PISTON AND CONNECTING ROD ASSEMBLY

RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application Serial

No. 61/025,080 filed on January 31, 2008, the entire content of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to internal combustion engines, and more particularly to pistons, connecting rods, and wrist pins incorporated in internal combustion engines.

BACKGROUND OF THE INVENTION

[0003] Steel piston or wrist pins typically interconnect a piston and a connecting rod in an internal combustion engine.

SUMMARY OF THE INVENTION

[0004] Wrist pins often experience oscillating movement with periodic stress cycles corresponding to the expansion, exhaust, intake, and compression cycles of the engine. For a brief period during the expansion cycle, the forces exerted on the wrist pin by the piston and the connecting rod often cause slight bending of the wrist pin, which is ultimately recovered during the subsequent exhaust cycle. When the wrist pin is bent during these brief periods, a non-uniform contact pressure along the length of the wrist pin in contact with the bearing surface of the "small end" of the connecting rod often develops. The contact pressure between the wrist pin and the bearing surface of the small end of the connecting rod is often highest at the edges of the bearing surface of the small end of the connecting rod. This "edge loading," in turn, may impart a highly concentrated load (resulting in an area of high stress) on the outer peripheral surface of the wrist pin. For modern engine designs having higher combustion pressures, these highly stressed areas on the outer peripheral surface of the wrist pins may scuff, potentially leading to engine seizure.

[0005] To increase the load-carrying capacity of a wrist pin and decrease the risk of scuffing or other damage to the outer peripheral surface of the wrist pin, the present invention

provides a wrist pin with a staged loading geometry and improved lubrication distribution. The staged loading geometry of the wrist pin decreases the contact pressure between the wrist pin and the bearing surface of the small end of the connecting rod by increasing the area of the wrist pin in contact with the bearing surface of the small end of the connecting rod as the bending of the wrist pin increases. The staged loading geometry of the wrist pin of the present invention also minimizes the amount of time that the wrist pin is exposed to high contact stresses, thereby decreasing the risk of scuffing on the outer peripheral surface of the wrist pin, and ultimately increasing the service life of the wrist pin.

[0006] The present invention provides, in one aspect, a piston and connecting rod assembly including a piston having a bore defined by an inner surface of the piston. The piston and connecting rod assembly also includes a connecting rod having a bore adjacent one end of the rod. The bore is defined by an inner surface of the connecting rod. The piston and connecting rod assembly further includes a pin received within the respective bores of the piston and the connecting rod to interconnect the piston and the connecting rod. The pin includes a first portion engaged with the inner surface of the connecting rod. The first portion has a first diameter. The pin also includes a second portion, having a second diameter, engaged with the inner surface of the piston. The pin further includes a third portion between the first and second portions. The third portion has a third diameter less than the first and second diameters.

[0007] Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is partial cross-sectional view of a first construction of a piston and connecting rod assembly of the present invention.

[0009] FIG. 2a is an exaggerated side view of a wrist pin incorporated in the assembly of FIG. 1.

[0010] FIG. 2b is an enlarged view of a portion of the wrist pin of FIG. 2a.

[0011] FIG. 3 is an exaggerated side view of an alternative construction of a wrist pin that may be incorporated in the assembly of FIG. 1.

[0012] FIG. 4 is an exaggerated side view of another alternative construction of a wrist pin that may be incorporated in the assembly of FIG. 1.

[0013] FIG. 5 is an exaggerated side view of yet another alternative construction of a wrist pin that may be incorporated in the assembly of FIG. 1.

[0014] FIG. 6 is an enlarged view of a portion of the assembly of FIG. 1 , illustrating the contact between the wrist pin and the connecting rod during a low-load condition of the piston and connecting rod assembly.

[0015] FIG. 7 is an enlarged view of a portion of the assembly of FIG. 1, illustrating the contact between the wrist pin and the piston during a low-load condition of the piston and connecting rod assembly.

[0016] FIG. 8 is a partial cross-sectional view of the piston and connecting rod assembly during a high-load condition.

[0017] FIG. 9 is an enlarged view of a portion of the assembly of FIG. 8, illustrating the contact between the wrist pin and the connecting rod during the high-load condition of the piston and connecting rod assembly.

