Abstract:
An anti-backlash nut assembly is on a reinforced lead screw having a threaded lead screw rotatable about a central axis. A hollow, elongated reinforcing rail extends lengthwise of and surrounding the lead screw. The reinforcing rail has a slot extending lengthwise of the central axis. The anti-backlash nut assembly has a nut moveable along the reinforcing rail. The anti-backlash nut has a pair of separate nut portions. Each nut portion has a tongue extending radially through the slot and having threads engagable with the threads of the lead screw for moving both nut portions in reciprocating motion lengthwise of the rail when the screw is rotated. A spacing mechanism forces the nut portions apart to urge the flanges of their threads into forcible engagement with the threads of the lead screw to prevent backlash while the anti-backlash nut translates laterally along the reinforcing rail.

Description:
RELATED APPLICATIONS 
     This application is a Continuation-in-Part of application Ser. No. 09/336,905 filed Jun. 21, 1999, now U.S. Pat. No. 6,202,500, which is a Continuation-in-Part of 08/842,849 filed Apr. 17, 1997 now U.S. Pat. No. 5,913,941, the contents of the which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 4,566,345, issued Jan. 26, 1986 to the present inventors and disclosed a carrier for mounting a tool for reciprocating motion along a pair of spaced, parallel guide rails. The carrier itself is reciprocated bilaterally by a rotating lead screw extending lengthwise between and parallel to the guide rails. The lead screw engages an internally threaded nut on the carrier. The nut may be an anti-backlash nut and will be so illustrated hereinafter. With the lead screw located between the rails, there are bearing supports extending laterally from the carrier, each mounting a bearing which slides on a guide rail much in the manner of outrigger pontoons. 
     The rails serve a number of purposes. They create a low-friction guideway for the carrier to ride on but, more importantly, they create an anti-rotation mechanism for the carriage. If there were no guide rails and the carrier were mounted on the lead screw only, the carrier probably would not reciprocate. This is because the mass of the carrier would create high frictional resistance between its threaded nut and the lead screw such that the carrier would rotate with the lead screw rather than traversing it. Secondly, again, if there were no guide rails and if the lead screw were oriented in a horizontal direction, the flexure of the lead screw, created by the weight of the carrier and the tool that it mounts, could cause the carrier to bind on the lead screw and/or cause the tool that it mounts to disengage from its workpiece. 
     Yet another problem can exist. Tools mounted on the carrier are normally offset from the central axis of the lead screw. In addition, the tools are mounted on posts in a cantilevered position relative to the carrier. Normal drag of the tool relative to its workpiece creates a torque which is imparted through the tool post to the carrier and, hence, to the supports riding on the rails which carry the bearings. This can cause unnecessary drag, monkey-biting, or can conceivably result in the lead screw&#39;s driving motor to stall out. 
     Another shortcoming in certain applications where space is limited is the size of the mechanism. Two guide rails spaced laterally of a lead screw creates a drive mechanism which is spread out, generally horizontally, and, in some machine operations, there is not sufficient space to do this. In addition, it is difficult to obtain initial alignment of the screw and two parallel rails. 
     It is to the solution of these problems that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     The invention resides in a reinforced lead screw assembly which includes a threaded lead screw rotatable about a central axis by a reversible motor. Surrounding the lead screw is a rigid, hollow, elongated reinforcing rail or tube which extends lengthwise end to end of the screw. A two-part anti-backlash nut is movable along the reinforcing rail and is driven by the screw. There is a slot in the reinforcing rail which extends lengthwise of the central axis of the lead screw. Projections or tongues on both portions of the nut extend in a radial direction through the slot in the reinforcing rail and are engagable with the walls of the slot to prevent rotation of the anti-backlash nut relative to the rail. The tongues or projections mount threads which are engagable with the threads of the lead screw. Consequently, when the lead screw is rotated in either direction, the nut will translate lengthwise of the screw. 
     The projections on the nut portions have either partial threads engagable with the threads of the lead screw or mounted circular hubs which are internally threaded to engage the lead screw. 
     The reinforcing rail may include at least one flat extending lengthwise which is engagable with mating flats on the nut portion to supplement the anti-rotation function of the tongue and the slot in the rail to prevent the nut from rotating relative to the reinforcing rail. The outer surface of the reinforcing rail may be coated with a low-friction material such as PTFE, Nylon or the like. The anti-backlash nut also may be made of self-lubricating material such as acetyl and may include PTFE carbon fiber additive or other lubricating additives for low frictional engagement with the lead screw. It may be molded around or attached to a ball bushing for added stiffness and lubricity. The sleeve bearing located within the lead screw may be made of Nylon, Delrin or like plastic material. 
     A spacing mechanism such as an advancing collar positions and retains the nut halves or portions apart so that the flanks of their threads are in contact with the flanks of the threads of the lead screw to prevent backlash while the nut translates laterally along the reinforcing rail in both directions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred 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 the principles of the invention. 
