Technical Information on Axles

Everything you ever wanted to know about axles

Axle shafts
Function & benefit of drive flange piloting
What causes axle shafts to break?
What is the most stressful thing on axle shafts?

We have been working with Land Rover drive train for almost 20 years, so we have acquired an unparalleled body of knowledge in this area. We design and manufacture our own axle shafts with the idea that they are being specifically designed, built and installed into a Land Rover. This compares to some of our competition, which either starts with somebody else’s generic blank shafts and adapts them to fit your Land Rover or just resells someone else’s product.

To develop the best axle shafts requires three things:
1) Design and Engineering
2) Destructive testing
3) Real world feedback

For design and engineering work, we employ a qualified engineer to assist in the design of our axle shafts using the latest computer technology and software. We use Solidworks, Cosmos and Nastran FEA (Finite Element Analysis) software packages.

For destructive testing we have our own custom built destructive torque equipment. It can precisely measure total torque and degree of twist of the component being tested. In addition several of our manufacturing partners employ this same procedure. On a side note, in our opinion, data generated thru destructive torque testing is often mis-used, usually touted as the be all/end all in axle durability development. It is not but having being said that, destructive torque testing it does provide some interesting information.

Real world feedback is gathered using a couple of different methods.
First we supply several military customers with various drive train products including upgraded axle shafts for heavily modified military Land Rovers. Military customers do not just take your word for it after a convincing “dog and pony show”. They do very vigorous field testing of equipment before entering into purchase contracts. Many of our axle shafts have gone thru this type of testing and certified for use in these applications.
Second, over the years we have also sponsored some select competition and rally teams and individuals with our products to compile some data under these arduous conditions. We have sponsored several vehicles in competitive events. These events include the Siberian Challenge, Ladoga, Australian Outback Challenge and the professional Rock Racing circuit in the United Sates.

After we engineer a specific axle shaft, it now requires quality manufacturing to get utilize the maximum potential for it’s design. We use manufacturing partners that we have worked with for years and have a proven record of manufacturing quality and technical knowledge. Most of the companies we select to manufacture our shafts – manufacture only axle shafts. We can also produce a limited number of axle shafts in house for custom applications.

Specific and accurate technical information enable us to answer almost any questions you have. We can also provide you with specific technical information regarding any of our competitor’s axle shafts. This allows you the customer to make valid competitive comparisons and permits you choose the best product for your application hence, hence insuring you get the best value for your money spent.

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Note the following discussion is intended as a very brief overview of some common technical terms, definitions and concepts. It is not intended as a comprehensive highly technical discussion. If you would like more in depth information, we would be happy to provide it – feel free to call.

Axle strength and durability is determined by several factors. The major ones are:
1) Size
2) Material
3) Design
4) Heat treatment

1) Size is the easiest concept to grasp. All things being equal i.e. material, design etc, a larger axle will be more durable. The easiest and most cost effective way to increase the durability is to make it larger. Larger refers to the diameter although the surface area of the cross section is what really counts. A perfect example is upgrading a Land Rover with 10 spline axles to 24 spline. This increases the diameter approximately 25% although the cross section surface area increases by almost 75%. In the real world, there are dimensional limitations to just making axles larger. Examples are the diameter of the spindle/stub axle and the carrier bearing jounal on the differential carrier. When you get to this point, it is usually more cost effective to increase the durability of the shafts by increasing the quality of the material, utilizing a better design, changing the heat treatment package or a combination of all or some of them. The reason is that building larger diameter custom spindles and machining drive hubs to accommodate larger wheel bearings becomes much more expensive than just building a better axle shaft. Using the above Land Rover example, a 24 spline axle shaft is the largest shaft that can fit in a stock spindle, without boring the spindle to a larger size. Although this seems like a minor modification, in reality you are removing material from a critically important part of the vehicle. A competitor of ours claims that boring the spindle does not weaken it. This defies logic. If the spindle breaks, the wheel/tire falls off! Making one part stronger by making another part weaker seems counterproductive to us.