[0018] FIG. 10 is an enlarged view of a portion of the assembly of FIG. 8, illustrating the contact between the wrist pin and the piston during the high-load condition of the piston and connecting rod assembly.

[0019] FIG. 11 is an enlarged view of a portion of a second construction of a piston and connecting rod assembly, similar to the portion shown in FIG. 6, illustrating the contact between a wrist pin and a connecting rod during a low-load condition of the piston and connecting rod assembly.

[0020] FIG. 12 is an enlarged view of a portion of the second construction of the piston and connecting rod assembly of FIG. 11, illustrating the contact between the wrist pin and a piston during a low-load condition of the piston and connecting rod assembly.

[0021] FIG. 13 is an enlarged view of a portion of the second construction of the piston and connecting rod assembly of FIG. 11, illustrating the contact between the wrist pin

and the connecting rod during a high-load condition of the piston and connecting rod assembly.

[0022] FIG. 14 is an enlarged view of a portion of the second construction of the piston and connecting rod assembly of FIG. 11, illustrating the contact between the wrist pin and the piston during a high-load condition of the piston and connecting rod assembly.

[0023] FIG. 15 is a schematic illustrating contact load on the wrist pin referenced to the crankshaft angle of an internal combustion engine incorporating the piston and connecting rod assembly of the present invention.

[0024] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms "mounted," "connected," "supported," and "coupled" and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, "connected" and "coupled" are not restricted to physical or mechanical connections or couplings.

DETAILED DESCRIPTION

[0025] FIG. 1 illustrates a first construction of a piston and connecting rod assembly

10 incorporated in an internal combustion engine. The assembly includes a piston 14 having a head 18, a skirt 22 extending from the head 18 (see FIG. 15), and aligned bores 26 extending through the head 18. Alternatively, other piston configurations may include aligned bores through the skirt of the piston (e.g., the skirt 22 illustrated in FIG. 15). The assembly 10 also includes a connecting rod 30 having a bore 34 at a "small end" 38 of the connecting rod 30. The other end of the connecting rod 30 (i.e., the "large end") is attached to a crankshaft in a conventional manner.

[0026] With continued reference to FIG. 1, the assembly 10 further includes a wrist pin 42 interconnecting the piston 14 and the connecting rod 30. Although the wrist pin 42 appears to include a cylindrical outer periphery, the wrist pin 42 includes a plurality of raised portions 46, 50, 54 and a plurality of recessed portions 58, 62 situated along the length of the wrist pin 42 (see FIG. 2a). In addition, although the wrist pin 42 is illustrated as a solid body, the wrist pin 42 may comprise a hollow body to reduce the reciprocating mass of the piston and connecting rod assembly 10. Further, the wrist pin 42 may be made from any of a number of different metals, metal alloys, or composite materials.

[0027] With continued reference to FIG. 2a, the raised portion 46 is positioned in the middle of the wrist pin 42, and the raised portions 50, 54 are positioned adjacent opposite ends 66 of the wrist pin 42. Consequently, the recessed portion 58 is positioned between the raised portions 46, 50 and the recessed portion 62 is positioned between the raised portions 46, 54. It should be understood that the raised portions 46, 50, 54 and the recessed portions 58, 62 of the wrist pin 42, as illustrated in FIG. 2a, are highly exaggerated to facilitate viewing of the raised portions 46, 50, 54 and the recessed portions 58, 62.

[0028] Each of the raised portions 46, 50, 54 includes a cylindrical outer peripheral surface 70 (with a substantially flat profile as shown in FIG. 2a) having a substantially similar outer diameter "Dl," and each of the recessed portions 58, 62 includes a cylindrical outer peripheral surface 74 (having a substantially flat profile as shown in FIG. 2a) having an outer diameter "D2" that is less than the outer diameter Dl of the raised portions 46, 50, 54. As shown in FIG. 2a, the edges of the raised portions 46, 50, 54 may include an arcuate profile for a more smooth transition to the adjacent recessed portion 58, 62 (see also FIG. 2b). The raised portion 46 of the wrist pin 42 is engaged with an inner peripheral surface 78 of the connecting rod 30 that defines the bore 34. The raised portion 46 coincides with the length dimension "A" in FIG. 1. The raised portions 50, 54 of the wrist pin 42 are engaged with inner peripheral surfaces 82 of the piston head 18 that define the respective aligned bores 26 (see FIG. 2a). The raised portions 50, 54 coincide with the length dimensions "B" in FIG. 1.

