Abstract:
A camshaft assembly and method of making a camshaft assembly is disclosed. Each of the cams includes a lobe boss portion that defines the cam lift profile, and a base portion that provides a surface for joining the cam to the shaft. In contrast to conventional ring-type cams, the base portion of the cam does not circumscribe the outer surface of the shaft, but instead extends only part way around the circumference or periphery of the shaft. This allows for radial mounting of the cams at virtually any timing angle, and permits the use of simple techniques for joining the cams to the shaft, including capacitance discharge welding. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 CFR 1.72(b).

Description:
CLAIM TO PRIORITY 
     This application claims benefit to provisional patent application No. 60/323,835, filed Sep. 20, 2001, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a camshaft for use in internal combustion engines, and more particularly, to a cam design and method of assembly. 
     2. Description of the Related Art 
     Conventional camshafts used to control valve motion in internal combustion engines include a shaft having axially spaced cams, which project outward from the surface of the shaft. The shaft and cams can be machined from a single casting or forging, but are usually assembled from separate parts. Each cam is mechanically coupled to one of the engine valves so that rotation of the shaft results in valve movement. In addition to the cams, conventional camshafts include journals, fittings, sensors, and balancing masses mounted to the shaft. 
     FIG.  1  and FIG. 2 show, respectively, a side view of a portion of a conventional camshaft  10 , and a cross-sectional view of the camshaft  10  through section line  2 . The camshaft  10  includes a tubular shaft  12  having inner  14  and outer surfaces  16  and an annular or ring-type cam  18  mounted on the outer surface  16  of the shaft  12 . As shown in FIG. 2, the cam  18  includes a lobe boss portion  20  and a ring portion  22  having respective inner  24 ,  26  and outer surfaces  28 ,  30 . The inner surface  24  of the lobe boss  20  and the inner surface  26  of the ring portion  22  of the cam  18  define a continuous mounting surface for joining the cam  18  to the outer surface  16  of the shaft  12 . During operation of the camshaft  10 , the outer surface  28  of the lobe boss  20  generates the desired valve lift, while the outer surface  30  of the ring portion  22  defines a base circle, which provides zero-valve lift. To assemble the conventional camshaft  10 , each of the cams  18  are positioned over an end of the shaft  32 , translated to a pre-defined axial position, and attached or joined to the outer surface  16  of the shaft  12 . 
     Although generally satisfactory, conventional camshaft designs can be improved. For example, to set the relative angular position of the cams around the periphery of the shaft (timing angle), conventional camshafts typically employ a ring-type cam having polygonal or spline mounting surfaces that interlock with matching surfaces on the outer surface of the shaft. As a result, any necessary adjustments in lift or timing—e.g., changes in the relative angular position of the cams—require costly changes to the shaft and cams. In addition, to reduce overall camshaft weight and cost, recent cam designs have sought to minimize wall thickness of the ring portion of the cam and the shaft. However, insufficient wall thickness may result in undesirable thermal distortion, severe cold working or thinning during assembly, and marginal mechanical performance. Furthermore, ring-type cams often require preprocessing of the shaft, such as forming and precision machining which increases costs and process variability. The wall thickness of the ring portion of the cam also limits the outer diameter of the shaft and journal, which may result in increased journal dynamic bearing loading and decreased camshaft service life. 
     In many cases, use of ring-type cams also requires complex joining or attachment methods, including shrinkage joining and hydroforming. Although used successfully to assemble camshafts, both techniques present difficulties. For instance, when using shrinkage techniques only a small percentage of the cam mounting surface contacts the outer surface of the shaft. As a result, shrinkage techniques require precision ground components that must be carefully positioned to prevent attachment failures. Although hydroforming may work well on thin wall cams subject to low stress, the method is impractical for relatively high stress loadings of most current automotive and diesel engines. In addition, hydroforming uses large and expensive equipment and tooling, and requires lengthy development time since iterative testing is often necessary to optimize material flow and strength characteristics. 
