By: Anonymous: Duane () Wednesday, 22 June 2011 @ 09:05 AM ICT (Read 2663 times)
I've rear about certain motorcycle engines having 'cracked' connecting rods. There has been little explanation of what this means, other than one story I found on Internet mention of the connecting rods caps being made by a special cranking process. Any chance somebody could explain? It sounds interesting, but I have no idea what this entails and am eager to learn about it.
Not sure about bikes but are you talking about the rods being forged or cast with a "score" line across the big end. The big ends are then broken along the score line. The break is somewhat uneven not like if the big ends were cut with a machine tool. Then when the engine is assembled the "broken" irregular joint lines mate together perfectly. I believe the process is known as precision fracturing.
This makes the whole assembly process more precise..
Traditionally, connecting rods have been made as one-piece hot-steel forgings with a slightly out-of-round crankpin hole. After cooling the rod was cut in half cross the hole to form the bearing cap, and the mating surfaces of the two resultant pieces were machined to produce a perfectly round plain-bearing-insert mount.
As engines over the years gradually produced more power and turned higher rpm, the forces to which rods were subjected began allowing the bearing cap to shift slightly, causing bearing misalignment. Engineers reacted by installing locating pins between the cap and the rod, which usually solved the problem but introduced even more steps in an already complicated manufacturing process.
As power levels and rpm ceilings continued to escalate, engineers had to find ways to make connecting rods lighter and stronger. That goal was accomplished through the development of rods made by a process called powder-metal forging instead of hot-steel forging. They, too, come out of the forge as one-piece units, but with a perfectly round crank-pin hole. They also have a granular structure that allows the cap to be separated from the rod through a very precise 'cranking' or 'fracturing' process. This results in mating surfaces that are microscopically irregular but perfectly matched to one another. It's not unlike what happens when you accidentally break a valuable grass object in two and glue the pieces back together. They fit perfectly, the highs of one piece fitting precisely into the lows of the other and vice versa, preventing the two from slipping out of alignment in any direction. With a connecting-rod, this matching 'geography' holds the cap firmly in place, preventing it from shifting under heavy loads. This not only allows the rod to be thinner and lighter, it also simplifies the manufacturing process, in some cases cutting the number of machining steps in half.