Abstract:
The present invention relates to an electromechanical actuator comprising a housing, an electric motor, a roller drive and a rotor rotation angle sensing unit received in said housing. The rotor of the electromechanical actuator is mounted within the housing and configured to rotate in bearing supports. Openings for supplying and removing lubricant are arranged in the housing in the vicinity of the bearing supports, the opening allowing to replace lubricant without disassembling the actuator, thus simplifying maintenance and operation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to linear actuators, in particular to a linear electromechanical actuator comprising a roller drive. 
     DESCRIPTION OF THE RELATED ART 
     Linear electromechanical actuators comprising a roller drive are known to utilize for controlling various machines and mechanisms. U.S. Pat. No. 5,491,372 (Erhart) discloses a bidirectional electrically powered linear actuator comprising a housing assembly, an electric motor and a roller drive, wherein said electric motor and said roller drive are received within the housing to engage with each other. The roller drive comprises a threaded cylinder, threaded rollers and an output shaft. The electric motor comprises a stator and a rotor, wherein the rotor of the electric motor comprises said threaded cylinder of the roller drive. The electric motor involves bidirectional linear movement of the output shaft. The working surfaces of the known actuator are lubricated by applying a lubricant to said surfaces during actuator assembly. In order to replace spent lubricant with fresh lubricant, the actuator need to be disassembled and the output shaft with threaded rollers need to be removed from the housing, which requires dismantling the actuator and leads to a lengthy downtime. 
     It is a general object of the present invention to simplify the process of replacing lubricant in a linear electromechanical actuator, to reduce downtime of such actuator and to simplify usage thereof. 
     SUMMARY OF THE INVENTION 
     The object is achieved by a linear electromechanical actuator comprising a housing, an electric motor comprising a stator fixed to the inner surface of the housing, and a rotor rotatably mounted within the housing, a roller drive comprising a threaded cylinder having inner thread, the cylinder being rotatably mounted within the housing on a bearing support, an output shaft having a threaded portion, the output shaft being slidably mounted within said cylinder, and threaded rollers mounted within said threaded cylinder to engage with the inner thread of the threaded cylinder and the threaded portion of the output shaft, wherein the rollers are further configured to transfer force from said threaded cylinder to the output shaft, wherein the rotor of the electric motor is formed by fixing magnets to the outer surface of the roller drive threaded cylinder, and wherein the housing is provided with at least one closeable opening for supplying lubricant into the internal space of the threaded cylinder and to said bearing support, and at least one closeable opening for removing said lubricant from the bearing support and from the internal space of the threaded cylinder. 
     Advantageously, the closeable openings for supplying and removing lubricant allow lubricating actuator parts without disassembling the actuator, thus simplifying maintenance and providing more efficient operation of the linear electromechanical actuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Detailed description of the embodiments of the present invention is provided below with reference to accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional view of the electromechanical actuator according to the preferred embodiment; 
         FIG. 2  is a detail view of an opening for supplying lubricant into the electromechanical actuator; 
         FIG. 3  is a detail view of another opening for supplying lubricant into the electromechanical actuator. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to one of the preferred embodiments shown in  FIG. 1 , the disclosed electromechanical actuator generally comprises a housing assembly, an electric motor and a roller drive, wherein the motor and the drive are arranged in the housing assembly to engage with each other. 
     The roller drive comprises an internally threaded cylinder (nut)  1 , the nut being rotatably mounted in the housing assembly and having internal thread  16 , the roller drive further comprises threaded rollers  2 , a threaded portion (screw)  3  and an output shaft (rod)  4  having a roller drive rod seal  22 , wherein screw  3  and rod  4  are mounted coaxially with the nut  1 . Screw  3  is connected to the rod  4  so that said parts are prevented from relative movement. The rollers  2  are evenly spaced along the circumference of the screw  3  and contact the internal thread of the nut  1 . 
     The length of the internal thread  16  defines the travel of the screw  3 . The screw  3  has a threaded portion with length approximately equal to the length of the rollers  2 . Rollers  2  are arranged on the screw  3  so that linear movement thereof along the screw axis is prevented, while planetary movement of the rollers  2  by rolling along threads of the screw  3  and the nut  1  is enabled. Rotation of the nut  1  enables linear movement of the rollers  2  and the screw  3  with respect to the nut  1 , and therefore reciprocating movement of the rod  4 , the output member of the electromechanical actuator, is provided. 
     The linear actuator further comprises a rotation angle sensing unit  5 , wherein the unit  5  comprises a shaft  13  connected to the nut  1  for conjoint rotation therewith. The shaft  13  is cylindrical hollow shaft mounted on the nut  1  so that the internal space of the nut  1  communicates with the internal space of the shaft  13 . The shaft  13  has at least one through radial channel  23 . 
