Abstract:
A ball screw assembly includes a ball nut or a ball screw having internal or external, respectively, helical curves, wherein two adjacent turns of the helical curves are joined by a crossover/crossunder path, wherein ends of the crossover/crossunder path and ends of the helical curves are integrally joined without interruption. A ball screw assembly may further include crossover/crossunder paths which are staggered about the screw or the nut.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to ball screw assemblies, and, more particularly, this invention relates to crossover-type ball screw assemblies which utilize continuous paths within a recirculating area for allowing balls to recirculate in their respective circuits.  
         BACKGROUND OF THE INVENTION  
         [0002]    Ball screw assemblies may be used to translate rotary motion to linear motion and linear motion to rotary motion. Most ball screw assemblies include an elongate ball screw, a number of balls, ball recirculation means, and a ball nut body. The ball nut body in cooperation with the ball screw direct a plurality of balls through an internal bearing race formed between the ball nut body and ball screw upon rotation of the ball screw. The balls translate rotary motion of the ball screw to relative linear motion of the ball nut body. Both the elongate ball screw and ball nut body commonly include a continuous helical groove which defines the internal bearing race.  
           [0003]    In ball screw assembly applications wherein the screw proper is held in a stationary manner, or at most allowed only to translate, the associated ball nut may be designed such that the balls are recirculated in their respective circuits by way of a so-called “crossover” or “deflector” such as shown in U.S. Pat. Nos. 3,261,224, 3,667,311, 4,841,796, and 4,945,781. This is a device which allows the balls to accomplish their recirculation without need of a separate return tube. Rather, they are allowed to “skip” over the crest of the screw thread and re-enter the load zone, the aforementioned helical passage, via a path created in some additional component. Assuring that the path allows unimpeded movement of the balls has been challenging in the past.  
           [0004]    There are challenges in designing the paths in the crossover or deflector and the corresponding ball nut. The balls must be provided a clear and continuous path via which they may be recirculated and the accurate location of an insert containing the recirculation passages is challenging.  
           [0005]    A multi-section ball nut has been proposed wherein a curve could be ground into the nut “faces” which would compliment additional ground curvature applied to the screw as well. This multi-component stacked ball nut and ground ball screw arrangement proved prohibitively expensive to manufacture.  
         SUMMARY OF THE INVENTION  
         [0006]    The above discussed and other drawbacks and deficiencies are overcome or alleviated by a ball screw assembly including a ball nut having an internal helical curve, wherein two adjacent turns of the helical curve are joined by a crossover path, wherein ends of the crossover path and ends of the helical curve are integrally joined without interruption.  
           [0007]    In an alternative embodiment, a ball screw assembly includes a ball screw having an external helical curve, wherein two adjacent turns of the helical curve are joined by a crossunder path, wherein ends of the crossunder path and ends of the helical curve are integrally joined without interruption.  
           [0008]    In an alternative embodiment, a ball screw assembly includes a ball nut having a plurality of ball paths, each ball path comprising an internal helical curve and a crossover path, wherein two adjacent turns of the helical curve are joined by the crossover path to form an unending recirculation path, wherein each crossover path is staggered radially about the longitudinal axis of the ball nut from a previous crossover path.  
           [0009]    In an alternative embodiment, a ball screw assembly includes a ball screw having a plurality of ball paths, each ball path comprising an external helical curve and a crossunder path, wherein two adjacent turns of the helical curve are joined by the crossunder path to form an unending recirculation path, wherein each crossunder path is staggered radially about the longitudinal axis of the ball screw from a previous crossunder path.  
       
    
    
       [0010]    The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 is a diagrammatic view of an exemplary ball path in a ball nut;  
         [0013]    [0013]FIG. 2 is a side plan view of a ball screw including an exemplary set of ball paths;  
         [0014]    [0014]FIG. 3 is a front perspective view of a ball screw including an exemplary set of staggered ball paths;  
         [0015]    [0015]FIG. 4 is a front plan view of the ball screw of FIG. 3;  
         [0016]    [0016]FIG. 5 is a front perspective view of a ball nut including an exemplary set of staggered ball paths; and,  
         [0017]    [0017]FIG. 6 is a front plan view of the ball nut of FIG. 5. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    As shown in FIG. 1, a ball screw assembly  100  includes a nut  112  that runs over a screw  116  wherein the screw thread  116  is cut smaller and the nut thread  112  is cut larger such that balls placed within the space between the screw  116  and the nut  112  may roll in contact with both the screw  116  and the nut  112 . A crossover/crossunder path  124 , also termed a ball recirculating path or ball return path, is then formed integrally with either the nut  112  or the screw  116 , not requiring an additional component or means of retaining same, such that travel of the nut  112  and screw  116  with respect to each other does not result in the balls traveling outside the confines of the nut/screw interface. It should be understood that the entire crossover/crossunder path  124  is not actually visible at any one time, but is shown in its entirety in FIG. I for an understanding of its course. Also, it should be understood that if formed in a ball nut, the path would be termed a “crossover path” and if formed in a ball screw the path would be termed a “crossunder path”.  
