Connecting rods are crucial and highly stressed parts of the automotive engine, subjected to alternating tensile and compressive stresses. Large numbers of them are needed. Conventionally, they are made of ferrous alloys and are made by casting or drop forging. The final dimensions and finish are obtained by machining.
With the need to reduce automobile weight, the engine also needs to be lighter, more power intensive, and with well balanced components. Lighter and well-balanced oscillating engine components like the connecting rods and pistons enable higher engine rpms, and lesser noise and vibrations. Often, the lighter materials with the requisite strength are costly, but lesser quantity of material may be needed because of more precise design and manufacturing possible with them. There may also be a reduction in manufacturing and finishing processes, reducing labor requirements, and also reducing the loss of the material as chips during finishing. These can often help offset the higher cost of these materials.
The paper reviews recent developments in materials and processes for automotive connecting rods, and the factors which motivate the use of the improved materials and processes. Many automotive manufacturers have switched from cast or drop-forged ferrous connecting rods to powder metal forged rods. Inspite of the higher cost of the P/M forged roughstock, these connecting rods are found to be economic, and have found commercial acceptance. The manufacturing cost is further reduced by fracturing the crankshaft-bearing cap from the rod shank, since the mating fractured face does not require machining. The larger surface area also permits higher loading.
The use of the lighter metals like aluminum, magnesium, and titanium, and of plastics, is under evaluation, in order to achieve weight reduction. Aluminum and magnesium matrix connecting rods using ceramic particulate and fiber reinforcement, and using P/M techniques, are being evaluated. Titanium connecting rods, so far used in racing engines, are moving to the passenger vehicles. To reduce cost, special titanium alloys are being developed. Another approach is to use hybrid connecting rod, using titanium in combination with graphite-fiber reinforced polyimide compression shank and tension loops. Some of these developments could become commercial in the coming years.