Abstract:
A method for forming a helical pinion gear ( 12 ) for a rack and pinion steering apparatus ( 10 ) comprises the steps of: providing a cylindrical first blank ( 60 ) made of a deformable material and having an outer surface ( 68 ); providing a cylindrical second blank ( 100 ) made of a deformable material and having an outer surface ( 108 ); forming a bore ( 116 ) extending at least partially through the second blank ( 100 ); forming helical teeth ( 52 ) on the outer surface ( 108 ) of the second blank; and interconnecting the second blank ( 100 ) with the first blank ( 60 ) to form the helical pinion ( 12 ). The helical teeth ( 52 ) on the pinion ( 12 ) mesh with rack teeth ( 44 ) on a rack ( 16 ) in a rack and pinion steering apparatus ( 10 ).

Description:
TECHNICAL FIELD 
     The present invention relates to a rack and pinion steering apparatus and a method for manufacturing a pinion, and is particularly directed to a method for forming a two-piece helical pinion for a rack and pinion steering apparatus. 
     BACKGROUND OF THE INVENTION 
     A typical rack and pinion power steering apparatus for use in a power-assisted vehicle steering system includes a rack operatively coupled with steerable vehicle wheels and a pinion operatively coupled with a vehicle steering wheel. Teeth on the pinion are meshed with teeth on the rack such that rotation of the pinion produces linear movement of the rack which, in turn, causes the steerable wheels to turn laterally of the vehicle. 
     The teeth on the pinion can extend parallel to the central axis of the pinion, or can alternatively extend at an angle relative to the central axis in a pattern such as a helical pattern. It is desirable to have helical teeth on a pinion which extend at an angle of greater than 15° because a higher angle accommodates a greater range of potential vehicle applications and creates a smoother feel to the vehicle driver when turning the vehicle steering wheel. 
     It is known to manufacture a pinion, including the forming of teeth in a helical pattern on the outer surface of the pinion, using a machining process. The machining process produces a relatively large quantity of waste material. It is also known to manufacture a pinion having helical teeth using cold forming processes. One known cold forming process begins with a single piece of a metal material which is first extruded to form some of the features of the pinion teeth, and which is subsequently placed into a hobbing machine to cut the helical teeth in the material into their final form. This known process is not capable of efficiently mass producing pinions with a helical tooth angle over 15° because the large forces required to eject the helical pinions from the cold forming press destroys the tooling in the machine. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for forming a helical pinion gear for a rack and pinion steering apparatus. The method comprises the steps of: providing a cylindrical first blank made of a deformable material, the first blank having an outer surface and oppositely disposed first and second ends; providing a cylindrical second blank made of a deformable material, the second blank having an outer surface and oppositely disposed first and second ends; forming a bore extending at least partially through the second blank; forming helical teeth on the outer surface of the second blank; and interconnecting the first blank with the second blank to form the helical pinion. 
     The present invention also provides a rack and pinion steering apparatus for turning steering wheels of a vehicle upon rotation of a vehicle steering wheel. The rack and pinion steering apparatus comprises a housing having a chamber, and a rack linearly movable in opposite directions in the chamber to effect turning of the steerable vehicle wheels in opposite directions. The rack has an outer surface portion which includes rack teeth. A pinion is operatively coupled for rotation with the vehicle steering wheel. The pinion has an outer surface which includes pinion teeth extending in a helical pattern. The pinion teeth are meshed with the rack teeth to cause the rack to move linearly upon rotation of the pinion. The pinion comprises coaxially disposed first and second members fixedly attached to one another. Each of the first and second members has an inner surface and an outer surface. The inner surface of the second member engages the outer surface of the first member. The helical pinion teeth are formed on the second member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, wherein: 
     FIG. 1 is a sectional view of a rack pinion steering apparatus having a helical pinion manufactured according to the method of the present invention; 
     FIGS. 2-7 schematically illustrate a first component of the helical pinion of FIG. 1 during consecutive steps of manufacture; 
     FIGS. 8-13 schematically illustrate a second component of the helical pinion of FIG. 1 during consecutive steps of manufacture; and 
     FIG. 14 is a side view showing the first and second components which have been joined together to form the helical pinion. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention relates to a rack and pinion steering apparatus and a method for manufacturing a pinion gear, and is particularly directed to a method for forming a two-piece helical pinion gear which may be advantageously used in a rack and pinion steering apparatus. The pinion gear described below could be used in either a manually driven steering apparatus or a power assisted steering apparatus. FIG. 1 illustrates a hydraulically assisted rack and pinion steering apparatus  10  having a pinion  12 . The steering apparatus  10  further includes a housing  14 , a rack  16 , an input shaft  18 , and a torsion bar  20 . 
