Abstract:
A ball screw and nut mechanism includes a screw having helical grooves with helical ridges therebetween. A nut is formed from at least two portions and has complementary grooves which in combination with the screw grooves define raceways for at least one closed loop of rolling balls. The nut defines at least first and second seams between the two nut portions. The rolling balls circulate in a closed loop to enable the screw to translate in a linear manner relative to the nut. In the region of at least one seam, the grooves in the nut portions are configured so as to at least partially unload the balls as they roll past the seam.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/063,721 filed Feb. 6, 2008. 
     
    
     BACKGROUND 
       [0002]    Various embodiments of a ball screw and nut assembly are described herein. In particular, the embodiments described herein relate to an improved ball screw and nut assembly. 
         [0003]    Screw and nut mechanisms with recirculating balls are commonly used to transform a rotational movement into a linear movement or a linear movement into a rotational movement. The nut can be made in two halves and assembled about the screw. Use of such a split nut may cause undesirable noise and vibration as the balls move across the seams between the two nut halves. 
         [0004]    One example of a recirculating ball screw and nut assembly is disclosed in U.S. Pat. No. 7,013,747, which is incorporated herein by reference. U.S. Pat. No. 7,013,747 discloses a unitary nut ( 2 ) and a screw ( 4 ) complementary threaded in a manner well known in the conventional technology. 
         [0005]    U.S. Pat. No. 4,364,282 discloses a screw and nut mechanism. The nut is made of two sheet metal halves. The groove in a cylindrical portion of the nut intermediate the edges of the halves is provided with a recessed portion constituting a return portion of the closed loop over a ridge between two adjacent groove turns in the screw. 
         [0006]    U.S. Pat. No. 4,474,073 discloses a spindle drive assembly with recirculating balls. The nut includes at least one compensating gap and a clamping means to apply a circumferential force to the nut to control the width of the compensating gap in such a way as to prevent the unloading of the balls as they traverse the gap. 
       SUMMARY 
       [0007]    The present application describes various embodiments of a ball screw and nut assembly. One embodiment of the ball screw and nut mechanism includes a screw having helical grooves with helical ridges therebetween. A nut is formed from at least two portions and has complementary grooves which in combination with the screw grooves define raceways for at least one closed loop of rolling balls. The nut defines at least first and second seams between the two nut portions. The rolling balls circulate in a closed loop to enable the screw to translate in a linear manner relative to the nut. In the region of at least one seam, the grooves in the nut portions are configured so as to at least partially unload the balls as they roll past the seam. 
         [0008]    Other advantages of the ball screw and nut assembly will become apparent to those skilled in the art from the following detailed description, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic plan view of a portion of the ball screw and nut assembly, showing one the screw in one half of the nut. 
           [0010]      FIG. 2  is a schematic cross sectional view taken along the line  2 - 2  in  FIG. 1 . 
           [0011]      FIG. 3  is an enlarged schematic cross sectional view of a portion of the ball screw and nut assembly illustrated in  FIG. 2 . 
           [0012]      FIG. 4  is an enlarged schematic cross sectional view of a portion of the ball screw and nut assembly taken along the line  4 - 4  in  FIG. 3 . 
           [0013]      FIG. 5  is an enlarged schematic cross sectional view of an alternate embodiment of the portion of the ball screw and nut assembly illustrated in  FIG. 4 . 
           [0014]      FIG. 6  is an enlarged schematic cross sectional view of a portion of the ball screw and nut assembly taken along the line  6 - 6  in  FIG. 3 . 
           [0015]      FIG. 7  is a sectional view of a portion of a known embodiment of a vehicle electric power steering assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring now to  FIG. 4 , there is illustrated a known embodiment of a vehicle electric power steering assembly, indicated generally at  100 . The illustrated vehicle electric power steering assembly  100  is a vehicle electric belt driven rack drive steering assembly and is associated with the front driven wheels (not shown) of the vehicle. The general structure and operation of the electric power steering assembly  100  is conventional in the art. 
         [0017]    The illustrated electric power steering assembly  100  includes a vehicle steering wheel  112  and a rotatable input shaft  114  which is operatively coupled in a manner not shown, to the steering wheel  112  for rotation therewith about a steering axis X 1 . A torque sensor  116  is located inside a pinion housing  118  and encircles the input shaft  112 . The torque sensor  116  includes coils (not shown) which respond to the rotation of the input shaft  112  and which generate over electrical lines (not shown) an electrical signal indicative of the direction and magnitude of the applied steering torque. 
         [0018]    A torsion bar (not shown) is provided to connect the input shaft  114  to a pinion  122  located inside the pinion housing  118 . The torsion bar  120  twists in response to the steering torque applied to the steering wheel  112 . When the torsion bar  120  twists, relative rotation occurs between the input shaft  114  and the pinion  122 . 
         [0019]    The pinion housing  118  is attached to a rack housing, indicted generally at  130 . A linearly movable steering member  132  extends axially through the rack housing  130 . The steering member  132  is linearly (or axially) movable along a rack axis X 2 . A rack portion  134  of the steering member  132  is provided with a series of rack teeth (not shown) which meshingly engage gear teeth (not shown) provided on the pinion  122 . The steering member  132  further includes a screw portion  140  having an external thread convolution  142 . The steering member  132  is connected with steerable wheels (not shown) of the vehicle through tie rods (not shown) located at the distal ends of the steering member  132 . Linear movement of the steering member  132  along the rack axis X 2  results in steering movement of the steerable wheels as is known manner. 
         [0020]    The rack housing  130  has a generally cylindrical configuration and includes a first section  150 , a second section  152 , and a third section  154 . The first section  150  is connected to the second section  152  by suitable means, such as for example by a plurality of bolts and nuts (not shown). Similarly, the second section  154  is connected to the third section  154  by suitable means, such as for example by a plurality of bolts and nuts (only the bolts shown in  FIG. 4  by reference numbers  270 ). The first section  150  is provided with a radially enlarged end  150 A, and the third section  154  is provided with a radially enlarged end  154 A. The enlarged ends  150 A and  154 A of the respective sections  150  and  154  cooperate with the second section  152  to define an annular chamber  156 . Alternatively, the structure of the rack housing  130  can be other than illustrated if so desired. For example, the rack housing  130  can include less than three sections or more than three sections if so desired. 
         [0021]    The steering assembly  100  further includes an electric motor  160  which is drivably connected to a ball nut assembly, indicated generally at  170  for effecting axial movement of the steering member  132  upon rotation of the steering wheel  112 . In the event of the inability of the electric motor  160  to effect axial movement of the steering member  132 , the mechanical connection between the gear teeth on the pinion  124  and the rack teeth on the rack portion  134  of the steering member  132  permits manual steering of the vehicle. The ball nut assembly  170  is located in the chamber  156  of the rack housing  130  and encircles the screw portion  140  of the steering member  132 . 
         [0022]    The ball nut assembly  170  further includes a plurality of force-transmitting members  260 . The force transmitting members  260  comprise balls (shown in  FIG. 4 ), which are disposed between the internal screw thread convolution of the ball nut and the external thread convolution on the screw portion  140  of the steering member  132 . The balls  260  are loaded into the ball nut assembly  170  in a known manner. The ball nut assembly  170  further includes a recirculation passage (not shown) for recirculating the balls  260  upon axial movement of the steering member  132  relative to the ball nut assembly  170 . 
         [0023]    As used herein, load is defined as force transferred from the screw  12  through the balls  34  to the nut  18 . Unloaded is defined as the condition wherein little if any force is transferred from the screw  12  through the balls  34  to the nut  18 . For example  FIG. 4  is a schematic illustration of a ball  34  in an unloaded state. A screw force F S  is transferred to the ball  34 , but the ball  34  does not transfer a force to the nut  18 , therefore placing the ball  34  in an unloaded condition. In the illustrated embodiment the ball  34  has a radius R B  smaller than a radius R 1  of the groove  26 . 
         [0024]      FIG. 5  is a schematic illustration of an alternate embodiment of the ball screw and nut assembly  10 ′, wherein the screw force F S  is transferred from the screw  12 , through the ball  34  to the nut  18 . In the illustrated embodiment the ball  34  has a radius R B  substantially equal to the radius R 3  of the groove  26 . 
         [0025]    It will be understood that in an unloaded state, the relative positions of the screw  12 , the ball  34 , and nut  18  may be other than illustrated in  FIGS. 4 and 5 , so long as little if any force is transferred from the screw  12  through the balls  34  to the nut  18 . 
         [0026]      FIG. 6  is a schematic illustration of a ball  34  in a loaded state. The screw force F S  is transferred to the ball  34 , but the ball  34  does not transfer a force to the nut  18 , therefore placing the ball  34  in an unloaded condition. In the illustrated embodiment the ball  34  has a radius R B  smaller than a radius R 1  of the groove  26 . 
         [0027]    Referring again to the drawings, there is illustrated in  FIG. 1  a sectional view of a portion of a first embodiment of a ball screw and nut or ball nut assembly, indicated generally at  10 . The illustrated embodiment of the ball screw and nut assembly  10  includes a worm or screw  12 . The screw  12  includes a helical groove  14  formed in an outer surface thereof. The helical groove  14  is limited by a helical land or ridge  16 . As best shown in  FIG. 2 , the screw  12  is surrounded by a nut  18  made of two halves or portions; a first portion  20  and a second portion  22 . The first and second portions  20  and  22  define halves of the nut  18 . 
         [0028]    The screw  12  may be formed from any suitable material. Examples of suitable materials include steel, brass, engineered plastics, and aluminum. Any other suitable metal and non-metal may also be used, the selection of which would be determined by the loads anticipated in the particular application, and by routine experimentation. 
         [0029]    The first and second portions  20  and  22  are substantially identical and are substantially semi-cylindrical in shape. The first and second portions  20  and  22  each cover or extend around about 180 degrees of the screw  12 . Grooves  24 ,  26  are formed in the inner surfaces of the first and second portions  20  and  22 , respectively. 
         [0030]    When assembled to form the nut  18 , the first and second portions  20  and  22  define first and second longitudinal splits or seams  28  and  30 , respectively. Additionally, the grooves  24 ,  26  of the first and second portions  20 ,  22  cooperate with the helical groove  14  to define raceways  32  for a plurality of rolling members or balls  34 , which circulate in one or more closed loops or circuits, such as shown at A, B, and C in  FIG. 1 . It will be understood that the number of circuits will be determined by the specific application, the space available, the load and life requirements, and the like. 
         [0031]    Referring to  FIGS. 1 and 2 , a first embodiment of a first recessed portion  36  is formed in the grooves  24  and  26  of the first and second nut portions  20  and  22 , respectively. When the first and second portions  20  and  22  are assembled to define the nut  18 , the recessed portion  36  provides a recirculation path for the balls  34  over the ridge  16  between two adjacent portions of the helical groove  14  in the screw  12 . In the exemplary embodiment illustrated a ball circuit A in  FIG. 1 , and in  FIGS. 2 and 3 , the recessed portion  36  is formed at the seam  28 . The recessed portion  36  includes first part  36 ′ formed in the groove  24  of the first nut portion  20  and a second part  36 ″ formed in the groove  26  of the second nut portion  22 , the functions for each will be described in detail below. The recessed portion  36  is formed deep enough to unload the balls  34  and provide clearance for the balls  34  as they pass over the ridge  16 . 
         [0032]    It will be understood however, that the recessed portion  36  is not required to be formed at a seam  28 ,  30 , as described above. As illustrated in a second embodiment of a ball circuit B in  FIG. 1 , an alternative location where the balls  34  may pass over the ridge  16  is shown at  50 . In the ball circuit B, the recessed portion (shown by the dashed line  136  in  FIG. 2 ) is formed in a groove  24  of the first nut portion  20 . Similarly, in a third embodiment of a ball circuit C in  FIG. 1 , another alternative location where the balls  34  may pass over the ridge  16  is shown at  52 . In the ball circuit C, the recessed portion (shown by the dashed line  236  in  FIG. 2 ) is formed in a groove  24  of the first nut portion  20 . 
         [0033]    A second recessed portion  38  is formed in the grooves  24  and  26  of the first and second nut portions  20  and  22 , respectively, such that the rolling balls  34  unload within the second recessed portion  38 . The second recessed portion  38  includes first part  38 ′ formed in the groove  24  of the first nut portion  20  and a second part  38 ″ formed in the groove  26  of the second nut portion  22 , 
         [0034]    As best shown in the embodiment illustrated in  FIG. 3 , the raceway  32  at second recessed portion  38  has a maximum depth D 1  slightly larger than a depth D of the remainder of the raceway  32 . As used herein, the depth D and D 1  are measured from the upper surface of the ridge  16 . At the second recessed portion  38 , the surface (lower surface when viewing  FIG. 2 ) of the nut groove  24 ,  26  is tapered to the maximum depth D 1  over a distance defined by an arc having an angle  42  of about 10 degrees, centered on the second seam  30 . If desired, the surface (lower surface when viewing  FIG. 2 ) of the nut groove  24 ,  26  of the second recessed portion  38  may be tapered to the maximum depth D 1  over a distance defined by an arc having any other desired angle  42 . It will be understood that the angle of the arc  42  may be determined through routine experimentation and may vary from application to application. Factors such as for example, the pitch of the screw  12 , the desired load, the speed of rotation, the size of the ball  34 , and the size of the nut  18 , may be considered to determine the appropriate angle of the arc  42 . 
         [0035]    Although the second recessed portion  38  is illustrated as being formed at the second seam  30 , it will be understood that the second recessed portion  38  may be formed at the first seam  28 , or at both the first and second seams  28  and  30  of any desired ball circuit, such as the circuits A, B, and C. 
         [0036]    In the embodiment illustrated in  FIG. 3 , the maximum depth D 1  is slightly larger than the depth D of the remainder of the raceway  32 . It will be understood that the maximum depth D 1  may be determined through routine experimentation and may vary from application to application. Factors such as for example, the pitch of the screw  12 , the desired load, the speed of rotation, the size of the ball  34 , and the size of the nut  18 , may be considered to determine the appropriate angle of the arc  42 . 
         [0037]    Referring again to  FIGS. 1 and 2 , balls  34  traveling a single circuit of the ball nut assembly  10 , enter into contact with both the nut  18  and the screw  12  at the start of an exemplary circuit A at the first part  36 ′ of the first recessed portion  36 . The balls  34  then travel under load in the raceway  32  defined by the helical groove  14  and groove  24 , around the outer periphery of the screw  12  to the second recessed portion  38 . 
         [0038]    The second recessed portion  38  allows the balls  34  to become unloaded while traversing the seam  30 . More specifically, the tapered first part  38 ′ of the recessed portion  38  allows for the gradual reduction of the load on each ball  34  as the ball  34  enters the recessed portion  38 . The load on the ball  34  is then gradually reapplied upon exiting the recessed portion  38  through the tapered second part  38 ″, and returning to the raceway  32  defined by the helical groove  14  and groove  26 . The balls  34  then come under load again while traveling the opposite side of the outer periphery of the helical groove  14  (the path of which is illustrated by the balls  34  shown in dashed line), until reaching the second part  36 ″ of the first recessed portion  36  of the nut  18 , wherein the balls  34  again become unloaded. 