     FIG. 1 is a plan view of a reinforced lead screw embodying the present invention; 
     FIG. 2 is a partial sectional view taken along the line  2 — 2  on FIG. 1; 
     FIG. 3 is a sectional view through the reinforced guide rail and the nut-bearing taken along the line  3 — 3  on FIG. 2; 
     FIG. 4 is a sectional view through the reinforced guide rail and the nut-bearing taken along the line  4 — 4  on FIG. 2; 
     FIG. 5 is a sectional view through an alternative nut-bearing with the lead screw and the reinforcing rail; 
     FIG. 6 is a view similar to FIG. 5 with the lead screw and the reinforcing rail removed and having one flat on the nut-bearing; and 
     FIG. 7 is a schematic sectional view of part of the guide rail and the nut-bearing illustrating another embodiment of the flats. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, where like numerals indicate like elements, there is illustrated a reinforced lead screw in accordance with the present invention, generally referred to as  10 . 
     The reinforced lead screw  10  is seen assembled in plan view in FIG.  1 . The reinforced lead screw  10  includes a screw  12 , a lead screw, rotatable about an axis α and having threads  14  formed along its length. A projection  16  is formed on one end of the lead screw  12  to connect it to a reversible driving motor (not shown) in order to rotate the lead screw  12  alternatively in clockwise and counterclockwise direction. 
     The reinforced lead screw  10  has surrounding the lead screw  12 , an elongated reinforcing rail  18  or tube which extends lengthwise of the screw  12 . The reinforcing rail  18  is hollow to accommodate the lead screw and includes a slot  20  extending lengthwise and parallel to the central axis α of the lead screw  12 . The slot  20  includes side walls  22  and  24 , as seen also in FIG. 3, which may constitute bearing surfaces as will be explained in further detail hereinafter. The lead screw  12  is journaled within the reinforcing rail  18  by bearings  26  and  28  for rotation relative to the reinforcing rail  18 . 
     The reinforced lead screw  10  can have an elongated sleeve bearing extending lengthwise of the reinforcing rail. The bearing can be made of a low-friction plastic such as PTFE, Nylon or Delrin and has a circular interior and engages the threads of the lead screw. It functions as a radial support for the lead screw preventing the screw from whipping or vibrating as it is rotated. An elongated sleeve bearing is described in U.S. patent application Ser. No. 09/336,905 filed on Jun. 21, 1999 which is incorporated by reference in its entirety. 
     In operation, the reinforcing guide rail  18  would be secured in a machine structure by any convenient means (not shown) with a reversible driving motor attached to the lead screw  12 . 
     An anti-backlash nut, generally indicated  36 , which may be metal or, preferably, moldable plastic such as acetyl with carbon fiber additives is movable in bilateral direction along the reinforcing rail  18 . The anti-backlash nut  36  includes a pair of separated nut halves  38  and  40 . The nut half  38  has a cylindrical sleeve portion  42  and a larger diameter flange portion  44 . The flange  44  may also be called a face plate. The flange  44  is illustrated as circular but may be constructed in any convenient shape depending on the tool or load to be attached. The load with the anti-backlash nut  36  is reciprocated back and forth by the reinforced lead screw  10 . 
     Referring to FIG. 2, the cylindrical sleeve portion  42  of the nut half  38  has an outer surface  48  which is threaded. The other nut half, nut half  40  has a cylindrical sleeve portion  46  with an outer surface  50 . A portion of the outer surface  50  is encircled by a slip collar  52 , which is rotatable relative to the nut half  40 . The antibacklash nut  36  has an advancing collar  54  that has an interior surface  56  which is threaded to be received by the threaded outer surface  48  of the nut half  38 . 
     The advancing collar  54  and the slip collar  52  are rotatably interlocked by each having an interlocking projection  62  received in an annular groove  64  in the other collar  52  and  54 . The advancing collar  54  is rotated on the threaded outer surface  48  of the nut half  38  to position or space the nut halves  38  and  40 . The anti-backlash nut  36  has a lock nut  68  that has a threaded interior surface which is received by the threaded outer surface  48  of the nut half  38 . The lock nut  68  abuts the advancing collar  54  as described below. 
     Referring to FIGS. 2 and 3, where FIG. 3 is a sectional view of nut half  38 , the nut half  38  includes a tongue  72  extending radially downward from the cylindrical sleeve portion  42  and inwardly, engaging the sidewalls  22  and  24  of the reinforcing rail  18 . A circular hub  74  is formed on the tongue  72  and has a plurality of internal threads  76  which are in mating engagement with the threads  14  of the lead screw  12 . While longitudinal movement is imparted to the nut half  38  by rotation of the lead screw  12 , engagement of the tongue  72  with the sides  22  and  24  of the slot  20  in the reinforcing rail  18  prevents the nut half from rotating. The lock nut  68  is shown encircling the cylindrical sleeve portion  42 . 
