Axles 101

In addition to the weak differential design a second surprisingly weak area of a Rover drive train are the axles, particularly the Series Land Rovers with part time 4 wheel drive. If your projected use includes extreme 4 wheeling, long term expedition use in remote areas, more powerful engines, oversize tires or, if you find field repairs one of your least favorite things to do, then upgrading to a superior strength axle will ensure a much more reliable vehicle. Land Rover axles are lacking strength or fatigue easily for several reasons: firstly, they are relatively undersized, secondly, they are produced from commercial grade materials and thirdly, they are designed with the usual compromises inherent in a mass produced mechanical part. There are basically three ways to increase the strength of an axle shaft:

1) Increase the size
2) Upgrade the material specifications
3) Upgrade the design specifications

Let’s look at these three areas in more detail but first let us mention that the following discussion is not intended to be definitive but rather a general overview of axle technology in layman terms.

1) INCREASE THE SIZE: Obviously bigger is stronger and surprisingly relatively small increases in axle diameter lead to proportionally large increases in axle strength. Remember high school algebra and the concept of pi, which equals approximately 3.1416. If you were like the average high school student like me, you probably wondered how some of these abstract concepts, like pi, could ever be applied to every day reality. Well here it is, the area of a circle (cross section of an axle) is the the radius squared multiplied by pi (3.1416). Still confused, in a nutshell here is an example. If you increase an axle diameter from 1 to 1.25, an approximate 25% increase, you increase the cross sectional area of the axle by at least 75%. Any diameter increase approximately triples the cross section of the axle and hence proportionally triples the strength of that axle. Your high school algebra teacher would be proud of you now! Note: To the worlds mathematicians and high school algebra teachers, we realize that this simple example is not perfectly mathematically correct because you are supposed to actually square the radius but on the other hand, from a practical stand point it does illustrate the point and actually understates the increase in axle strength.

Most Land Rovers have one of two axle sizes and designs, either a 1/ 10 spline design or a 1.25/24 spline design. Series L/R's usually have the 1/10 spline design. The only exceptions are later 109's which had the Salisbury rear axle (1.25/24 spline) and the post July 1980 series lll's, which had a 10 spline inner/24 spline outer axle. Early Range Rovers, Discoverys and Defenders to mid 1993 also had 1/10 spline axles. The exception again is the Defender 110, which has Salisbury rear axles (1.25/24 spline). Another exception is the axle size on the diff side of some CV joints which is 1.030/32 spline. Increasing axle size is an effective way to increase the strength of your axles but you are frequently limited by the dimensions of the differential carrier bearing jounal diameter on the diff side and on the other side by your spindle diameter. Dimensionally it is very easy to upgrade the size of 1/10 spline axles to 1.25/24 spline.

2) UPGRADE THE MATERIAL SPECIFICATIONS: Land Rovers like every other mass produced vehicle in the world have various production compromises. One of them is the material chosen to produce the driveline components. The primary tradeoffs are purchase and production costs. Land Rover uses a common commercial grade steel for their axles usually 862 or XK1360. It is relatively inexpensive, easy to machine and heat treat. It is a good steel and perfectly adequate for the normal use and abuse that a stock Land Rover will see on an every day basis. But it is easy to substantially increase axle strength by using better alloy steels. For the sake of simplicity there are two basic types, normal alloy steels and aviation grade alloy steels. Common materials added to an alloy steel include Carbon, Silicon, Manganese, Nickel, Chromium and Molybdenum. These ingredients add desirable characteristics such as flexibility and better wear qualities. Aviation grade steels are used extensively in the aviation industry for obvious reasons - major mechanical failures in airplanes lead to big problems - such as the planes going down in a less than controlled manner! Aviation grade steels differ from normal steels in that they are double refined. They are also referred to as double melt steels. The advantage of this extra refining is that it removes more impurities from the steel. This is very important because weak points and stress cracks frequently start with impurities because they interrupt the grain structure of the material. Aviation steels can be much stronger and more flexible than normal steels. We say "can be" because it is much harder to correctly heat treat an aviaton alloy than non aviation alloy. Heat treating is a process that substantially increases the strength of steel. If the heat treatment is not correctly done to an aviation steel you lose most of the advantages of using that material.

3) UPGRADE THE DESIGN SPECIFICATIONS: The design of an axle is another area where production compromises can be easily improved and hence a much stronger axle results. There are three areas that can easily be improved - the spline design, the overall shape of the axle and the surface finish. We can also add custom features such as "gun boring" Let's look at spline design first. Splines can be generated in one of three ways - hobbing, rolling or shaping. Hobbing is the fastest, least expensive and most common way to add splines; the downside is that you remove material, interrupt the grain structure and end up with an axle that can be much more prone to stress fatique. Rolled splines, on the other hand, have none of these disadvantages although the process doesn't lend itself to mass production quite as easily because it is very time consuming. The splines are formed by actually pressing the splines into the axle in a rolling motion. With rolled splines no material is removed, the grain structure is not interrupted and the spline area is actually forged in this process. Shaping is a specialized form of hobbing and has some advantages over both of these processes. The biggest one is that the splines are totally stress relieved and perfectly shaped. Shaping is time consuming because you generate one spline at a time. Also, as a general rule, within limits the more splines that an axle has the stronger the spline section of the axle. Another advantage of a larger spline count, is that there is less backlash between the axles, side gears and drive flanges reducing shock loads in these areas. All factory Land Rover axles, both 10 and 24 splines have hobbed splines.

Now let's look at axle shape. Ideally, the shaft of an axle should taper down narrower than the smallest depth of the splines at some point. This is referred to as "waisting down". The reason for this is that when the axle shaft flexes, it will flex uniformly along the entire length of the shaft without concentrating stress at any particular point. By allowing this to occur your axle essentially becomes a torsion bar, hence the advantage of a more flexible material becomes obvious! When you have the axle shaft larger than the spline depth, the shaft will obviously not flex uniformly and will create stress points usually at or near the splines. These stress points lower your peak overload measurement significantly. This stress point will start to fatique and fail over time or if you exceed this smaller peak overload point, it will fail suddenly. This is why oftentimes an axle will start to visibly twist before failure.

The last area is surface finish - believe it or not the smoother and finer the outer surface area of an axle the less likely that stress cracks will start on the surface. This is because carbon atoms tend to concentrate on sharp edges during the heat treating process resulting in uneven surface stress's, which eventually leads to surface cracks and ultimately premature axle failure. Some of our axles are bead blasted to reduce this problem. This is a process that is rarely applied to production axles. As noted earlier we can also supply on a, special order basis, "gun boring". This is a process where a hole is bored thru the center of a axle. This results in a much stronger axle because now the axle has two surface areas that need to break for a axle failure - this is the same reason that drive shafts are constructed from hollow tubes.