[0029] As shown in FIG. 2a, each of the recessed portions 58, 62 of the wrist pin 42 is at least partially received within the bore 34 in the connecting rod 30 and a respective bore 26 in the piston head 18. Because the outer diameter D2 of each of the recessed portions 58, 62 is less than the outer diameter Dl of the raised portions 46, 50, 54, each of the recessed portions 58, 62 is spaced radially inwardly from the inner peripheral surface 78 of the

connecting rod 30 and the inner peripheral surfaces 82 of the piston head 18. Consequently, when the wrist pin 42 is not loaded or lightly loaded, neither of the recessed portions 58, 62 engages the respective inner peripheral surfaces 78, 82 of the connecting rod 30 and the piston head 18. Alternatively, the recessed portion 58 of the wrist pin 42 may be at least partially received only within one of the bores 78, 82 in the connecting rod 30 and the piston head 18, respectively, or the recessed portion 62 of the wrist pin 42 may be at least partially received only within one of the bores 78, 82 in the connecting rod 30 and the piston head 18, respectively.

[0030] Because the raised portions 46, 50, 54 are narrow, a tighter fit between the wrist pin 42 and the connecting rod 30 can be achieved with a smaller pre-load compared to a conventional wrist pin. The tighter fit can reduce engine noise caused by loading zone reversals of the wrist pin 42.

[0031] FIG. 3 illustrates an alternative construction of the wrist pin 42 of FIG. 2a that may be incorporated in the piston and connecting rod assembly 10 of FIG. 1 in place of the wrist pin 42 of FIG. 2a. Like features are labeled with like reference numerals, with the letter "a." FIG. 3 illustrates a wrist pin 42a having a raised portion 46a configured to engage the inner peripheral surface 78a of the connecting rod 30a, raised portions 50a, 54a configured to engage the respective inner peripheral surfaces 82a of the piston head 18a, and stepped recessed portions 58a, 62a (each having a stepped profile as shown in FIG. 3) positioned between the raised portions 46a, 50a and the raised portions 46a, 54a, respectively. The stepped recess portions 58a, 62a facilitate fine-tuning the distribution of contact pressure exerted on the wrist pin 42a (described in more detail below). Each of the raised portions 46a, 50a, 54a includes an outer diameter Dl, and each of the recessed portions 58a, 62a includes an outer diameter D2 corresponding with the narrowest section of each of the recessed portions 58a, 62a. It should be understood that the raised portions 46a, 50a, 54a and the recessed portions 58a, 62a of the wrist pin 42a, as illustrated in FIG. 3, are highly exaggerated to facilitate viewing of the raised portions 46a, 50a, 54a and the recessed portions 58a, 62a.

[0032] FIG. 4 illustrates another alternative construction of the wrist pin 42 of FIG.

2a that may be incorporated in the piston and connecting rod assembly 10 of FIG. 1 in place of the wrist pin 42 of FIG. 2a. Like features are labeled with like reference numerals, with the letter "b." FIG. 4 illustrates a wrist pin 42b having a substantially cylindrical raised

portion 46b configured to engage the inner peripheral surface 78b of the connecting rod 30b, substantially cylindrical raised portions 50b, 54b configured to engage the respective inner peripheral surfaces 82b of the piston head 18b, and arcuate, concave recessed portions 58b, 62b (having an arcuate or concave profile as shown in FIG. 4) positioned between the raised portions 46b, 50b and the raised portions 46b, 54b, respectively. Each of the raised portions 46b, 50b, 54b includes an outer diameter Dl corresponding with the widest point of each of the raised portions 46b, 50b, 54b, and each of the recessed portions 58b, 62b includes an outer diameter D2 corresponding with the narrowest point of each of the recessed portions 58b, 62b. It should be understood that the raised portions 46b, 50b, 54b and the recessed portions 58b, 62b of the wrist pin 42b, as illustrated in FIG. 4, are highly exaggerated to facilitate viewing of the raised portions 46b, 50b, 54b and the recessed portions 58b, 62b.

