Abstract:
A method of manufacture of a power assisted steering control valve including a pinion shaft, an input shaft and a torsion bar comprising following steps: receiving into clamping fixtures of an assembly machine a pre-assembled loose assembly of said pinion shaft, input shaft and torsion bar, establishing and recording a predefined rotation clearance between a shoulder of said pinion shaft and an end of said input shaft, rotating said input shaft in opposing directions between rotation limiting stops to said pinion shaft, pressing a first end of said torsion bar into said input shaft to form a locked together assembly of said input shaft and said torsion bar, and axially advancing said assembly of said input shaft mid said torsion bar into said pinion shaft so as to press a second end of said torsion bar into a locked together assembly with said pinion shaft.

Description:
The present invention relates to power assisted steering for motor vehicles and, more particularly, to the assembly of elements of a control valve for an electric power assisted steering system. 
     BACKGROUND 
     Typically, larger and more expensive passenger motor vehicles have used hydraulically activated and controlled power assisted steering systems. These systems relied on a continuously available hydraulic power source generated by a hydraulic pump, typically belt driven from the crankshaft pulley. When the vehicle is travelling at speed and negotiating relatively gentle turns, no or very little hydraulic force is called upon, with the full available hydraulic force only coming into effect during parking or very low speed maneuvering. 
     The degree of torque applied to the steering column in these systems is transferred to a hydraulic control valve system in which a torsion element allows a degree of differential rotation between that of the steering column and the pinion driving the steering rack commensurate with the torque applied. The greater the degree of twist of the torsion bar, the greater the flow of hydraulic force made available to the rack of the steering system and hence to turning of the road wheels. 
     While these systems have been very effective, they are considerably wasteful of energy. The pumping of hydraulic fluid must be continuous whether required or not, with attendant frictional losses from the fluid itself as well as those of the pump and its drive belt/pulley system. In addition the valve block controlling different levels of hydraulic fluid flow requires quite complex machining so that the system is both complex and expensive. 
     While Electrical Power Assisted Steering (EPAS) overcomes many of the above disadvantages, there remains however a precise and rather complex assembly process associated with the location of the torsion bar sensing of torque common to both hydraulic and EPAS systems. Typically the torsion bar is “locked” into its default centred position by a pinning operation. This requires the simultaneous drilling and reaming of an input shaft body and the torsion bar, followed by the insertion of a locking pin. In the automated assembly machines which carry out the centring and pinning operation, this requires additional indexing stations for drilling and reaming equipment with automated supply and insertion of pins, adding considerably to the complexity and time of the assembly process and the cost of the assembly machine. 
     It is an object of the present invention to address or at least ameliorate some of the above disadvantages. 
     Notes 
     
         
         1. The term “comprising” (and grammatical variations thereof) is used in this specification in the inclusive sense of “having” or “including”, and not in the exclusive sense of “consisting only of”. 
         2. The above discussion of the prior art in the Background of the invention, is not an admission that any information discussed therein is citable prior art or part of the common general knowledge of persons skilled in the art in any country. 
       
    
     BRIEF DESCRIPTION OF INVENTION 
     Accordingly, in a first broad form of the invention, there is provided a method of manufacture of a power assisted steering control valves said control valve including a pinion shaft, an input shaft and a torsion bar; said method including the steps of:
         (a) receiving into clamping, fixtures of an assembly machine a pre-assembled loose assembly of said pinion shaft, said input shaft and said torsion bar,   (b) establishing and recording a predefined rotation clearance between a shoulder of said pinion shaft and an end of said input shaft,   (c) rotating said input shaft in opposing directions between rotation limiting stops so as to establish a midpoint of rotation of said input shaft relative to said pinion shaft,   (d) pressing a first end of said torsion bar into said input shaft to form a locked together assembly of said input shaft and said torsion bar,   (e) axially advancing said assembly of said input shaft and said torsion bar into said pinion shaft so as to press a second end of said torsion bar into a locked together assembly with said pinion shaft.       

     Preferably, in said pre-assembled assembly, said torsion bar is located as a free sliding fit in said input shaft; a said first end of said torsion bar projecting from an outer end of said input shaft. 