2) Material is a very effective method for increasing the durability of an axle shaft. Most OEM axle shafts are designed to cope with the conditions that the designers of the vehicle envisioned the vehicle would be used. This usually doesn’t include aggressive recreational off roading with modified vehicles, which results in limited life spans of stock axle shafts. Most stock axle shafts are made with “decent” material but there is certainly a lot of room for improvement. Most stock axle shafts are manufactured using commercial grade medium carbon case hardening steel such as 1040 or 1050. Higher quality steels start by adding certain alloys to these base steels. An example is 1541H. It contains a healthy dose of manganese, which increases the “toughness” of the steel. The next level are what we call the alloy steels. An example is your Chrome Moly steels such as 4140 or 4340. The ultimate materials are the Aerospace Material Specifications (AMS) steels. These are alloy steels that have additional manufacturing steps added to them along with additional elements such as Silicon, Chrome, and Vanadium etc. Examples of the additional treatments are things like a second refining, vacuum melt refining. Part of the objective of these additional steps is to reduce the impurities in the material, which dramatically improves the fatigue qualities. Making the heat treatment more consistent is another objective and the reason for the addition of materials like Vanadium. Examples of these types of material are Hy-Tuf (AMS 6418) and 4340 Modified 300 M, which is usually referred to as 300 M (AMS 6419).

3) Design is another very effective method of increasing axle shaft durability. The two different designs are waisted and non-waisted. In most applications a waisted design is a superior design. What waisted means is having a section of the shaft that has a smaller diameter than the minor spline diameter. The objective of this is to give the axle a larger section that it can twist instead of concentrating this twisting on a very small section of the shaft. The ability to twist allows the shaft to absorb greater torque and shock loads. We have done a substantial amount of development using both computer modeling and destructive testing and have discovered that the design of the waisted portion of the axle shaft is critical to gaining the maximum benefit of from it. Factors such as the length, diameter and radiuses have a significant impact on the effectiveness.

Other design features that can be integrated into a shaft design are one piece models (piloted integral drive flanges) and extended spline lengths for two piece shafts. There are some poorly designed axle shafts on the market that utilize inferior designs such as unpiloted integral flanges.

4) Heat treatment – The heat treating process basically involves two different steps.

1) Heating the material to a very high level. Metal has a crystalline grain structure, which in a un-heat treated state are large and inconsistent, known as Ferrite for steel alloys. As you heat the material the grain structure changes. The Ferrite grain structure evolves into a state know as Austinite, which is tighter, more consistent, medium sized crystals. Eventually it turns into Martinsite, which has the smallest, tightest and most consistent grain structure.

2) Rapidly cooling the material to “lock in” the desired grain structure. If you don’t rapidly cool it and let the material cool slowly, the Martinsite returns to an Austinite state and it eventually returns to the Ferrite state.

The object of a heat treating is to alter the grain structure of a metal to change its properties to enhance a specific performance characteristic. Usually the reason is to increase the components durability and/or wear characteristics.
Heat treating is generically referred to as hardening as if there is one process that is applied uniformly to all axle shafts. Or what makes an axle shaft – “heavy duty” is to “harden” it as if a stock axle shaft in unhardened or un-heat treated. In reality all axle shafts are heat treated or hardened, including stock axle shafts. If they weren’t, they probably wouldn’t make it out of your driveway. The question becomes what heat treatment processes you utilize and to what extent. Different materials require or respond different types of heat treatments. You can even combine different types of heat treatments. The two major types of heat treatments are thru hardening and case hardening. There are also different methods of case hardening, such as induction or quenching. Some materials can be heat treated in either way and some materials can only be heat treated, one way or the other. As an example, to case harden a material, requires a minimum percentage of carbon to be in the material, hence some low carbon specialty alloy steels can only be thru hardened.

Although you can measure the level or hardness of a material using several different scales, the most commonly used one is the Rockwell C scale. It is expressed as an example RC 55. Another important measurement of the case hardening heat treatment process is Depth of Penetration of DP. This refers to how deep the target heat treatment is into the material. It can either be expresses as a % or a specific distance.

Generally speaking, a harder steel is a stronger steel although you try to achieve a balance between the hardness and the ductility because harder steel can be more brittle. You also want to avoid certain ranges with certain materials as the resulting characteristics are not favorable. One such condition is referred to tempering embrittlement. We utilize several different types and combinations of heat treating in our axle shafts to maximize their durability and utility.
As you have probably gathered by now, heat treatment is a very involved and complex subject.

