial direction. As a result, the slide table 142 moves along the guide rail 144 in response to the energization of the electric motor 128, for thereby feeding a workpiece supported on the slide table 142. At this time, the feed screw 134 moves out of the hole 124, but remains surrounded by the tubular member 138 because the tubular member 138 also moves out of the hole 124. The encoder 154 generates a pulse signal depending on the angular displacement of the electric motor 128, and transmits the generated pulse signal to a control system (not shown), which can detect the position of the slide table 142 with respect to the body 122 based on the pulse signal. In the third embodiment, since the feed screw 134 is surrounded by the tubular member 138, it is not exposed to the exterior, and hence is protected from the deposition thereon of dust, dirt, or other foreign matter. Furthermore, the feed screw 134 is held in threaded engagement with the internally threaded inner wall surface 130a of the rotor 130, and movable within the rotor 130 upon energization of the electric motor 128. Consequently, the length of the electric actuator 120 may be relatively small while the slide table 142 may be movable in a relatively large range. As the electric actuator 120 requires no feed nut, it may be made up of a reduced number of parts and can be manufactured at a reduced cost. FIG. 5 shows an electric actuator according to a fourth embodiment of the present invention. As shown in FIG. 5, the electric actuator, generally designated by the reference numeral 156, differs from the electric actuator 120 according to the third embodiment in that it has no slide table, and employs a rod 158 as a guide means in place of the guide rail 144 and the guide members 148a, 148b. Details of only those parts of the electric actuator 156 which differ from those of the electric actuator 120 according to the third embodiment will be described below. The electric actuator 156 has an elongate body 160 which has a first hole 162 and a second hole 164 which are defined longitudinally therein and laterally spaced from each other. The first hole 162 houses an electric motor 128, a rotor 130, a feed screw 134, a support 136, and a cover 126 in the same manner as with the electric actuator 120 according to the third embodiment. A bushing 166 is mounted in an end portion of the second hole 164, and the rod 158 is slidably inserted in the bushing 166. The support 136 is mounted on ends of the feed screw 134 and the rod 158. The rod 158 and the second hole 164 jointly serve as a guide means for guiding the support 136 to move with respect to the body 160. An open end of the second hole 164 remote from the support 136 is closed by a cover 170. When the electric motor 128 is energized by a power supply (not shown), the feed screw 134 is linearly displaced. Therefore, the support 136 moves in the longitudinal direction of the electric actuator 156 in response to the energization of the electric motor 128. At this time, the feed screw 134 moves out of the first hole 162, but the tubular member 138 also moves out of the first hole 162 in surrounding relationship to the feed screw 134. FIG. 6 shows a modification of the electric actuator 156 shown in FIG. 5. As shown in FIG. 6, the modified electric actuator, generally designated by the reference numeral 172, has two rods 158a, 158b inserted respectively in two second holes 164a, 164b defined in a body 174 and slidably supported by respective bushings 166a, 166b inserted in respective end portions of the second holes 164a, 164b. The two rods 158a, 158b, which are connected at ends thereof to the support 136, are effective in guiding the support 136 more stably for more stable and accurate linear movement with respect to the body 174 than the single rod 158 shown in FIG. 5. In the fourth embodiment, since the feed screw 134 is surrounded by the tubular member 138, it is not exposed to the exterior, and hence is protected from the deposition thereon of dust, dirt, or other foreign matter. Furthermore, because the feed screw 134 is movable within the rotor 130 upon energization of the electric motor 128, the length of the electric actuators 156, 172 may be relatively small while the support 136 may be movable in a relatively large range. The electric actuators 156, 172 may be made up of a reduced number of parts and can be manufactured at a reduced cost. Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein.

Claims: An electric actuator for displacing a support (96, 108) with linear motion converted from rotation of an electric motor (88) by a drive force transmitting shaft (94), comprising: a body (84, 112) having a hole (86, 114) defined therein; a drive force transmitting shaft (94) disposed in said hole (86, 114); a tubular member (100) having a portion movable into and out of said hole (86, 114), and surrounding said drive force transmitting shaft (94); said tubular member (100) having an end fixed to said support (96, 108), characterized in that said drive force transmitting shaft comprises a feed screw (94) with a feed nut (92) threaded thereover, said electric motor (88) having a hollow rotor (90), said feed nut (92) being coaxially fixed to said hollow rotor (90). An electric actuator according to claim 1, further comprising guide means (102, 104, 110a, 110b) mounted on said body (84, 112) parallel to said feed screw (94), for holding said support (96, 108) against rotation upon energization of said electric motor (88). An electric actuator according to claim 1, further comprising a slide table (96) coupled to said support (96). An electric actuator according to claim 3, wherein said slide table (96) is integral with said support (96). An electric actuator for displacing a support (136) with linear motion converted from rotation of an electric motor (128) by a drive force transmitting shaft (134), comprising: a body (122, 160, 174) having a hole (124, 162) defined therein; a drive force transmitting shaft (134) disposed in said hole (124, 162); characterized by a tubular member (138) having a portion movable into and out of an similar hole (140) surrounding said hole (124, 162), and surrounding said drive force transmitting shaft (134); said tubular member (138) having an end fixed to said support (136), wherein said electric motor (128) has a cylindrical rotor (130) having an internally threaded inner wall surface (130a), said drive force transmitting shaft comprising a feed screw (134) fixed to said support (136) and held in threaded engagement with said internally threaded inner wall surface (130a). An electric actuator according to claim 5, further comprising guide means (144, 148a, 148b, 158, 158a, 158b) mounted on said body (122, 160, 174) parallel to said feed screw (134), for holding said support (136) against rotation upon energization of said electric motor (128). An electric actuator according to claim 5, further comprising a slide table (142) coupled to said support (136). An electric actuator according to claim 7, wherein said slide table (142) is integral with said support (136).

Inventors: HASEGAWA MASAHISA, KAWAMOTO TADASU, NAGAI SHIGEKAZU, SAITOH AKIO, SUGIYAMA TORU, HASEGAWA, MASAHISA, KAWAMOTO, TADASU, NAGAI, SHIGEKAZU, SAITOH, AKIO, SUGIYAMA, TORU, Hasegawa, Masahisa, c/o SMC K.K., Tsukuba Gijutsu, Kawamoto, Tadasu, c/o SMC K.K., Tsukuba Gijutsu, Nagai, Shigekazu, c/o SMC K.K., Tsukuba Gijutsu, Saitoh, Akio, c/o SMC K.K., Tsukuba Gijutsu, Sugiyama, Toru, c/o SMC K.K., Tsukuba Gijutsu
Applicants: SMC KK, SMC KABUSHIKI KAISHA
Application year: 1996
Main classification: B23Q5
IPC codes: B23Q11, B23Q5