Abstract:
Fastener joints induce static pre-stresses into a fastener that clamps the joint in a magnitude equal and direction opposite from bending stresses induced in the fastener by application loading. Such joints include angling the bolts seats of a connecting rod bearing cap inwardly, creating a relief at the bolted interface between the cap and the rod adjacent to the main bore of the connecting rod or skewing the threaded holes in the rod body inwardly as they extend from the bolted joint faces.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This claims the benefit of U.S. Provisional Patent Application No. 60/516,488 filed Oct. 31, 2003.  
       Not applicable. 
    
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to fastened joint design, such as a bolted joint, and in particular to a joint that pre-stresses a fastener to result in a more uniform stress in the fastener at maximum application load.  
       BACKGROUND OF THE INVENTION  
       [0003]     Bending stress is induced across the shank  11  of a fastener such as a bolt in the plane of bending when the joint is non-symmetric and when the fastener loading is not on the centerline of the fastener.  FIGS. 1A and 1B  schematically illustrate two fasteners with the same axial application load (i.e., the load at the axis of the shank of the bolt).  FIG. 1A  illustrates a bending stress superimposed in the plane of the paper (in the plane of bending, so as to make the fastener shank  11  convex left) on the axial pre-stress so as to subject the shank  11  of the fastener  10  to bending and axial loads.  FIG. 1B  illustrates the shank  11  of the fastener  10  subjected to only an axial load (no bending load), equal in magnitude to the axial load  12  at the center axis of the bolt in  FIG. 1A , so that the average stress in both fasteners is the same.  
         [0004]     The bending stress in the fastener in  FIG. 1A  reduces the load carrying capacity of the fastener and joint. One side  14  of the fastener shank  11  in the plane of bending has higher stress than the other side  16  because of the induced bending. This is not a desirable condition because the stress distribution across the fastener shank causes high stress on side  14  of the fastener shank. A more desirable stress condition at maximum loading would be to have a uniform stress distribution across the fastener shank  11  in the plane of bending at maximum loading conditions as illustrated in  FIG. 1B , where the stresses  12 ,  14  and  16  are substantially equal. In some cases, fastened joints cannot be designed to eliminate bending stresses in the fastener under all conditions, such as in a connecting rod joint where the application load is dynamic and therefore changes.  
         [0005]     The load being carried by the fastener is related to the average stress in the fastener. In  FIGS. 1A and 1B , both fasteners  10  have the same average stress  12  but the fastener in  FIG. 1A  has a higher maximum stress  14 , as a result of the bending stress. If failure occurs it would occur at the point of highest stress along side  14 . Thus, bending stress added to the axial stress reduces the load carrying capability of a fastener compared to a fastener subjected to the same average stress but with a uniform stress distribution.  
         [0006]     Referring to  FIG. 2A , when fastening a joint, an initial axial pre-stress is applied as a result of tightening or tensioning the fastener. This is represented by the uniform pre-stress components  18 . If the joint is non-symmetric, it will compress more on one side than the other side of the fastener hole. This causes the fastener shank  11  to be subjected to bending stresses and the load to be applied in a non-uniform fashion across the fastener shank  11 . This is represented by the non-uniform components  20 . In addition, if the application load is applied off-center to the fastener centerline, additional bending will occur in the fastener. The stress components  12 ,  14  and  16  in  FIG. 1A  are the sum of the uniform components  18  and the non-uniform components  20  at maximum application load. Fastener joint design is limited by the highest stress level in the fastener including the bending stress, which makes a uniform stress profile as illustrated in  FIG. 1B  a more desirable choice.  
         [0007]     The word “fastener” as used herein is any type of fastener having a shank that is subjected to tensile forces when applied to a joint, such as bolts, rivets, rods (threaded, pinned, welded, etc.), screws, etc. The word “bending stress” refers to a non-uniform stress across the fastener shank. This invention includes the use of a nut in the joint system, which could act like a bolt head and therefore “head” of a fastener includes a nut, a bolt head or screw head, a rivet head or rivet flange, etc.  
       SUMMARY OF THE INVENTION  
       [0008]     This invention provides a bolt joint that at the maximum loading conditions in the service application of the joint the maximum stress will be reduced across the bolt shank. It does this by the joint inducing a bending stress in the fastener shank in the plane of bending of the application bending stress when the fastener is assembled to the joint. The bending stress induced by the joint is substantially inversely proportional to the bending stress induced in the plane of bending by the maximum application load that the fastener shank is subjected to in service so as to reduce the maximum stress when the maximum application load is applied.  