There are several other less common features you can utilize during the design and manufacturing of an axle shaft. These include:

a) Gun boring – this is drilling a hole down the center of an axle shaft. You essentially end up with a hollow tube instead of a solid bar. From a structural standpoint tubes have some advantages over solid bars. It is called gun boring because it needs to be done within very precise tolerances (.0001”/ft) and it should be done with the same equipment that is used to bore the barrels on rifles, hence the name. It is very expensive and is not usually found in the recreational 4X4 market for this reason. A second reason to use hollow axles is weight reduction, which is not an important factor for off road vehicles.

b) Cryogenic treatments – cryogenics came out of the space program. Objects in outer space are subjected to very low temperatures. When they came back people started to notice that certain materials and components exhibited different characteristics. The primary benefit of cryogenics is to remove residual Austinite from heat treated steel. No heat treatment process is perfect so essentially cryogenics is helping to reduce these imperfections. This makes it more consistent with better fatigue qualities. The benefit of cryogenic treatments is inversely proportional to the quality of the material and quality of its processing i.e. superb material, heat treated perfectly gets almost no benefit from it whatsoever. On the other had components such as cast iron brake rotors receive dramatic benefits. The cryogenic industry is unregulated hence there are many providers making wild claims of durability increases in the 200% + range for axle shafts. We think that is nonsense. Our research indicates durability increases more in the range of 10%. This may not sound like much but on the other hand it may be the difference between a component breaking or not.

c) Polishing/grinding vs lathe turning – if you watch an axle shaft be turned down to size on a lathe, it can look like and effortless process sort of like a hot knife thru butter. In reality, on a microscopic level, the material is being torn from the bar! This creates small stress fractures or stress risers. Under heavy usage such as an axle shaft twisting under high torque and shock loads, these small stress cracks can turn into medium sized stress cracks and eventually large stress cracks! The vast majority of axle shaft breakage is the result of fatigue not overload shear. Stress crack/risers are one of the first place fatigue starts.

The benefit of grinding an axle shaft to size is that grinding does not create these microscopic tears in the material, hence giving the shaft better fatigue qualities. The disadvantage of grinding an axle shaft to size is that it takes a larger quantity of machine time hence it is very expensive. A ground axle shaft has a mirror finish because it essentially has been polished. The very best axle shafts are manufactured using this method.

d) Shot peening – This is an external stress relieving process that compacts the surface making it denser and smoother. It is of most benefit when performed before heat treatment because it makes for a more consistent heat treatment.

e) Bead Blasting – similar to shot peening but not quite as effective since it does not have as much compaction.

f) Finer spline counts – If you take two different axle shafts with the same outer diameter (major spline diameter) but one has a finer spline count, it will have a larger minor spline diameter. Having a larger minor spline diameter is a good thing especially for an un-waisted design of axle shaft since it will be the shafts smallest diameter. It even has implications on a waisted design because it effects the diameter of the waisted portion of the axle shaft i.e. it can be larger.

A finer spline count also has more surface area so spline wear is reduced.

A common misconception is that the larger the spline count, the larger the diameter of the shaft. This is not the case at all. In the Land Rover universe, a 32 spline is the smallest major spline diameter axle that they make, being smaller than both 10 and 24 spline.

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We have made reference to piloted vs non piloted drive flanges. First a definition, it is the raised portion that of the drive flange that recesses into the drive hub.

It has several functions. First, it centers the flange to the hub although this is actually a minor function. If everything is properly machined the drive flange bolts can provide the same function. Its primary function is to take the shear stress off of the drive flange bolts in the case of the drive flange contacting an immovable object such as a rock. This is CRITICALLY IMPORTANT in a 4 wheel drive vehicle because of the likelihood of this happening. If you do not have a piloted flange, the drive flange bolts take most of the shock load which can fracture them. This frequently results in broken drive flange bolts either immediately or at some point in the future. We earlier mentioned a competitors axles that had this poor design. This brand of axles not only does not have a piloted drive flange, they also utilize a spacer that creates a second shear plane, making the entire problem even worse. As we noted earlier, broken drive flange bolts are common with this brand of aftermarket axle shafts, which is the direct result of their poor design.

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The answer is fatigue! Contrary to popular belief, axle shafts rarely from peak overload. The vast majority of the time, breakage is the cumulate result of fatigue. In the normal course of use, axle shafts twist although it is not of a sufficient amount to permanently deform the shaft. You can apply a torque load strong enough to cause the shaft to exceed the yield point. The yield point is the point at which the shaft does not return to its original position. When this occurs, the shaft loses some strength, which means that it will hit the yield point easier the next time a large torque load exceeds this point. This process goes on for hundreds and/ or thousands of times. Where the shaft fatigues depends upon the design of the shaft. A non-waisted design will fatigue at the first small diameter of the shaft i.e. the minor spline diameter immediately outside of the differential side gear.