[0033] FIG. 5 illustrates yet another alternative construction of the wrist pin 42 of

FIG. 2a that may be incorporated in the piston and connecting rod assembly 10 of FIG. 1 in place of the wrist pin 42 of FIG. 2a. Like features are labeled with like reference numerals, with the letter "c." FIG. 5 illustrates a wrist pin 42c having an arcuate or crowned raised portion 46c configured to engage the inner peripheral surface 78c of the connecting rod 30c, arcuate or crowned raised portions 50c, 54c configured to engage the respective inner peripheral surfaces 82c of the piston head 18, and substantially cylindrical recessed portions 58c, 62c (each having a flat profile as shown in FIG. 5) positioned between the raised portions 46c, 50c and the raised portions 46c, 54c, respectively. When employed in the piston and connecting rod assembly 10, the crowned raised portions 46c, 50c, 54c are each deformable to increase the load-carrying surface area of the raised portions 46c, 50c, 54c (i.e., by flattening the raised portions 46c, 50c, 54c) in response to loads experienced by the wrist pin 46c during the low load condition or the high load condition. Each of the raised portions 46c, 50c, 54c includes an outer diameter Dl corresponding with the widest point of each of the raised portions 46c, 50c, 54c, and each of the recessed portions 58c, 62c includes a cylindrical outer peripheral surface 74c having an outer diameter D2. It should be understood that the raised portions 46c, 50c, 54c and the recessed portions 58c, 62c of the wrist pin 42c, as illustrated in FIG. 5, are highly exaggerated to facilitate viewing of the raised portions 46c, 50c, 54c and the recessed portions 58c, 62c.

[0034] With reference to FIG. 6, the inner peripheral surface 78 of the connecting rod

30 includes a shallow, tapered portion 86 (only one of which is shown in FIG. 6) adjacent

each edge of the connecting rod 30 surrounding the bore 34. As a result, the spacing between the recessed portion 58 and the inner peripheral surface 78 of the connecting rod 30 increases (i.e., forms a progressively increasing gap, shown as gap "Gl" in FIG. 6) along the length of the wrist pin 42 from the raised portion 46 toward the raised portion 50, and the spacing between the recessed portion 62 and the inner peripheral surface 78 of the connecting rod 30 increases (i.e., forms a progressively increasing gap) along the length of the wrist pin 42 from the raised portion 46 toward the raised portion 54. In one construction of the assembly 10, the gap Gl is between about 5 microns and about 25 microns. In another construction of the assembly 10, the wrist pin 42 and the connecting rod 30 may be configured such that the gap Gl is between about 10 microns and about 20 microns. The purpose of the tapered portions 86 of the inner peripheral surface 78 of the connecting rod 30, and the resultant progressively increasing gaps between the wrist pin 42 and the connecting rod 30, is explained in more detail below.

[0035] With reference to FIG. 7, each of the inner peripheral surfaces 82 of the piston head 18 includes a shallow, tapered portion 90 (only one of which is shown in FIG. 7) adjacent the innermost edge of the piston head 18 surrounding the bore 26. As a result, the spacing between the recessed portion 58 and the inner peripheral surface 82 of the piston head 18 increases (i.e., forms a progressively increasing gap) along the length of the wrist pin 42 from the raised portion 50 toward the raised portion 46, and the spacing between the recessed portion 62 and the inner peripheral surface 82 of the piston head 18 increases (i.e., forms a progressively increasing gap, shown as gap "G2" in FIG. 7) along the length of the wrist pin 42 from the raised portion 54 toward the raised portion 46. In one construction of the assembly 10, the gap G2 is between about 5 microns and about 25 microns. In another construction of the assembly 10, the wrist pin 42 and the piston head 18 may be configured such that the gap G2 is between about 10 microns and about 20 microns. The purpose of each of the tapered portions 90 of the inner peripheral surfaces 82 of the piston head 18, and the resultant progressively increasing gaps Gl, G2 between the wrist pin 42 and the respective connecting rod 30 and piston head 18, is explained in more detail below.