         [0009]     By so doing, the invention also reduces the cyclical mean stress to which the fastener shank is subjected. This is especially useful to increase the fatigue life of the fastener.  
         [0010]     In a useful aspect of the invention, the bending stress induced by the joint is of a magnitude and direction to produce a substantially uniform stress distribution across the fastener shank in the plane of bending when the maximum application load is applied, to obtain the full advantage of the invention.  
         [0011]     In one form of the invention, the joint has a seat that the fastener bears against to induce tension in the shank and the seat is skewed at an angle other than 90 degrees to an axis of a fastener hole in the parts through which the shank extends. The seat is angled in a direction so as to induce bending stresses in the fastener opposite in direction to the bending stresses induced by the maximum application load. Thereby, the bending stresses induced by the joint cancel the bending stresses induced by the application load to reduce the maximum application load on the shank of the fastener and to reduce the cyclic mean stress to which the fastener shank is subjected.  
         [0012]     In another way of practicing the invention, the joint has joint faces that face one another and are held together by the fastener, a portion of the joint faces defining between them an unsupported gap that induces bending stresses in the shank of the fastener opposite in direction to bending stresses induced by the maximum application load.  
         [0013]     In another ways of practicing the invention, a hole that extends in the parts and receives the fastener shank has a first portion in one of the parts and a second portion in the other part, with the first portion skewed relative to the second portion so as to induce bending stresses in the fastener opposite in direction to bending stresses induced by the maximum application load.  
         [0014]     These different ways of practicing the invention can be practiced alone or in any combination with one another.  
         [0015]     In an especially useful form, the joint is a joint in a connecting rod connecting a bearing cap to a rod portion of the connecting rod. A bearing cap joint is an especially useful application of the invention because the fastener shank is subjected to a cyclic bending stress by the cyclic motion of the connecting rod, such that prestressing the fasteners using the invention can reduce the maximum application stress and the cyclic mean stress in the fastener shanks.  
         [0016]     The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1A  is a typical prior art bolt stress distribution diagram illustrating the stress distribution in a bolt subjected to bending and axial loading;  
         [0018]      FIG. 1B  is a typical prior art bolt stress distribution diagram illustrating the stress distribution in a bolt subjected to only axial loading, with the magnitude of axial loading equal to the loading at the axis of the bolt in  FIG. 1A ;  
         [0019]      FIG. 2A  is a view like  FIG. 1A  with the stress diagram illustrating the components of total stress as pre-stress and maximum stress;  
         [0020]      FIG. 2B  is a view of a fastener comparable to  FIG. 2A , but with a pre-stress and maximum stress distribution produced by a joint incorporating the invention;  
         [0021]      FIG. 3  is a view of a connecting rod bearing cap joint with an angled bolt seat according to the invention, the angle being exaggerated for illustrative purposes;  
         [0022]      FIG. 4  is a view like  FIG. 3  but of a typical prior art connecting rod bearing cap joint;  
         [0023]      FIG. 5  is a view of a connecting rod bearing cap joint with angled joint faces according to the invention, the angles being exaggerated for illustrative purposes; and  
         [0024]      FIG. 6  is a view of a connecting rod bearing cap joint with the threaded fastener holes in the rod being angled inwardly according to the invention, the angles being exaggerated for illustrative purposes. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     Referring to  FIG. 2B , the present invention provides a fastener clamped joint design that provides a substantially uniform stress load distribution in the fastener shank  11  at the maximum application loading conditions. In  FIG. 2B , the stress chart shows the initial pre-stress with a lower set of vector arrows  22 ,  24  and  26  and the application stress by the upper set of vector arrows  28 ,  30  and  32 . In both  FIG. 2A  and  FIG. 2B  the average pre-stress and the average maximum stress are the same so each case would be handling the same system load; however using the invention results in a lower maximum stress under the application load. In the case where the joint application loads are cyclic as in a connecting rod bearing cap joint, the mean cyclic stress would be lower also. The total cyclic stress swing would remain the same.  
         [0026]     The schematic stress charts of  FIGS. 2A and 2B  are simplified in that they do not show any incidental or accidental joint bending pre-stress. If there was joint bending pre-stress, the horizontal set of pre-stress vectors in  FIG. 2A  would be non-uniform (at some angle) and the corresponding pre-stress vectors in  FIG. 2B  would need to be adjusted to compensate for the bending pre-stress.  
