Ask Kevin: How Are Fracture-Split Connecting Rods Made?

Question: I'm curious how a fracture-split connecting rod is made. I suspect a sintered steel alloy is used, and that the material is somehow frozen or made cold enough to be rendered brittle. What exactly is the "wedge" used for the break? And I guess scribing marks are made on the inner diameter for the break to be straight across. Can conventional bearing shells be used as long as the break is uniform and where desired? But can that be completely controlled?

Peter R. Del Nagro

Poland, OH

Answer: You can read an excellent article on this at "Yamaha Motor Technical Review," which reveals that con-rods for YZF-R1 have been made in this way since 2003.

Advantages—First comes a 30-percent manufacturing cost reduction from not having to machine and dowel the mating surfaces of rod and cap. Second are greater roundness and more accurate, longer lasting registration of the two pieces, which brings benefits in bearing capacity and life.

History—Fracture-splitting of races for rolling bearings is decades old, rendered easier to accomplish by most rolling bearing races being through-hardened (same hardness all the way through). Roller rods for chainsaws and outboards have been made this way for a long time, and the inner main bearings on the Yamaha 500 GP bikes Kenny Roberts rode to three world titles were made in this way (to eliminate the weakness of press-fitting flywheels onto main shafts, the two inner wheels were forged in unit with the mainshaft, like dumbells).

I had the opportunity to look at main bearings from MV factory race engines, which likewise had fracture-split outer races. In their case, the fracture was guided by a series of drilled holes. In other cases, the races are grooved on the inner diameter where the split is desired.

How it works—The Yamaha paper explains that while modern automotive con-rods are sinter-forged from powder, resulting in a material that is uniformly brittle enough to fracture accurately, the rods of higher performance engines such as those of sport motorcycles must be made of higher-fatigue-strength forged-from-solid steel. Such rods are case-hardened but have less brittle cores. When attempts to fracture-split such rods were made, the fracture was brittle only in the hard surface layer, with the inner material—being three times tougher—displaying the plastic deformation of ductile fracture. The presence of such deformation prevented the cap from fitting as it should.

Therefore, the research group studied conditions of temperature and rate of strain application to discover reliable means of separating the cap entirely by brittle fracture (older readers may recall that some “Liberty Ship” cargo vessels of WWII suffered brittle fracture of their normally ductile steel hulls in the cold water of the North Atlantic).

The fracture apparatus consists of a two-piece cylinder (split along the desired fracture plane), fitting closely into the rod’s big-end bore. It is pierced for a wedge. The wedge carries a striker cap, against which a falling weight impacts to rapidly strain the rod cap, resulting in fracture. Measures must be taken to guide the fracture and to ensure that only a single fracture surface is initiated on each side.

More information—
You can read more at global.yamaha-motor.com/about/craftsmanship/technical_review/, or by just Googling Yamaha Motor Technical Review.

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