     Preferably, in said pre-assembled assembly, a pinion end of said input shaft and a second end of said torsion bar are loosely engaged in a socket and central bore of said pinion shaft. 
     Preferably, said pinion shaft is rigidly clamped into a fixed pinion shaft clamping element of said assembly machine. 
     Preferably, said input shaft is rigidly clamped into an input shaft clamping element of said assembly machine; said input shaft clamping element provided with axial and rotational degrees of freedom. 
     Preferably, in a first axial motion of said input shaft clamping element, a locating shoulder of said input shaft is brought into contact with an end of said pinion shaft; a linear encoder recording a first position of said input shaft clamping element. 
     Preferably, a rotation clearance is calculated based on said first position of said input shaft clamping element to establish a second position of said input clamping element. 
     Preferably, said input shaft clamping element is alternately rotated between said rotation limiting stops between said pinion and said input shaft; limits of said rotation recorded by a rotary encoder; said input shaft then rotated by said input shaft clamping element to said midpoint of rotation. 
     Preferably, said input shaft clamping element is axially translated to create a predetermined gap between said end of said pinion shaft and said locating shoulder of said input shaft. 
     Preferably, laterally moving input shaft support collets are inserted into said predetermined gap between said pinion shaft end and said locating shoulder of said input shaft. 
     Preferably, an axially driven press in a first advance acts on said projecting first end of said torsion bar to drive said first end into said outer end of said input shaft. 
     Preferably, splines at said first end of said torsion bar engage with the internal bore of said input shaft outer end so as to rotationally lock together said first end of said torsion bar and said outer end of said input shaft. 
     Preferably, said input shaft support collets are withdrawn from said gap. 
     Preferably, said axially driven press in a second advance acts on said input shaft and said torsion bar so as to drive said second end of said torsion bar into said central bore of said pinion shaft; said second advance limited to locate said input shaft relative said pinion shaft at said recorded rotation clearance. 
     Preferably, splines at said second end of said torsion bar engage with said central bore of said pinion shaft so as to rotationally lock together said second end of said torsion bar and said pinion shaft. 
     In another broad form of the invention, there is provided an automated assembly and calibration method of a power assisted steering control valve; said assembly and calibration method restricted to steps of axial translation and rotation of components of said control valve; said steps including:
         (a) placing a pinion shaft into a pinion shaft clamping element of an assembly machine,   (b) placing an input shaft with loosely inserted torsion bar in an input shaft clamping element of an assembly machine,   (c) determining and recording a rotation clearance between said pinion shaft and said input shaft,   (d) determining and recording a mid-point of rotation of said input shaft relative said pinion shaft,   (e) pressing a first end of said torsion bar into an outer end of said input shaft to form an assembly of said torsion bar and input shaft in which said outer end of said input shaft and said first end of said torsion bar are rotationally locked together,   (f) pressing said assembly of said torsion bar and said input shaft to force a second end of said torsion bar into a central bore of said pinion shaft so as to rotationally lock together said second end of said torsion bar and said pinion shaft.       

     Preferably, a section at each end of said torsion bar is provided with splines; said splines arranged to engage respectively with bores of said input shaft and said pinion shaft as interference fits. 
     In still another broad form of the invention, there is provided a method of securing a torsion bar between an input shaft and a pinion shaft of a power assisted steering control valve; said method including the steps of:
     (a) in a first pressing operation, driving a section at a first end of said torsion bar into an upper section of a bore of said input shaft,   (b) in a second pressing operation, driving a section at a second end of said torsion bar into a central bore of said pinion shaft.   

     Preferably, in a pre-assembly operation, a friable washer is placed in a socket of said pinion shaft. 
