Abstract:
An actuator ( 1 ) for the linear displacement of a member to be controlled comprising a motor part ( 2 ) and an actuating device part ( 3 ) including a threaded screw ( 11 ) with linear displacement, and a rotary member ( 9 ) provided with a threaded portion ( 10 ) matching the screw thread, the rotary member capable of being driven in rotation by the motor part and being supported by bearings ( 15,16 ) in the form of roller bearing stops with four contact points arranged on either axial side of said threaded portion to guide the rotary member axially and radially. The raceways of the balls are directly in the rotary member and integral with parts of the body ( 14 ) or the cover ( 13 ) of the actuator.

Description:
The present invention concerns a linear or rotary actuator comprising a member rotated by an electric motor for the displacement of a member to be controlled. The member may more particularly be a nut in a screw and nut mechanism for the linear displacement of a member to be controlled. 
     BACKGROUND OF THE INVENTION 
     Linear actuators are very common and are used in many different applications, with examples being described in the European patent applications EP-A-468 920 and EP-A-987 477. The devices described in these publications comprises a step motor driving a screw and nut mechanism for the linear displacement of a shaft integral with a screw. The step motor allows the screw shaft to be rapidly displaced and positioned with few mechanical carts and using relatively simple controls. 
     In EP-A-468 920, a pinion mounted on the motor axle engages with a wheel that is integral with the nut. A hub of the wheel is sandwiched between flanges of a housing for axial positioning of the wheel. This axial bearing is not very rigid and performs poorly owing to friction. In certain applications, on the other hand, the electric motor must be separated from the member to be displaced, for reasons of sealing or safety such as are present in supply systems for combustible gases or liquids. A device such as that described in EP-A-468 920 cannot be used, since the separation between motor and screw is insufficient. 
     In EP-A-987 477, permanent magnets are mounted on the nut which thus is driven directly by the magnetic field created by the coils of the motor. The nut is mounted in the motor on just one bearing. The axial and radial support of the nut is very loose and thus unstable, both with respect to static and with respect to dynamic forces. Yet in many applications, stability is an important criterion, since the screw is coupled to the nut that at the same time serves as a guide determining the stability and the axial and radial positioning of the screw. 
     It is necessary not only to improve the efficiency and reliability of an actuator of the type cited, but also to reduce the price and size of these devices. 
     The device described in EP-A-987 477 is provided with a partition wall between the coils of the motor and the magnets mounted on the nut, for applications where a seal between the motor and the member to be controlled is necessary. This partition wall comprises a flange resting on a housing of the motor&#39;s stator part on one hand, and on a seal of the type of an O-ring sandwiched between the flange and a cover part of the actuator on the other hand. The reliability of sealing thus depends on the quality of the seal, which may deteriorate with time, may move when exposed to impacts, or may be poorly installed, for instance at the time of manufacturing of the motor. It will be advantageous to improve the reliability and safety of such devices, particularly in applications involving combustible liquids. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to realize an actuator that is efficient, reliable and not very expensive. 
     It will be advantageous for applications where sealing or separation is required between the electric motor and the member to be controlled, to realize an actuator that has reliable sealing. 
     It will be advantageous, moreover, to realize a device having few parts so as to reduce manufacturing and assembly costs. 
     It will also be advantageous to realize a linear or rotary actuator that is compact yet rigid and precise. 
     Objects of the invention have been realized by an actuator according to claim  1 . 
     In the present invention, an actuator for the linear displacement of a member to be controlled comprises a motor part and an actuating device part including a screw moving linearly, and a rotary member having a threaded portion matching the screw&#39;s thread that can be rotated by the motor part, the rotary member being supported by bearings in the form of thrust ball bearings having four contact points located on each axial side of a threaded portion of the rotary member so as to axially and radially guide this member, the raceways of the balls being directly integrated, on one hand into the rotary member and on the other hand into the housing or housing parts of the actuator. 
     The raceways are preferably formed from stamped steel metal that may be hardened by a tempering operation. The raceways may be attached to the rotary member and to the portions of the housing or flange by elastic tongues that allow parts to be snapped in an axial direction corresponding to the direction of the axis of rotation of the rotary member while the screw, the balls and the flanges may also all be mounted in an axial direction on the motor part. The raceways may also be glued, welded or molded to said nut and to the housing or housing parts. 
     This design advantageously allows the costs of manufacture of the parts and of assembly to be appreciably reduced. The thrust ball bearings with four contact points present on each side of the rotary member yield a highly rigid, stable, precise and compact design. 