         [0039]    The first recessed portion  36  is designed in such a way as to allow sufficient space for the balls  34  to simultaneously move radially outward from the screw  12 , to travel over the ridge  16  of the screw  12 , and axially along the screw  12  a distance approximately equal to one helical pitch, such that each ball  34  is again at the start of a circuit; i.e., at the first part  36 ′ of the first recessed portion  36 . 
         [0040]    It will be understood that the balls  34  travel in the circuits B and C in the same manner as described above regarding circuit A. Because the balls  34  pass over the ridge  16  at the first recessed portion  136  in circuit B and over the ridge  16  at the first recessed portion  236  in circuit C, both circuits B and C may include the second recessed portion  38  formed at both the first and second seams  28  and  30 . 
         [0041]    The embodiments of the ball screw and nut assembly  10  described herein may have any desired pitch, and it will be understood that the optimum pitch may be determined through routine experimentation. The embodiments described and illustrated herein include single start worms  12 , wherein the recirculation of the balls  34  back to the start of the raceway  32  defines one pitch. Although not illustrated, the features of the improved ball screw and nut assembly  10  described herein may be used with multiple start worm assemblies. 
         [0042]    It will be understood that it is the loading of the screw  12  against the nut  18  through the balls  34  that forces the movement of the balls  34  through the advancing raceways  32 . The recessed portion  38  are deep enough to provide clearance of the balls  34  as they pass over the ridges  16 . 
         [0043]    The nut  18  may be formed from any suitable material. Examples of suitable materials include steel, brass, engineered plastic, and aluminum. Any other suitable metal and non-metal may also be used. The nut  18  may be formed by any suitable method, such as for example; forging or cold forming. Forming the nut  18  in two halves  20  and  22 , allow for the grooves  24 ,  26  and the recessed portions  36 ,  38 ,  136 , and  236  to be exposed and easily accessible for inspection to ensure compliance with dimensional tolerances and for modification if necessary. 
         [0044]    The balls  34  may be loaded into the nut  18  before the first and second portions  20  and  22  are assembled. The first and second portions  20  and  22  may then be assembled around the screw  12  and fastened together by any desired means, such as with fasteners  40 . 
         [0045]    One or more fasteners  40 , such as a threaded fastener, may be provided to attach the first and second portions  20  and  22  together. In the illustrated embodiment the fastener  40  is shown at the first and second longitudinal seams  28  and  30 , respectively, between the first and second portions  20  and  22 . 
         [0046]    Alternatively, fastening of the first and second portions  20  and  22  at one of the longitudinal seams  28 ,  30  may be accomplished by means of interlocking tabs (not shown) or a tab and slot arrangement (not shown), whereby the interlocked features position the two portions  20  and  22  of the nut  18  and may act as a pivoting hinge. The two portions  20  and  22  of the nut  18  are then secured in place around the worm by the fastener  40  at the other of the longitudinal seams  30 ,  28 . 
         [0047]    It will be understood that the position of the first portion  20  relative to the second portion  22  may be adjusted during assembly to achieve a desired pre-load or lash (i.e., movement without load) characteristic of the ball screw and nut assembly  10 . 
         [0048]    It will be also understood that the second recessed portion  38  may be formed in one or more seams of a nut in a ball screw and nut assembly wherein the nut has more than two component parts, such as for example, three parts or four parts. 
         [0049]    It will be further understood that the second recessed portion  38  may be formed in one or more seams of a nut in a ball screw and nut assembly wherein the ball return path is external to the nut. One example of such a ball screw and nut assembly with a ball return bath external to the nut is disclosed in U.S. Pat. No. 7,207,234. 
         [0050]    The principle and mode of operation of the ball screw and nut assembly have been described in its various embodiments. However, it should be noted that the ball screw and nut assembly described herein may be practiced otherwise than as specifically illustrated and described without departing from its scope.