     Preferably, an end of said input shaft is inserted into a socket of said pinion shaft such that said input shaft is at a mid-point of rotation relative said pinion shaft; said end of said input shaft pressed into said friable washer so as to retain said input shaft at said mid-point of rotation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the present invention will now be described with reference to the accompanying drawings wherein: 
         FIG. 1  is a perspective view of a power steering control valve assembly according to the invention in a pre-assembled state, 
         FIG. 2  is an exploded view of the components of the assembly of  FIG. 1 , 
         FIG. 3A  is a side view of a torsion bar component of the assembly, 
         FIG. 3B  is a sectioned side view of an input shaft component of the assembly, 
         FIG. 3C  is a sectioned side view of a pinion shaft component of the assembly, 
         FIG. 3D  is an input shaft end view of the pinion shaft of  FIG. 3C   
         FIG. 3E  is a pinion shaft end view of the input shaft of  FIG. 3B , 
         FIGS. 4A to 4D  are sectioned views of the stages of a preferred embodiment of an assembly process of the components of  FIGS. 2 and 3 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The term “EPAS valve” in this specification is refers to an assembly of the main mechanical components of an Electrical Power Assisted Steering (EPAS) system by means of which differential rotation between the pinion of a rack and pinion steering arrangement and a steering column can be electrically monitored. 
     First Preferred Embodiment 
     With reference to  FIGS. 1 and 2 , the components of the EPAS valve  10  of the invention comprise a pinion shaft  12 , an input shaft  14  and a torsion bar  16 . Additionally, a bearing sleeve  18  is pre-assembled into a socket  20  (see  FIGS. 3C and 3D ) of pinion shaft  12 , and an “O” ring  22  is pre-assembled into the groove  24  of torsion bar  16 . 
     In two further pre-assembly operations, torsion bar  16  is inserted into input shaft  14  as a free sliding fit with the splined first end  26  projecting from the outer end  28  of input shaft  14 , and the pinion end  30  of input shaft  14  and second end  32  of torsion bar  16  are loosely located in the socket  20  and central bore  34  of the pinion shaft  12 . 
     These pre-assembly operations may be automated with suitable component orienting and presenting, and pick and place arrangements, or may be performed manually. 
     When the components have been assembled to this stage, as shown in  FIGS. 1 and 3A , they are presented to an automated final assembly machine. Preferably the assembly takes place with the axis of the components, in vertical orientation. Pinion shaft  12  placed into a fixed pinion shaft support and clamping element (not shown). Pinion shaft  12  is located in the support and clamping element at lower bearing journal  36  and clamped, preferably around upper main bearing journal  38 . 
     An input shaft clamping element (not shown) locates on the main location journal  40  and clamps around the upper end  42  of input shaft  14 . The input shaft clamping element is provided with axial and rotational degrees of freedom and is provided with linear and rotational encoders and torque and axial force measuring equipment. Linear and rotational movements, as well as torque and axial force, are controlled and monitored by a programmable logic controller and computer software. 
     With reference to  FIGS. 4A to 4D , assembly now proceeds as follows. Firstly the input shaft  14  is driven by its clamping element axially in the direction of the pinion shaft  12  (as indicated by the arrow in  FIG. 4A ) to bring the locating shoulder  44  of input shaft  14  into contact with the end  46  of pinion shaft  12 . The reading of the linear encoder at this point is recorded and used to calculate an encoder reading defining a rotation clearance (indicated in  FIG. 4D ) between the locating shoulder  44  of the input shaft  14  and end  46  of pinion shaft  12 . 
     Secondly, input shaft  14  is rotated about its axis alternately in clockwise and anti-clockwise directions (as indicated in  FIG. 4A ) between the stop limits provided by the walls  48  of socket  20  of pinion shaft  12 , and the angled flats  50  (see  FIG. 3E ) at the pinion end  30  of input shaft  14 . The rotary encoder records the angular position of the input shaft at each limit of rotation and from these two values calculates the mid-point of rotation. The input shaft  14  is then rotated to this mid-point value and retained in this rotated position for the remaining operations of  FIGS. 4B to 4D . 
     The input shaft  14  and torsion bar  16  are now axially withdrawn from the pinion shaft  12  sufficient to open up a predetermined gap  52  between the end  46  of pinion shaft  12  and locating shoulder  44  of input shaft  14  (as shown in  FIG. 4B ). Support collets  54  are moved into position in gap  52  to provide support for locating shoulder  44  against the end  46  of pinion shaft  12 . 
     A press (indicated by the arrow in  FIG. 4C ) acting along the axis of the components, now engages first end  26  of torsion bar  16  and drives it into the upper portion of the bore  56  of input shaft  14 . Preferably, the face  58  of first end  26  is driven to just below the rim  60  of outer end  28  as shown in  FIGS. 4C and 4D . The force exerted by the press is recorded. 