     Referring to FIGS. 2 and 4, the nut half  40  is constructed in a similar fashion. The nut half  40  has a tongue  78  depending from the cylindrical sleeve portion  46  to a central hub  80  formed on the tongue  78 . The central hub  80  has the internal threads  82  which are in mating engagement with the threads  14  of the lead screw  12 . As with the tongue  72 , rotation of the nut half  40  is prevented by the tongue  78  engaging the sides or walls  22  and  24  of the slot  20  of the lead screw  12 . 
     Referring back to FIG. 2, the nut halves  38  and  40  are positioned so that they are urged in opposite directions by the advancing collar  54  so that the threads  76  and  82  of the nut halves engage the flanks of the threads  14  of the lead screw  12  which faces the other nut half. The positioning and retaining the threads  76  and  82  of the nut halves  38  and  40  into firm engagement with the flanks of the threads  14  of the lead screw  10  prevents backlash while the anti-backlash nut translates bilaterally along the reinforcing rail  18 . A further description of the anti-backlash contact is described in reissued Patent No. 31,713 issued on Oct. 30, 1984 and patent application No. 09/336,905 filed on Jun. 21, 1999 which are incorporated by reference in their entirety. 
     To use the anti-backlash feature of the anti-backlash nut  36 , the user rotates the lock nut  68  away from the advancing collar  54  on the cylindrical sleeve portion  44  of the nut half  38 . The advancing collar  54  is then rotated to move the advancing collar  54  lateral relative to the nut half  38  and move the nut halves  38  and  40  apart until the threads  76  and  82  of the nut halves  38  and  40  are in firm engagement with the flanks of the threads  14  of the lead screw  10 . The lock nut  68  is then rotated into engagement with the advancing collar  54  to prevent rotation of the advancing collar  54 . 
     The user can periodically move the lock nut  68  away from the advancing collar  54  and rotate the advancing collar  54  further apart to compensate for wear of the threads  76  and  82  of the nut halves  38  and  40 . The lock nut  68  is then rotated into engagement with the advancing collar  54  to prevent rotation of the advancing collar  54 . 
     The slip collar  52  is an interposed connection between the nut half  40  and the advancing collar  54  in the embodiment shown for ease of manufacturing. It is recognized that the advancing collar  54  can be attached in a rotatable manner directly to the nut half  40 . 
     Referring to FIGS. 5 and 6, an alternative anti-backlash nut  86  and an alternative reinforcing rail  88  are shown. The nut halves  90  of the anti-backlash nut  86  have an optional internal flat  94  on the cylindrical sleeve portion  92 . The internal flat  94  is engagable with an optional mating flat  96  formed on the bottom of the reinforcing rail  88 , as seen in FIG.  5 . 
     The anti-backlash nut  86  includes a projecting portion or tongue  98  extending radially inward. The tongue  98  has parallel sides  100  which are engagable with the sides  22  and  24  of the aligned slots  20  formed in the guide rail  18 . This engagement also assures that, in addition to the functioning of flats  46 ,  48 , the nut will not rotate relative to the guide rail  18  when the lead screw is rotated. 
     In contrast to the previous embodiment, the tongue  98  does not have a cylindrical sleeve portion but rather a plurality of arcuate threads  102 , as seen in FIG. 6, at the innermost end of the projection  98 . The threads are engagable with the threads  14  of the lead screw  12  such that when the lead screw is rotated, either clockwise or counterclockwise, the anti-backlash nut  86  will translate relative to the reinforcing rail  88 . Functionally, the FIGS. 5 and 6 embodiment of the nut operates in the same manner as the FIGS. 1-4 embodiment. As seen in FIG. 7, a plurality of flats  104  and  106  (hereinafter illustrated as only 2) may be formed in the nut bearing  86  which are engagable with mating flats  108  and  110  on the reinforcing rail  88 . 
     The purpose of the optional flats  104 ,  106 ,  108 , and  110 , be there one or a plurality, is to supplement the auto-rotational function of the tongue  98  and the slots  20  preventing the nut  36  from rotating relative to the reinforcing rail  18  when the lead screw  12  is rotated. This could be the case where the frictional engagement between the threads of the lead screw and the nut is greater than the frictional engagement between the nut and the exterior of the guide rail. 
     Referring to FIG. 5, an elongated sleeve bearing  112  is shown within the reinforced rail  88 . The bearing can be made of a low-friction plastic such as PTFE, Nylon or Delrin and has a circular interior and engages the threads of the lead screw. 
     With the present invention, there is no need for lateral guide rails for the nut, or if a carriage is attached, to ride on, since the reinforcing rail  18  is generally sufficient except in very large tool requirements. Since the breadth of the single rail construction is smaller, there is less chance for canting or skewing of the nut because its load can be mounted closer to the axis α. Circular loads can also be attached to the nut  44 . 
     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.