     Other complex methods of attachment, such as ballizing, sinter brazing, and liquidous-type expansion joining, also present difficulties. For example, ballizing is an expansion technique requiring the use of highly controlled tube wall and outside shaft geometry as well as an expensive die arrangement for assembly. Common problems with ballizing include part distortion and inconsistent material properties. Sinter brazing uses a filler agent, which adds expense and material coverage problems. It also requires the use of a high temperature furnace and lengthy heating and cooling cycles to process the camshaft assembly, which may lead to thermal distortion of the camshaft. Like shrinkage joining and ballizing, sinter brazing requires precision components to optimize joining characteristics. Finally, liquidous-type expansion techniques employ concentric tubes and a liquid crystalline polyester resin, which is injected into an annular gap between the tubes. Since multiple tubes are used, the method is costly. 
     The present invention is directed to overcoming, or at least minimizing, one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention provides a camshaft assembly for transmitting and controlling valve motion in an internal combustion engine. The camshaft assembly includes a shaft having an outer surface and a longitudinal axis, and a cam that is mounted on the shaft. The cam includes a lobe boss portion having a pair of side walls and a transverse surface. The transverse surface of the lobe boss portion of the cam bridges the pair of side walls and defines a cam profile that provides the requisite valve lift and valve velocity during operation. The cam also includes a base portion that provides a surface for joining the cam to the shaft at a predetermined position along the longitudinal axis of the shaft. In contrast to ring-type cams, the base portion or the mounting surface of the cam does not circumscribe the outer surface of the shaft, but instead extends only part way around the circumference or periphery of the shaft. This allows for radial mounting of the cams at virtually any relative angular displacement or timing angle. Because the cams of the present invention lack a ring portion, the cam width adjacent to the base portion can be made narrower, which allows for greater flexibility in the design of the cam profile shape and the resulting cam lift curves. 
     Another aspect of the present invention provides a method of assembling a camshaft. The method includes providing components that make up the camshaft, such as a shaft and cams, and radially mounting at least one of the cams on the shaft. The mounting step includes positioning the cam at a pre-mounting location that is spaced away from an outer surface of the shaft and located between ends of the shaft, and placing the cam on the outer surface of the shaft at a mounting angle of about 90°. A mounting angle of 90° corresponds to placing the cam on the shaft normal to a plane containing a longitudinal axis of the shaft. In contrast to assembling ring-type cams, which require complicated joining or attachment methods, radial mounting can use simpler joining methods such as capacitance discharge welding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a side view of a portion of a conventional camshaft. 
     FIG. 2 is a cross-sectional view of a conventional camshaft through section line  2  of FIG.  1 . 
     FIG. 3 is a perspective view of a portion of a lobe boss camshaft. 
     FIG. 4 is a cross-sectional view of a lobe boss camshaft though section plane  4  of FIG.  3 . 
     FIG. 5 is a side view of a portion of a flat-bottom lobe boss camshaft. 
     FIG. 6 is a cross-sectional view of a flat-bottom lobe boss camshaft through section line  6  of FIG.  5 . 
     FIG. 7 is a flow chart of an assembly method for a low cost camshaft. 
     FIG. 8 is a top view of a portion of a shaft during assembly. 
     FIG. 9 is a cross-sectional view of a shaft through section line  9  of FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG.  3  and FIG. 4 show, respectively, a perspective view of a lobe boss portion of a camshaft  50 , and a cross-sectional view of the camshaft  50  through section plane  4 . The camshaft  50  includes a tubular shaft  52  having inner  54  and outer surfaces  56  and having adequate torsion resistance and stiffness for valve-train actuation. An optional base plate  58  is mounted on the outer surface  56  of the shaft  52 , providing additional joining strength. The base plate  58  includes an inner surface  60  and an outer surface  62 . The inner surface  60  of the base plate  58  shown in FIG. 4 defines an arcuate mounting surface, through generally, the inner surface  60  conforms to the outer surface  56  of the shaft  52 . 