One of the disadvantages of this axle design is that when the shaft fatigues and eventually breaks, you usually need to remove the differential to retrieve the broken piece of the axle from the side gear. Various schemes are promoted to retrieve this piece such as extendable magnets although our experience is that this rarely works. The broken end of the shaft usually flares out and the piece is firmly stuck in the side gear/bearing journal requiring some serious pounding to remove it.

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One of the most common misconceptions in the 4WD sport is that the most stressful thing you can do to an axle shaft is to install larger tires. This is surprisingly and totally incorrect. People occasionally claim that since everyone believes this – it must be true. In reality, it only means that everyone is wrong. Why is this so? Before I get into an explanation, First here are some definitions to make the discussion more understandable:

Torque – force applied to an arm at some distance from the center of rotation which will create a radial movement or turning force.

Velocity – radial speed

Momentum – something moving or stored energy

Inertia – momentum in a radial motion

The key concept to understand is TRACTION. Here is an example. Visualize this, a vehicle with an open diff is attempting to climb a very steep, very high traction incline in Moab. Part way up the incline, there is a patch of gravel that the left tire will hit. To make the example simpler lets say it’s a 2 wheel drive and again it has an open diff. As long as you have TRACTION all of the torque being generated is being split 50/50 between the right & left axle shafts. At the point, you hit the gravel and you lose traction with the left tire, most of the torque is converted to velocity, momentum and inertia but the torque split remains the same 50/50 even though there is technically very little torque being generated at this point because it has been converted to something else (inertia). This is because an open differential can never technically exceed a 50/50 torque split. The point where forward motion of the vehicle ceases is where there is no longer any torque being generated. To summarize what happened to either of the individual axle shafts – the torque being applied never exceeded 50% of what was available. The same concept applies i.e. no more than 50% of the available torque can ever go to an individual axle shaft no matter what size tires you have. You can increase the TRACTION by increasing the tire size (larger footprint) or reducing the tire pressure (larger footprint) or even better – both but again never more than 50% of the torque ever goes to an individual shaft.

Now lets look at the exact situation with the only difference being that the vehicle has a fully locking differential. At the point the left tire hits the gravel and loses all traction, as long as the right tire retains TRACTION, at this instant 100% of the torque being generated in this situation is being borne by the right axle shaft. What happens if you added larger tires? The larger tires allow you to retain TRACTION longer and hence allows 100% of the torque to be applied to the right axle shaft – longer. So what’s adding the additional stress to the situation – the locking differential or the larger tires. We will answer the question – a fully locking differential!

Many people like simple explanations that a certain axle shaft is good up to certain size tire but most of these simple minded explanations are rubbish.

In conclusion, a larger tire will add more stress to axle shafts but it will always be substantially larger when you add a locking differential to the equation because a locking differential will potentially increase your traction by as much as two times.

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Minor spline vs major spline diameter – the minor spline diameter is the smallest part of the spline, the root. The major spline diameter is the largest part of the spline the peak.

Mean polar diameter – the smallest cross section of an axle shaft, which ideally should not be the minor spline diameter but rather a special waisted section of the shaft.

Square root vs involute spline – Involute spines are V shaped splines as opposed to square splines which and straight sided. Involute splines are a much better design.

Fillet root vs square root spline. – A fillet root spline comes to a point at the root. A square root spline does not and as it name indicates is square. Square root splines are generally used for high torque applications for two reasons. First the minor spline diameter is larger and second, the point at the root of a fillet root spline is an unnecessary stress riser.

Pressure angle – the angle at which the teeth of a splined joint contact. It can be anything from 90 degrees for a straight sided square spline to 30 degrees for an involute spline. Other common angles are 37.5 and 45.

Waisted vs non-waisted axle design – A non-waisted design means that the smallest diameter of the shaft is the minor spline diameter. a waisted design means that a section of the axle shaft is a smaller diameter than the minor spline diameter. The issue with a non-waisted design is that the twisting stress on a shaft is concentrated in a very small area usually the spot immediately outside of the differential side gear. A waisted design is considered to be a superior design because the twisting stress is spread out over a larger area. Our own research has determined that the diameter and length of the waisted portion of an axle shaft is very important to optimizing the waisting.

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