1. Technical Field
This invention relates to the art of making annular bearing assemblies with separable journalizing parts that permit reception of a shaft other than along an axial direction of the bearing, and more particularly to the art of splitting connecting rods for use in automotive internal combustion engines using techniques which more readily propagate the splitting.
2. Description of the Prior Art
Automotive connecting rods usually have one end (large end) with separable cap and body portions to form an annular bearing assembly which permits insertion of a complex configured crankshaft from a direction not along the axis of the bearing. A simple ring bearing assembly is at the opposite end (small end) of the connecting rod which is connectable to a piston by a pin; the simplicity of this attachment is permitted by insertion of the pin along the axis of its bearing.
The advent of more compact engines, delivering higher horsepower at increasingly higher rpm's, has placed increased stress on the connecting rod and its bearings. The bipartite rod must act as a unitary piece to transfer dynamic forces with better bearing life. To meet this challenge, the manufacture of automotive connecting rods has undergone evolutionary changes.
Connecting rods were originally made by casting or forging separate attachable cap and body portions. These parts were usually made of medium carbon wrought steel and were separately machined at both joining and thrust faces; they were then separately drilled with holes to accept fasteners.
A first evolutionary step was to cast or forge the connecting rod as a single steel piece, followed by drilling of holes to accept fasteners. The single piece was sawed to obtain cap and body portions which were separately rough-machined at the thrust and contacting surfaces; the two portions were then bolted together for finish-machining. The separate steps of machining and sawing were not only cumbersome and expensive, but they also did not ensure a perfectly matched cap and body under all operating forces. Under some operating conditions, inherent diametrical fastener clearance permitted slight shifting of the cap and body portions which in turn affected bearing life.
As the next step in this evolutionary change, a single-piece connecting rod was split or cracked into cap and body portions in the hope of providing nonsliding surfaces where the cap and the body portions are bolted together. The hope was that if the surfaces were properly remated, the remate would prevent any microshifting and assure accurate operating alignment. To split the single piece into two, it was initially struck on one side with a sharp blow. This met with little success because of the uncontrollability of the cracking plane and possible damage to the connecting rod.
Another early attempt inserted a wedge-expandable mandrel into the large bore of the rod (see U.S. Pat. No. 2,553,935). The big end of the rod would fracture at the two weakest sides of the yoke; such cracking was carried out at room temperature even though the rod was made of a strong, nonbrittle, high carbon wrought steel. Cutting deep radial reductions at the intended cracking plane by sawing, milling and drilling, or a combination of all three, reduced the crackable section and weakened the material to assist cracking. This did not assure distortion-free cracked surfaces of such a tough material.
Another approach to splitting was disclosed in U.S. Pat. No. 3,751,080, which recognized the difficulty of fracturing strong high carbon steels at room temperature when they were formed in large sizes adequate for automotive engine applications. An electron beam was moved along a desired path in an undulating fashion which separated the rod to render a pair of rippled interfacing surfaces. This technique is undesirable not only because a high energy electron beam imparts a deleterious effect upon material performance but also because it is slower and more costly than previous techniques.
Yet another attempt to provide for cracking of ductile strong steel connecting rods is shown in U.S. Pat. No. 3,994,054, wherein tension forces were provided mechanically by conical pins forced into bolt holes at each side of the big end of the connecting rod. The bolt holes reduced the split plane section and the tapered pins provided a more equalized cracking impact. Unfortunately, this technique resulted in wear on the sides of the bolt openings causing distortions and thus inhibited accurate remating.
More recent attempts at splitting are disclosed in U.S. Pat. Nos. 4,569,109 and 4,768,694, which suggest that the rod can be composed of either cast iron, aluminum or steel, and made brittle by freezing or heat treatment. The connecting rod is fractured by applying high impact tension forces (i.e., 90,000 psi or greater) across a cracking plane defined by two notches in the internal surface of the large end bore while limiting relative movement of the cap and body portions to avoid ductile bending or incomplete fracture. The exact direction of the cracking plane cannot always be assured even though the notches are presented in the internal surface to provide such direction. Embrittlement by freezing or heat treatment leads to this indefiniteness of direction of the crack. As much as 25% of a production run of cracked rods with this method may have to be scrapped because the final crack planes are improperly placed.
A primary object of this invention is to provide an improved and more economical process for making split connecting rods with a greater consistency of accurate cracking at higher production levels and with improved cracked surface remating.