[0036] FIG. 11 illustrates a portion of a second construction of a piston and connecting rod assembly 1Od. Like components are labeled with like reference numerals, with the letter "d." In this construction, the connecting rod bore 34d is cylindrical (i.e., the tapered portions 86 of the inner peripheral surface 78 are omitted), and a cylindrical bushing

94 is positioned in the connecting rod bore 34d. The bushing 94 may be made from any of a number of different metals (e.g., leaded bronze) or other materials. The wrist pin 42d includes raised portions 46d, 5Od, 54d (only the raised portion 46d being shown in FIG. 11) and recessed portions 58d, 62d similar to those shown in the wrist pin 42b of FIG. 4. The outer peripheral surface 74d of each of the recessed portions 58d, 62d is concave, such that the spacing between the recessed portion 58d and a bearing surface 98 of the bushing 94 increases (i.e., forms a progressively increasing gap, shown as gap "Gl" in FIG. 11) along the length of the wrist pin 42d from the raised portion 46d toward the raised portion 50d, and the spacing between the recessed portion 62d and the bearing surface 98 of the bushing 94 increases (i.e., forms a progressively increasing gap) along the length of the wrist pin 42d from the raised portion 46d toward the raised portion 54d. In one construction of the assembly 1Od, the gap Gl is between about 5 microns and about 25 microns. In another construction of the assembly 1Od, the wrist pin 42d and the piston head 18d may be configured such that the gap Gl is between about 10 microns and about 20 microns.

[0037] Similarly, the spacing between the recessed portion 58d and a cylindrical inner peripheral surface 82d of the piston head 18 increases (i.e., forms a progressively increasing gap, shown as gap "G2" in FIG. 12) along the length of the wrist pin 42d from the raised portion 54d toward the raised portion 46d, and the spacing between the recessed portion 62d and the inner peripheral surface 82d of the piston head 18 increases (i.e., forms a progressively increasing gap) along the length of the wrist pin 42d from the raised portion 5Od toward the raised portion 46d. In one construction of the assembly 1Od, the gap G2 is between about 5 microns and about 25 microns. In another construction of the assembly 1Od, the wrist pin 42d and the piston head 18d may be configured such that the gap G2 is between about 10 microns and about 20 microns. The purpose of the concave outer peripheral surfaces 74d of the respective recessed portions 58d, 62d, and the resultant progressively increasing gaps between the wrist pin 42 and the respective connecting rod 30 and piston head 18, is explained in more detail below.

[0038] As discussed in the summary of the invention section above, the wrist pin 42 experiences cyclic loading during engine operation. Specifically, for a brief period during the expansion cycle, the forces exerted on the wrist pin 42 by the piston 14 and the connecting rod 30 often cause the wrist pin 42 to bend. In an effort to increase engine efficiency, the wrist pin 42 is made as slim as possible, ultimately decreasing the overall stiffness of the

wrist pin 42. As a result, the slimmer the wrist pin 42, the more the wrist pin 42 is likely to bend. In conventional wrist pin designs, bending of the wrist pin is generally viewed as a detrimental effect because it reduces the usable life and load capacity of the wrist pin.

[0039] FIG. 15 illustrates this cyclic loading relative to a rotational angle of the crankshaft (i.e., the crank angle θ). As shown in the graph in FIG. 15, 720 degrees of crankshaft rotation is required to complete a single expansion-exhaust-intake-compression cycle. The wrist pin 42 experiences a relatively high load between a crank angle θ of about 0 degrees to about 135 degrees (during the expansion cycle), and again between a crank angle θ of about 675 degrees to about 720 degrees (during the compression cycle). As such, the wrist pin 42 experiences a relatively high load for about 180 degrees of crankshaft rotation for each expansion-exhaust-intake-compression cycle, and a relatively low load for the remaining 540 degrees of crankshaft rotation for each expansion-exhaust-intake-compression cycle. The phrase "high load condition" is used below to indicate the loading on the wrist pin 42 during the 180 degrees of crankshaft rotation at high loading, and the phrase "low load condition" is used below to indicate the loading on the wrist pin 42 during the remaining 540 degrees of crankshaft rotation at low loading.

