Patent Publication Number: US-2010107808-A1

Title: Method for increasing torsional fatigue strength in crankshafts

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to processes for increasing fatigue strength in materials for use on engines, and more particularly to processes for increasing the torsional fatigue strength of cast iron materials to permit use of such materials in the production of engine crankshafts. 
     BACKGROUND OF THE INVENTION 
     Modern engines, such as diesel engines for vehicles, produce significant power through fuel combustion, which causes reciprocal motion of pistons, which in turn cause rotation of a crankshaft. The rotating crankshaft is coupled to various systems on the vehicle, including the transmission, to cause rotation of wheels and motion of the vehicle. 
     The stresses on the crankshaft are severe, and increase with engine output power. One of the stresses imparted to the crankshaft is torsional stress, which is a type of shear stress resulting from the forces urging the crankshaft to twist substantially along its longitudinal axis during operation. It is known that one of the areas of the crankshaft most susceptible to failure as a result of torsional stress is the area of the crankshaft oil holes. Oil holes are generally formed at various locations along the length of the crankshaft to distribute oil or other lubricant onto bearing surfaces, thereby decreasing friction. The absence of material at the oil holes may make these areas of the crankshaft especially weak, particularly after an extended period of use. In other words, the torsional fatigue strength of the crankshaft in the vicinity of the oil holes may limit the power output of the crankshaft. 
     One approach to providing crankshafts with high torsional fatigue strength is to manufacture the crankshafts from high strength steel. Steel, however, is relatively expensive. 
     As such, the cost of the engine is increased as a result of its use. Another approach is through use of inductive hardening as explained in U.S. Pat. No. 3,623,128. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present disclosure, the torsional fatigue strength of a cast iron crankshaft is increased through roller burnishing the interior of the oil holes formed in the crankshaft from the openings of the holes to at least past the transition of a hardened area of the cast iron material through which the holes extend. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features of this invention and the manner of obtaining them will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the present invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a partially fragmented side view of a crankshaft. 
         FIG. 2  is a fragmented, cross-sectional view of a portion of a crankshaft, depicting an oil hole processed according to one embodiment of the present disclosure. 
     
    
    
