CARBON Axle Tech
Carbon is found virtually everywhere, and is the second most abundant element on earth after oxygen. Besides being the chemical basis of all known life (kind of a big deal), Carbon has specific properties important to metallurgy. The amount of Carbon in a metal or alloy determines which type of heat treating to use, and ultimately the hardness of a finished metal product.
Materials (the real info)
Common factory axles are made to work with small OE tires, see very little real off roading, and simply don't hold up to hard use. These axles are often made from 1039 or 1040 material. An inexpensive but good upgrade for rear shafts is 1541, or 1541H (a bit of Manganese and Boron added, which allow harder surface heat treat). The benefit of 1541h is strength, cost and ease of machine and heat treating. A big step up in strength is the Chromoly family of material, commonly 4140 or 4340. Some companies will say chromoly is "brittle", which is far from the truth. The material is very ductile, or has a lot of "give' to it before deforming. It's the heat treat process that is more complicated and expensive, keeping some companies unwilling to offer it's advantage. We use 4140 in Rear shaft applications, as 4140 we can heat treat efficiently while maintaining high surface hardness which is important for the non-steering application. In front axle kits, we use 4340 chromoly, stronger still, and even more complicated on proper heat treatment. We developed the dual heat treat in the late 90's (see below) for the BEST combination of strength and ductility. What that means is we heat treat the entire front shaft, including the yoke (OE and some aftermarket shafts have ZERO heat treat in the yoke area). We then go back and re-heat treat the shaft portion only, giving the high strength where we need it, and the ductility where required. Here is the most common and most perpetuated "fake news" about material: US material is better than import. Simply not true. The people saying that have their reasons, or just are repeating what others have told them. The truth is it's just metal - if the steel plant is good and follows basic metallurgy protocols, 4340 made here or in a plant outside of our border is the same. I know, I've tested more chromoly steel than every competitor combined, from the US and abroad. There is good here and crap here, same elsewhere. Know who to source from, and confirm via testing. I prefer using US sources if it makes sense, but often you get no better for a lot more. There are very good metallurgists around the world - you see examples every weekend in Formula1, Moto GP, Dakar, etc. Here's the last bit of information the competitors probably don't know: The largest US axle manufacturers by FAR, is also the largest axle manufacturer around the world. They do not ship to their OE customers, they build plants nearby. The technology and information is out there, you just have to know where to go.
There are many types of heat treatment, and the metal alloy and application determines which type to use. For axle shafts, there are two types used almost exclusively.
Induction Heat Treat: Most Original Equipment factory axles are induction heat treated, which uses electromagnetic heat generated by high frequency alternating currents generated by the electromagnet. The shaft is heated and quenched immediately to create hardness on the surface and penetrating up to .300. This type of heat treatment is quick, but limited to the shaft area only, as the induction coil must be just larger than the shaft diameter, and will not accommodate a flange or yoke
Through Hardening: Certain types of alloys with higher Carbon content, along with manganese and molybdenum such as Chromoly, are able to be through hardened. Like the name implies, the shaft is hardened all the way through, creating a very strong axle shaft. However, in order to achieve the correct surface hardness, through hardening can result in a tough shaft that doesn't live in repeated shock cycles. We developed the Dual Heat treat system for front chromoly shafts around 2000, and have perfected it. We Through Harden just enough for strength in the core and ears, while allowing high ductility, or "give", then induction heat treat the shaft to get the surface hardness where we need it. It takes longer, costs more, but creates the best combination of strength and ductility.
To allow suspension travel, the front axle housing must be offset to clear the engine oil pan. This offset results in significantly different axle lengths. The longer shaft can handle more twist angle than the much shorter shaft (math below). That's why typically, the short side OE shaft is the most likely to fail. We counter this with proprietary heat treat and what we call EquiTorque technology. Rather than make each shaft the largest possible, which makes a very strong shaft but not a reliable assembly, we make specific changes to left and right diameters on a per vehicle basis. The diameter changes are designed to "equalize" the torque across both shafts, with the right balance of strength and ductility to keep your axle assembly alive in the toughest conditions.
a = 584 x T x L ÷ D4
a = angle of twist in degrees
T = torque in inch lbs.
L = length of shaft in inches
D = major shaft diameter