[0040] With reference to FIG. 6, the recessed portions 58, 62 of the wrist pin 42 are spaced from the inner peripheral surface 78 of the connecting rod 30, such that the contact between the connecting rod 30 and the wrist pin 42 during the low load condition occurs only between the interface of the raised portion 46 of the wrist pin 42 and the inner peripheral surface 78 of the connecting rod 30 (see also FIG. 2a). Likewise, with reference to FIG. 7, the recessed portions 58, 62 of the wrist pin 42 are spaced from the inner peripheral surfaces 82 of the piston head 18, respectively, such that the contact between the piston head 18 and the wrist pin 42 during the low load condition occurs only between the interface of each of the raised portions 50, 54 of the wrist pin 42 with the respective inner peripheral surfaces 82 of the piston head 18 (see also FIG. 2a). Consequently, any wear that may occur on the wrist pin 42 during the low load condition is limited to the raised portions 46, 50, 54 of the wrist pin 42.

[0041] By providing the recessed portions 58, 62 on the wrist pin 42, a lubricant film may accumulate within the annular space between the recessed portions 58, 62 and the connecting rod 30, and the annular space between each of the recessed portions 58, 62 and the piston head 18. Such a lubricant film may be sufficient to allow omission of a bushing

(e.g., the cylindrical bushing 94 shown in FIG. 11) between the connecting rod 30 and the wrist pin 42.

[0042] The raised portions 46, 50, 54 of the wrist pin 42 bear the contact load with the connecting rod 30 and the piston 14 during the low load condition. The resultant contact pressure at the low load condition is small and substantially uniformly distributed along the length of the wrist pin 42 in contact with the connecting rod 30 and the piston 14. The relatively narrow raised portions 46, 50, 54 of the wrist pin 42 receive lubrication from the adjacent recessed portions 58, 62, essentially acting as a lubricant reservoir. A sufficient supply of lubricant and low contact pressure substantially prevents scuffing of the wrist pin 42.

[0043] As discussed above, the wrist pin 42 often resiliently deforms or bends during the high load condition. As shown in FIG. 8, the piston 14 exerts a downward force (indicated by arrow F) on each end 66 of the wrist pin 42 via the piston head 18, and the connecting rod 30 exerts an upward reaction force (indicated by arrow N) on the wrist pin 42. As a result, the wrist pin 42 experiences a bending load on each of its ends 66, in addition to a double-shear load. The direction of the loading on the wrist pin 42 is such that the wrist pin 42 assumes an upside-down "U" shape during bending.

[0044] By providing the substantially cylindrical recessed portions 58, 62 on the wrist pin 42 and the tapered portions 86 of the inner peripheral surface 78 of the connecting rod 30, a portion of the outer peripheral surface 74 of each of the recessed portions 58, 62 engages the tapered portions 86 of the inner peripheral surface 78 of the connecting rod 30 when the wrist pin 42 bends during the high load condition (see FIG. 9). As a result, the amount of load-bearing area of the wrist pin 42 in contact with the connecting rod 30 increases when transitioning from the low load condition to the high load condition, thereby decreasing the overall pressure on the wrist pin 42 exerted by the connecting rod 30 and more uniformly distributing the load on the wrist pin 42 across the length of the connecting rod 30 in contact with the wrist pin 42. This reduces the risk of scuffing that may otherwise occur on a conventional cylindrical wrist pin that is received in a cylindrical bore of a conventional connecting rod, due to the highly concentrated edge-loading on the wrist pin by the small end of the connecting rod when the conventional wrist pin is bent during the high load condition. Furthermore, the lubricant film accumulated within the respective recessed portions 58, 62

facilitates slippage and provides additional cushioning between the wrist pin 42 and the connecting rod 30 when the wrist pin 42 bends during the high load condition.

[0045] Likewise, by providing the recessed portions 58, 62 on the wrist pin 42 and the tapered portions 90 of the respective inner peripheral surfaces 82 of the piston head 18, a portion of the outer peripheral surface 74 of each of the recessed portions 58, 62 engages the tapered portions 90 of the respective inner peripheral surfaces 82 of the piston head 18 when the wrist pin 42 bends when transitioning from the low load condition to the high load condition (see FIG. 10). As a result, the amount of load-bearing area of the wrist pin 42 in contact with the piston head 18 increases when transitioning from the low load condition to the high load condition, thereby decreasing the overall pressure on the wrist pin 42 exerted by the piston head 18 and more uniformly distributing the load on the wrist pin 42 across the length of the piston head 18 in contact with the wrist pin 42. This reduces the risk of scuffing that may otherwise occur on a conventional cylindrical wrist pin that is received within aligned cylindrical bores of a conventional piston, due to the highly concentrated edge- loading on the wrist pin by the piston when the conventional wrist pin is bent during the high load condition.