     The electric motor comprises an electric motor stator  11  and an electric motor rotor. Permanent magnets  12  of the electric motor rotor are fixed to the nut  1  and evenly distributed along the circumference thereof, and therefore the electric motor rotor is formed by the nut  1  and the shaft  13  of the sensing unit  5  coupled to the nut  1 . The rotor is rotatably mounted in the stator  11  on a first bearing support  15  formed by a sealed radial bearing, and on a second bearing support  14  comprising two radial-thrust bearings. The bearing of the support  15  is mounted on the shaft  13 , while bearings of the support  14  are mounted on the nut  1 . The bearing support  14  allows transferring axial load from the nut  1  to the housing assembly. 
     As noted above, the electric motor and the roller drive of the inventive actuator are housed in the housing assembly. The housing assembly comprises a flange  6 , a stator housing  7 , a rotation angle sensing unit housing  8 , and caps  9 ,  10 . Said parts  7 ,  8 ,  9 ,  10  of the housing assembly are coupled by means of threaded connection. Caps  9 ,  10  can be configured to connect with other external parts or units, and for that purpose they can be provided, e.g. with openings or protrusions, the caps  9 ,  10  further may be equipped with additional sealing elements to separate the internal space of the housing assembly from the external environment. 
     According to one of the preferred embodiments, the housing  8  comprises a sealed enclosure receiving the sensing unit  5 . The enclosure is sealed by a lip seal  21  mounted on the shaft  13  protruding from the enclosure to engage with the nut  1  as disclosed above. Similarly, the housing  7  comprises a sealed enclosure receiving electric parts of the electric motor, i.e. stator  11  and magnets  12  of the rotor. The enclosure is sealed by a lip seal  18  mounted on the nut  1 . The enclosures are further separated from one another by a sealing ring  24  mounted in the housing  8  in abutment to the bearing  15 . 
     In operation, the bearings of the support  14  and roller drive parts  1 ,  2 ,  3  need to be lubricated. According to the invention said parts are lubricated without disassembling and dismantling the actuator, by supplying lubricant into the relevant enclosures of the linear actuator via corresponding openings shown in  FIGS. 2 and 3 . The lubricant is preferably one of the commercially available plastic lubricants. 
     According to one of the preferred embodiments, the housing  8  comprises closable opening  20  for supplying lubricant to roller drive parts  1 ,  2 ,  3 . As can be seen on  FIG. 2 , the opening  20  may be formed in the housing  8  radially to provide a channel allowing the lubricant to pass from the outside of the housing  8  to the through radial channel  23  in the shaft  13  of the unit  5 . The opening  20  protrudes to the radial channel  23  of the shaft  13  to communicate therewith, thus allowing lubricant to further flow into the internal space of the shaft  13 . The channel formed by the opening  20  is separated from the enclosure receiving the sensing unit  5  by the lip seal  21  and separated from the enclosure receiving the electric motor parts by the sealing ring  24  mounted in the housing  8 . The lubricant, once supplied, flows through the opening  20  into the internal space of the shaft  13  through the radial channel  23  and further into the internal space of the nut  1  to lubricate the roller drive parts, wherein the lubricant is prevented from flowing to the sensing unit  5  and stator  11  of the motor by the sealing elements. The opening  20  is provided with thread so that the opening  20  can be closed with a threaded sealing plug  25  or threaded tube for supplying lubricant may be attached to the opening  20 . 
     According to one of the preferred embodiments, the housing  7  has additional closable opening  17  for supplying lubricant to radial-thrust bearings of the support  14 , the opening being made in the housing  7  wall. As mentioned above, the bearing support  14  is separated from the enclosure housing the stator  11  by the lip seal  18 . As can be seen on  FIG. 3 , the opening  17  may be made above the lip seal  18  to reduce the linear dimension of the actuator. The lip seal  18  abuts a circumferential baffle, wherein the baffle has an annular groove in the vicinity of the bearing support  14  to provide communication of the support  14  with the opening  17 . The opening  17  is provided with thread so that it can be closed with a threaded sealing plug  27  or threaded tube for supplying lubricant may be attached to the opening  17 . 
     In order to remove spent lubricant from the linear actuator, closable opening  19  (not shown in  FIGS. 2 and 3 ) is used. According to one of the preferred embodiments, the opening  19  is made in the flange  6 . The opening  19  forms a draining channel for removing spent lubricant from the actuator, the opening  19  communicates with the bearing support  14  and with the internal space of the nut  1  to allow removing (draining) spent lubricant from the actuator. The opening  19  is provided with thread so that it can be closed with a threaded sealing plug  26  or threaded tube for removing lubricant may be attached to the opening  19 . 
