Have you ever tried to sit down with a representative from an axleshaft manufacturer and talk steel? We suggest you bring a notebook, pocket recorder, a sandwich, and an open mind. Not only do many opinions vary in the land of steel, but often you will run into engineering minds that will slam more information onto your noggin than you ever thought possible.
For most of us, when the time comes to replace a broken axleshaft, we simply hit the forums, search for what others have done, and follow suit. Then, the age-old saying, “If your friend jumps off a bridge, would you?” comes to mind. So, we distill some of the knowledge a few experts gave us to help you the next time your axleshaft needs replacing.
First, what is an axleshaft and what does it do? An axleshaft is a central shaft that is used to turn a set of wheels. These axles transmit torque to the wheels and maintain the position of the wheels relative to each other and the vehicle body. The easiest way to put it is, power is made in the engine, sent through the transmission, pushed through the driveline, into the gear cluster, and out to the axles.
The two most common reasons axleshafts break are due to too much load from big engines and oversized tires. Both cause huge amounts of stress, applying it all through the axleshafts.
For all things load bearing, strength is the first thing that comes to mind. More specifically, how strong is the material being used to create the axle, does the size and profile of the shaft make a difference, and do the number and shape of the splines count.
Steel is the best choice for making axleshafts because it is strong, very stiff, and is elastic in nature. You can heat treat steel in many different ways, including induction hardening and through hardening. The process which aftermarket axleshaft companies harden the steel is what sets them apart from the typical OEM axles, which cannot be through hardened.
To save on cost, OEMs will use SAE 1055 or 1541 steel, which work fine for most of us, but the extreme wheelers among us that overly modify our toys require a little more strength, such as SAE 4340 or 300M steel.
Let’s use an example all of us can understand: You are out wheeling with your buddies and get stuck trying to tackle a huge boulder in your way. Your tires are wedged against the rocks and you slam your gas pedal to the floor, hoping that power will overcome your lack of driving skills. This is the No. 1 place 4x4 guys break axleshafts. This is where the type of steel your axleshafts are made from comes into play.
All axles, given the same size and profile shape, will twist exactly the same amount under the same load. However, an axle made from 1030 carbon steel will have exceeded its maximum strength and will most likely break. Your stock OEM 1541 axles may not have passed their maximum strength point, but will most likely never fully return to their original shape. The far stronger 4340 axle has much higher yield strength and will not have reached its maximum strength point, which will allow it to return to its original size and shape allowing you to continue on the trail.
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.
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.