     Although the drawings represent embodiments of various features and components according to the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplification set out herein illustrates embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings, which are described below. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. The invention includes any alterations and further modifications in the illustrated device and described method and further applications of the principles of the invention, which would normally occur to one skilled in the art to which the invention relates. Moreover, the embodiments were selected for description to enable one of ordinary skill in the art to practice the invention. 
     Referring now to  FIG. 1 , a portion of a crankshaft  10  is depicted as including a crank pin bearing  12  and a pair of main bearings  14 . As is well understood by those skilled in the art, crank pin bearing  12  is coupled to a connecting rod (not shown) at the opposite end of a piston (not shown). Main bearings  14  are supported for rotation by bearings (not shown) held by the engine block (not shown). Crankshaft  10  is formed of ductile cast iron material such as any of the nodular irons according to German Standard DIN 1693. Crankshaft  10  also includes a plurality of oil holes  16  which extend through portions of crankshaft  10  to distribute oil or other lubricant to the bearing surfaces. As shown, oil holes  16  may be drilled at an angle relative to the longitudinal axis of crankshaft  10 , and may branch off to a plurality of openings  18  at the bearing surfaces. While oil holes  16  are depicted as being formed at angles other than 90 degrees relative to the crankshaft longitudinal axis, it should be understood that oil holes  16  may be formed at any angle relative to the longitudinal axis. 
       FIG. 2  depicts a portion of a main bearing  14  with an oil hole  16  extending from the bearing surface  20  (i.e., the exterior surface) into the main bearing  14 . It should be understood that oil holes  16  extending into crank pin bearings  12 , while not described herein, may similarly be processed according to the principles of the present disclosure. Main bearing  14  includes a hardened portion  22  and an interior portion  24 . Any of a variety of different known processes may be used to harden the cast iron material in hardened portion  22 . 
     Oil hole  16  includes an opening  18  and an interior bore  26  formed in a conventional manner using a drill or other suitable tool. Adjacent opening  18 , oil hole  16  includes a chamfered portion  28  formed in a conventional manner. Oil hole  16  further includes a burnished portion  30  which is formed by roller burnishing. As is known in the art, the specific requirements (e.g., speed, pressure, etc.) for roller burnishing vary from burnishing tool to burnishing tool, and depend upon the material being burnished. Burnished portion  30  extends from chamfered portion  28  to interior portion  24  of main bearing  14  and terminates at a terminating end which is shown in the depicted embodiment in flow communication with another oil hole extending from another bearing of the crankshaft. As should be apparent from the foregoing, burnished portion  30  extends, at a minimum, beyond the transition of hardened portion  22  and interior portion  24 . Burnished portion  30  may, in another embodiment of the present disclosure, extend the entire length of interior bore  26 . As shown, burnished portion  30  has a diameter that is somewhat larger (e.g., 2%) than the diameter of the portion of interior bore  26  that has not been burnished. 
     The process of burnishing burnished portion  30  improves the surface finish of oil hole  16 . An improved surface finish at burnished portion  30  may impact the life of crankshaft  10  as it undergoes cyclical loading in service. A surface with fewer flaws provides fewer locations from which cracks are likely to originate. As such, a reduction of surface defects may increase the service life of crankshaft  10 . 
     In many situations, surface finish has a relatively minor influence on fatigue strength. The production of residual compressive stresses in burnished portion  30  of main bearing  14  is primarily responsible for the increased torsional fatigue strength. Residual compressive stresses reduce the effects of tension during cyclic loading. Nearly all formed surfaces are in some state of residual stress (compression or tension). Surfaces in high tension often crack and fail quicker than if there were no stresses at all, as a result of the higher tension produced during a loading cycle. This tension pulls at the surface of the material and weakens it. Oscillating tension eventually causes damage at some small point on the surface, usually a defect or a stress concentration location such as a corner. Cracks leading to failure generally originate at (and grow from) such surface locations. It follows that a surface in compression experiences less tension during loading. As such, cracks are less likely to form at the surface and the component lasts longer and/or can withstand greater forces. 
     In one embodiment of the present disclosure, a roller burnishing tool is used to create burnished portion  30 . Such tools generally include steel or carbide rollers that rotate at high speeds while being placed in contact with the surface (such as interior bore  26 ) with a slight interference. The resulting plastic deformation of interior bore  26  leaves residual compressive stresses at burnished portion  30  in addition to smoothing the surface finish. In another embodiment, multiple passes are performed to cold-work the surface, which may increase the material&#39;s tensile strength. Appendix 1 shows data for rolled and non-rolled oil holes, and a comparison of the resulting fatigue strength. 
     One method for processing cast iron to form a crankshaft according to the principles of the present disclosure is as follows: First, the crankshaft is cast and prepared for oil hole drilling in a conventional manner. The oil holes are then drilled into the crankshaft at the desired locations. The openings of the oil holes may be chamfered. The crank pin bearings  12  and main bearings  14  are then treated using any of a variety of conventional hardening techniques to achieve the desired hardness. After hardening, the oil holes are roller burnished to a diameter that is fractionally larger than the drilled diameter of the oil holes. The roller burnishing is performed into the oil holes beyond the transition of the hardened portion of the bearings  12 ,  14  to the interior portion. Although roller burnishing is described herein, it should be understood that other mechanical and non-mechanical techniques for inducing compressive stress in a non-through hole that must remain open to oil flow may be employed. For example, and without limitation, autofrettage may be used to induce residual stress by subjecting interior portion  30  to very high pressure fluid. 
     While this invention has been described as having exemplary designs, the present invention may be further modified within the spirit and scope of this disclosure. For example, although steel crankshafts are stronger than cast iron crankshafts, the process of the present disclosure may nonetheless be used to increase the torsional fatigue strength of steel crankshafts as well. Indeed, the present process may be used with any of a variety of different types of metal. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.