High-strength steel rod unveiled

Wednesday, February 16, 2011

Tags :  con-rods

con-rodsAt the recent Autosport Show in England there was a lot to interest the motorsports enthusiast, but during the two trade days preceding the public show days, there is a simultaneous show, Autosport Engineering, where the latest developments in motorsports components are shown, manufacturing technologies are highlighted and much new technology is discussed. Many specialist engine component manufacturers from Europe and the US are represented here.

One engine component represented such a significant development that it won the Autosport Show award for Best Technical Innovation 2011. Given that the con rod has enjoyed more than a century of development, it might be a surprise to find that this component beat many other new technologies.

Engine component design specialist Zzuhl has a technical partnership with British crank and rod specialist Arrow Precision, with Arrow producing con rods in a material in which rods have not been available before. Zzuhl has developed an ultra-high strength steel with the specific aim of producing con rods, although the material has properties that make it an attractive proposition for other engine components.

The new material, called AZZ-2 .1/M2000, has extremely high strength for a rod material, with a UTS in excess of 2100 MPa and high fatigue strength. While many materials of similarly high strength suffer from a lack of ductility, AZZ-2 .1/M2000 retains a significant degree of plastic elongation, coupled with unusually low notch sensitivity for a material of this strength.

Notch sensitivity, as the name suggests, is a measure of how a material is affected by notches and other stress-raisers such as threads and so on. High-strength materials have a higher notch sensitivity than those with lower strength, and if highly notch-sensitive materials are selected, the increase in fatigue strength in the region of a high-stress concentration is significantly derated. Therefore a combination of high strength and lower than usual notch sensitivity is a very attractive combination in a cyclically loaded component with significant stress concentrations. This is a good description of a con rod.

con-rods zzuhl-rod

A number of people report that a steel con rod can be used satisfactorily without a small end bush, especially where DLC-coated piston pins are employed, and the Zzuhl rods displayed at Autosport follow this pattern. In addition to dispensing with the mass of the bush, the small end is made smaller and it no longer has to contain interference-fit loading resulting from the fit of the bush.

So, where might we see this material used in the 2011 season? Zzuhl considers this a good candidate material for high-boost applications. With the continuing trend toward highly turbocharged production engines and a raft of new race engine regulations that either allow or mandate the use of turbocharged engines, the material seems well placed to allow engine designers to produce durable rod designs.

In general, a material with an increased fatigue limit where stress concentrations exist can be used to produce lighter components by shedding mass where existing designs are strength-limited. For an existing design of con rod, this higher strength material can be used at higher load, or simply to give a greater factor of safety against failure.

Fig. 1 - Acclaimed as Best Technical Innovation at this year's Autosport Engineering Exhibition, this rod uses ultra-high strength steel (Courtesy of Zzuhl)

Written by Wayne Ward

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A high rpm, N/A four-stroke conrod design is all about buckling and bending in the beam section.  And to maximize buckling and bending strength, while minimizing mass, you want to push the material out to the extreme boundaries of the available area.  Any material at the beam section's neutral axis doesn't contribute to strength.

So rather than the conventional forged and machined H-beam or I-beam conrod, I would propose a hollow section rod.  Ideally, the rod should be made from vacuum melt, wrought metal alloys with controlled grain orientations.  Normally, this is only achieved with forged blanks.  But I would propose making the rod body blank from three pieces rough machined from wrought material (two thin outer skins and a center spacer) that get diffusion bonded together.  There would be no material in the center section of the rod body.

Diffusion bonding is a wonderful "solid state" joining process that produces finished parts with metallurgical properties as good as the materials they are made from.  In essence, the diffusion bonded joints don't exist.

A diffusion bonded conrod body would put the material where it's most effective, thus minimizing stress, weight and inertias, and maximizing stiffness in bending, compressive buckling, and torsion.

What do you think/

There was chatter about Honda experimenting with a hollow rod in their MotoGP bikes a few years back. I haven't heard rumors of the process used though.
I have a couple hollow con rods. used in formula 1 back in 2006. We found no benefit between them or an I beam rod. Expensive to manufacture.