Engine connecting rods will be used to illustrate today’s issue. Utilized as key motion imparting components, these rigid steel connectors move in long, linearly oriented strokes. From piston to crankshaft, the linear relationship is maintained. One other thing, the rods have to be hardened by a heat-treatment process. And therein lies the problem. You see, thermally active processes can introduce stress and unpredictable grain alterations, which then cause the rod metal to bend.

A Rod and Pinion Straightening Review

When it comes to engine parts, manufacturers seek out the hardest, most rigid alloys. They need steel grades that won’t crack under the pressure, and there’s no shortage of power transmission stress inside a working engine. There’s a heat treatment operation to conduct here, there’s no question about that, but the operation doesn’t end after the parts are imbued with additional tensile strength. Above and beyond this feature, the thin and possibly long rods also need to retain their singularly straight outlines. Again, that’s not a straightforward proposition. With grain irregularities and work stress deeply rooted inside the slender workpieces, they begin to bend. The distortion stress can be partially controlled by regulating the heat source and the quenching station, but a slight amount of rod curvature is still likely.

Cold Drawn Steels Eliminate Pin Distortion

With a secondary stage added to the process, the pins and linkages are exposed to enough heat to trigger microstructure annealing. Softer after the steel alloy cools to room temperature, a drawing machine stretches the rods. Of some benefit here, high-tensile steels, although exhibiting heat induced distortion, immediately straighten up and lose those ingrained stresses that have initially caused the rod to curve. The annealing process conducted beforehand helps, of course, but the bulk of the work is still left to the cold drawing machinery. Continuing with the benefits, the metal is straight, its surface has retained a polished finish, and its rigid strength hasn’t been leached by some supplemental heat treatment process, one that just adds more internalized grain changes.

There’s an even bigger payoff to mention before bringing this post to a close. Cold drawn steels exit the process as high-tolerance workpieces. The rods and pins don’t shrink or gain a metal oxide film, one that would impact their functional outlines. That means there’s no additional machining to carry out after the precision straightening work concludes. Furthermore, this is a customizable straightening operation. Sure, it works well on engine pins and power transmitting rods, but it can also be scaled to process thin wires, valve stems, sprockets and spindles, and all manner of slender but rigid workpieces.