     As can be seen in FIG.  3  and FIG. 4, the camshaft  50  includes a cam  64  that is radially mounted on the outer surface  62  of the base plate  58 , though the cam  64  can be mounted directly on the outer surface  56  of the shaft  52 . The cam  64  includes a lobe boss portion  66  and a base portion  68 . The base portion  68  of the cam  64  provides a mounting surface for joining the cam  64  to the outer surface of the base plate  62  or to the outer surface of the shaft  56 . In contrast to the ring-type cam of FIG. 1, the base portion  68  or the mounting surface  70  of the cam  64  does not circumscribe the outer surface  56  of the shaft  52 , but instead extends only part way around the circumference or periphery of the shaft  52 . As described below, this allows radial mounting of the cams at virtually any relative angular displacement. The lobe boss portion  66  of the cam  64  includes a pair of generally planar faces or side walls  72 ,  74  and a transverse surface  76 , which bridges the pair of faces  72 ,  74  and defines a cam profile  78 . During operation, the cam profile  78  generates the requisite cam lift curve and velocity, and an exposed portion  80  of the outer surface  56  of the shaft  52  defines a base circle that provides zero-valve lift. Because the camshaft  50  shown in FIG.  3  and FIG. 4 lacks the ring portion  22  of conventional cam  10 , the camshaft  50  width adjacent to the base portion  68  can be made narrower than in conventional ring-type cams  10 . This allows for greater flexibility in the design of the cam profile  78  shape and the resulting cam lift curves. The camshaft  50  is generally made of ferrous alloys, such as steel, but can also be made of aluminum, polymeric composites, and other materials known in the art. 
     To reduce mass and cost, the cam  64  may include a hollow portion or cavity  82  located within the lobe boss  66 . Alternatively or additionally, the cam  64  may include one or more apertures (not shown) extending through the cam  64  between the faces  72 ,  74  of the lobe boss  66 . Ordinarily, such mass saving structures can be used whenever camshaft surface life and loading requirements permit. 
     Another embodiment is shown in FIG.  5  and FIG. 6, which provide, respectively, a perspective view of a camshaft portion  50 ′, and a cross-sectional view of the camshaft portion  50 ′ through section line  6 . The camshaft  50 ′ includes a tubular shaft  52 ′ having inner  54 ′ and outer surfaces  56 ′, and a cam  64 ′ that is radially mounted on the outer surface  56 ′ of the shaft  52 ′. As can be seen in FIG. 6, the cam  64 ′ includes a lobe boss portion  66 ′ and a base portion  68 ′. Like the embodiment shown in FIG.  3  and FIG. 4, the base portion  68 ′ of the cam  64 ′ does not circumscribe the outer surface  56 ′ of the shaft  52 ′ but leaves exposed a portion  80 ′ of the outer surface  56 ′ of the shaft  52 ′ that serves as a base circle. The lobe boss portion  66 ′ of the cam  64 ′ also includes a pair of generally planar faces or side walls  72 ′ and  74 ′, and a transverse surface  76 ′ that bridges the pair of faces  72 ′ and  74 ′ and defines a cam profile. In contrast to the embodiment shown in FIGS. 3 and 4, however, the base portion  68 ′ of the cam  64 ′ fits into a notch  90  having a substantially flat mounting surface  92  formed on the outer surface  56 ′ of the shaft  52 ′. The camshaft  50 ′ may include optional pin  94  and locator holes  96  on the base portion  68 ′ of the cam  64 ′ and the mounting surface  92  of the notch  90 , respectively. The pin  94  and corresponding locator hole  96  help position and secure the cam  64 ′ in the notch  90  during assembly. The pin  94  may also serve as a weld stud for joining the cam  64 ′ to the shaft  52 ′, depending on the pin&#39;s  94  response to heat, pressure, electrical current, and the like, that can be applied during assembly of the camshaft  50 ′. In one respect, the notched camshaft  50 ′ is a less flexible design than shafts having constant radius mounting surfaces (FIG.  3  and FIG. 4) since each notch  90  sets the timing angle for a given cam, making it difficult to effect changes in the cam lift curve or valve timing. 
     FIG. 7, FIG.  8  and FIG. 9 illustrate a method  110  of assembling a camshaft for use in a valve train assembly of an internal combustion engine. As noted in the flow chart shown in FIG. 7, the method  110  includes providing  112  components that comprise the camshaft, including a shaft having the requisite torsion resistance, stiffness, and strength for valve train actuation, and cams having base portions that allow radial mounting on the shaft. Other components may include base plates—if needed to provide additional joining strength between the cams and the shaft—and any gears, fittings, journals, sensors, balancing masses, end fittings, and the like. Suitable components include shafts, cams, and base plates shown in FIG.  3 -FIG.  6 . 