         [0027]     Uniform stress distribution at maximum application loading can be accomplished in any number of ways. Currently, typical connecting rod bearing cap joints are made as illustrated in  FIG. 3 , with each bolt joint seat  36  oriented 90° to the corresponding bolt hole  37  and threaded hole  39  centerline  38 , the unthreaded hole  37  being in the bearing cap  42  and the threaded hole  39  being in the connecting rod body  44 . This yields a stress distribution substantially as in  FIG. 2A , with the vectors  18  representing the static pre-stress and the vectors  20  representing the dynamic application loading. Note that in this case, the maximum stress occurs on the inner side (toward the crankshaft bore  40  of both seats  36 .  
         [0028]     One way to practice the present invention would be to skew each joint bolt seat  36  to the bolt hole  37  and threaded hole  39  centerline  38  by some small amount, chosen based on the maximum application loading that is to be cancelled or offset. Typically, the angle would be less than one degree, for example 0.125 degrees, depending on the magnitude of application loading. The angle must also be in the correct direction so that it cancels the bending stress at the maximum application (dynamic) loading condition, which is induced by the joint and application load. This is illustrated in  FIG. 3 . Both seats  36 , which are flat as illustrated, are machined or formed so as to both angle or skew inwardly in the direction of the plane of bending, so as to induce bending stresses in each bolt  10  that are counter to the bending stresses induced by the application load. In other words, the bolts  10  tend to bow outwardly (convex-out relative to the axis of the main bore  40 ) in the plane of the paper as a result of the skewed seats  36 , whereas the application load tends to bow the bolts  10  inwardly (convex-in relative to the axis of the main bore  40 ). The magnitude and direction of the angle of the seats  36  is chosen, and also the torque to which the bolts  10  are tightened is chosen, so as to produce a substantially uniform stress distribution in the shank of the fastener  10  at the maximum application load, as illustrated in  FIG. 2B .  
         [0029]     If in  FIG. 4  the bolt hole  37 ,  39  and bolt-joint seat are machined along the same spindle centerline  38 , the seat and bolt centerline would be 90 degrees to each other by virtue of the manufacturing process, like the typical joint shown in  FIG. 3 . An additional or different process is needed to create the required bolt seat  36  skewness. This could be done in many different ways. For example, the bolt seat  36  skewness of  FIG. 4  could be forged into the bearing cap  42 . Another way would be to machine the bolt hole with one spindle along axis  38  and machine the bolt seat with another spindle at a small angle to the hole-drilling spindle. Yet another way would be to create the angle of the seats  36  by using the powder metallurgy process to form the skewness of each bolt seat  36  in the bearing cap  42 .  
         [0030]     Another way to create a uniform stress across the bolt shank  11  in the plane of bending at maximum application load is to make the joint faces, where they face each other near the center of the main bore  40 , at a small angle to each other tapering outwardly so as to create a small unsupported gap  48  between each set of the joint faces in the area adjacent to the bore  40 . This is illustrated in  FIG. 5 . One or both facing surfaces could be angled so as to create the gap  48 . This small angle (greatly exaggerated in  FIG. 5 ; may be less than one degree depending on the magnitude of the application load to be cancelled) could be machined on the faces, formed by forging or powder metallurgy, or the joint could be plastically deformed to create the gap, which latter method could be incorporated into an otherwise typical fracture splitting production process of a rod and cap of a connecting rod. This allows the cap  42  to flex toward the rod member  44  in the areas of the gaps formed by the angles, which has the effect of subjecting the shanks  11  of the fasteners  10  to bending stresses so as to bow them outwardly. When the bolts  10  are tightened, the gap  48  may be closed or substantially closed, or not. The size of the gaps  48  and the torque to which the bolts  10  are tightened are chosen so as to produce a substantially uniform stress distribution in the shank  11  of the fastener  10  in the plane of bending at the maximum application load, as illustrated in  FIG. 2B .  
         [0031]     Yet another way to create a uniform stress in the plane of bending across the bolt shank  11  at maximum application load would be to create the centerline  38 A of the threaded hole  39  at a small angle to the bolt hole  37  and (unbent) bolt  10  centerline  38 B as illustrated in  FIG. 6 . Again, the angles of the axes  38 A are greatly exaggerated and may be less than one degree relative to the axes  38 B. These bow the shanks  11  of the bolts  10  outwardly, as in the previously described embodiments, to yield a uniform stress distribution across the bolt shank in the bending plane at maximum application load, with a reduced cyclical mean stress and reduced maximum stress in the bolt shank  11 . The angles of the axes  38 A and the torque to which the bolts  10  are tightened are chosen so as to produce a substantially uniform stress distribution in the bending plane in the shank  11  of the fastener  10  at the maximum application load, as illustrated in  FIG. 2B .  
         [0032]     Preferred embodiments of the invention have been described in considerable detail. Many modifications and variations to the preferred embodiments described will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiments described.