Abstract:
A linear actuator having particular application for reconfigurable fixtures used to hold large area workpieces. The linear actuators include a thrust rod connected to a translatable nut member that is driven by a rotatable drive screw that has a longitudinal axis off-set from the longitudinal axis of the thrust rod that preferably is centrally disposed within the housing of the actuator. The off-set axis of the drive screw enables the drive screw per unit size to handle greater loads and run at higher speeds. Also, the use of a solid thrust rod enables the thrust rod per unit size to handle greater loads and be less expensive as opposed to being hollow when the drive screw is coaxial with the thrust rod. This arrangement also enables less material to be used to manufacture the linear actuator per unit size. The linear actuators may be used as an alternative to existing assemblies with central thrust rods.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/871,469 filed Aug. 29, 2013, which is hereby incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates generally to actuators, and more particularly to linear actuators having particular application in a holding fixture, and more particularly in a holding fixture capable of holding a predetermined contoured workpiece such as an aircraft skin panel. 
       BACKGROUND 
       [0003]    Reconfigurable fixtures heretofore have been used for holding large area workpieces in a proper position for manufacturing. Such fixtures have included a planar array of linear actuators including thrust rods that can be controllably extended collectively to define a contoured support surface for a large area workpiece such as an aircraft skin panel. The ends of the thrust rods may be equipped with holding devices such as suction cups for holding the workpiece to the thrust rods. 
         [0004]    The linear actuators typically included a screw and nut drive assembly for extending and retracting a tubular thrust rod relative to an actuator housing. The tubular thrust rod and screw were coaxial, with the drive screw extending into the hollow interior of the tubular thrust rod. In another known linear actuator, the tubular thrust rod was replaced by a circumferential arrangement of four smaller but solid thrust rods connected between the nut member that was driven by rotation of the screw and an end plate at the opposite end of the thrust rods. The drive nut member was guided by a plurality of guide rods mounted between opposite end plates of the housing. The four smaller thrust rods were each separately sealed, in contrast to a single seal for the tubular thrust rod. 
       SUMMARY OF INVENTION 
       [0005]    The present invention provides a linear actuator having particular application for reconfigurable fixtures used to hold large area workpieces. The linear actuators include a thrust rod connected to a translatable nut member that is driven by a rotatable drive screw that has a longitudinal axis off-set from the longitudinal axis of the thrust rod that preferably is centrally disposed within the housing of the actuator. The off-set axis of the drive screw enables the drive screw per unit size to handle greater loads and run at higher speeds. Also, the use of a solid thrust rod enables the thrust rod per unit size to handle greater loads and be less expensive as opposed to being hollow when the drive screw is coaxial with the thrust rod. This arrangement also enables less material to be used to manufacture the linear actuator per unit size. The linear actuators may be used as an alternative to existing assemblies with central thrust rods. 
         [0006]    According to one aspect of the invention, a linear actuator including a housing, a thrust rod disposed centrally within the housing and having an end portion extending from an end of the housing, the thrust rod having a longitudinal axis, a rotatable drive screw disposed within the housing and having a longitudinal axis parallel to the longitudinal axis of the thrust rod, and a translatable nut member disposed within the housing in threaded engagement with the drive screw and connected to the thrust rod for longitudinal movement therewith, the translatable nut member being driven upon rotation of the drive screw along the longitudinal axis of the drive screw, and wherein the drive screw is radially outwardly offset from the centrally disposed thrust rod. 
         [0007]    According to another aspect of the invention, a linear actuator including a housing having a longitudinal center axis, a thrust rod disposed within the housing and having an end portion extending from an end of the housing, the thrust rod having a longitudinal axis, a rotatable drive screw disposed within the housing and having a longitudinal axis parallel to and offset from the longitudinal center axis of the housing, and a translatable nut member disposed within the housing in threaded engagement with the drive screw and connected to the thrust rod for longitudinal movement therewith, the translatable nut member being driven upon rotation of the drive screw along the longitudinal axis of the drive screw. 
         [0008]    According to another aspect of the invention, a linear actuator including a housing having a longitudinal center axis, a thrust rod disposed within the housing and having an end portion extending from an end of the housing, the thrust rod having a longitudinal axis, a rotatable drive screw disposed within the housing and having a longitudinal axis parallel to the longitudinal axis of the thrust rod, a translatable nut member disposed within the housing in threaded engagement with the drive screw and connected to the thrust rod for longitudinal movement therewith, the translatable nut member being driven upon rotation of the drive screw along the longitudinal axis of the drive screw, and an anti-rotation guide extending along a length of the housing parallel to the longitudinal axis of the drive screw, the anti-rotation guide being located at a side of the thrust rod diametrically opposite the drive screw. 
         [0009]    Optionally, opposite ends of the drive screw are rotatably supported by respective bearings fixed in the housing. 
         [0010]    Optionally, further including an anti-rotation guide configured to prevent rotation of the translatable nut member about the longitudinal axis of the drive screw relative to the housing. 
         [0011]    Optionally, the anti-rotation guide includes a rod extending between opposite ends of the housing and on which the translatable nut member slides. 
         [0012]    Optionally, the anti-rotation guide is disposed on a side of the thrust rod diametrically opposite the drive screw. 
         [0013]    Optionally, the translatable nut member includes a thrust plate to which the thrust rod is attached and a nut attached to the thrust plate. 
         [0014]    Optionally, the longitudinal axis of thrust rod is concentric with the longitudinal axis of the housing. 
         [0015]    Optionally, the thrust rod is movable between a fully retracted position and a fully extended position, and wherein the extent of the thrust rod that is housed within the housing when in the retracted position of the thrust rod has a solid cross-section. 
         [0016]    Optionally, there is only a single said thrust rod. 
         [0017]    Optionally, further including a motor coupled to the drive screw, preferably with the rotational axis of the motor coaxial with the longitudinal axis of the drive screw. 
         [0018]    Optionally, a reconfigurable fixture comprising a base and a plurality of the linear actuators of any above aspect mounted to the base, preferably with the actuators disposed parallel with respect to one another. 
         [0019]    Optionally, the thrust rods of a plurality of the linear actuators are equipped with holding devices such as suction cups for holding the workpiece to the thrust rods. 
         [0020]    Optionally, the drive screw includes a reduced diameter portion at an end of the drive screw. 
         [0021]    The foregoing and other features of the invention are hereinafter described in greater detail with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a partial perspective view of a reconfigurable fixture supporting an aircraft wing, which fixture includes an array of linear actuators. 
           [0023]      FIG. 2  is a perspective view of an exemplary linear actuator useful in the fixture of  FIG. 1 . 
           [0024]      FIG. 3  is perspective view similar to  FIG. 2 , but with part of the actuator&#39;s housing cut-away to show internal components. 
           [0025]      FIG. 4  is a top view of the linear actuator of  FIG. 2 , with part of the housing cut-away to show internal components. 
           [0026]      FIG. 5  is a side view of the linear actuator of  FIG. 2 , with part of the housing cut-away. 
           [0027]      FIG. 6  is a cross-sectional view taken along the line  6 - 6  of  FIG. 5 . 
           [0028]      FIG. 7  is a perspective view of another exemplary linear actuator. 
           [0029]      FIG. 8  is a cross-sectional view of the actuator of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    The principles of the present application have particular application to linear actuators, such as electromechanical linear actuators, for effecting movement of manufacturing components, such as manufacturing workpieces, tools, or tool holders, and thus will be described below chiefly in this context. Such a linear actuator may be suitable for raising, lifting, or otherwise moving or supporting a manufacturing component. It will of course be appreciated, and also understood, that principles of this invention may be applicable to other applications where it is desirable to effect movement, such as in construction equipment, gate operation, etc. 
         [0031]    Referring now in detail to the drawings, and initially to  FIG. 1 , a manufacturing set up  10 , in particular a reconfigurable fixture, is illustrated. The fixture includes a plurality of linear actuators  12  for raising and/or supporting a workpiece  14 , such as an aircraft wing. The linear actuators  12  are secured, such as bolted, to a base surface, such as a work table  16 . A portion of the actuators extend vertically therefrom to raise and thereafter maintain a raised height of the workpiece  14 , or more particularly respective regions of the workpiece  14  for various manufacturing operations, such as cutting or drilling operations. The extension of the linear actuators can be varied to fit the contour of the workpiece  14  or to determine the contour of the workpiece  14 . As is known in the art, the rod ends of the linear actuators may be equipped with suction devices for holding respective portions of the workpiece surface to the rod ends. 
         [0032]    Turning now to  FIGS. 2-6 , an exemplary one of the linear actuators  12  is illustrated. The linear actuator  12  includes an actuator housing  20 , a thrust rod  22  for extending and retracting in relation to the actuator housing  20 , a drive screw assembly  24  disposed within the actuator housing  20 , and an anti-rotation guide  26  for preventing rotation of the thrust rod relative to the drive screw assembly  24 . Rotational movement of the drive screw assembly  24  is translated into axial movement of the thrust rod  22  relative to the actuator housing  20 . Particularly, the thrust rod  22  is caused to move axially along a longitudinal axis, such as longitudinal axis A of the thrust rod  22  and the actuator housing  20 . 
         [0033]    As seen in  FIGS. 3-6 , the actuator housing  20  has a tubular portion or sleeve  32  that extends axially between a first end  34  and a second end  36  of the actuator housing. Located at the first and second ends  34  and  36  of the housing are a proximal end wall  40  and a distal end wall  42 , respectively. In the illustrated embodiment, the end walls  40  and  42  are formed by end caps. The end caps  40  and  42  are secured to and close the ends of the tubular portion  32 . In the illustrated embodiment, the end caps  40  and  42  are held to the ends of the tubular portion by tie bars  43  and associated nuts  44 , although other suitable attachment means may be used. The sleeve  32  may be circular in cross-section and the end walls may be generally square in cross-section such that the corners of the end walls project beyond the sleeve. These projecting corners may have formed therein holes through which the ends of the tie bars extend. 
         [0034]    The actuator housing  20  may be made of aluminum, steel, or any other suitable material. The actuator housing  20  may be of a length, width, and height to receive a substantial extent of the thrust rod  22  when the thrust rod  22  is retracted into the housing interior  32 . The thrust rod  22  extends axially along the longitudinal axis A through an opening  54  in the distal end wall  42  concentric with the longitudinal axis A for securing the thrust rod  22  from lateral movement and allowing axial movement therethrough. 
         [0035]    The thrust rod  22  has a proximal end  60  and a distal end  62  that may include a mount for a holding component, such as a suction cup. The length of the thrust rod  22  from the proximal end  60  to the distal end  62  is illustrated as a solid (i.e., non-hollow) rod. The thrust rod  22  being solid allows less expensive manufacturing of the thrust rod  22  because manufacturing steps can be avoided. For example, the manufacturing steps to hollow-out a substantial portion of the thrust rod  22 , as well as the manufacturing steps to provide a precise and smooth surface inside the thrust rod  22  for a radial bearing to slide within are not necessary. Preferably, the proximal end  62  does not include any opening concentric with the thrust rod  22 . More preferably, less than 10% of the thrust rod  22  is hollow. 
         [0036]    The thrust rod  22  may be a cylindrical rod, entirely solid (i.e., non-hollow), substantially solid (i.e., substantially non-hollow), hollow, or of any other suitable shape and cross-section. Further, the thrust rod  22  may be made of steel or any other suitable material. 
         [0037]    The distal end  62  of the thrust rod  22  is extendable toward and away from the distal end  42  of the actuator housing  20 . An intermediate portion of the thrust rod  22  is supported during axial movement of the thrust rod  22  to prevent lateral movement by a bearing  64  coaxial with the longitudinal axis A. The bearing  64  is illustrated as a bushing disposed in the distal end wall  42  of the actuator housing  20  and may be any other suitable bearing, such as a radial rod bearing, a slider bearing, or another type of bushing. The bearing  64  may slidably engage an outer circumference  66  of the thrust rod  22 , thus supporting the thrust rod  22  about its outer circumference  66 . The bearing  64  may seal against the outer circumference  66  to prevent contaminants from entering the actuator housing  20 . Otherwise, a seal or wiper may be used to prevent contamination during actuation. The thrust rod  22  is driven into and out of the housing  20  by the drive screw assembly  24 . 
         [0038]    The drive screw assembly  24  is rotatably attached to the proximal and distal end walls  40  and  42 . The drive screw assembly  24  is attached at locations laterally spaced from the longitudinal axis A and the thrust rod  22 . Laterally spacing the drive screw assembly allows the thrust rod  22  to be solid (i.e., non-hollow) at the proximal end  60  because the no portion of the drive screw assembly  24  needs to be located within the thrust rod  22 . 
         [0039]    As the drive screw assembly  24  axially drives the thrust rod  22 , outward and inward of the actuator housing  20 , the drive screw assembly may urge the thrust rod  22  to rotate relative to the actuator housing  20  or the drive screw assembly  24 . Maintaining axial movement allows the thrust rod  22  to more easily extend and retract into a desired position without jamming the thrust rod  22  against bearing  64  or jamming the drive screw assembly  24 . 
         [0040]    The drive screw assembly  24  includes a rotatable drive screw  70  disposed in the actuator housing  20  and a translatable nut member  72  with inner threads that engage with the drive screw  70 . The drive screw  70  and translatable nut member  72  may be components of any suitable rotational movement to axial movement device, such as a ball screw assembly or of a roller screw assembly. As the drive screw  70  rotates the translatable nut member  72  is driven axially and thereby drives the thrust rod  22  axially. 
         [0041]    The drive screw  70  extends axially along a longitudinal axis B of the drive screw  70  and includes outer threads  74  between the first end  86  and the second end  90  of the drive screw  70 . Rotational movement of the drive screw  70  causes the translatable nut member  72  to be translated along the center longitudinal axis B of the drive screw  70  relative to the rotational movement of the drive screw  70 . As illustrated, the longitudinal axis A, the thrust rod  22  and the actuator housing  20  may be coincident and the longitudinal axis B is off-set from the thrust rod  22 . Thus, rotational movement of the drive screw  70  may cause the translatable nut member  72  to be driven axially along the longitudinal axis A of the actuator housing  20 . Thereby, axial movement of the thrust rod  22  coupled to the translatable nut member  72  may be effected, the axial movement of the thrust rod  22  also being along the center axis of the actuator housing. 
         [0042]    As shown, the drive screw  70  extends between the proximal and distal end walls  40  and  42 . A first end  86  of the drive screw  70  is disposed within the proximal end wall  40  and a second end  90  is disposed within the distal end wall  42 . In an embodiment, a portion of at least one of the first end  86  or the second end  90  abuts a portion of the actuator housing  20 . The drive screw  70  is radially outwardly spaced from the thrust rod  22  and the longitudinal axis A relative to the actuator housing  22 . Additionally, the thrust rod  22  is radially inwardly spaced from the drive screw  70  relative to the actuator housing  22  to allow the thrust rod  22  to be concentric with the actuator housing  20  and the drive screw  70  to be adjacent the tubular portion  32  of the actuator housing  20 . The drive screw  70  is adjacent to the tubular portion  32  and journaled in the proximal end wall  40  and the distal end wall  42  by suitable means such as a bearing  76  and a bearing  78 . 
         [0043]    The first end  86  of the drive screw  70  may be supported against radial and/or axial movement by the bearing  76 , such as a radial bearing, a radial screw bearing, an angular contact bearing, or a thrust bearing. The bearing  76  is disposed within the proximal end wall  40 . Alternatively, the end wall may form at least a portion of the bearing  76 . In an embodiment, the first end  86  is coupled to a drive shaft of a motor  52 . The bearing  76  can be mounted in the proximal end wall  40  or otherwise in the actuator housing  20 . The second end  90  of the drive screw  70  may be supported by the bearing  78 . 
         [0044]    The bearing  78  may be coupled, such as rotatably journaled, to an axially extending reduced diameter portion  88  of the drive screw  70  at the second end  90 . The drive screw  70  may be rotatably connected to the bearing  78  disposed at the second end  86  within the distal end wall  42  to facilitate holding the drive screw  70  in tension to counter a buckling force caused by load on the thrust rod  22  exerting force on the translatable nut member  72  and buckling of the drive screw  70 . Alternatively, the end wall may form at least a portion of the bearing. The bearing  78  may be any suitable type of bearing, preferably an angular contact bearing or a thrust bearing, less preferably a radial bearing, bushing, or any other suitable bearing. Use of a bearing type able to support an axial load allows the drive screw  70  to be put into tension by holding the second end  90  of the drive screw  70  with a bearing that allows the drive screw  70  to be held. 
         [0045]    Holding the drive screw  70  in tension prevents buckling of the drive screw  70  and allows better performance and longer life. The tension also allows a drive screw  70  with a smaller diameter to operate at the same load and speed as a larger diameter drive screw without such tension. For example, the motor  52  may attach to the distal end wall  42 , which may be located lower relative to the center of the earth than the proximal end wall  40  to put the drive screw  70  in tension. 
         [0046]    The motor  52  drives the drive screw  70  and may be a rotary motor or any other suitable type of prime mover. The motor  52  may be coupled to the proximal end wall  40  for securing the motor  52  against rotational movement relative to the actuator housing  20 . The proximal end wall  40  may have a first opening  50  for allowing coupling of the drive screw assembly  24  to the motor  52 . In an embodiment, the motor has a drive shaft that is parallel with, but not coaxial with the drive screw. Alternatively, rotational motion of the drive screw  70  may be provided by any other suitable method at the first end  86  opposite the second end  90 . 
         [0047]    At the second end  90 , the reduced diameter portion  88  extends axially away from the first end  86  and is coaxial with the longitudinal axis B. The reduced diameter portion  88  engages the bearing  78  and allows positioning the drive screw  70  closer to the thrust rod  22 . Positioning the drive screw  70  closer allows the overall footprint of the linear actuator  12  to be reduced. The reduced diameter portion  88  may be less than 50% an average diameter of the drive screw  70  to reduce the footprint of the linear actuator  12  while still allowing the drive screw  70  to drive the thrust rod  22  through the translatable nut member  72 . 
         [0048]    The translatable nut member  72  includes a thrust/guide plate  92  (which may be referred to as a “guide ring” or “piston”) and a nut  94  with inner threads that engage with the drive screw  70 . The outer threads  74  of the drive screw  70  allow threaded engagement with the nut  94 . 
         [0049]    The nut  94  includes inner threads  108 , shown in  FIG. 6 , for engaging the drive screw  70 . The inner threads  108  may be formed of fixed threads or formed by ball bearings. For example, the nut may be a ball screw nut that circulates ball bearings to smoothly translate along the drive screw  70 . The nut may couple the thrust rod  22  to the drive screw  70 , either directly or indirectly, the thrust rod  22  to translate axially in response to rotational movement of the drive screw  70 . The nut  94  and the thrust rod  22  may be directly coupled, such as fixedly attached via threaded engagement, welding, adhesives, or other suitable method. Alternatively, the thrust/guide plate  92  may have threads for engaging the drive screw. 
         [0050]    The thrust/guide plate  92  is a cylindrical portion with a radially outward facing surface facing the actuator housing  20  and includes an axially facing surface  96  connected to the nut  94 , a drive screw passage  100  parallel to the longitudinal axis A, and an anti-rotation guide passage  102  parallel to the longitudinal axis A. The thrust/guide plate  92  forms one-piece with the thrust rod  22 . In an embodiment, the thrust rod is connected to the thrust/guide plate  92  in any suitable manner, such as bolting or welding. In another embodiment, the drive screw assembly  24  is slidably coupled to the anti-rotation guide  26  without abutting the actuator housing to prevent rotational movement of the thrust rod  22 . 
         [0051]    The anti-rotation guide  26  prevents rotation of the thrust rod  22  and allows the thrust rod  22  to maintain alignment with the longitudinal axis A and only move axially along the longitudinal axis A. The anti-rotation guide  26  extends axially between the proximal and distal end walls  40  and  42 . The anti-rotation guide  26  is connected to the first and second ends  34  and  36  at locations laterally spaced from the longitudinal axis A and diametrically opposite the locations connecting the drive screw assembly  24 . Alternatively, the anti-rotation guide may extend partially between the first and second ends  34  and  36 . In an embodiment, the anti-rotation guide  26  is not diametrically opposite the locations connecting the drive screw assembly  24  to the actuator housing  20 . 
         [0052]    The anti-rotation guide  26  may be laterally spaced from the longitudinal axis A and opposite the locations connecting the drive screw assembly  24  in any suitable location to prevent rotation of the thrust rod  22  relative to the drive screw assembly  24 . In an embodiment, an anti-rotation guide and the corresponding anti-rotation guide passage are not present and the drive screw alone prevents rotation of the thrust tube relative to the drive screw assembly. The proximal end  60  of the thrust rod is supported against radial movement via a portion of the thrust/guide plate  92  abutting an inner wall of the actuator housing  20  and the anti-rotation guide  26  slidably coupling to the thrust/guide plate  92 . During use the anti-rotation guide passage  102  slides along the anti-rotation guide  26  while the drive screw passage  100  slides along or adjacent to the drive screw  70 . 
         [0053]    The drive screw passage  100  is illustrated as generally cylindrical, axially extending, without threading, and laterally off-set from the thrust rod  22  to engage the drive screw  70 . A screw bearing may be disposed within the drive screw passage  100  to support the drive screw  70 . In an embodiment, the drive screw passage has inner threading to engage the drive screw. Threading the drive screw passage allows the drive screw  70  to drive the thrust rod  22  without a nut. Threading the drive screw passage  100  also allows the drive screw passage  100  to receive the force otherwise applied to the nut  94  when being driven parallel to the anti-rotation guide passage  102  by the drive screw  70 . 
         [0054]    The anti-rotation guide passage  102  is illustrated as generally cylindrical, axially extending, and laterally off-set from the drive screw passage to slidably connect with the anti-rotation rod  26 . A bearing  106  is disposed within the anti-rotation guide passage  102 . The bearing  106  allows a tight slidable connection between the anti-rotation guide passage  102  and the anti-rotation rod  26 . In an embodiment, the anti-rotation guide passage  102  is disposed at a radially outward portion of the thrust/guide plate  92  to engage an axially extending radially inward facing protrusion connected to the actuator housing to prevent rotational movement of the thrust rod  22 . In another embodiment, the anti-rotation guide passage is part of a radially inward facing portion of the actuator housing and engages a radially outward extending protrusion of the thrust/guide plate  92  to prevent rotational movement of the thrust rod  22 . 
         [0055]    During operation, the thrust rod  22  extends and retracts to a desired position. The motor  52  drives the drive screw  70 , which in turn drives the translatable nut member  72 . The translatable nut member  72  moves axially along the drive screw  70  to drive the thrust rod  22 . The anti-rotation guide  26  prevents rotation of the translatable nut member  72  relative to the anti-rotation guide  26  and allows the translatable nut member  72  to move axially along the anti-rotation guide  26  to position the thrust rod  22 . The thrust rod  22  may statically support a workpiece. For example, when the thrust rod  22  is in a desired position a workpiece may be placed at the distal end  62  of the thrust rod to support the workpiece in place. 
         [0056]    As shown in  FIG. 1 , a plurality of linear actuators  12  may each include a thrust rod that is positioned based on a relative position of each linear actuator of the plurality of linear actuators  12 . Thus, the thrust rods of each of the plurality of linear actuators may be positioned collectively form the shape of a contour of the workpiece  14 . Alternatively, the thrust rod  22  may be extended or retracted after the workpiece  14  is placed at the distal end  62 . In an embodiment, the thrust rod  22  is dynamically loaded. For example, the thrust rod  22  may be moved to re-position an already supported workpiece or additional load may be placed on the thrust rod  22  during manufacturing. In another embodiment, the thrust rod  22  is used to position components outside of the manufacturing context. For example, the thrust rod  22  may retract or extend a door or wing on a vehicle while the vehicle is in use. 
