by Wayne Scraba

Here’s the spline end (35 spline) of a Mark Williams Hi-Torque axle for a 9-inch Ford. This is actually a pro street setup. As noted in the text, the axle tapers down slightly after the spline.
 

In the past segment of our Axles 101 series, we looked at the material choices (and finishing options) for axles. If you recall, there’s much more than meets the eye. But that’s far from the end of it. You see one of the most important factors when it comes to axle strength is actually the splined end.

According to Mark Williams, when you examine a set of axle splines, the outer edge of the spline defines the major diameter. The lowest portion of the groove within the spline defines the minor diameter. The minor diameter is what determines the strength of the axle shaft. The included angle of the spline (commonly called the “pressure angle”) is important as well. Some pressure angles measure 30-degrees (which works out to a 60-degree included angle). But for racing applications, a 45-degree pressure angle (90-degree included angle) is superior. The reason is, the spline depth is actually shorter, which allows for a larger minor axle diameter (on a specific axle outside diameter).

If you view the profile of the Hi-Torque axle, you’ll see that the entire axle is tapered. There’s good reason for this: Triangulation (which, as most know is the strongest of geometric shapes). Williams builds axles that are designed to work in a fashion similar to a torsion bar.

Williams goes on to point out that most modern axles are manufactured with a 24-pitch. If an axle has a 1-inch circular pitch diameter (the mid-point between the major and minor diameters), it would have exactly 24 splines (or teeth). The distance between the centerline of adjacent splines remains constant, so as the diameter of the shaft increases, so does the number of splines. As an example, a 35-spline axle has a major diameter of 1.500-inches while a 40-spline axle measures 1.708-inches in diameter.

We’re not quite done yet. The actual shape of the spline is important too. Original equipment axles are manufacturer with what is called an “Involute Spline”. This means the face of each spline is slightly curved. This type of spline provides for an optimum contact patch along with even pressure while engaged. But there’s a catch: You can’t recreate an involute spline by way of flycutting (resplining). That results in a straight cut spline. To manufacture an involute spline, you have to hobb the spline. What’s hobbing? Hobbing is a machining process which incorporates a special type of mill. The teeth or splines are progressively machined into the component by a series of cuts made by a cutting tool, which is called a hob. It is possible to fit (and use) a flycut flat axle spline into a spool or differential designed for an involute spline. But what happens is the spline on the axle(s) is stressed – often considerably. Reliability diminishes. That’s why it’s not a good idea to mix and match splines.
 

This is the flange side of the Pro Street axle (it’s being fitted with Drive Studs in this photo – we’ll spend some time on these down the road). Different applications mandate different flanges. A Pro Street piece will be thicker (heavier) than a dedicated drag race axle (for example, no worries about curbs on the strip).

A pair of axles for the same car. What’s going on with the length? Simple. It’s pinion offset in the Ford housing. Our car has a centered pinion housing too. The result is a different axle length for each side of the car. Keep this mind when buying axles (more on measuring for axles in Part 4 of this series).