i dont understand your logic here... with cut splines the minor diameter of the shaft is usually at the root of the splines....most people prefer the shaft not to break at the splines because it can screw some expensive stuff up.

i dont feel like typing anymore...you might want to check out billavistas axle tech article on pirate...he explains it alot better than i can

 

The diamater of the axle shaft on the bottom of the splines is going to be the same weather its cut splines or rolled splines?
The difference I was stating is with rolled splined the diamater of the axle shaft is smaller then a shaft with cut splines. The shaft has to be thinner so when the splines are rolled in its "squezed" out to the right diamater

 

Summary

So to briefly recap:

* Energy trasmitted into a shaft has to go somewhere - it can't just dissappear
* Stress likes to flow, and when it bunches up, it concentrates - possibly to levels so much greater than the input stress that the part breaks
* Axle shafts, by necessary design, will have section changes - resulting in stress concentrations.
* Shaft diameter has huge effect on torsional deflection of the shaft (How much a solid steel shaft will twist under load is dependant on the diameter of th shaft raised to the power of 4.

What does all this have to do with the "ultimate" shaft?

Well, ultimately, how "strong" a shaft is, how much abuse it can stand, how much load it can carry is very much affected by the shafts profile - specifically in the following areas:

* The Spline Stress Raiser
* The working diameter of the shaft
* The equalization of torque

Let's examine each in turn:

The Spline Stress Raiser and Working Diameter

Every splined axle shaft has at least one huge stress raiser by virtue of it's design. The point at which the splines end is natural stress raiser. Why should this be so? Imagine a wheel wedged between 2 rocks, but the throttle pressed and torque applied to the axle shaft. The splined portion of the shaft, engaged in the carrier, is, in a manner, held fast. Because the wheel is held steady, the shaft will immediately twist. Because the force is input via the carrier, we can instinctively imagine how the stress would concentrate where the splines end. The concept would be similar to the following example: chuck a thin stick of wood vertically in your vice, sticking up a couple of feet. Apply a force to the top of it by pushing, and you already know where it will snap - right where the jaws of the vice clamp it. Same deal with the splines.

Now, admittedly the torsional stress an axle shaft experiences is significantly more complicated (being a combination of tension, compression, and shear) than the simple tension or shear stress examples we have been using to illustrate concepts - however, we can still use these simple models to understand the concepts.

Here's what Carroll Smith has to say, in his usually extremely blunt fashion, about splined shafts profile, particularly in the area of the end of the splines.

"Splines MUST be placed on a diameter that is greater then the operating diameter of the part, or they must be eliminated from the design. Spline roots must never be allowed to blend into the operating diameter"

So, in order to deal with the spline stres raiser, the textbook way to build an axle shaft is to make the operating diameter of the shaft smaller than the diameter of the splines. This not only serves to reduce the stress raiser at the root of the splines, but, if we re-examine the equation for torsional deflection, we can easily see how this reduced diameter also allows the axle shaft to twist more. Recalling how energy is converted, this twisting allows the shaft to convert the energy (mainly into heat through internal friction) as opposed to just transferring it where it would break u-joints, ring and pinions, driveshafts, transmission outputs, etc. As we dicussed under material - the key is then to build the shaft from a high enough quality material to be able to handle this twisting comfortably withing the elastic range.

This "reduction of shaft diameter after the splines" is often commonly referred to (and often with a negative connotation) as "neckdown". This proper neckdown IS NOT to be confused with what the OEM's do (e.g. notorious D44 front axle neckdown). That OEM Spicer neckdown is done either as a manufacturing convenience (to facilitate quick, easy splining) or to introduce an intentional design weak point (so that if the ring and pinion fails/siezes in 4wd the shaft breaks and the wheels remain free and steerable as opposed to the front wheels locking up) - depending on who you ask.