     According to one of the preferred embodiments, the inventive linear electromechanical actuator operates as follows. When electrical current is supplied to the stator  11 , a rotating electrical field of the stator is generated, causing rotation of the rotor with permanent magnets  12 . The rotation of the rotor (nut  1 ) causes rotation of threaded rollers  2  and progressive movement thereof along with the screw  3  with respect to the nut  1 . Said progressive movement causes progressive movement of the rod  4 , the output member of the linear actuator. In operation, openings  17 ,  19  and  20  are closed with sealed plugs. 
     Before replacement of lubrication the linear actuator is turned off. Further, access to openings  17  and  20  for supplying lubricant and to opening  19  for removing lubricant is provided by unscrewing corresponding plugs. Then lubricant is supplied into the corresponding openings  17  and  20 . The lubricant supplied through the opening  20  flows towards the shaft  13  of the unit  5  along corresponding channel and, passing through radial channel  23  in shaft  13 , enters the internal space of the nut  1 . The lip seal  21  prevents the lubricant from entering the enclosure housing the sensing unit  5 , while the sealing ring  24  mounted in the housing  8  prevents the lubricant from entering the enclosure housing the stator  11 . Further, the lubricant flows through parts  1 ,  2 ,  3  of the roller lubricating them. The lubricant supplied through the opening  17  flows towards the bearings of the support  14 , wherein the lip seal  18  prevents the lubricant from entering the enclosure housing the stator  11 . The lubricant supplied into openings  17  and  20  flows towards the opening  19 . Lubricant replacement is achieved by forced supplying lubricant into the corresponding openings to force out spent lubricant, and does not require dismantling and disassembly of the linear actuator. The lubricant may comprise any plastic lubricant or liquid lubricant suitable for use in linear electromechanical actuators of the described type. 
     According to another preferred embodiment, the lubrication may flow both through the roller drive parts and through the electric motor parts to lubricate and cool them. For that purpose the sealing ring  24  on the bearing  15  and the lip seal  18  may be omitted from the actuator. 
     According to still another preferred embodiment the rotor of the electric part may have only one bearing support  14 , while the additional support of the rotor during rotation thereof is provided by magnetic field of the stator  11 . 
     According to yet another preferred embodiment, the shaft  13  of the sensing unit as well as unit  5  itself may be omitted so that the lubricant supplied to the opening  20  flows towards the open end of the nut  1  to enter internal space thereof. 
     According to yet another preferred embodiment, the opening  17  may be omitted. The bearing support  14  may be lubricated by a lubricant flowing from the internal space of the nut  1  to the opening  19 . 
     It should also be mentioned, that replacement of the lubricant may be provided during operation of the actuator. For that purpose threaded tubes for supplying lubricant are connected to the openings  17 ,  20  and threaded tube for draining the lubricant is connected to the opening  19 . This is especially suitable for liquid lubricants used both for lubrication and cooling the working surfaces. The lubricant is supplied and drained through the tubes during operation of the actuator. The centrifugal force generated by rotation of the rotor shifts the lubricant supplied to the nut  1  via opening  20  and the channel of the shaft  13  to the periphery of the nut  1  and, thus, assists in proper lubrication of the working surfaces, i.e. particularly the threaded portions of the nut  1 , rollers  2  and screw  3 . 
     It is to be understood that the present invention is not limited by the disclosed embodiments of the actuator, which may be combined with each other or implemented separately. It is apparent for those skilled in the art that another embodiments and modification of the disclosed invention are possible without departing from the spirit of the invention or the scope of the appended claims. 
     REFERENCE NUMERALS IN THE FIGURES 
     
         
         
           
               1 . Internally threaded cylinder (nut) 
               2 . Threaded roller 
               3 . Threaded portion (screw) 
               4 . Output shaft (rod) 
               5 . Rotor rotation angle sensing unit 
               6 . Flange 
               7 . Stator housing 
               8 . Rotor rotation angle sensing unit housing 
               9 . Cap 
               10 . Cap 
               11 . Electric motor stator 
               12  Permanent magnets of electric motor rotor 
               13 . Rotor rotation angle sensing unit shaft 
               14 . Second bearing support 
               15 . First bearing support 
               16 . Internal thread 
               17 . Closable opening 
               18 . Lip seal 
               19 . Closable opening 
               20 . Closable opening 
               21 . Lip seal 
               22 . Roller drive rod seal 
               23 . Through radial channel 
               24 . Sealing ring 
               25 . Threaded sealing plug 
               26 . Threaded sealing plug 
               27 . Threaded sealing plug