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Super-Shaft

Following a crank from forging to fantastic

Jon Barrett - January 30, 2014 10:00 AM

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This is one of Lunati’s forging die sets. It is between these two halves that the 4340 steel ingot is squeezed into shape under intense pressure.

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Here is a rough forging fresh out of the forging dies. As you can see the throws are in the right place but there is a lot of excess material that needs to be removed.

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The three-axis vertical CNC machine is used to define the shape of the counterweights, whether that be rounded or “knife-edged.”

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Lunati’s skilled operators program these machines to precisely remove exactly the right amount of material with each pass.

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When finished, the counterweights now have their aerodynamic “knife-edge” profile that helps maintain proper crankcase windage and improve the oiling of the engine.

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After the crankshafts have their counterweights profiled on the CNC, they are then gun-drilled with lightening holes to reduce rotating inertia and oiling holes to improve lubrication.

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Here you can see the finished lightening holes running through each throw as well as the oiling holes that transect the main and rod journals.

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Lunati’s skilled craftsmen hand-blend all of the CNC work on the counterweights to create a smooth, aerodynamic profile.

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Next, it is onto the lathe to have the rod and main journals turned down to their specified size.
The final stroke of the crank can be adjusted up to 3/8-inch within the concentric circle as the journals are turned down to size.

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These crankshafts are now ready to be hardened in the next step of the process.

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Lunati plasma nitrides its crankshafts inside of this vacuum chamber to case harden the surface for extra strength and durability. Nitriding is performed as a cold process so that there is no risk of warping the parts.

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These are the gasses that are ionized and mixed to create the “plasmas” which are introduced into the vacuum chamber to perform the nitriding process.

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Fresh out of nitriding, the crankshafts are ready for a final micro-polishing of the journals.

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Once the journals are polished and perfectly smooth, the crankshaft is ready for balancing (if it is ordered as internally balanced) and then shipped to the customer.

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The journey from an ingot of 4340 steel to a raw forging to finished crankshaft is wrought with pressure, heat, and work, but it all goes into making one of the most crucial pieces of any performance engine.

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There are few components that affect your engine’s strength, longevity, and power potential more than the crankshaft.

Try to imagine what a crankshaft must go through during the average commute to work; long, arduous highway miles; or a 12-second quarter-mile pass on a dragstrip. Spinning at an average speed of 5,000 rpm for 12 seconds means that crankshaft is dealing with the forces generated by 4,000 controlled explosions.

It is because of this violent environment that Lunati developed its line of forged 4340 steel crankshafts. What exactly goes in to making a quality crankshaft that can stand up to years of abuse? Join us for an in-depth look at the manufacturing and machining process of the backbone of your engine.

Born Under Pressure

All of Lunati’s cranks start life as an ingot of 4340 steel. But what is 4340 steel and why is it used? “4340” is a number assigned by the American Society for Metals (ASM) that refers to a particular steel alloy’s metal content (mostly a mixture of iron and carbon). It is the specific combination of these metals that determines the overall tensile strength ratings. In the world of steel alloys, 4340 is the top dog, boasting a tensile strength of around 145,000 psi, far surpassing the average factory cast iron crank’s 70,000 psi rating.

The first step of the manufacturing process is called forging, which gives the crank its rough shape. The forging process begins when the steel ingot is placed into a forging die, which is then subjected to pressures up to 240,000 psi, squeezing the metal into voids of the die, compacting its molecules, and aligning and strengthening the grain structure.

There is something truly unique about the forging process that Lunati uses, which is known as cold forging. Rather than pre-heating the steel to 2,200 degrees Fahrenheit to make it pliable, a cold forging just uses immense pressure to form room-temperature metal. This results in a forging that is “work hardened” for added strength. The Lunati forgings are also a non-twist type of forging, in which each crankshaft throw is forged in its final position in the forging die.

The benefit of the non-twist forging over a twist forging (where all of the throws are forged inline and then rotated into position while the crank is still hot) is that the grain structure of the metal remains uninterrupted, which maintains the high tensile strength of the 4340 alloy.

 Taking Shape

After the forging process has been completed, the machining process begins. During this initial machining, the counterweights are cut and the crank flanges are located. The main journals and rod journals are then machined 0.100-inches oversized so that they can later be sized to a dimension specific to the crankshaft’s final part number or customer request. This initial machining has now removed most of the excess material, leaving the crankshaft actually looking close to its final form. The crankshaft is now known as a “semi” and is ready to be machined into its intended final specifications.

The next stop for these “semi” cranks is the CNC three-axis Vertical Machining Center. It is at this highly specialized machine that the original cuts are further refined, giving the counterweights their “knife-edge” profile. This aerodynamic shape is an especially important feature for high-horsepower and high-rpm wet-sump engines, because the improved windage in the crankcase leads to greater efficiency, more horsepower, and more consistent oiling.

In addition to profiling the counterweights, the Vertical CNC machine removes excess material to further reduce the weight of the crank and also smoothes out the sides of the journals. At this point, the main and rod journals are still oversize and the final stroke of the crank can vary up to 3/8-inch within the concentric circle. Finally, lightening holes to reduce inertia and oiling passages to lubricate the rod and main bearings are gun-drilled into the journals. Now the crank is ready for the most hands-on step of the process – a hand detailing of the counterweight profiles.

Best by Hand

In this age of automated CNC machining and digital designing, it may seem counter-intuitive to still hand finish the counterweight profiles rather than having it done by a machine. However, it is tough to replace the years of experience, passion, and attention to detail of a skilled artisan’s finely honed talents. This is why all Lunati crankshafts are still hand detailed as part of the machining process. After the cranks have been thoroughly inspected and detailed, they are sent off to have the main and rod journals turned down to their final size on a lathe before the cranks are sent off to be shot peened.

Shot peening is a process during which the surfaces of the crankshaft are bombarded with round metallic shot as a way to relieve the tensile stresses that are built up during the machining/grinding process. Those tensile stresses are replaced with more beneficial compressive stresses, which extends the fatigue life of the metal up to 1,000 percent. After the shot peening process is complete, the cranks are ready to be nitrided.

The nitriding process creates a “case hardened” surface on the crank for added strength and durability. Lunati uses what is known as Pulsed Plasma Nitriding on all of their crankshafts. This process is accomplished with a mix of ionized gasses (mostly nitrogen) called plasmas that are introduced inside of a vacuum chamber. By performing the nitriding as a cold process, the parts do not need to be heated so there is no risk of them getting warped.

After the nitriding has been finished, the main and rod journals receive a final micro-polishing to ensure a uniform surface that will rotate within the bearings without interference. As a final step, the crankshafts that are to be sold as internally-balanced are put on a balancer and have heavy metal balancing material added to their counterweights to offset the weight of the matched rod and piston combinations.

For Your Information:

Lunati, (662) 892-1500, www.lunatipower.com

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