Note that there is a discrepancy between this neckdown rule, and making the diameter of the shaft as large as possible for maximum strength. Like all design - there is a trade off and a compromise must be reached. With this in mind, consider that when building a stock-replacement axle to fit into existing carriers, the manufacturer is limited to an existing, exact spline size - thay can't make it larger or smaller - the splines have to fit the carrier. So, in order to obey the neckdown rule, they would have to make the shaft, in the case of a D60, smaller than 1.50". This may well cross the line of diminishing return. In other words, when the final engineering alanlysis is done, it may turn out that keeping the shaft operating diameter at 1.50" actually imparts greater overall load carrying capacity/strength to the shaft than would obeying the neckdown rule.

Add in the strength of the materials, as well as the cost/complexity factor of manufacturing an axle with a neckdown and we begin to get a clearer picture.

There are, in fact, no aftermarket companies that I'm aware of making replacement shafts for Dana axles with this particular profile feature, the proper neckdown - not even Superior. I asked aorund amongst all my contacts and tried to find out why - nobody's talking. I suspect that the added complexity and expense of this design, especially when balanced aginst the wide design margin afforded by the high quality materials (heat treated 4340) in use amongst the top companies, and the trad-off in strength vs shaft diameter, when coupled with an analysis of the shafts required duties (in a rock-crawler, as opposed to darg car etc.) that the decision is that it's just not worth it.

I do know, however, that certain axles built by Sandy Cone and other suppliers to the big $$ trophy trucks have the textbook neckdown profile - it's just not common on front D60 4x4 shafts.

The concept of equal torque.

There is one more important aspect of axle shaft profile we should examine before moving on, and that is the concept of equal torque.

Imagine climbing a steep, rocky climb. You need to get your foot into it, and as result you're hopping and skidding all over the place with the weight bouncing around and the tires alternatly spinning and then suddenly grabbing traction. This is a nightmare for axle shaft survival - but a scenario we commonly encounter and expect our parts to survive! In this scenario, or any other high-load situation, the axle as an assembly has the greatest chance of survival if both shafts survive. For both shafts to survive - the best scenario is for them to share the abuse / stress; to participate equally in sharing the load.

However, there's one big gotcha - almost always one shaft is dramatically shorter than the other. In my own GM Dana 60, the long side shaft is 35" long, or about 50% longer than the 18" short side shaft. The problem with this becomes evident when we recall once agaibn our friend the angular deflection equation (math is soooo cool :-)

alpha=[584(T)(l) ] / D^4 * G

Where:

alpha = the torsional deflection in degrees (how much it twists)
T = Torsional or twisting moment in inch pounds (torque or load placed on the axle)
l = length of the shaft in inches
D = diameter of shaft in inches
G= torsional modulus of elasticity (a constant for all steel shafts at 11,500,000 psi )

In my front axle in any given "all wheels driving" sitaution, for each shaft G and T are equal, or about equal, and l is quite different: meaning that if D is constant between the 2 shafts, the longer shaft will twist more than the short. This will normally allow the energy to be dissapated better in the longer, greater twisting shaft than in the short shaft - this explains why quite often we see more broken short side axles than long. The converse is also possible, depending on the nature of the loading on the axle (magnitude, axle material, etc), the long side axle may fail prematurly because it twists so much more than the short side. Either way this explains why some rigs perpetually break one side or the other.

Regardless, the optimum situation is to have both shafts share the load and twist equally - therby creating the most reliable and predictable assmbly. This concept is the concept of equal torque. On last look at the equation, and we see that for either shaft of any given axle we can't change l or G, and we have no real way to predict or control T (except let up on the loud pedal - and what fun is that?) so the only way to achieve equal torque loading of the shafts is to alter D between the shafts. Note that D is raised to the fourth power so that small changes in shaft diameter have relatively large effects on how much the shaft will twist.

A well made axle shaft will take this into account. Again, it will come as no surprise that Superior do, in a process they call their "exclusive (patent pending) Torque Equalizing Diameter Profile"

OK, so if we understand material, heat treatment, neckdown, operating diameter, and torque equalization, it's time to consider the last factor in shaft design - splines.

i tried to lpost a link to this article, bu tit wouldnt let me, this is a section of billavistas tech article on axle shafts from pirate