Patent Publication Number: US-2020300343-A1

Title: Linear actuators

Description:
FOREIGN PRIORITY 
     This application claims priority to European Patent Application No. 19164773.4 filed Mar. 23, 2019, the entire contents of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to linear actuators and in particular to electromechanical linear actuators. Such actuators may be used to actuate control and other surfaces in aircraft, for example slats, flaps, thrust reverser doors and so on. 
     BACKGROUND 
     Presently such actuators typically comprise a ball screw shaft which is driven by an electric motor, for example a brushless DC motor. The ball screw shaft drives an output piston through a ball nut which is mounted to the output piston. The balls of the ball screw are recirculated through recirculation passages formed in the nut. Lubricant is typically retained within the ball nut by scraper seals formed between the grooves of the ball screw shaft and the ball nut. 
     While such a construction provides satisfactory operation, the radial dimensions of the actuator may be relatively large and the actuator may need regular servicing to maintain lubricant in the region of the ball nut. In the aerospace industry at least, size is a significant factor, as is the desire to reduce the mean time between overhaul of components. 
     SUMMARY 
     In accordance with the disclosure there is provided a linear electromechanical actuator which comprises an electric motor, a ball screw shaft driven by the electric motor and a tubular output shaft receiving the ball screw shaft and rotationally coupled thereto. The ball screw shaft has at least one helical groove formed on a radially outer surface thereof and the output shaft has at least one helical groove formed in a radially inner surface thereof. A plurality of ball elements is received within the grooves for rotationally coupling the ball screw shaft to the output shaft. The ball screw shaft further comprises a ball recirculation element mounted in the radially outer surface of the ball screw shaft. The ball recirculation element interrupts the helical groove in the ball screw shaft and has one or more ball recirculating passages. Each ball recirculating passage comprises an inlet portion a recirculation portion and an outlet portion. The inlet portion deflects balls into the recirculation portion from a first portion of the helical groove and the outlet portion deflects balls from the recirculation portion back into a second portion of the helical groove. 
     The recirculation portion may comprise a passage which extends around a radially outer circumferential portion of the ball screw shaft radially inwardly of the radially outer surface of the ball screw shaft to thereby recirculate the balls from the first portion of the helical groove to the second portion of the helical groove through a radially outer portion of the ball screw shaft. 
     The ball recirculation element may be mounted in a slot or groove formed in the radially outer surface of the ball screw shaft. 
     The ball recirculation element may be press fitted, bonded or fastened into the slot or groove. 
     The inlet and outlet portions of the ball recirculation passage may project into the groove of the output shaft to deflect balls therefrom into the recirculation passage. 
     The inlet and outlet portions of the ball recirculation passage may be curved. 
     The recirculation portions of the ball recirculation passage may, in projection, be straight and have an axis which arranged at an angle to the longitudinal axis of the ball screw shaft. 
     The ball recirculation element may be formed as a unitary, one piece body. 
     In an alternative arrangement, the ball recirculation element may comprise two components joined together, the ball recirculation passage being formed at the interface of the two components. 
     The output shaft may comprise a distal end remote from the motor and a proximal end closer to the motor. The distal end of the output shaft may be closed, optionally by a connecting eye for attaching the output shaft ( 8 ) to an element to be actuated. The actuator may further comprise a seal between the proximal end of the output shaft and a cylindrical, ungrooved portion of the ball screw shaft to retain lubricant in a chamber formed between the seal and the closed end of the output shaft and in which the grooved portion of the ball screw shaft is arranged. 
     The seal may be mounted in a groove formed in a radially inner surface of the proximal end of the output shaft. 
     The seal may be a garter seal. 
     The actuator may further comprise a housing receiving the motor, the output shaft being slidably mounted within a bore of the motor housing. 
     A scraper seal and/or a linear bearing may be mounted between the housing and a radially outer surface of the output shaft. 
     A torque reactor may be provided between the housing and the output shaft. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some embodiments of the disclosure will now be described by way of example only with reference to the accompanying drawings in which; 
         FIG. 1  shows an actuator in accordance with the disclosure; 
         FIG. 2  shows a detail of the ball screw shaft of the actuator of  FIG. 1 ; 
         FIG. 3  shows a schematic cross-section along the line A-A of  FIG. 2 ; 
         FIG. 4  shows a schematic cross-section of a first embodiment of actuator in accordance with the disclosure taken along a line corresponding to line B-B of  FIG. 2 ; 
         FIG. 5  shows a schematic cross-section a second embodiment of actuator in accordance with the disclosure taken along a line corresponding to line B-B of  FIG. 2 ; and 
         FIG. 6  shows an alternative construction of ball recirculation element. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , an electromechanical actuator  2  comprises a motor  4 , a ball screw shaft  6  driven by the motor  4  and an output shaft  8  driven by the ball screw shaft  6 . 
