Abstract:
A motorized anti-backlash linear actuator comprises a motor; a hollow shaft that is rotated about a central axis by the motor, an anti-backlash nut that is nested at least partially inside the hollow shaft; and an externally threaded rod or lead screw that engages with the anti-backlash nut such that the rotation of the hollow shaft and the nut imparts a linear motion to the rod that is substantially free of backlash. Nesting the anti-backlash nut at least partially within the hollow shaft provides greater stability to the actuator, and permits high speed operation. A mechanical element, such as a nut or a bushing, comprises a first cylindrical element comprising a first material; a second cylindrical element comprising a second material, the second cylindrical element having a different coefficient of thermal expansion relative to the first cylindrical element, the second cylindrical element being disposed inside of and in contact with the first cylindrical element; and at least one interlocking structure at an interface between the first cylindrical element and the second cylindrical element to prevent the second cylindrical element from separating from the first cylindrical element under thermal effects. The mechanical element can be a plastic nut secured to the interior of a hollow shaft of a motorized anti-backlash linear actuator.

Description:
BACKGROUND 
     Anti-backlash nuts, per se, are commercially available today in many forms and sizes. One of their important uses is to drive an element of a machine in a linear path with accurate positional repeatability and constant drag torque control in both the forward and reverse directions. For example, data printers and XY tables, used as peripheral equipment in the computer industry, have such requirements. Generally speaking, the anti-backlash nut is a nonrotatable member physically attached to a machine element. It is driven linearly in both forward and reverse directions by the rotation of a lead screw to which it is threadably attached. Such anti-backlash mechanisms will be found in our earlier U.S. Pat. Nos. 4,131,031, 4,249,426 and RE 32,433. We have found that such mechanisms are ideal for the creation of anti-backlash linear motion, not of the nut, per se, but of the lead screw which heretofore drove the nut. 
     There are numerous requirements today for very accurate linear reciprocation, as for example, a piston of pumping mechanism for chemical or medical analysis apparatus. Accuracy is also required in raising and lowering of apparatus in a predictable and repeatable sequence in robotic applications. 
     One example of a motorized anti-backlash linear actuator is disclosed in the present inventors&#39; U.S. Pat. No. 4,974,464. This patent describes an actuator having a motor in a casing and an anti-backlash nut extending out from the motor casing. In such a device, the motorized components can result in large temperature disparities between operating and non-operating conditions, which can have unpredictable consequences on the various components of the device, particularly if they have different thermal properties. Also, the fact that the anti-backlash nut extends a significant distance outside of the motor casing can make the device unstable, particularly when the anti-backlash nut is rotated at high speeds. 
     SUMMARY OF THE INVENTION 
     A motorized anti-backlash linear actuator comprises a motor; a hollow shaft that is rotated about a central axis by the motor, an anti-backlash nut that is nested at least partially inside the hollow shaft; and an externally threaded rod or lead screw that engages with the anti-backlash nut such that the rotation of the hollow shaft and the nut imparts a linear motion to the rod that is substantially free of backlash. 
     The motor and hollow shaft are preferably housed in a stationary motor housing. The hollow shaft and the anti-backlash nut rotate together with respect to the motor housing. The rod reciprocates linearly, but does not rotate. In a preferred embodiment, the anti-backlash mechanism comprises a first, movable nut that is at least partially nested inside the hollow shaft and a second, fixed nut located inside the hollow shaft. The fixed nut can comprise a plastic material secured to the interior of the shaft. The hollow shaft is preferably aluminum, bronze, brass or another metal material. The first and second nuts can be made of an injection moldable thermoplastic. 
     The moveable nut is housed within the hollow shaft such that the nut can move axially within the shaft but cannot rotate independently of the shaft. In one embodiment, the shaft comprises an interior splined surface and the movable nut comprises a plurality of grooves that mate with the splined surface of the shaft. The linear actuator comprises a mechanism that biases the fixed and movable nuts in an axial direction to maintain intimate contact between the threads of the nuts and the threads of the rod. This reduces backlash between the nut and the rod as the nut rotates. 
     The mechanism for reducing backlash between the nut and the rod preferably includes a torsion spring. In certain embodiments, the movable nut comprises a cylindrical portion containing both the internal threads that engage with the rod, and external threads that contact an internally-threaded collar. The torsion spring is disposed over the cylindrical portion of the movable nut, and extends between a first end of the movable nut and the collar. The collar is biased by the torsion spring to rotate on the external threads of the cylindrical portion of the movable nut, and this rotation forces the internal threads of the movable nut into contact with the threads of the rod to reduce backlash. 
     The movable nut can comprise a mechanism for adjusting the amount of drag torque between the nut and the rod, such as an adjustment ring on the nut for varying the torsional bias between the nut and the collar. 
     An advantage of the present invention is that by having at least a portion of the anti-backlash nut nested within the motor housing, the profile and mass moment of inertia of the anti-backlash nut is reduced compared to, for example, the embodiments described in U.S. Pat. No. 4,974,464. The linear actuator with an anti-backlash nut is therefore characterized by greater stability during operation, even when the motor is operated at high speeds. Another advantage over the embodiments of U.S. Pat. No. 4,974,464 is that spring torque adjustments are readily accessible. 
     In another aspect of the invention, a mechanical element, such as a nut or a bushing, comprises a first cylindrical element comprising a first material; a second cylindrical element comprising a second material, the second cylindrical element having a different coefficient of thermal expansion relative to the first cylindrical element, the second cylindrical element being disposed inside of and in contact with the first cylindrical element; and at least one interlocking structure at an interface between the first cylindrical element and the second cylindrical element to prevent the second cylindrical element from separating from the first cylindrical element under thermal effects. The first cylindrical element can comprise a metal material, and the second cylindrical element can comprise a plastic material, for example. 
     The interlocking structure can comprise a re-entrant portion of the first cylindrical element that mates with the outer surface of the second cylindrical element. In one embodiment, the interlocking structure comprises a plurality of dovetail joints around the interface of the first and second cylindrical elements. The interlocking structures lock the interior cylindrical element to the outer housing, and prevents the inner cylindrical element from shrinking away and separating from the outer cylinder. This is particularly advantageous, for example, in the context of a motorized linear actuator, where the outer cylindrical element comprises a hollow metal tube, and the interior cylindrical element comprises an internally threaded plastic nut. The structure is preferably formed by an injection molding process. 
     In another aspect, a motorized linear actuator comprises a motor; a hollow shaft that is rotated about a central axis by the motor, at least a portion of the hollow shaft comprising an internally threaded surface; a rod having external threads, the threaded portion of the hollow shaft engaging the threads of the rod, such that the rotation of the hollow shaft imparts a linear motion to the rod relative to the hollow shaft; and a motor housing that contains the motor and the hollow shaft such that axial motor play is substantially eliminated. 
     The above and other features of the invention including various novel details of construction and combinations of parts will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular motorized anti-backlash linear actuator embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
         FIG. 1  is a front perspective view of a motorized anti-backlash linear actuator in accordance with one aspect of the invention; 
         FIG. 2  is a rear perspective view of the motorized anti-backlash linear actuator of  FIG. 1 ; 
         FIG. 3  is a side view of a motorized anti-backlash linear actuator and lead screw assembly; 
         FIG. 4  is a cross-sectional side view of the assembly of  FIG. 3 ; 
         FIG. 5  is an exploded front perspective view of an anti-backlash nut and rotating hollow tube; 
         FIG. 6  is an exploded rear perspective view of the anti-backlash nut and rotating hollow tube of  FIG. 5 ; 
         FIG. 7  illustrates the components of a motorized anti-backlash linear actuator in accordance with one embodiment of the invention; 
         FIG. 8  is a cross-sectional view of a bushing using materials with dissimilar thermal properties in accordance with one aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the invention follows. 
       FIGS. 1 and 2  shows a motorized linear actuator in accordance with one aspect of the invention. The actuator comprises a motor assembly  10  having a motor housing  11 , a first nut  17  on one end of the housing  11 , and a second nut  21  on the opposite end of the housing  11 . The nuts  17  and  21  are internally threaded and configured to mate with a lead screw  12  having threads extending along the length of the screw, as is shown in  FIG. 3 . 
     In a preferred embodiment, moveable nut  17  comprises an anti-backlash biasing mechanism, and nut  21  comprises a fixed non-anti-backlash nut. 
     As shown in  FIG. 4 , the motor housing  11  includes a drive mechanism that imparts a rotary motion to nut  17  and nut  21  relative to the housing, in the direction of arrow  55 . The threads of nuts  17 ,  21  engage the threads of lead screw  12 , so that the rotation of the nuts causes a translation of the screw  12  relative to the housing  11  in the direction of arrow  56 . Thus, the controlled rotational movement of the nuts  17 ,  21  by the motor results in a controlled linear reciprocation of the lead screw  12 . The lead screw  12  is preferably attached to a load (not show), and the actuator  10  can precisely control the linear motion and position of the load. Examples of a motor assembly for imparting linear motion to a lead screw are described in U.S. Pat. No. 4,974,464 to Erikson et al., the entire teachings of which are incorporated herein by reference. 
     A linear actuator  10  according to one embodiment is shown in the cross-sectional side view of  FIG. 4 . In this embodiment, the drive mechanism comprises one or more stationary field coils  14  that rotate a rotor  13  within the motor housing  11 . The rotor  13  is connected to, and rotates, a hollow tube  15  on bearings  31 . 
     The hollow tube  15  is preferably made of a metal material, such as brass or stainless steel, and comprises a first portion  20 , a second portion  22 , and optionally a third portion  23 . The second portion  22  of the tube  15  includes an internal surface, and comprises the fixed second nut  21 . The second portion  22  can comprise, for example, a plastic material secured to the interior surface of the hollow tube  15 , and having internal threads that engage with the threaded outer surface of the lead screw  12 . 
     The first portion  20  of hollow tube  15  houses the movable nut  17 . The moveable nut  17  is nested within the first portion of the tube so that the nut  17  is able to move axially relative to the tube  15 , but cannot rotate relative to the tube, as is described in further detail below. 
     The third portion  23  of the tube comprises the middle portion of the tube and is fixed to a rotor. 
     Turning now to  FIGS. 5 and 6 , the anti-backlash nut mechanism and rotating hollow tube  15  are shown in front and rear exploded perspective views. As can be seen in  FIG. 5 , the movable nut  17  comprises a tubular portion  16 , an adjustment ring  29 , a collar  19  and a torsion spring  25 . The tubular portion  16  of the nut  17  comprises a threaded external surface  61  and a series of evenly-spaced grooves  63  around the outer circumference of the tubular portion  16 . The threaded external surface  61  and the grooves extend along the length of the nut and terminate at a shoulder portion  24  at the face of the nut, which has a series of evenly-spaced teeth  64  around the outer circumference of the shoulder  24 . The movable nut  17  has a hollow interior having internal threads  66  for engaging with a threaded lead screw. The movable nut  17  can comprise a plastic material. 
     The movable nut  17  is at least partially housed within the first portion  20  of the hollow tube. The first portion  20  of the hollow tube  15  includes a series of evenly-spaced splines  65  that are configured to mate with the grooves  63  on the surface of the nut  17 . In this way, the nut  17  is permitted to move in an axial direction, in-and-out of the hollow tube  15 , but the nut  17  is not permitted to rotate with respect to the tube  15 . Thus, as the hollow tube  15  rotates within the motor housing  11 , the anti-backlash nut is forced to rotate simultaneously. 
     The splines  65  on the hollow tube  15  and the mating grooves on the nut  17  can have any profile (e.g., quadrangular, triangular, etc.), and in a preferred embodiment, the splines and grooves have a dovetail profile. 
     In operation, as shown in  FIG. 4 , the movable nut portion  17  of the anti-backlash mechanism is partially nested within the hollow tube  15 . As previously discussed, the nut  17  can slide axially within the tube  15 , but cannot rotate independently of the tube  15 . Collar  19  contains internal threads, and is threaded over the threaded external surface  61  of the nut  17 . The collar  19  has a larger diameter than the diameter of the tube  15 . The collar  19  acts as a stop, defining the distance between the end of the hollow tube  15  and the face of the nut  17 . By rotating the threaded collar  19  on the nut  17 , the lateral separation between the face of the nut  17  and the hollow tube  15  is adjusted. 
     The torsion spring  25  is disposed over the nut  17  between the shoulder  23  of the nut and the collar  19 . The torsion spring  25  is secured at one end to the collar  19 . In the embodiment shown in  FIGS. 5 and 6 , the torsion spring  25  is secured to an adjustment ring  29 . The adjustment ring  29  has a series of evenly-spaced notches  67  around its inner circumference, which mate with the teeth  64  on the shoulder  24  of the nut  17 . In other embodiments, the torsion spring  25  can be secured directly to nut  17 , such as to the shoulder  24 . 
     Referring now to  FIG. 4 , the operation of the linear actuator  10  with an anti-backlash mechanism is described. The lead screw  12  extends through the hollow tube  15  of the actuator. Nut  21  and nut  17  include internally threaded surfaces that engage with the threads on the lead screw  12 . Collar  19  is biased by torsion spring  25  to rotate on the threaded outer surface of the nut  17 . The collar  19  is forced against the end of the hollow tube  15 , which prevents the collar from moving axially. Preferably, a neoprene washer  109  is located between the collar  19  and the end of the tube  15 . The rotation of the collar  19  on the threaded outer surface of the nut  17  forces the nut  17  to move in an axial direction away from the hollow tube  15  and nut  21 . The threads of the nut  17  are thus biased against the mating threads of the lead screw  12 , and away from the threads of the fixed nut  21  in the hollow tube  15 , thus taking up any backlash or slack as the lead screw reciprocates with respect to the motor assembly  10 . The collar  19  will continue to rotate on the threaded outer surface of the nut  17  as the interior threads of the nut wear down over time. Thus, the anti-backlash mechanism is able to compensate for wear and maintain intimate contact between the threads of the nut and the threads of the lead screw  12 . 
     In the embodiment shown in Figs,  5  and  6 , the amount of torque and resulting drag of the anti-backlash bias can be controlled by the adjustment ring  29 . As shown in  FIG. 5 , rotating the notches  67  of the adjustment ring  29  to different teeth  64  on the shoulder  24  of the nut will alter the torsional bias force between the collar  19  and the nut portion  17 , and can therefore be used to modify the drag force between the threads of the nut and the threads of the lead screw. 
     In the embodiments shown and described, it will be apparent that at least a portion of the anti-backlash nut  17  is nested within the motor housing  11 . In this way, the profile and inertia of the anti-backlash nut is reduced compared to, for example, the embodiments described in U.S. Pat. No. 4,974,464. Consequently, the present motorized linear actuator with an anti-backlash nut is characterized by greater stability during operation, even when the motor is operated at high speeds. 
     An embodiment of a linear actuator according to one aspect of the invention is shown in exploded view in  FIG. 7 . In addition to the components previously described, this embodiment also illustrates one example of a suitable motor assembly  10 , including motor coil  14 , rotor  13 , front end cap  101 , rear end cap  103 , torque ring  105 , washer  111 , curved washer  113  and fastening screws  107 . The motor assembly  10  can also include insulating material, such as the plastic insulator  115  between the motor coil  13  and the end cap  103 . An advantage of this embodiment is that the motor assembly is designed to minimize internal axial motor play. The bearings  31  are held solid against the hollow tube  15  which serves as the motor shaft. As is shown in  FIG. 7 , the rear bearing  31  is fixed between the rear end cap  103  and a rear shoulder  116  on the hollow tube  15 . The front bearing  31  is held between the front shoulder  116  of the hollow tube  15  and the torque ring  105 . A 0.030 washer  111  and a curved 0.015 spring washer  113  are located between the bearing  31  and the torque ring  105 . The torque ring  105  is mounted within the front end cap  105 , and can be tightened to substantially eliminate axial play of the hollow tube  15  and rotor  13  as the motor operates. The curved washer  113  is preferably maintained in a compressed shape, as shown in  FIG. 7 . In this way, if any gap develops between the components of the motor assembly (i.e. the torque ring  105 , bearings  31 , hollow tube  15  and rear end cap  103 ), such as from thermal effects, the curved washer  113  will become sufficiently uncompressed to take up any slack that develops in the motor assembly. 
     Turning now to  FIG. 6 , the motorized linear actuator is shown in a rear view, which illustrates a thermally-insensitive nut  21 . As previously mentioned, the hollow tube  15  that rotates within the motor housing  11  is generally made from a metal material, such as aluminum or brass. The nut  21  that is secured to the interior of the tube  15  is made from a plastic material, such as an injection moldable thermoplastic. The expansion coefficient of plastic is generally higher than that of a metal, such as aluminum or brass, so that with changes of temperature, the plastic nut  21  expands and shrinks at a different rate than does the metal tube  15  surrounding the nut. Both the outer diameter of the plastic nut and the inner diameter of the metal tube to which the nut is attached vary by an amount proportional to the thermal expansion coefficient (α) of each material multiplied by the change in temperature (ΔT). Since the plastic material of the nut has a larger thermal expansion coefficient, it is more sensitive to temperature change than the metal tube that surrounds it. Under cooler temperature conditions, the plastic nut  21  will shrink more than the metal tube  15 , which can result in a gap forming between the nut and the tube, and can ultimately cause the nut to become dislodged from the tube and tighten around the lead screw. 
     In one aspect, the present invention relates to a design for a nut or bushing that overcomes these limitations. For example, in the cross-sectional view of a plastic nut  21  and hollow tube  15  shown in  FIG. 8 , the interface between the nut  21  and the tube  15  includes a plurality of interlocking structures  91 . In the embodiment shown in  FIG. 8 , the interlocking structures  91  comprise a plurality of evenly-spaced dovetail joints between the nut  21  and the tube  15 . Even when plastic nut  21  shrinks at a faster rate than the outer metal tube  15  due to thermal expansion effects, the interlocking dovetail joint will hold the nut in position within the tube. Preferably, the interlocking structure(s)  91  extend around the entire interface between the inner and outer cylinders. In the embodiment of  FIG. 8 , the dovetail structures do not extend completely around the interface, as a portion  95  of the interface has been left without any interlocking structures to illustrate the gap  93  that can form between the nut and tube due to thermal effects. 
     The interlocking structures  91  are configured to hold the plastic nut  21  in place within the metal tube  15 , and render the device largely insensitive to changes in temperature and differential thermal expansion effects. Outer cylinder includes a re-entrant portion that mates with the outer surface of the inner cylinder. In the embodiment shown, the interlocking structures comprise a dovetail joint, though it will be understood that other similar interlocking structures could be used, such as a “T”-shaped joint, an “L”-shaped joint, or a rounded or ball-shaped joint. 
     Although the embodiments illustrated herein relate to a threaded nut, it will be understood that the principles of the invention equally apply to unthreaded elements, such as a bushing, within a cylindrical outer tube, where the bushing and outer tube have dissimilar thermal properties. 
     In one embodiment, the interlocking structures are produced by forming a series of evenly-spaced trapezoidal splines that extend along the length of the hollow tube  15 . These splines can advantageously comprise the same splines  65  at the first portion  20  of the tube  15  that mate with the grooves  63  on the anti-backlash nut  17 . The second, non-anti-backlash nut  21  can be formed by injection molding a plastic material directly into the second portion of the metal tube  15  (i.e. on the opposite end from the movable nut  17 ). 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.