         [0019]    This crossover/crossunder path  124  whether placed in the nut  112  or the screw  116 , should be designed such that the balls may be urged past the crest, e.g. at points  126  or  128 , at the thread of the opposing element, then urged back into the helical space  122  in the previous “turn” of the nut or screw. The crossover/crossunder path  124  and the helical path  122  together combine to form a ball path  120 .  
         [0020]    Although only one ball path  120  is demonstrated in FIG. 1, it should be understood that a plurality of ball paths  120  may and probably would be employed within either the ball screw  116  and the ball nut  112 , and that the other of the ball screw  116  and the ball nut  112  would contain a plurality of helical grooves. Only one exemplary ball path  120  is shown in FIG. 1 in its entirety for clarity. The ball path  120  is “unending” in the sense that a ball traveling along the ball path may be recirculated about the ball path over and over again non-stop, until the screw and nut stops. The ball path  120  is in itself unending in the sense that there is no beginning nor end to the ball path  120  as it forms an uninterrupted loop.  
         [0021]    The crossover/crossunder path  124  further should be of such a girth and form that the balls never become pinched between the crests of corresponding nut  112  and screw  116  threads. The girth of the path  124  may not be so large as to allow the balls to deviate significantly from the desired path.  
         [0022]    Still further, the balls preferably follow a crossover/crossunder path  124  which is continuous with the helix  122  at the thread in question, in its departure and its approach  126 ,  128 , is tangent to the helix  122  of that same thread at these same points  126 ,  128 , and allows the balls crossing over to be carried to a height/depth sufficient to clear the crest at the thread of the opposite element without necessarily impacting it.  
         [0023]    When formed in the ball nut  112 , the paths  120  are created internally and integrally to the ball nut  112 . This may be accomplished by setting the tangency points  126 ,  128  of the crossover path  124  to the ball nut helix path  122  sufficiently far apart that they may be created by turning methods with, for example, the tool of a computerized milling machine holding coordinates of the ball path  120 , entering the ball nut  112  from one (or either) end. Of course, other methods of creating the paths are within the scope of this invention. The intersections between the crossover path  124  and the helix path  122  are preferably smooth for improved transfer between the paths  124 ,  122 . Also, the intersection between the crossover path  124  and the helix path is seamless, that is, there are no joints, welds, or other obstructions within the paths that may spoil the continuity of the paths. This ball nut  112  and crossover path  124  provides an advantage to the designer/manufacture of ball screw assemblies  100  by allowing for a more economical method of screw manufacture such as rolling. Similar manufacturing techniques may be modified for forming the crossunder path  124  in a ball screw.  
         [0024]    Turning now to FIG. 2, the integrated ball path  120  may be applied to ball screws  116  (of sufficiently limited travel) as well as ball nuts  112 . Crossunder paths  224 , which are so named due to their location on the ball screw  116  rather than the ball nut  112 , may be integrated directly into ball screws  116 , thus also eliminating the need for separate crossunder inserts and their attendant alignment and retention details. Crossunder paths  224  combine with helical paths  222  to form ball paths  220  similar to ball paths  120  described above in FIG. 1. Although three ball paths  220  are shown, it is within the scope of this invention to employ more or less ball paths  220  within the ball screw  116 . An advantage of this embodiment is that the external thread form existing on the exterior of the ball screw  116  is far more easily subject to many different manufacturing techniques, e.g. turning (in the soft or hardened state), milling, chasing or even rolling.  