     The motor  4  is, in this embodiment, a brushless DC motor comprising a stator  10  mounted in a bore  12  of a motor housing  14  and a rotor  16  mounted on a portion  18  of the ball screw shaft  6 . The ball screw shaft  6  is therefore the motor shaft in this embodiment. 
     One end  20  of the ball screw shaft  6  is supported in the motor housing  14  by means of a bearing  22 . The bearing  22  may be a dual row angular contact bearing which may act as a thrust bearing to carry thrust from the output shaft  8  and may be preloaded to eliminate axial play in the actuator  2 . The bearing  22  is located against a shoulder  24  of the motor housing  14  and fixed in position by a cap  26  which is mounted to an end  28  of the motor housing  14  by fasteners such as bolts  30 . The bearing  22  is retained on the ball screw shaft  6  by means of a nut  32 , tab washer  34  and spacer  36 . 
     The cap  26  has an eye  38  which may typically comprise a spherical bearing for attachment to a static structure (not shown) of an aircraft or other structure. 
     The output shaft  8  is slidably received in the bore  12  of the motor housing  14 . To seal the output shaft relative to the motor housing  14 , a scraper seal  40  is mounted in a groove  42  at a distal end  44  of the motor housing bore  12 . Such types of seal are well known in the art and need not therefore be described in further detail here. 
     To facilitate sliding of the output shaft  8  in the motor housing bore  12 , a linear bearing  46  is provided inboard of the scraper seal  40 . In this embodiment, the linear bearing  46  is received in a further groove  48  formed in the motor housing bore  12 . The linear bearing  46  may, for example, comprise a sleeve of low friction material such as PTFE. 
     The output shaft  8  is a tubular element having a helical groove  50  extending along an internal surface of the internal bore  52  of the output shaft  8 . A distal end  54  of the internal bore  52  of the output shaft  8  is closed by an eye element  56 . The eye element  56  is received in a threaded end portion  58  of the internal bore  52  of the output shaft  8  and the end of the output shaft sealed by an O-ring or similar seal  60 . The eye element  56  is locked in position by means of a tab washer or similar  62 . The eye element also comprises an eye  64  which may also comprise a spherical bearing for attachment to an element such as a flap, slat or other movable element to be actuated. 
     A hinged link type torque reactor  66  extends between the distal end  44  of the motor housing  14  and the eye element  56  to prevent the output shaft  8  rotating relative to the motor housing  14 . Other forms of torque reactor may be provided. 
     The output shaft  8  is extended from and retracted into the motor housing bore  12  in response to rotation of the ball screw shaft  6  by the motor  4 . As can best be seen in  FIG. 2 , the ball screw shaft  6  comprises a grooved portion  68  which comprises at least one helical groove  70  formed on a radially outer surface  72  of the ball screw shaft  6 . The ball screw shaft  6  further comprises an ungrooved, cylindrical portion  74 . The pitch and the helix angle of the helical grooves  70 ,  50  provided on the ball screw shaft  6  and the output shaft  8  are the same. 
     A plurality of balls  76  are received in the channel formed between the respective helical grooves  50 ,  70 . Rotational movement of the ball screw shaft  6  is transmitted to the output shaft  8  via the balls  76 . However, due to the presence of the torque reactor  66 , the output shaft  6  cannot rotate and therefore moves linearly into and out of the motor housing  14 , depending on the direction of rotation of the ball screw shaft  6 . The position of the output shaft  8  may be monitored by one or more sensors (not shown). 
     The balls  76  must be recirculated to allow proper functioning of the actuator. In existing actuators, this recirculation is normally effected through a nut which is mounted within the output shaft  8 . However, in the actuator  2  of the present disclosure, no such nut is provided and recirculation occurs in the ball screw shaft  6 . This arrangement is potentially advantageous in that it allows for a reduction in the diameter and therefore size and weight of the actuator  2 . 