     The housing  14  has a hydraulic valve section  30  and a transversely extending rack section  32  through which the rack  16  extends. A rack chamber  34  is defined in the rack section  32  of the housing  14 . Hydraulic lines  36  provide fluid communication between the rack chamber  34  and the valve section  30  of the housing  14 . Hydraulic conduits  38  provide fluid communication between the valve section  30  and a power steering pump (not shown). 
     A piston  40  is connected to the rack  16  and is disposed in the rack chamber  34 . The rack  16  includes a section  42  having rack teeth  44 . The rack teeth  44  are meshed with helical teeth  52 , described further below, on the pinion  12  inside the housing  14 . Opposite ends of the rack  16  are connected with steerable vehicle wheels (not shown) by pivotable tie rods  46  and  48  as is known in the art. 
     The pinion  12  is located inside the housing  14  and has an outer surface  50 . The outer surface  50  includes the teeth  52  which extend in a helical pattern. The torsion bar  20  and the input shaft  18  are non-rotatably connected to a first end  54  (FIG. 14) of the pinion. The first end  54  of the pinion includes first and second recesses  56  and  58 , respectively. The second recess  58  extends axially from the first recess  56  toward an oppositely disposed second end  59  of the pinion  12 . The input shaft  18  is received in the first recess  56  and the torsion bar  20  is received in the second recess  58  in the pinion  12  as may be seen in FIG.  1 . 
     The helical pinion  12  is manufactured using cold forming processes. The pinion  12  is made from two separate pieces of a deformable material which are cold formed separately, and subsequently joined together to form the final product shown in FIG.  14 . According to a preferred embodiment of the present invention, first and second blanks  60  (FIG. 2) and  100  (FIG.  8 ), respectively, are cut from steel bar stock (not shown), preferably SAE 4140 coil stock. Alternatively, the blanks  60 ,  100  could be made of a powdered metal material, or a plastic material. The first and second blanks  60 ,  100  may be cut from the same bar stock or from different bar stocks. The first blank  60  is cut to a first length X 1  (FIG. 2) to create a “preform” pinion blank. 
     The first blank  60  has a cylindrical outer surface  62  and first and second ends  64  and  66 , respectively. The first blank  60  is inserted into a cold heading machine. In the cold heading machine, the ends  64  and  66  of the first blank  60  are squared and the first blank is centered on a first axis  68  (see FIG.  3 ). The first blank  60  is placed into a die (not shown) having a desired interim shape for the first blank. The first end  64  of the first blank  60  is then upset, by pressing against the first end, to form a radially enlarged section  70  (FIG. 4) adjacent the first end. A first frustoconical surface  71  forms a portion of the radially enlarged section  70 . In addition, a second frustoconical surface  72  is formed at the second end  66  of the first blank  60  during this step in the manufacturing process. A cylindrical shaft section  74  lies between the radially enlarged section  70  and the frustoconical surface  72  at the second end  66 . The upsetting of the first blank  60  lengthens the first blank to a second length X 2 . 
     The next step in the manufacture of the helical pinion  12  is to forward extrude, by applying a press force to a punch (not shown) as is known in the art, the first recess  56  in the radially enlarged section  70  of the first blank  60  (see FIG.  5 ). The forward extruding process, which causes the first blank  60  to move into a female die (not shown) in the cold header in the same direction as the punch, lengthens the first blank to a third length X 3 . 
     The second recess  58  in the radially enlarged section  70  of the first blank  60  is then formed by a second forwarding extruding step (see FIG.  6 ). This second forward extrusion, in which a punch is forced farther into the radially enlarged section  70 , of the first blank  60  lengthens the first blank to a fourth length X 4 . 
     The first blank  60  is next subjected to another extrusion process in which the first blank is forced through a die (not shown) to form splines  80  on the outer surface  62  of the shaft section  74  of the first blank. The forming of the splines  80  further increases the length of the first blank  60  to a fifth and final length X 5 . Alternatively, it should be understood that a different drive connection feature than the splines  80 , such as a D-flat or hexagonal shape, could be formed on the outer surface  62  of the first blank  60 . 