[0046] When the wrist pin 42a of FIG. 3 is employed to interconnect the piston 14 and the connecting rod 30, the stepped recessed portions 58a, 62a provide an incremental or stepped increase in load-bearing area of the wrist pin 42a during the transition from the low load condition to the high load condition. As a result, the distribution of the contact pressure on the wrist pin 42a can be fine-tuned.

[0047] The wrist pin 42 is configured to increase its loading capacity with no change to the overall dimensions of the wrist pin 42. The heavier the load on the wrist pin 42, the more the wrist pin 42 will bend and more of the contact load exerted on the wrist pin 42 shifts to the recessed portions 58, 62 of the wrist pin 42. This "shifting" of the contact load on the wrist pin 42 substantially protects the raised portions 46, 50, 54 of the wrist pin 42 from being over-stressed.

[0048] During times of extreme overloading that may cause scuffing initiation or micro-scuffing on the recessed portions 58, 62 of the wrist pin 42, the brief high load condition is immediately followed by a longer (e.g., at least 3 times longer) low load condition. The spring-back of the bent wrist pin 42 during the transition from the high load

condition to the low load condition relieves the contact and allows time for any damage to the outer peripheral surfaces 74 of the respective recessed portions 58, 62 to be repaired or "heal" with the inflow of lubricant. The piston and connecting rod assembly 10 is operable to limit the contact time during which the wrist pin 42 experiences its peak stress, thereby prolonging the useful life of the wrist pin 42. In other words, the piston and connecting rod assembly 10 benefits from a reduced contact stress, minimized contact exposure to peak stress, and improved lubricant distribution.

[0049] The portion of the connecting rod assembly 1Od of FIGS. 11-14 is functional in a similar manner as the connecting rod assembly 10 of FIGS. 1-10. By providing the concave recessed portions 58d, 62d on the wrist pin 42d and the cylindrical bearing surface 98 of the connecting rod bushing 94, a portion of the outer peripheral surface 74d of each of the concave recessed portions 58d, 62d engages the bearing surface 98 of the connecting rod bushing 94 when the wrist pin 42d bends during the high load condition (see FIG. 13). As a result, the amount of load-bearing area of the wrist pin 42d in contact with the connecting rod 30d (via the bushing 94) increases when transitioning from the low load condition to the high load condition, thereby decreasing the overall pressure on the wrist pin 42d exerted by the connecting rod 30d and more uniformly distributing the load on the wrist pin 42d across the length of the bushing 94 in contact with the wrist pin 42d. This reduces the risk of scuffing that otherwise may occur on a conventional wrist pin received within a cylindrical bore of a conventional connecting rod, due to the highly concentrated edge-loading on the wrist pin by the small end of the connecting rod or the edges of a bushing in the conventional connecting rod. Furthermore, the lubricant film accumulated within the respective recessed portions 58d, 62d facilitates slippage and provides additional cushioning between the wrist pin 42d and the connecting rod bushing 94 when the wrist pin 42d bends during the high load condition.

[0050] Likewise, by providing the concave recessed portions 58d, 62d on the wrist pin 42d and the cylindrical inner peripheral surfaces 82d of the piston head 18d, a portion of the outer peripheral surface 74d of each of the concave recessed portions 58d, 62d engages the respective inner peripheral surfaces 82d of the piston head 18d when the wrist pin 42d bends during the high load condition (see FIG. 14). As a result, the amount of load-bearing area of the wrist pin 42d in contact with the piston head 18d increases when transitioning from the low load condition to the high load condition, thereby decreasing the overall pressure on the wrist pin 42d exerted by the piston head 18d and more uniformly distributing

the load on the wrist pin 42d across the length of the piston head 18d in contact with the wrist pin 42d. This reduces the risk of scuffing that otherwise may occur on a conventional wrist pin received within aligned cylindrical bores of a conventional piston, due to the highly concentrated edge-loading on the wrist pin 42d by the piston when the conventional wrist pin is bent during the high load condition .

[0051] Various features of the invention are set forth in the following claims.