     In order to effect recirculation of the balls  76  in the ball screw shaft  6 , the ball screw shaft  6  is provided with a ball recirculation element  80  which is mounted in the radially outer surface  72  of the grooved portion  68  of the ball screw shaft. Two embodiments of ball recirculation element  80  are disclosed herein. The first is illustrated in  FIGS. 2, 3 and 4  and the second in  FIGS. 2, 3 and 5 . The two embodiments are generally similar and differ only in certain details which will be discussed further below. 
     In the embodiments illustrated, the ball recirculation element  80  is mounted in a groove or slot  84  formed in the radially outer surface  72  of the grooved portion  68  of the ball screw shaft  6 . The slot or groove  84  may, for example, be machined into the ball screw shaft  6 . The ball recirculation element  80  may for example be press fitted bonded or fastened, into the slot or groove  84 . 
     The ball recirculation element  80  interrupts the helical groove  70  of the ball screw shaft  6  and has, in this embodiment, two ball recirculating passages  86 . Depending on the particular actuator  2 , more or fewer ball recirculating passages  86  may be provided. 
     Each ball recirculating passage  86  comprises an inlet portion  88 , a central recirculation portion  90  and an outlet portion  92 . The inlet portion  88  acts to deflect the balls  76  into the recirculation portion  90  from a first portion of the helical groove  70  of the ball screw shaft  6 . The outlet portion  92  deflects the balls  76  from the recirculation portion  90  back into a second portion of the helical groove  70 . The recirculation element  80  therefore creates a closed recirculating path for the balls  76 . In the disclosed embodiment, there are therefore two closed recirculation paths for the balls  76  and the balls  76  will not enter the central groove portions  94 , for example. 
     The recirculation portion  90  of the recirculation passage  86  comprises a passage  98  which extends around an outer circumferential portion of the ball screw shaft  6  radially inwardly of the radially outward surface  72 . This can be seen most clearly from  FIG. 3 . This recirculates the balls  76  from the first portion of the helical groove  70  to the second portion of the helical groove  70  through a radially outer portion  100  of the ball screw shaft  6 . It will therefore be seen in the embodiments of this disclosure, that the ball recirculation insert  80  is arranged only in the radially outer portion  100  of the ball screw shaft  6 . This avoids weakening the ball screw shaft  6  and considerably facilitates manufacture of the ball screw shaft  6  as no bores need to be formed through the ball screw shaft  6  to accommodate the ball recirculation insert  80  or to form recirculation passages within the ball screw shaft  6 . 
     As can be seen in  FIG. 3 , the insert  80  may be formed as a unitary body  102  having the recirculation passage  86  formed therein. The recirculation passage may be open on its radially outward side as shown to allow lubricant access. For example a slot  104  may be formed in the insert  80  as shown, with a smaller width that the diameter of the balls  76  to retain the balls  76  in the recirculation passage  86 . In other embodiments, however, the recirculation passage  86  may be closed on its radially outward side. 
     In an alternative embodiment illustrated in  FIG. 6 , the insert  80  may be formed from two components  106 ,  108  joined together, the ball recirculation passage  86  being formed at the interface between the two components  106 ,  108 . Dowels  110  may be provided to accurately locate the two components  106 ,  108  relative to each other. The two components  106 ,  108  may be joined by any suitable technique for example bonding or by using fasteners. Although shown as closed, the recirculation passage  86  may be radially outwardly open as in the embodiment of  FIG. 3 . 
     The insert  80  may be made of any appropriate material. Example materials include plastics, aluminium and bronze, depending on the application. The material may be a low friction material such as PTFE. 
     The insert  80  may be made by any suitable technique such as moulding, casting, machining or additive manufacturing. 
     The recirculation passage  86  extends in both circumferential and axial directions around the ball screw shaft  6 . As shown, the central recirculation portion  90  of the passage  86  may have an axis A which in projection is a straight line arranged at an angle α to the axis X of the ball screw shaft  6 . The angle α may be between 0° and 60°. The inlet and outlet portions  88 ,  92  curve relative to that axis as shown. The recirculation passage therefore has a shallow S shape in this embodiment. In other embodiment, the recirculation portion  90  may be curved. 
     In the embodiment illustrated in  FIG. 4 , the inlet portion  88  and outlet portion  92  of the recirculation passage  86  open into the radially outer surface  72  of the ball screw shaft  6 . In other words, the inlet portion  88  and outlet portion  92  do not protrude into the helical groove  50  of the output shaft  8 . The balls  76  are deflected into the recirculation passage  86  by the upper corner  120  of the wall  122  of the inlet portion  88  facing the balls  76 . The wall  122  is advantageously inclined at an angle β relative to an axis  124  normal to the radially outer surface  72  of the ball screw shaft  6  to facilitate deflection of the balls  76  into the recirculation passage  86 . In various embodiments, the angle may be up to 45°. 