         [0057]    Turning now to  FIGS. 7 and 8 , another exemplary embodiment of the linear actuator is shown at  212 . The linear actuator  212  substantially the same as the above-referenced linear actuator  12 , and consequently the same reference numerals but indexed by  200  respectively are used to denote structures corresponding to similar structures in the linear actuator  212 . In addition, the foregoing description of the linear actuator  12  is equally applicable to the linear actuator  212  except as noted below. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the linear actuators may be substituted for one another or used in conjunction with one another where applicable. 
         [0058]      FIGS. 7 and 8  illustrate a linear actuator  212  including an actuator housing  220 , a thrust rod  222  for extending and retracting in relation to the actuator housing  220  and having outer threads  320 , a drive screw assembly  224  disposed within the actuator housing  220 , an anti-rotation guide  226  for preventing rotation of the thrust rod relative to the drive screw assembly  224 , and a bearing  264  for slidably engaging thrust rod  222 . 
         [0059]    The drive screw assembly  224  includes a rotatable drive screw  270  disposed in the actuator housing  220  and a translatable nut member  272  with inner threads that engage with the drive screw  270 . 
         [0060]    The drive screw  270  engages a bearing  276  that is held in place by a nut  318 . The bearing  276  may be any suitable bearing, such as an angular contact bearing. The nut  318  engages the drive screw  270  to axially abut the bearing  276  and prevent axial movement relative to the drive screw  270 . The nut  318  may be any suitable nut, such as a lock nut. In an embodiment, an axially facing portion of a motor  252  abuts the bearing  276  to hold the bearing  276  in place. As the drive screw  270  rotates the translatable nut member  272  is driven axially and thereby drives the thrust rod  222  axially. 
         [0061]    The translatable nut member  272  includes a thrust/guide plate  292  (which may be referred to as a “guide ring” or “piston”) with inner threads  322  that engage with the outer threads  320  of the thrust rod  222  and a nut  294  with inner threads that engage with the drive screw  270 . The inner threads  322  extend concentrically with the outer threads  320  to prevent axial movement of the thrust rod  222  relative to the thrust/guide plate  292 . 
         [0062]    The thrust/guide plate  292  engages with and axially translates with the nut  294 . The thrust/guide plate  292  has inner threads  324  that engage with and are coaxial with outer threads  326  of the nut  294  to prevent the thrust/guide plate  292  from relative axial movement relative to the nut  294 . 
         [0063]    The thrust/guide plate  292  further includes a bearing  330  coaxial with the anti-rotation guide  226 . The bearing  330  allows the thrust/guide plate  292  to translate axially along the anti-rotation guide  226  and also engages the anti-rotation guide  226  to prevent radial movement of the thrust/guide plate  292  relative to the anti-rotation guide  226 . 
         [0064]    As best shown in  FIG. 8 , the anti-rotation guide  226  includes a first engagement portion  332  and a second engagement portion  334  at opposite ends of the anti-rotation guide  226  for radially fixing the anti-rotation guide  226  to the actuator housing  220 . The first and second engagement portions  332  and  334  are illustrated as axially extending cavities. Alternatively, at least one of the first and second engagement portions may include a protruding portion that engages a corresponding cavity within the actuator housing. As the translatable nut member slides along the anti-rotation guide  226  during retraction or extension, the thrust rod  222  slides along the bearing  264 . 
         [0065]    Referring again to  FIGS. 7 and 8 , the bearing  264  extends axially outside of the actuator housing  220 . Extending the bearing  264  axially outside allows greater support of the thrust rod  222  to prevent radial bending of the thrust rod  222 . Greater than 60% of a length of the bearing  264  extends axially outside of the actuator housing  220 . In an embodiment, 60% or less of the bearing  264  extends axially outside of the actuator housing  220  to allow greater displacement length of the thrust rod  22 . The bearing  264  may be any suitable bearing, such as a bushing, preferably a two-piece bronze bushing. 
         [0066]    Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.