Abstract:
A ball relief feature for a ball screw allows multiple turn ball circuits with reduced ball bearing stack-up within the ball screw assembly. Reliving bearing stack up allows for ball circuits having more turns without reducing the number of load carrying ball bearings of a typical ball nut. The ball relief feature reduces ball bearing stack-up by allowing the balls to re-distribute relative to one another and space themselves evenly in oversized portions of the ball circuit track other than the ball return mechanism.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims priority to U.S. Provisional Patent Application No. 62/309,278 filed on Mar. 16, 2016, which is incorporated herein by reference in its entirety for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to the field of ball screws. More particularly, the present invention relates to a ball nut that includes a relief portion to prevent stack-up of ball bearings traveling through multiple turn ball circuits. 
       BACKGROUND 
       [0003]    A ball screw is a mechanical device that translates rotational motion into linear motion. Typical ball screws include a leadscrew portion that is threaded to include at least one raceway through which a series of ball bearings travel. Each raceway has a pitch, which is defined as the axial distance between threads and is typically measured in threads per inch or millimeter. Leadscrews may have one raceway, or may alternatively include multiple raceways that follow alongside each other. The balls are captured inside a ball nut that includes its own raceways that correspond to the raceways of the leadscrew. 
         [0004]    The above mentioned raceways combine to form a circuit through which the ball bearings travel. To complete the circuit, the ball nuts include a mechanism for removing the ball bearings from one end of the circuit and transferring them to the other end of the circuit. A circuit can comprise anywhere from one to several complete turns around the screw before the balls are re-directed back to the beginning of the circuit. A ball screw and nut assembly may comprise anywhere from one to several circuits. 
         [0005]    Currently the number of turns that can comprise a circuit is limited by a phenomenon known as ball stack up in which the ball bearings may jam or bind against one another as they roll through the ball track formed by the screw and nut with different amounts of slippage. If nothing is done to prevent ball stack up, the maximum number of turns that comprise a circuit is typically between  5  and  10  turns, depending on a number of factors. One common method of dealing with this problem is incorporating spacer balls amongst the load carrying balls. The spacer balls are smaller in diameter than the loading carrying balls, allowing the load carrying balls to slip more readily and preventing the load carrying balls from binding against one another. The problem with using spacer balls is that they significantly reduce the load carrying capacity of the ball screw assembly, because the load is being carried by fewer balls. 
         [0006]    It is also desirable to produce a ball screw assembly having as few circuits and as many turns per circuit as possible for several reasons. First, each circuit requires its own ball return mechanism; therefore, limiting the number of ball returns reduces the cost to manufacture the ball screw. In addition, there must also be some amount of space between each circuit. This space is essentially wasted space, as there can be no load carrying balls within that unused space. 
         [0007]    Finally, typical ball screws having internal ball return systems must be installed on a ball screw at an end of the screw because that is the only position where the raceway is accessible such that ball bearings loaded into the ball nut may be installed on the leadscrew. However, in some applications, the leadscrew raceway may not extend all the way to both ends of the leadscrew, making it impossible to install a ball nut having an internal ball return system. 
         [0008]    Hence there is a need for a ball nut assembly that prevents ball bearing stack-up while also increasing the number of turns that a given circuit can handle. In addition, there is a need for a ball nut assembly having an internal ball return system that may be installed on a leadscrew away from either end of the leadscrew. 
       SUMMARY 
       [0009]    The present invention relates to a ball screw assembly that includes a leadscrew having at least one leadscrew raceway. A ball nut having a body capable of surrounding the leadscrew includes an internal raceway capable of aligning with the leadscrew raceway. A plurality of ball bearings are captured between the internal raceway and the leadscrew channel. The internal raceway further includes an oversized portion that allows the ball bearings to re-distribute relative to one another as they pass through the oversized portion. 
         [0010]    The present invention further relates to a method of installing a ball nut onto a leadscrew having at least one leadscrew raceway that does not extend to an end of the leadscrew. The method includes providing a ball nut having a body and at least one internal raceway corresponding to the at least one leadscrew raceway, where the ball nut further includes at least one cut-out portion on the body that allows access to the leadscrew raceway through the body. Next, a user moves the ball nut over the leadscrew until the cut-out portion aligns with the leadscrew raceway. Next, the user inserts ball bearings through the cut-out portion until the raceway is filled with ball bearings. Finally, after the raceway is filled, the user installs an insert having an oversized ball track into the cut-out and secures the insert into the cut-out. 
         [0011]    It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can lead to certain other objectives. Other objects, features, benefits and advantages of the present invention will be apparent in this summary and descriptions of the disclosed embodiment, and will be readily apparent to those skilled in the art. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanying figures and all reasonable inferences to be drawn therefrom. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of a leadscrew and ball nut including one embodiment of a ball bearing relief in accordance with the invention; 
           [0013]      FIG. 1A  is a top view of the leadscrew and ball nut of  FIG. 1 ; 
           [0014]      FIG. 2  is an exploded perspective view of the ball nut shown in  FIG. 1 ; 
           [0015]      FIG. 3  is a section view of the leadscrew and ball nut of  FIG. 1  taken generally along the line  3 - 3  in  FIG. 1A ; 
           [0016]      FIG. 4  is a section view of the ball nut of  FIG. 1  taken generally along the line  4 - 4  in  FIG. 3 ; and 
           [0017]      FIG. 5  is a perspective view of the section view of  FIG. 4  showing the path of ball bearings through the ball nut of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    As shown in  FIGS. 1-5 , the present invention relates to a ball nut  12  for installation on a leadscrew  14 . Ball nut  12  further includes a ball track  13  that corresponds to a raceway  15  on leadscrew  14  and forming a circuit  17  within the ball nut through which the ball bearings, or balls,  16  circulate. The invention provides a relief feature  10  in the ball nut  12 , so as to allow balls  16  traveling through the ball nut to re-distribute relative to one another at a point in circuit  17  other than a ball return mechanism  18 . The ball return mechanism  18  shown in the present embodiment includes an end cap  19  that lifts the balls  16  out of raceway  15  and redirects them back through the ball nut via longitudinal tube  21 . Rotating leadscrew  14  moves the balls  16  along the raceways  15  and through circuit  17 , thereby causing ball nut  12  to move longitudinally along the leadscrew. 
         [0019]    In the embodiment shown, relief feature  10  includes an insert  20  that is inserted into a cut-out  22  in the wall  23  of ball nut  12 . Insert  20  may be made of plastic (or any other suitable material) and includes a section of oversize ball track  24 . In the embodiment shown, insert  20  is slightly undersized compared to cut-out  22 , which allows the insert to float slightly within the cut-out. Insert  20  is retained in the cut-out  22  by a threaded fastener  26 . In the embodiment shown, threaded fastener  26  has a thickness and includes threads  28  around the exterior of the fastener. The threads  28  engage with a threaded portion  30  of cut-out  22 . Fastener  26  also includes a faceted inside opening  29  for wrenching, such as a six-sided opening shown for use with a wrench such as an Allen wrench. In some embodiments, adhesive or other material such as epoxy resin may be used to further secure insert  20  in cut-out  22 , although the use of such additional material is not always necessary. Alternatively, relief feature  10  may be formed by machining a portion of oversized ball track integral to the ball nut  12  itself 
         [0020]    As shown in  FIGS. 3-5 , relief feature  10  allows the ball bearings  16  to slide freely relative to one another as they pass through oversize ball track  24  before being tightly clamped between the ball tracks  13 ,  15  of the screw and nut once again. In the embodiment shown, oversize ball track  24  is approximately 0.001″ larger than ball track  13 , but any suitable amount, such as the next larger drill size from the nominal ball size, may alternatively be used without departing from the invention. It is important not to made the oversize ball track  24  too large, however. The oversize ball track  24  should be just large enough so that the balls  16  are not under load as they pass through the oversize section. Making the oversize ball track  24  too much larger than ball track  13  may create a step in the track, which can affect how smoothly balls circulate through the ball nut  12 . 
         [0021]    Allowing the balls  16  to slide freely relative to one another as they pass through the relief feature  10  effectively relieves any pressure or binding that may have developed as the balls roll, tightly clamped, between the leadscrew  14  and ball nut  12  for the portion of the circuit  17  upstream of the relief feature. Thus, the balls  16  are able to re-distribute and space themselves evenly again as they pass through the relief feature  10  before entering the downstream part of circuit  17 . 
         [0022]    Although the relief feature  10  is shown here as an oversize ball track  24 , the ball relief can alternatively be accomplished through any suitable method of providing a portion of ball track  13  that releases the balls  16  from the normally heavy clamping pressure between leadscrew  14  and ball nut  12 . 
         [0023]    The multiple turn circuit ball relief is best employed with the use of end cap returns  18  to create a single circuit that runs the entire length of the ball nut  12 . One or more ball relief features  10  may be employed to prevent stack-up of the balls  16  throughout the circuit. The ball relief feature  10  could potentially be used with other return mechanisms but is most advantageous with end cap returns because end cap returns are typically used in internal ball return nuts, which do not provide access to the balls or the raceway from the outside of the nut. 
         [0024]    In addition to the benefit of providing relief to the balls  16  as they circulate through the ball nut  12 , insert  20  also allows for installation of ball nut  12  having end cap returns on a leadscrew having a raceway  15  that does not extend to either end of the leadscrew. Typical leadscrews with internal ball return systems are installed at the end of the leadscrew and require the use of a loading arbor or other additional part to prevent balls  16  from falling out of the ball nut  12  during installation. The need for the loading arbor or other additional part is eliminated by using the relief feature  10  of the present invention to assemble the ball nut  12  in a position away from the ends of the leadscrew  14 . To assemble the ball nut  12 , a user moves the ball nut over the leadscrew  14  without the ball bearings  16  installed. Once the ball nut  12  is in position over the raceway  15 , a user can insert the ball bearings  16  through cut-out  22 . After the balls  16  have been inserted into the ball nut  12 , the user installs insert  20  by placing it in cut-out  22  and securing it using threaded fastener  26 . 
         [0025]    Although the invention has been herein described in what is perceived to be the most practical and preferred embodiments, it is to be understood that the invention is not intended to be limited to the specific embodiments set forth above. Rather, it is recognized that modifications may be made by one of skill in the art of the invention without departing from the spirit or intent of the invention and, therefore, the invention is to be taken as including all reasonable equivalents to the subject matter of the appended claims and the description of the invention herein.