Abstract:
An anti-backlash nut includes a body having an axially-extending bore and internal threads for engaging a lead screw inserted into the bore. The internal threads are divided circumferentially into three inner segments each movable in a radial direction. The anti-backlash nut also includes springs for independently biasing the inner segments radially inward to contact the lead screw. The anti-backlash nut provides wear compensation as well as anti-backlash to improve positioning repeatability when used with an open loop positioner.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an anti-backlash nut for engaging a lead screw. 
     2. Description of the Relevant Art 
     The use of lead screw mechanisms for providing linear motion is well known. A typical lead screw mechanism has a rotating lead screw and a non-rotating traveler that is coupled to the lead screw with a nut. The traveler is typically guided or constrained so that rotation of the lead screw results in linear motion of the traveler. 
     Fairly high precision positioning can be accomplished with a lead screw mechanism by accurately controlling the rotation of the lead screw. An essential component for precision positioning is an anti-backlash nut or other coupling between the lead screw shaft and the traveler, which reduces or, ideally, eliminates positioning hysteresis resulting from bidirectional rotation of the lead screw. 
     There are tradeoffs in the design of anti-backlash nuts in that a greater precision (less backlash) may be achieved by reducing the clearance and increasing the contact area and/or forces between the lead screw and the anti-backlash nut, but at the expense of higher frictional forces and wear. It is desirable to have the nut apply a uniform pressure to the lead screw so that there is the same torque load on the lead screw regardless of the direction of travel. It is desirable to have a torsionally stiff nut that avoids or minimizes any wind-up or torsional distortion that could cause positioning inaccuracies. It is desirable to be able to adjust the pressure applied by the nut to optimize it for a particular application. It is also desirable to provide some float or compliance in the nut to avoid binding or high torque, but at the same time not compromising positioning accuracy. 
     SUMMARY OF THE INVENTION 
     In accordance with the illustrated preferred embodiment, the present invention is an anti-backlash nut that includes a body having an axially-extending bore and internal threads that engage a lead screw inserted into the bore. The internal threads are divided circumferentially into at least two, and preferably three, inner threaded segments each independently movable in a radial direction. The anti-backlash nut also includes springs that bias the inner threaded segments radially inward to contact the lead screw. The anti-backlash nut provides wear compensation as well as anti-backlash to improve positioning repeatability when used with an open loop positioner. 
     In a preferred embodiment, the body has an outer shell, and each of the three inner threaded segments is compliantly attached to the outer shell by two axially-extending webs that permit radial movement of the inner threaded segment by bending of the webs. An axially-extending pocket is defined between an outside surface of an inner threaded segment, its two webs, and an inside surface of the outer shell. One of the springs is mounted in each pocket. The springs are preferably fabricated from flat pieces of stainless steel that are bent so that a central portion contacts the outside surface of the inner threaded segment (pushing it radially inward) and two ends contact the inside surface of the shell. 
     Also preferably, the body and its outer shell, inner threaded segments, and webs are a monolithic structure, composed of a low-friction, self-lubricating material, such as DSM Delrin AF Blend or Turcite. The outer shell preferably further includes a tubular portion spaced axially from the inner threaded segments, where the tubular portion has an inner diameter slightly greater than an outer diameter of the lead screw. The tubular portion has external threads that engage an internally-threaded mounting plate for coupling the nut to a traveler. 
     The inner threaded segments are independently movable in a radial direction, which provides some degree of radial compliance to avoid binding between the nut and the lead screw. By varying the shape and/or thickness of the springs, the spring force can be varied to adjust the contact force between the inner threaded segments and the lead screw. The radial compliance provides wear compensation and accommodates lateral run-out of the lead screw. 
     The features and advantages described in the specification are not all inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side sectional view of an anti-backlash nut according to the present invention as installed on a lead screw. 
     FIG. 2 is a side sectional view of the anti-backlash nut. 
     FIG. 3 is a side view of the anti-backlash nut. 
     FIG. 4 is a side sectional view of inner threaded segments of the anti-backlash nut. 
     FIG. 5 is an end view of the anti-backlash nut, and illustrates section line A—A for the sectional views of FIGS. 1,  2 , and  4 . 