         [0025]    An added advantage of the crossover/crossunder paths  124  described with respect to FIG. 1 is that the crossover/crossunder paths  124  no longer must be located in the same “clock position”, as would be the case in an insert of some type, and therefore the ball nut&#39;s  112  ability to carry radial load may be greatly improved by “staggering” the crossover/crossunder paths  124 , even to the point where fewer crossover/crossunder circuits are necessary to carry design loads. Turning now to FIGS.  3 - 4 , a ball screw  316  having a center  330  through which a longitudinal axis  117 , such as shown in FIG. 2, passes may include one or a plurality of ball paths  320 , each having a helical path  322  and an integrally combined crossunder path  324 . Also shown in FIGS.  3 - 4  are an exemplary set of ball bearings  118  travelling through the ball paths  320 . Although only three ball paths  320  are shown, it should be understood that any number of ball paths  320  would be within the scope of this invention. Also for exemplary purposes, FIG. 4 shows that three separate ball paths  301 ,  302 , and  303  have their crossunder paths  324  located at varying “clockwise” positions about the exterior surface  332  of the ball screw  316 . That is, the substantially cylindrical and grooved outer surface  332  of the ball screw  316  may include a first ball path  301  having a crossunder path  324  located in a first position, a second ball path  302  having a crossunder path  324  located in a second position, approximately 120 degrees, measured radially from the center  330 , away from the first position, and a third ball path  303  having a crossunder path  324  located in a third position, approximately 120 degrees away from the second position. Thus, in the example shown, the crossunder paths  324  from the first, second, and third ball paths  301 ,  302 , and  303  are equally spaced about the outer surface  332  of the ball screw  316  and thus the loads carried by the ball screw assembly are efficiently balanced. While three ball paths are shown, it should be understood that a wide variety of combinations of numbers of paths and spacing between crossover paths could be employed for equally balancing the loads. For example, six paths could utilize crossunder paths which are spaced apart  60  degrees from each adjacent path, or alternatively spaced apart 120 degrees from each adjacent path such that the first set of three paths and the second set of three paths are identical. Alternatively, it is also within the scope of this invention to provide for uneven spacing of the crossunder paths  324  from each other. For example, a second crossunder path may be spaced 60 degrees from a first crossunder path, and a third crossunder path may be spaced 50 degrees from the second crossunder path. Of course, the examples given are only a sampling of the possibilities for arranging the crossunder paths in a staggered fashion about the ball nut or ball screw, and any pattern of arrangement, including even and uneven spacing, is within the scope of this invention. It should be further noted in FIGS. 3 and 4 that the crossunder paths  324  employ more deeply cut grooves into the outer surface  332  of the ball screw than the helical paths  322 . This allows the balls  118  to skip over the threads of the outwardly surrounding ball nut  112  so that the balls  118  can continue along with the helical paths  322 .  
         [0026]    Turning now to FIGS.  5 - 6 , the same concept discussed above with respect to FIGS.  3 - 4  is employed within a ball nut  412  instead of the ball screw  316 . A ball nut  412  having a center  430  through which a longitudinal axis passes may include one or a plurality of ball paths  420 , each having a helical path  422  and an integrally combined crossover path  424 . Also shown in FIGS.  5 - 6  are an exemplary set of ball bearings  118  travelling through the ball paths  420 . Although only three ball paths  420  are shown, it should be understood that any number of ball paths  420  would be within the scope of this invention. Also for exemplary purposes, FIG. 6 shows that three separate ball paths  401 ,  402 , and  403  have their crossover paths  424  located at varying “clockwise” positions about the interior surface  432  of the ball nut  412 . That is, the substantially cylindrical and grooved inner surface  432  of the ball nut  412  may include a first ball path  401  having a crossover path  424  located in a first position, a second ball path  402  having a crossover path  424  located in a second position, approximately  120  degrees, measured radially from the center  430 , away from the first position, and a third ball path  403  having a crossover path  424  located in a third position, approximately  120  degrees away from the second position. Thus, in the example shown, the crossover paths  424  from the first, second, and third ball paths  401 ,  402 , and  403  are equally spaced about the inner surface  432  of the ball nut  412  and thus the loads carried by the ball screw assembly are efficiently balanced. Again, while three ball paths are shown, it should be understood that a wide variety of combinations of numbers of paths and spacing between crossover paths could be employed for equally balancing the loads. For example, six paths could utilize crossover paths which are spaced apart 60 degrees from each adjacent path, or alternatively spaced apart 120 degrees from each adjacent path such that the first set of three paths and the second set of three paths have are identical. It should be further noted in FIGS. 5 and 6 that the crossover paths  424  employ more deeply cut grooves into the interior surface  432  of the ball nut  412  than the helical paths  422 . This allows the balls  118  to skip over the threads of the inwardly placed ball screw  116  so that the balls  118  can continue along with the helical paths  422 .  
         [0027]    While the ability to stagger the crossover/crossunder paths is simplified by the integral construction of the screws and nuts as described above, it is also within the scope of this invention to provide the staggered crossover or crossunder paths within separate inserts, although the manufacture of such an embodiment may be prohibitively expensive.  
         [0028]    Using the integrated ball nut/ball screw and crossover/crossunder as described above, separate components employed to contain the ball crossover/crossunder paths  124 ,  224 ,  324 ,  424  are unnecessary, thus reducing the total parts count and assembly complexity. Solitary integral screws and nuts assure smooth progress of balls within the ball paths. Another advantage is that the ball paths  120 ,  220 ,  320 ,  420  may now be made continuous, with no interruptions at all between the helical path  122 ,  222 ,  322 ,  422  and the crossover/crossunder path  124 ,  224 ,  324 ,  424 .  
         [0029]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.