     The points of contact between the balls and the surfaces inclined radially inward and radially outward are arranged so as to satisfy the relation: A/B=C/D, where D and C are the radii of the contact points radially inward and radially outward from the axis of rotation of the rotary member, and B and A are the diameters of the trajectories of the contact points radially inward and radially outward on the ball. This arrangement of the contact points ensures that the balls will roll on the raceways without slipping. 
     According to another aspect of the present invention, an actuator for the linear or rotary displacement of a member to be controlled comprises a motor part, an actuating device part with a rotary member having magnetized portions, the rotary member being able to be driven by the motor part, an air gap being formed between the magnetized portions and the motor part, the actuator in addition comprising a continuous partition wall between the motor part and the actuating part and having a portion located in said air gap, characterized by the fact that a portion of the partition wall has the shape of an outer flange that can be mounted onto a support or onto a assembly wall of a device to be controlled. 
     This economic design has the advantage of providing a reliable separation between the motor part and the actuating part constituting a seal, not only with respect to fluid leakage but also from an electric point of view. Electric arcs originating in the electric motor part are thus highly effectively and reliably separated from the actuating part. 
     The actuating part can be designed as part of a screw and nut mechanism comprising a screw with linear displacement driven by a nut forming the rotary member coupled to the motor. 
     The magnetized portions may comprise one or several magnets fixed to or integral with the nut. The magnets on the rotary member preferably are permanent magnets but may also be electromagnets. 
     The partition wall or at least the portion located in the air gap may be made of a magnetic material so as to boost the magnetic flux across the air gap. 
     The partition wall may be made of an electro-conductive material such as a steel sheet that on one hand is strong and inexpensive and on the other hand ensures an electric and physical separation of the motor part from the screw and nut mechanism. 
     The partition wall may advantageously function as a structural element for the assembly and positioning of the motor part and of the actuating part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantageous aspects of the invention will become apparent from the claims and description and from the appended drawings in which: 
         FIG. 1  is a sectioned view of a linear actuator according to a first embodiment of the invention; 
         FIG. 2  is a detailed sectioned view of one of the bearings of an actuator according to the invention; 
         FIG. 3  is a detailed sectioned view of another embodiment of a bearing of an actuator according to the invention; 
         FIG. 4  is a perspective view of the bearing according to  FIG. 3 ; and 
         FIG. 5  is a detailed sectioned view of a variant of a bearing according to FIG.  3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , an actuator  1  comprises an electric motor part  2 , an actuating part  3  and a partition wall  4 . The electric motor part  2  in these examples comprises a step motor having aspects similar to those of conventional step motors, such as the stator  5  with two portions  6  with coils separated by an air gap  7  of permanent magnets  8  mounted on a rotary member  9  of the actuating part  3 . The use of a step motor is advantageous inasmuch as it allows an easy, rapid setting of the position of the member to be controlled in a compact, not very expensive design. Nevertheless, other types of reversible motors may be used in the present invention. 
     The actuating part  3  comprises the rotary member  9  provided with a threaded portion  10  engaging with a matching member in the form of a screw  11  having a threaded portion  12 , a cover part  13 , a housing part  14  and bearings  15 ,  16  supporting the rotary member  9  when rotating. 
     In the embodiment illustrated, rotation of the rotary member  9  causes axial displacement of the screw  11  that is provided with axial guiding elements  17  cooperating with a matching axial guiding shape or elements of cover part  13  blocking rotation of the screw. The screw  11  may be coupled to a member to be displaced (not illustrated), for instance by fastening it to a threaded portion  18  of the screw protruding beyond the cover part  13 . 
     Other variants are possible, however, without going beyond the scope of the invention. For example, the rotary member may be integral with a screw that linearly displaces a nut coupled to the member to be controlled, or being itself part of the member to be controlled. In another variant the screw may be replaced by other members, for instance an axle for rotary actuation of the member to be controlled. 
     In the variant illustrated, screw  11  may for instance be coupled to a member controlling the flow rate of a fuel gas in a fuel supply or draining system, a specific example being a system for adjusting the flow rate in gas burners. 
     For reasons of safety, it will be important in such applications to separate the electric motor part from the part actuating the valve or other member in or near a combustible fluid. 
     In the present invention, the partition wall  4  arranged between the actuating part  3  and the motor part  2  is continuous, and extends to the outside of the actuator, thus forming a very efficient and reliable separation between these parts. 