     The second blank  100 , which was previously cut from steel bar stock, has a first length Y 1  (FIG.  8 ). The second blank  100  has a cylindrical outer surface  102  and first and second ends  104  and  106 , respectively. The second blank  100  is inserted into a cold heading machine to be used in cold forming of the second blank. The cold heading machine to be used in cold forming of the second blank  100  may be the same machine in which the first blank  60  was cold formed, or may be a different cold forming machine. 
     In the cold heading machine, the ends  104  and  106  of the second blank  100  are squared and the second blank is centered on a second axis  108  (FIG.  9 ). The first end  104  of the second blank  100  is then forward extruded, by applying a press force to a punch (not shown) as is known in the art, to form a first cavity  110  (FIG. 10) at the first end of the second blank. This extruding process, which causes the second blank  100  to move in the cold header in the same direction as the punch, lengthens the second blank to a second length Y 2 . 
     A second cavity  112  (FIG. 11) is next formed in the second blank  100  at the second end  106  of the second blank. The second cavity  12  has approximately the same diameter as the first cavity  110  in the second blank  100 . The second cavity  112  is formed by a reverse or backward extrusion process in which the blank  100  is either stationary or travels against the movement of a punch (not shown) which creates the second cavity. The backward extrusion of the second blank  100  lengthens the second blank to a third length Y 3  and leaves a dividing wall  114  in the second blank which separates the first and second cavities  110  and  112 , respectively. 
     It should be understood that the order of the forward extruding step, which forms the first cavity  110 , and the backward extruding step, which forms the second cavity  112 , could be reversed. 
     The dividing wall  114  in the second blank  100  is then pierced by a punch (not shown) to form a continuous opening or bore  116  (FIG. 12) through the second blank. The bore  116  is centered on the second axis  108  and is defined by a cylindrical inner surface  118 . The dividing wall  114  which was removed from the second blank  100  represents the entire scrap material generated by the manufacture of the helical pinion  12 . This quantity of scrap material is less than 10% of the total material used to manufacture the helical pinion  12 . In addition, the bore  116  could be further extruded as required to mate with an alternative drive connection feature on the first blank  60 . 
     Next, a mandrel (not shown) is inserted into the bore  116  in the second blank  100  at the first end  104  of the second blank where the punch presses against the second blank. The second blank  100  is then forward extruded through a die (not shown) which forms helical teeth  52  into the outer surface  102  of the second blank and which lengthens the second blank to a fourth length Y 4  (FIG.  13 ). In accordance with a preferred embodiment of the present invention, the helical teeth  52  are formed at a helical angle of at least 15°. It should be understood, however, that the disclosed process could be used to manufacture helical pinion gears having helical teeth formed at a helical angle of less than 15°. The mandrel rotates as the helical teeth  52  are formed into the second blank  100 . The mandrel pushes the second blank  100 , with its newly cut helical teeth  52 , out of the cold header. In accordance with the preferred embodiment of the invention, a third blank (not shown), which follows the above-described second blank in the cold heading machine and which is being cold formed in the cold heading machine, pushes against the mandrel to cause the second blank to be expelled from the cold heading machine. 
     The first and second blanks  60  and  100 , respectively, pressed together to form the helical pinion  12  (FIG.  14 ). The first axis  68  of the first blank  60  is aligned with the second axis  108  of the second blank  100 , and the second blank is forced onto the first blank. The cylindrical inner surface  118  defining the bore  116  through the second blank  100  is pressed into engagement with the splined outer surface  62 , or other alternative drive connection feature, on the shaft section  74  of the first blank  60 . 
     The two-piece helical pinion  12  is then subjected to an induction hardening process to harden the helical teeth. The final step in the manufacture of the helical pinion  12  is to grind the surfaces of the helical pinion as needed to bring the surface dimensions within their respective tolerance limits. 
     The disclosed cold forming method for manufacturing of the helical pinion  12  provides an efficient and cost effective method for mass producing helical pinions with helical teeth  52  having an angle of greater than 15°. It should be understood that the above cold forming method for manufacturing of a helical pinion can also be used to manufacture helical pinions with helical angles of less than 15°. The method according to the present invention overcomes the problems in the prior art of ejecting a helical pinion having greater than 15° helical teeth from a cold forming machine, and thereby increases tool life. Further, the method described above results in a low quantity of scrap material being generated during the manufacturing process. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. For example, the order of the cold forming of the first and second blanks  60  and  100  could be switched so that the second blank is formed before the first blank. Alternatively, the first and second blanks  60  and  100  could be formed simultaneously in the same cold heading machine, or in different machines. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.