     The first end  26  of torsion bar  16  and the outer end of input shaft  26  are now rotationally locked together as a result of the splines  62  engaging with the bore  56  as an interference fit. 
     Preferably, the driving element of the press is arranged so that the torsion bar  16  is contacted by projecting boss of slightly smaller diameter than that of the torsion bar, and with a larger diameter portion of the press&#39;s driving element just touching the rim  60  when the torsion bar has reached the position shown  FIGS. 4C and 4D . 
     The support collets  54  are now withdrawn and the press driving element, now acting on the rim  60  of the input shaft (but with the projecting boss maintaining contact with the end of the torsion bar), drives the input shaft  14  and torsion bar  16  in the direction of the pinion shaft  12 . The splined second end  32  of torsion bar  16  is thus pressed into the central bore  34  of the pinion shaft  12  until the linear encoder reads the previously calculated value defining the rotation clearance  47 . Again the force exerted by the press to perform this operation is monitored and recorded. 
     This completes the assembly process, with the splines  64  of the second end  32  now engaged as an interference fit within the central bore  34  of the pinion shaft  12 , rotationally locking the second end  32  to the pinion shaft and precisely establishing the rotation clearance  47 . 
     Torque readings and torque profiles are now generated by rotating the input shaft alternately to the rotation stops as previously described, but now with the resistance to rotation provided by the torsion bar  16  acting between the pinion shaft  12  and input shaft  14 . These readings firstly test if the torsion bar resistance is within design parameters and provide inputs for later calibration of the electrical power assistance to be applied at various vehicle speeds and conditions. 
     The assembly is now unclamped and removed from the assembly machine ready for the subsequent fitting of the electrical components. 
     It will be understood by those skilled in the art that although the process described above relies on splines for the rotational locking together of the torsion bar with the input shaft and pinion shaft, the same procedure may be applied if the ends of the torsion bar are not splined. Thus the ends may be smooth or treated with some texturing process. However a disadvantage of smooth ends of the torsion bar is that a higher degree of size tolerancing is essential for both torsion bar and the bores of the input and pinion shafts. 
     Second Preferred Embodiment 
     In a second preferred embodiment of a method of assembly, the components to be assembled are as described above. In this embodiment however, there is an additional step in the pre-assembly process. 
     Before inserting the input shaft  14  and torsion bar  16  into the socket  20  and central bore  34  of the pinion shaft  12 , a friable washer (not shown) is placed into the socket  20  abutting the surface surrounding the central bore  34 . The input shaft  14  and torsion bar  16  are now inserted, but with the angled flats  50  positioned so as to have the input shaft at approximately the mid point of rotation of the input shaft relative to the pinion shaft. 
     The input shaft  14  is pressed “home” to embed the edges of the flats  50  into the friable washer. The washer deforms to accept the shape of the flats  50  and so retain temporarily the desired mid point position of the input shaft relative to the pinion shaft. 
     The assembly is now introduced into the assembly machine which, as previously described, after clamping of the pinion and input shafts into their respective clamping elements, brings the locating shoulder  44  of input shaft  14  into contact with the end  46  of pinion shaft  12 . However, by means of the friable washer, the step of establishing the mid-point of rotation of the input shaft is eliminated and the process of the two stages of pressing can immediately proceed. 
     Once the pressing steps are completed, rotation of the input shaft tests the torsion bar and provides the data for later use as previously described. 
     In Use 
     In use, the assembly method of the present invention allows for a major simplification of an automated assembly machine for assembly of a control valve. The method, requiring only the axial and rotational movements of one of the shafts of the valve and a two-stage pressing operation, thus eliminates the complexities of drilling and reaming of the input shaft and torsion bar, and the insertion of a locking pin. It also of course eliminates the machining and then feeding and handling of pins in the machine. A further benefit of eliminating the pin method of locking shaft and torsion bar together, is a considerable saving of time in the assembly process. 
     The above describes only some embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention. Thus, although the components shown in the drawings are those of an electrical power assisted steering assembly, the assembly process of the invention may equally be applied to the assembly of similar components of an hydraulic power assisted steering, system.