     In advantageous embodiments, the partition wall  4  is formed by a conducting wall such as metal sheet that can be connected electrically to the installation being controlled (and to ground) so that there will be no electric potential difference between this wall and the installation being controlled. 
     The wall allows the motor part to be separated from the screw and nut mechanism by a physical and electric seal, particularly when the wall is electrically conducting. 
     The partition wall  4  comprises a cylindrical portion  19  in the air gap  7  between stator  5  and the magnets  8  on the rotary member  9 , a bottom part  20  and an external part  21  in the form of a flange having a surface  32  to be mounted on a support or on a wall of a device to be controlled. The partition wall  4  or at least the portion  19  in the air gap may be made of a material having a good magnetic permeability, so as to boost the magnetic flux between the stator  5  and the magnets  8 . 
     The stator  5  of the motor is disposed around the partition wall  4 , and the actuating part  3  is disposed inside the cylindrical portion  19  through an axial and a radial positioning surface  33 ,  34  of the housing part  14  and through positioning surfaces  35  of the cover part  13 , all these surfaces resting against the partition wall. 
     The cover part  13  is retained by mechanical fastening means such as an elastic ring or a check spring  36  engaged, on one hand with the cover part and on the other hand with the partition wall. 
     Advantageously, the partition wall is at once a structural element allowing the motor part to be assembled with the actuating part  3 . 
     Bearings  15 ,  16  of the rotary member are thrust ball bearings with three or four contact points for the axial and radial positioning of the rotary member, one bearing being located on each side of the threaded portion  10  of the rotary member. One of the bearings  15  is located between the cover part  13  and the rotary member  9 , the other bearing  18  is located between the rotary member and the housing part  14  that is mounted to the partition wall  4 . The raceways  22 ,  23  of the ball bearing  15  are integral with the cover part  13  and rotary member  9 , respectively, while the raceways  24 ,  25  of the thrust ball bearing  16  are integral with the rotary member  9  and housing part  14 , respectively. 
     The raceways are advantageously made of stamped steel metal. They may then be welded, glued, molded, or mechanically attached to the cover part, rotary member or housing part. Advantageously, this yields a particularly compact and rigid design having economic manufacturing costs. In fact, the actuator consists of few parts that are easy to assemble, which strongly reduces the manufacturing costs. It should be noted, for instance, that parts such as the ball bearings, the rotary member, the cover part, the partition wall and the motor may be assembled in an axial direction, thus facilitating the automation of the actuator assembly procedures. 
     Referring now to  FIG. 2 , the inclined walls radially inward  26  and radially outward  27  of the raceways are arranged so as to satisfy the relation: A/B=C/D, where D and C are the radii of the contact points radially inward  28  and radially outward  29  from the axis of rotation  30  of the rotary member, while B and A are the diameters of the trajectories of the contact points radially inward  28  and radially outward  29  on the ball. When this relation is obeyed, the balls  31  will roll on the raceways  22 ,  24  and  23 ,  25  without slipping, and hence with a minimum of wear and resistance. 
       FIGS. 3 and 4  show another embodiment of the bearings in which the axis of rotation  37  of the balls  31  is inclined relative to the plane  38  passing through the centers of the balls. One of the raceways  22 ′ is located radially outward from the balls, the other raceway  23 ′ is located radially inward from the balls. In this embodiment, the inclined walls radially inward  26 ′,  26 ″ and radially outward  27 ′,  27 ″ of the raceways are again arranged so as to satisfy the relation: A/B=C/D, where D and C are the radii of the contact points radially inward  28 ′ and radially outward  29 ′ from the axis of rotation  30  of the rotary member, while B and A are the diameters of the circles travelled by the contact points radially inward  28 ′ and radially outward  29 ′ on the ball  31 . When this relation is obeyed, the balls will roll on the raceways without slipping. This arrangement of the raceways, which leads to the inclination of the axis of rotation  37  of the balls, allows the bearings to sustain higher radial forces than the bearings according to FIG.  2 . In other words, the bearings according to  FIGS. 3 and 4  have a higher radial rigidity than the bearings according to FIG.  2 . 
     The inner raceway  23 ′ can be replaced by a spherical surface such as that shown in  FIG. 5 , or by a conical surface, so that there will be no more than three contact points between each ball and the raceways. The conical or spherical surface may be realized by a raceway, for instance made of stamped steel metal and hardened by tempering, or machined or otherwise formed, directly on the rotary member  9 .