Things To Look For In An Axle

A shaft getting the end turned to make it fit into the spline machine

The Structure
When discussing the profile of an axleshaft we have to first realize that all of the torque that gets sent into the shaft has to go somewhere. Bigger is better when talking about steel. Most strong shafts are made with the same outside diameter all the way to the end in one piece. The splines are then rolled (involute) on to the steel, thus keeping the profile consistent throughout. This is important: Imagine sticking a piece of metal in a vise and applying pressure from the top. Where do you think the metal will break? This is why you don’t see many “wasted body” or “neckdown” axles being made by the top aftermarket companies.

These days, most axles are made with rolled splines. Cut splines are often reserved for custom applications where the vehicle is using vintage components where pressure angles and special carriers are still being used. The cold-working process of rolling a spline is done using a hydraulically-powered machine that presses the splines into the shaft. A few of the reasons rolled splines are superior to cut include: couplings have the ability to transmit torque better, the splines can be produced using the same techniques and equipment used when cutting gears, the slightly curved profile provides optimum contact and even pressure distribution during engagement.

Bare 4340 shafts waiting their turn to have their splines rolled.

If you take anything away from this article, we hope you learned that buying a quality shaft (most often made in the USA) makes all the difference. Some of you are probably scratching your heads wondering why we didn’t discuss the differences between 4340 and 300M. We realized that comparison would further include discussing what people “should” use and “don’t need” for most rigs. We will leave that for another issue.

Beware of Some Foreign Steel!
Unfortunately, some manufacturers these days are getting a number of their parts and steel overseas. We have seen both quality and crap come off these boats over the years—but more of the latter. When choosing your replacement axleshaft, beware what some will say is comparable foreign steel. EN24 is common steel used to make axleshafts from India. Although many will argue that on paper this steel is just as strong as SAE 4340, it does not go through the same quality control and processes a typical piece of American made 4340 does and many experts will attest to them holding a higher failure rate. Bottom line: if you want the strongest stuff for your rig, limit the overseas items in your shopping cart to cell phones and alarm clocks. When you need to buy the ultimate strength, buy American steel.

A New Level In U-Joints?
Lately, a new mid-level U-joint has popped up, giving more strength than the typical needle-bearing U-joint, while still offering everyday drivability. Superior’s new super U-joint has a center cross made in the USA from 4340 chromoly steel, but still utilizes conventional Spicer-style needle bearing caps instead of solid bushing caps. This makes it a little weaker than a bushing-style cap U-joint like the OX joint or CTM joint, but longer lasting like a typical needle bearing cap U-joint. Its strength (and price) fall in between a premium typical needle-bearing U-joint and a competition-style high-end solid bushing U-joint.