Input Shaft
The input shaft is commonly one of the weakest links in the transmission. Its shape and design make it an ideal breaking point when caught between a high torque diesel engine and the weight of an 8000-pound truck. UT made the shaft diameter as large as possible and increased the spline count where the input shaft mates with the torque converter. UT calls their design the Fat Shaft. The Fat Shaft is a billet shaft. We hear billet a lot these days in this industry. It means that the part was machined out of a solid piece of material, rather than a casting. Casting is less expensive, but often contains flaws that can occur as the molten metal cools in the casting mold. Billet material is generally a chunk of metal that is forged under heat. This process forces the molecules in the metal to align, which make it stronger. UT also added a radius shoulder to the shaft where it meets the direct clutch hub. This may seem like a little detail, but if you look closely, it adds quite a bit of additional metal to this 90-degree shear point. The real industry secret here is the special alloy used. Sorry, they wouldn't share it with us. UT's engineer is actually an aerospace engineer and metallurgist with 20 years of experience under his belt. Fortunately for us, he has a passion for diesel trucks and designs stuff as a hobby. He uses an extremely strong alloy for these shafts and then has them heat treated in a process that he's specified just for these shafts. If you're feeling like an average Joe reading about this, it gets better. Even if you were to figure out what alloy this was, you still couldn't get your hands on it. It would take about two years to fill the order. Unless you were an aerospace engineer with some good connections. Finally, the shafts are shot peened to relieve the tensile stress that comes from cutting and machining the part.