     The upper corner  126  of the wall  128  of the input portion  86  opposite the wall  122  may, as shown, lie generally flush with the root diameter  130  of the helical groove  68  formed in the ball screw shaft  6 . The corner  126  may be curved or smooth to avoid adverse forces being exerted on the balls  76  as they enter the recirculation passage  86 . 
     The radially outer surface  132  of the insert  80  may, as illustrated, lie flush with the radially outer surface  72  of the ball screw shaft  6 . 
     The geometry of the outlet portion  92  of the recirculation passage  86  is, in effect, a mirror image of that of the inlet portion  88 , since when the direction of rotation of the ball screw shaft  6  is reversed, it will act as the input portion  88  of the recirculation passage  86 . 
     In the embodiment of  FIG. 5 , to encourage deflection of the balls  76  into the recirculation passage  186  of an insert  180 , the inlet portion  188  and outlet portion  192  thereof protrude from the radially outer surface  72  of the grooved portion  68  of the ball screw shaft  6  into the helical groove  70  of the output shaft  8 . 
     The protruding sections  194 ,  196  of the inlet and outlet portions  188 ,  192  have curved surfaces  198 ,  200  so as to provide a smooth transition from the helical groove  70  into the recirculation portion  190  of the passage  186 . The remainder of the radially outer surface  232  of the insert  180  may, as illustrated, lie flush with the radially outer surface  72  of the ball screw shaft  6 . 
     The upper corner  226  of the wall  228  of the input portion  186  opposite the protruding section of the inlet portion  188  may, as shown, lie generally flush with the root diameter  130  of the helical groove  68  formed in the ball screw shaft  6 . The corner  226  may be curved or smooth to avoid adverse forces being exerted on the balls  76  as they enter the recirculation passage  186 . 
     As in the earlier embodiment, the geometry of the outlet portion  192  of the recirculation passage  186  may be, in effect, a mirror image of that of the inlet portion  188 , since when the direction of rotation of the ball screw shaft  6  is reversed, it will act as the input portion  188  of the recirculation passage  186 . 
     The insert  180  of this embodiment may, other than as described above, include the other features of the insert  80  of the first embodiment. 
     Returning to the overall assembly, it will be seen in  FIG. 1  that a seal  112  is provided at a proximal end  114  of the output shaft  8 . The seal  112  is received in a groove  116  in output shaft  8 . The seal  112  makes sealing contact with the cylindrical, ungrooved portion  74  of the ball screw shaft  6 . This forms a chamber  118  between the distal and proximal ends  54 ,  114  of the output shaft  8  within which the grooved portion  68  of the ball screw shaft  6  rotates. A lubricant  120  is retained in the chamber  118  to lubricate the balls  76  between the ball screw shaft  6  and output shaft  8 . The disclosed seal  112  is advantageous compared to earlier constructions as it is made on the ungrooved portion of the ball screw shaft  6  rather than on a grooved portion of the shaft. This retains lubricant more reliably, leading to the need for less maintenance to be performed on the actuator  2 . 
     From the above, it will be seen that the disclosed actuator has a number of significant advantages over conventional actuators. By recirculating the balls  76  through the ball screw shaft  6 , a separate nut may be dispensed with, allowing an actuator with fewer parts a smaller diameter and lower weight to be produced. Moreover, the number of components is reduced compared to conventional actuators, thereby providing increased reliability. The inertia of the output shaft is also reduced compared with conventional actuators. This may make the actuator suitable for high frequency operations (for example up to 32 Hz) and small stroke applications (for example up to +/−2.5 cm). 
     Recirculating the balls through a radially outer region of the ball screw shaft  6  does not compromise the strength of the ball screw shaft  6 . In addition, it allows the recirculation path for the balls  76  to be provided by an element  80  which is mounted to an external surface of the ball screw shaft  6  only, considerably facilitating assembly of the actuator. Retention of lubricant is also improved by virtue of the sealing arrangement disclosed, leading to less need for maintenance of the actuator. 
     It will be appreciated that the description above is of an exemplary embodiment of the disclosure and that modifications may be made to that embodiment within the scope of the disclosure. For example, while a single recirculating insert  80  having multiple recirculation passages is illustrated, individual inserts  80  each providing just one recirculation path may be provided.