     As described in FIG. 7, the method  110  also includes radially mounting  114  the cams  64 ″ at predetermined positions on the outer surface of the shaft  52 ″ and, once mounted  114 , joining  116  the cams  64 ″ to the shaft  52 ″. 
     This process can best be seen in FIG.  8  and FIG. 9, which show, respectively, a top view of a portion of a shaft  52 ″ during assembly, and a cross-sectional view of the shaft  52 ″ through section line  9 . Radially mounting  114  the cams  64 ″ includes positioning  118  one or more of the cams  64 ″ at a desired pre-mounting location  130  and then placing  120  the cam  64 ″ on the outer surface  56 ″ of the shaft  52 ″. The pre-mounting location  130  is spaced away from the outer surface  56 ″ of the shaft  52 ″ and located between the ends  32 ″ of the shaft  52 ″. As shown in FIG.  8  and FIG. 9, the pre-mounting location  130  can be represented by longitudinal distance  132 , x, timing angle  134 , θ, and radial distance  136 , r, although any suitable coordinate system can be used (including a cylindrical coordinate system employing a different origin). Positioning  118  can be accomplished using a device capable of moving the cam  64 ″ or the shaft  52 ″ or the cam  64 ″ and the shaft  52 ″. One useful device includes a computerized numerically controlled (CNC) machine having a translation stage adapted to move the cam  64 ″ (or other camshaft parts) in three dimensions and a rotary fixture adapted to rotate the shaft  52 ″ about its longitudinal axis  138 . Positioning  118  can occur by successive translation and rotation of the cam  64 ″ and shaft  52 ″, respectively, or by simultaneous translation and rotation of the cam  64 ″ and the shaft  52 ″. 
     Once the cam  64 ″ is at the pre-mounting location  130 , it is placed  120  or mounted on the outer surface  56 ″ of the shaft  52 ″ at a mounting angle  140 , α, that is about normal to a plane containing the longitudinal axis  138  of the shaft  52 ″. A mounting angle  140  of about 0° or 180° corresponds to mounting conventional ring-type cams  18  that are slipped over an end of the shaft  34  and translated to a predefined position along the longitudinal axis  138  (cf. FIG.  1  and FIG.  8 ). The mounting step  114  can be performed in a reducing or inert atmosphere, which helps to produce a higher quality joint. 
     Once mounted  114 , the cams  64 ″ can be joined  116  to the shaft  52 ″ using any number of techniques, including resistance welding, which comprises applying weld energy to the parts to be joined for specified time interval. Resistance welding can produce at least three different bonds: brazed or soldered bonds, forged welds, and fusion welds. To produce brazed or soldered bonds, resistance heating of the cam and the shaft melts a third metal, such as silver solder alloy or tin/lead solder, which bonds to both parts. To produce forged welds, a short weld-time current is used to forge the parts together without melting them, which is useful when the cams and shaft are made of different materials. To produce fusion welds, a longer pulse is used to melt the cam and the shaft along their points of contact. Fusion welding is useful when the cams and shaft are made of two similar materials. 
     Resistance welding systems are distinguished by the method of applying energy to the parts, i.e., direct energy (alternating current), stored energy (capacitance discharge), and high-frequency direct-current (HFDC). Of these, capacitance discharge welding (CDW) is particularly advantageous because it can be used to join materials that are susceptible to thermal fracturing or undesirable phase formation, and because, compared to other welding techniques, CDW results in relatively thin welds and narrow heat-affected zones. Most CDW systems provide weld energy as a series of current pulses, resulting in high cooling rates in excess of 10 2  K/s. Dual or multi-pulsing is especially useful for joining coated or plated materials: a first pulse displaces surface oxides and a second pulse welds the underlying materials. Multiple pulses can also preheat or postheat the cam and shaft and can control overall temperature profiles to prevent material expulsion and cracking. Moreover, capacitance discharge systems can reverse the polarity of the sequential pulses, which is useful for welding dissimilar or polarity-sensitive parts. 
     It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments will be apparent to those of skill in the art upon reading the above description. The scope of the invention should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, if any, including patent applications and publications, are incorporated herein by reference for all purposes. 
     While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit.