Abstract:
A crankshaft has a non-homogenous structure is unitarily formed in a powder metallurgy process with at least two different metallic constituents providing dissimilar characteristics at discrete locations of the structure.

Description:
FIELD OF THE INVENTION 
   The present invention relates to components formed by powder metallurgy and, more specifically, to a method and apparatus for forming components by powder metallurgy. 
   BACKGROUND OF THE INVENTION 
   Powder metallurgy is a common manufacturing method used to produce components of high quality for applications such as engines. Powder metallurgy is often employed in the manufacture of engine components because it is economical, flexible, and can produce a finished part that requires less machining or secondary processing than other methods of forming components. Powder metallurgy allows for a component to be formed of a wide variety of alloys, composites, and other materials to provide the finished component with desirable characteristics. Powder metallurgy is well suited to manufacture parts of a wide range of sizes and shapes. Also, powder metallurgy can reliably produce parts with consistent dimensions and advantageous physical properties. 
   Referring to  FIG. 1 , a process chart for the conventional powder metallurgical component forming process  30  is shown. First, the metal powders  32  that comprise the component are provided. Often, lubricants are added to the metal powders to decrease the wear of pressing machinery. Next, the base powders are mixed  34  to form a homogenous mixture. The finished part is ultimately a homogeneous alloy of the constituent metal powders. 
   A mold or die is then filled  36  with the mixed powders. The die, when closed, has an internal cavity somewhat similar in shape to the final part. The powder is compressed  38  within the die to form a so-called “green part.” The compaction  38  is usually performed at room temperature and at pressures, for example, in the range of 30-50 tons per square inch. The green part, also referred to as a “green compact,” has the desired size and shape for the next operation when ejected from the die. After compaction  38 , the green part has sufficient strength for further processing. 
   The green part is subjected to a sintering process  40 . A variety of secondary operations  42  may be performed on the part after sintering  40 , depending on its intended use, the process yielding a finished part  44 . 
   Generally, sintering  40  involves subjecting the green part to a temperature, for example, of 70-90% of the melting point of the metal or alloy comprising the green part. The variables of temperature, time, and atmosphere are controlled in the furnace to produce a sintered part having improved strength due to bonding or alloying of the metal particles. The sintering process  40  generally comprises three basic steps conducted in a sintering furnace: burnoff  46 , sinter  48 , and cooling  50 . Continuous-type sintering furnaces are commonly used to perform these steps. The burnoff chamber is used to volatize the lubricants used in forming green part  46 . The high-temperature chamber performs the actual sintering  48 . The cooling chamber cools the sintered part prior to handling  50 . 
   The parts that exit the sintering furnace  40  after cooling  50  may be considered complete. Alternatively, they may undergo one or more secondary operations  42 . Secondary operations include, for example, re-pressing (forging) the component  52 , machining  54 , tumbling  56 , and joining the component with additional components  58  as part of an overall assembly. The secondary operations  42  may also include the impregnation of oils or lubricants  60  into the part for conveying self-lubricating properties. The sintered component may also undergo heat treatment  62  to provide certain characteristics and properties to the component, such as strength. Those skilled in the art will recognize that other secondary operations may be performed. The secondary operations  42  may be performed individually or in combination with other secondary operations. Once all the secondary operations  42  are performed, the component or part  44  is finished. 
   U.S. Pat. Nos. 5,303,468, 5,195,398, and 3,748,925 disclose crankshafts for use in an internal combustion engine. 
     FIG. 2  illustrates the internal detail of a conventional internal combustion engine to illustrate the use of a crankshaft  72 . A connecting rod  64  is pivotally connected to a piston  66  and the crankshaft  72 . The connecting rod  64  is connected to the crankshaft  72  at a large or crank end  76 . The large end  76  of the rod  64  receives a shaft portion (“crank pin”)  78  of the crankshaft  72 . The connecting rod  64  is further connected to a piston  66  at a small or piston end  70  of the rod  64 . The crankshaft  72  comprises a counterweight  74  disposed between the crankpins  78 . 
   Referring to  FIG. 3  and  FIG. 4 , a conventional crankshaft  72 , manufactured according to conventional methods, is shown. Crankshaft  72  comprises a longitudinally extending body  83  between a first end  80  and a second end  82 . The body  83  defines an axis or rotation  84  for the crankshaft, when rotating in the engine. A main journal  86  is provided at each of the first end  80  and second end  82  for supporting the shaft  72  in the engine block. The body  83  includes a plurality of bearing journals  88 , crank pin journals  90  and counterweights  74 . 
   The mass of the crank pin journals  90  when coupled with a connecting rod  64  defines an offset balance axis  92 . The balance axis  92  is the axis of rotation through which the forces generated by rotation of the shaft and connecting rod assembly are balanced. The axis of rotation  84  is offset from the axis of balance  92 . The offset creates a moment when the crankshaft is rotating. The moment is undesirable because it increases loading on the shaft bearings and minimizes oil film thickness between the journals of the crankshaft and their respective bearings. This limits the load carrying capacity of the main journals. 
   A conventional solution is to provide a plurality of counterweights  78  to the shaft  72  to shift the axis of balance  92  towards the axis of rotation  84 . Ideally, the counterweights  78  are located 180 degrees opposite each crankpin journal  90 . Such configuration results in an undesirably large crankshaft  72 . Engine designers are constantly striving to minimize engine size and increase engine efficiency. Larger crankshafts necessitate larger engine size. Moreover, larger crankshafts also increase the rotational inertia of the engine, thereby reducing efficiency. 
   U.S. Pat. No. 5,195,398 discloses one method of addressing the balance versus crankshaft size issue. This patent discloses offsetting one or more of the counterweights with respect to the crankpin journals to form an asymmetric counterweight configuration. The asymmetric arrangement strikes a balance between oil film thickness, crankshaft mass and packaging constraints. The asymmetric design, however, suffers from packaging limitations. The design also has the same disadvantages present in the unbalanced crankshaft and connecting rod assembly, albeit to a reduced degree. 
   Therefore, there is a need for a method of providing crankshaft that minimizes costs while providing for adequate balance. 
   SUMMARY OF THE INVENTION 
   A crankshaft includes a non-homogenous structure unitarily formed in a powder metallurgy process with at least two different metallic constituents providing dissimilar characteristics at discrete locations of the structure. A method of forming the crankshaft is provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a process flowchart for a powder metallurgy manufacturing process according to the prior art; 
       FIG. 2  is a partial cutaway perspective view of a vehicular engine according to the prior art; 
       FIG. 3  is side view of a crankshaft according to the prior art; 
       FIG. 4  is a partial side view of a crankshaft according to the prior art; 
       FIG. 5  is a process flowchart for fabricating a non-homogenous component using the powder metallurgy manufacturing process in accordance with the present invention; 
       FIG. 6  is a side cutaway view of the green part forming apparatus in accordance with the present invention; 
       FIG. 7  is a front view of a green part forming apparatus in accordance with the present invention; 
       FIG. 8  is a top view of a green part forming apparatus in accordance with the present invention; 
       FIG. 9  is a partial top cutaway detailed view of a feed valve for a green part forming apparatus in accordance with the present invention; 
       FIG. 10  is a partial cutaway side detailed view of a powder egress in the open position in accordance with the present invention; 
       FIG. 11  is a partial cutaway side detailed view of a powder egress in the closed position in accordance with the present invention; 
       FIG. 12  is a side view of a non-homogeneous crankshaft formed by powder metallurgy in accordance with the present invention; 
       FIG. 13  is a partial cutaway view of the crankshaft of  FIG. 12 ; 
       FIG. 14  is a side cutaway view of the green part forming apparatus in accordance with the present invention; and 
       FIG. 15  is a side cutaway view of the green part forming apparatus in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 5  illustrates a process for manufacturing a non-homogenous powder metallurgical manufactured component, including a crankshaft  280 . One or more metal powders are introduced into a mold at step  106 . Two, three, or more metal powders may be introduced simultaneously (in parallel), at different times (in series), or in some combination thereof. Each metal powder may be a mixture of constituents. The metal powders may be mixed prior to introduction, except as desired for a non-homogeneous result. At step  108 , the powder in the mold is pressed to form a green part. The green part is sintered at step  110 . Optionally, one or more secondary operations, such as forging, machining, heat treating, finishing, and so forth are performed at step  112 . Those skilled in the art will recognize that additional layering of powdered metals and/or process steps may be performed without deviating from the spirit and scope of the present invention. 
   One embodiment of a green component forming apparatus  120  is shown in  FIG. 6 . The green part forming apparatus  120  may be referred to generally as a feedshoe apparatus  120 . The feedshoe apparatus  120  generally comprises a powder filling vessel  122  actuatable by an actuator cylinder  134 , an upper punch  140 , a lower punch  142 , and a powder hopper  148 . More particularly, a first vessel  122  is rigidly connected to a second vessel  126  by one or more connection members  138 . The second vessel  126  is connected to an actuator cylinder  134  via a piston  136 . The actuator cylinder  134  may be a hydraulic or pneumatic cylinder for urging the piston  136  in or out, thereby guiding first  124  and second  125  vessels in a controlled movement. Each vessel  124 ,  126  comprises side walls  125  defining an interior cavity  124 ,  128  therein. The side walls  125  have sloped portions  129  for directing powder towards a powder outlet valve  146 . A top opening  127  in the vessel  122 ,  126  is sized to receive a chute  152 ,  154  connected to hopper  148 ,  150 . The hoppers  148 ,  150  receive a respective first and second powdered metal that are provided to a respective first interior cavity  124  and second interior cavity  128 . The first chute  152  and second chute  154  comprise a flexible tube configured to allow for the linear movement of the first vessel  122  and second vessel  126 . Both first and second vessels  122 ,  126  move linearly by sliding on bridge member  132 . Each of the bridge member  132  and actuator cylinder  134  are mounted on a die table  130 . 
   A side view of the feedshoe apparatus  120  is shown in  FIG. 7 . One or more locking mechanisms  160  are provided to the die table  130 . The locking mechanisms  160  allow for registration of the vessels  122 ,  126  during a die cavity  144  filling operation. The locking mechanism  160  may be a magnet or other locking means such as a male-female socket or equivalent thereto. 
   The bridge member  132  is slidably disposed on the guides  166 . Each guide  166  is further disposed upon a rail  168 . An elevation cylinder  162  is disposed on each bridge member  132  and configured to elevate the bridge member  132  above the guides  166  by extension of an elevation piston  154 . The separation shown in  FIG. 2  between the first vessel  122  and the die cavity  144  allows the actuator cylinder  162  to move the vessel  122  transverse to the cavity  144 . The vessels  122 ,  126  are advantageously moved away from the punches  140 ,  142  such that the vessels  122 ,  126  do not interfere with the pressing process. 
   Referring to  FIG. 8 , a top view of the feedshoe apparatus  120  is shown. Each vessel  122 ,  126  is depicted in a partial cutaway to illustrate interior detail. A dashed outline of the die cavity perimeter  172  is shown for reference purposes. One or more powder egresses  170  are disposed in the bottom surface of each vessel  122 ,  126 . The powder egresses  170  include the valves  149  for controlling the passing of the powder metal into the die cavity  144 . The egresses  170  may be sized to control the relative amount of flow through a particular egress  170  during a filling operation. The first vessel  122  is shown with a single egress  170 . The second vessel  126  is shown as having three egresses  170  with differing sizes. Various polygonal or eccentric shapes or varying size may be employed in place of the circular-shaped egresses without departing from the scope of the present invention. 
   The size and placement of the powder egresses  170  are advantageously chosen to correspond with the provision of predetermined characteristics for the finished part. The crankshaft may advantageously include counterweight material at a predetermined location of the shaft opposite the crankpins. The counterweight material may be in the form of a heavy alloy powder, such as one containing tungsten, or in the form of metal slugs introduced to the die cavity. 
   A conventional method for manufacturing a crankshaft is to forge the shaft and then machine it to final tolerances, as one single piece. Alternatively, the crankshaft may be formed from several component parts that are joined together as disclosed in U.S. Pat. No. 5,303,468. The apparatus and method disclosed herein provide for a powder egress advantageously positioned at the precise location for the desired counterweight material of the crankshaft. 
   The feedshoe apparatus shown in  FIG. 8  additionally includes a liquid injection apparatus  174 . The liquid injection apparatus  174  injects liquids to the first interior cavity  124  during a forming process. An inlet to the injection apparatus  176  is connected to a liquid conduit  178 , which supplies a liquid solution. The apparatus may comprise a solenoid valve, such as a zero dead leg volume solenoid valve. A variety of suitable dripless valves may be used without departing from the scope of the present invention. Those of skill in the art will recognize that the present invention may also be practiced with a second liquid injection apparatus provided to the second vessel, or alternatively, one liquid injection apparatus in communication with both of the first and second vessels. 
   The liquid solution may include aqueous solutions, lubricants, surfactants, or activation solutions for cleaning metal particulates for cold welding. The liquid solution may also include any solution that is intended to be incorporated into the material, such as a hardener, or solvent. The injection of lubricants may be employed to reduce wear to the die cavity of the apparatus. 
     FIG. 9  illustrates a valve assembly  149  that comprises the powder egress  170  of the vessel  122 ,  126 . A housing surface  182  in conjunction with slide hole  124  define an open position P 1  and a closed position P 2  for the powder egress  170 . The slide hole  184  moves between positions P 1  and P 2  as the actuator  134  linearly translates the vessel  122 ,  126 . The open condition permits metal powder to freely exit the vessel and enter the die cavity. The closed position blocks the transfer of powder to the cavity. Other methods or devices for cutting off the flow of powder from the feedshoe to the die cavity may utilized without departing from the scope of the present invention. 
     FIG. 10  and  FIG. 11  depict an alternative embodiment of an apparatus and method for controlling the flow of metal powder into the die cavity  144 . A feedtube  186  communicates between the interior cavity  124 ,  128  of the vessel  122 ,  126  and the die cavity  144 . The feedtube  186  is comprised of a flexible material, such as rubber. The bottom sidewall of the vessel  122 ,  126  defines a channel  188  therein as shown in the figures. A pincher or crimper device  190  is disposed within the channel  188 . The feedtube  186  is in the open position, as shown in  FIG. 10 , when the crimping devices  190  are withdrawn or not pressing on the tube  186 . 
     FIG. 11  shows the tube  186  in a closed position wherein the crimping devices  190  press on the tube sidewalls until the sidewalls contact, thereby blocking powder flow. The crimpers  190  are urged towards the feedtube  186  by way of pneumatic control. High pressure is presented to the channel  188 , which urges the crimpers  190  towards the tube  186 . The removal of this high pressure condition causes the natural resiliency of the tube  186  to re-open, thereby permitting powder flow. Mechanical means, such as a linkage, may be used instead of the pneumatic drive means without departing from the scope of the present invention. 
   A method and apparatus for manufacturing a non-homogeneous article with powder metallurgy are described in  FIG. 5  through  FIG. 7  and the associated text. The following description is more particularly directed towards manufacturing a crankshaft for an internal combustion engine wherein the shaft has unitary balancing material formed as part of a single forming procedure. A first metal powder, such as tungsten or other highly dense material, is placed in the first hopper  148  and a second metal powder, such as steel or less dense material, is placed in a second hopper  150 . The first vessel  122  is also centered over the die cavity  144  by either expanding or retracting the piston  136  of the actuator cylinder  134  as necessary. Alternatively, the first powder may be heavy metal (dense) slugs delivered to the mold cavity. Those of skill in the art will recognize that a variety of materials may be used without departing from the scope of the present invention. 
   The first metal powder is introduced to the first interior cavity  124 . The first powder fills the mold or die cavity  144  through the powder egress  170  with a predetermined amount of powder to form the weighted sections of the shaft. The flow of first powder is stopped by the valve  149  at the powder egress  170 . The piston  136  is extended until the second vessel  126  centers over the die cavity  144 . Note that the powder egress  170  is advantageously not centered over the die cavity, allowing the second powder to deposit at the desired discrete locations where the shaft material is formed. A predetermined amount of the second powder is filled into the die cavity  144 . 
   The piston  136  is retracted until the first vessel  122  is clear of the upper  140  and lower  142  punches. The powder in the die cavity  144  is pressed to form a green part, advantageously once the clearance has been established. The green part is placed in a sintering oven and cooled. The cooled sintered crankshaft  280  is machined to final tolerances. The machining operations may refine the balance imparted to the shaft by removing some of the counterweight material until desired characteristics are achieved. Other secondary operations may be performed without departing from the scope of the present invention. A finished crankshaft  280  results from the completion of any other secondary operations. 
   A crankshaft  280  is shown in  FIG. 12  and  FIG. 13 . The crankshaft  280  comprises a crankshaft body having a first end  282 , and a second end  284 . A plurality of spacers  298  and journals  286 ,  288 ,  290  are disposed between the first and second ends  282 ,  284 . The first end  282  and the second end  284  each include a main bearing journal  286 . Spacers  298  separate a plurality of bearing journals  288  and crankpin journals  290 . The number of pistons in the engine dictates the number of journals  288 ,  290 , and spacers  298 . Each journal  286 ,  288 ,  290  forms a hydrodynamic bearing with a respective bearing surface when rotating with an oil film between respective surfaces. The width of the spacers  298  will vary based upon clearance of other engine components, such as the bore of the block. 
   The ends  282 ,  284 , crankpin journals  290 , and bearing journals  286 ,  288  are comprised of the second material, such as steel. The spacers  298  are also partially comprised of the first material. The spacers  298  may include a counterweight material portion  292  comprised of the first material. The first material is advantageously heavier (more dense) than the second material in order to provide a counterweight in the shaft. The first material may be tungsten. Those of skill in the art will recognize that different materials of varying densities may be used without departing from the scope of the present invention. 
   The counterweight material portion  292  is advantageously located opposite respective crankpin journals  290  in order to offset moments that may be otherwise generated. Such offset material portion  292  allows the rotational axis  294  to be the same axis as the balance axis  296 . The counter-weighting portion  292  may be fine-tuned by machining to achieve a desired balance characteristic. Those of skill in the art will recognize that the counterweight material may be provided to other portions of the shaft, such as the journals, separately or instead of the spacers, without departing from the scope of the present invention. 
     FIG. 14  depicts an alternative apparatus for forming a green part in accordance with the method of  FIG. 6 . The feedshoe apparatus in accordance with this embodiment comprises a single vessel  222 . The vessel  222  comprises sidewalls  223  and a center divider  224 . The sidewalls  223  and center divider  224  define a first section or chamber  226  and a second section or chamber  228 . The first section  226  receives a first metal powder from a first hopper  230  and the second section  228  receives a second metal powder from a second hopper  232 . A first powder egress  234  is provided to the first chamber  226  and a second powder egress  226  is provided to the second chamber  228 . 
   In operation, the first and second powders  200 ,  202  may be provided to the die cavity at the same time. The respective powder egresses  234 ,  236  are located and sized to promote the filling of the cavity  238  with the first and second powders in their desired locations before pressing. Alternatively, the piston  240  may move the vessel  222  in a linear direction to place a respective first  234  or second  236  egress over a portion of the die cavity  238  prior to filling with a respective metal powder  200 ,  202 . As a further alternative, the powder egresses  234 ,  236  may be selectively opened and closed to create density gradients in the part or to further place a first material  200  within the second material  202 . Additionally, a combination of the above alternatives may be employed as part of the same forming operation. 
     FIG. 15  depicts another alternative embodiment of the green part forming (feedshoe) apparatus  250 . This embodiment again comprises a single vessel  252 . The vessel comprises first  256  and second  254  dividers for defining a first chamber or section  258 , a second chamber  260 , and a third chamber  262 . Each chamber  258 ,  260 , and  262  receives a respective first  264 , second  266 , or third  268  powder egress and is in communication with a respective first  270 , second  272 , or third  274  hopper. Those of skill in the art will appreciate that the present invention may be practiced with more than three chambers without departing from the scope of the present invention. Moreover, a single hopper may be in communication with two or more chambers. 
   The use of three chambers  258 ,  260 , and  262  allows a first of two different powders to be introduced to the die cavity  276  in two places simultaneously. Alternatively, the three chambers  258 ,  260 , and  262  allow three different density powders to be introduced to the die cavity  276  as part of a single forming operation. The embodiment of  FIG. 15  is operated in substantially the same manner as set forth above for the two-chamber embodiment. 
   The above process is performed to provide a component with dissimilar characteristics at discrete locations in the part. For example, the crankshaft for an internal combustion engine may be provided with discrete balancing weights by way of the forming operation. The method provides for a balanceable crankshaft with lower mass and machining costs. This method of manufacturing a crankshaft provides the ability to minimize crankshaft size. Other advantages include minimizing manufacturing steps, cost, time, labor, and complexity, and minimizing the offset between the axis of rotation and the axis of balance. 
   Although the present invention has been described with reference to the above embodiments, those skilled in the art will recognize changes may be made in form and detail without departing from the spirit and scope of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description.