     FIG. 6 is a side view of a spring used in the anti-backlash nut. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawings and the description herein depict various preferred embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     As shown in FIGS. 1-6, a preferred embodiment of the present invention is an anti-backlash nut  10  that includes a shell  12 , three inner threaded segments  14 , and three springs  16  (FIGS.  5 - 6 ). The inner threaded segments  14  have internal threads  18  that engage a lead screw  20 . The nut  10  has an internal bore that extends axially. 
     As shown in FIGS. 1-3, the shell extends axially to the left of the segments  14 , as viewed in those figures, forming a tubular portion  22 . The inner diameter  28  of the tubular portion  22  is slightly greater than the outer diameter of the lead screw  20  to provide clearance between the nut  10  and the lead screw. The left end of the nut  10 , opposite the segments  14 , has an external thread  24 . The external thread  24  of the nut  10  engages an internal thread of a mounting plate  26  (FIG. 1) to provide a means for mounting the nut to a traveler (not shown) to be driven by rotation of the lead screw  20 . When the lead screw  20  rotates, the nut  10  and attached traveler translates axially. 
     As shown in FIGS. 1 and 4, the inner threaded segments  14  have thread profiles  18  that match that of the lead screw  20 . Angled portions  30  of the inner threaded segments  14  contact corresponding angled portions  32  of the lead screw  20 . The major diameter  34  and minor diameter  36  of the threads  18  provide a slight clearance to the threads of the lead screw  20 . 
     FIG. 5 is an end view of the nut  10  showing that there are three inner threaded segments  14  independently movable in a radial direction and biased toward the central axis by the springs  16 . Each of the inner threaded segments  14  is attached to the shell  12  by two webs  40  that extend axially the entire length of the segments  14 . The webs  40  have cross sections that are significantly less in area than that of the segments  14 , so that radial-forces on the segments causes the webs  40  to bend, thus allowing the segments to move radially relative to the shell  12 . The inner threaded segments  14  are isolated from the tubular portion  22  of the shell  12  by a circumferential relief  48 , as shown in FIG. 2, so that the only attachment between the segments and the shell are the webs  40 . The segments  14  are thus spring biased into intimate contact with the lead screw  20  to minimize or eliminate backlash, yet have some radial compliance to accommodate wear, radial run-out in the lead screw, and tolerances in the threads that would otherwise affect the contact between the lead screw and the nut  10 . 
     The segments  14  are rigidly coupled to the shell  12  by virtue of the webs  40 , so that axial forces on the segments causes little or no axial movement of the segments relative to the shell. Three pockets  42  are formed between the radially-outer surfaces  44  of the segments  14 , the two webs  40 , and the radially-inner surfaces  46  of the shell  12 . 
     The springs  16  are located in the pockets  42  and exert a radially inward force to the radially-outer surfaces  44  of the segments  14 , thereby forcing the segments into contact with the lead screw. The springs  16  are preferably fabricated from flat pieces of stainless steel, about 0.005 to 0.006 inch thickness, bent into shape. The springs  16  have a central portion  50  that contacts the radially-outer surfaces  44  of the segments  14  and two outer portions  52  that are bent to an angle relative to the central portion and that contact the radially-inner surfaces  46  of the shell  12 . The precise shape of the springs  16  depends upon the amount of force to be applied by the springs. More force can be applied by bending the outer portions  52  to a steeper angle relative to the central portion  50  (or by using a thicker material), causing more deflection of the spring when installed and, consequently, more force applied to the segment. 
     The body of the nut  10  is preferably fabricated from a monolithic block of material, i.e., is a one piece structure. Preferably, the body of the nut  10  is composed of a low-friction, self-lubricating material such as DSM Delrin AF Blend or Turcite. The machining of the segments proceeds from the inside out, so that the inner threads  18  are machined before the segments  14  are cut and the pockets  42  and webs  40  are formed. Alternatively, the nut  10  may be fabricated as a multiple piece structure. 
     As an alternative to a monolithic construction, it may be desirable to make the tubular portion  22  of the nut  10  from one material and the lead-screw-contacting segments  14  from a different material. That way the beneficial characteristics of a low-friction, self-lubricating material can be used for contacting the lead screw and a stiffer, low-thermal-expansion material, such as stainless steel or ceramic, can be used to attach the threaded portion of the nut to the mounting plate  26 . 
     From the above description, it will be apparent that the invention disclosed herein provides a novel and advantageous anti-backlash nut assembly. The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.