Axle Strength Comparison

[rbattelle]

The January 2005 issue (isn't it only November?) of Four Wheeler has a nice table of strength ratings for some common axles. I didn't scan it as an image because I'm not entirely sure how copyrighting works and whether I'm allowed to scan a magazine image and post it here.



Anyway, the data for the axles we're interested in is (MOT is max output torque, COT is continuout output torque):



AAM 9. 25 Front - 4663 ft-lb MOT (no COT given)

AAM/GM 10. 5 FF Rear - 6242 ft-lb MOT (no COT given)

AAM/GM 11. 5 FF Rear - 8321 ft-lb MOT (no COT given)

Dana 60 Front - 5550 ft-lb MOT (1500 COT)

Dana 70 HD Rear - 8800 ft-lb MOT (2000 COT)

Dana 80 Rear - 10000 ft-lb MOT (2500 COT)

Ford 10. 25 FF Rear - 8300 MOT (2000 COT)

Ford 10. 5 FF Rear - 10660 MOT (2900 COT)



There are some interesting things in this data. First of all, none of the AAM axles had a COT rating. I take that to mean that the COT and MOT for AAM axles is the same (they are rated for continuous maximum torque output). If that's true it would be VERY impressive given how much lower the COT ratings are for every other axle in the list. Second, the list indicates the AAM 11. 5 inch rear is weaker than the Dana 80 and the Ford 10. 5 inch rear. I had been operating under the assumption that the 11. 5 was at least as strong as the Dana 80. Third, in the table in the magazine there's a little "+" next to the AAM 11. 5 and 10. 5 rear ends that, according to a footnote, indicates that higher-rated versions of these axles are available from the manufacturer. Perhaps we have higher rated versions on our trucks?



-Ryan



P. S. FF means "Full Floating". "Full Floating" means the axle shafts don't carry any of the vehicle weight. (I think... someone correct me if I'm wrong please!).

 

[Tony T.]

Full Floating ?

Well Ryan I'll take a stab at it, I also may be wrong, but it would'nt be the first time for me :-laf . I thought full floating might be a different term some use for full time, with out use of manual lock in hubs. I don't know for sure either.





Tony

 

[tgbol]

Ryan

You are right. Full floating axles do not carry any of the weight. They are bolted or splined to the hub. The semi-floating axles have a flange that the wheels bolt onto and support the weight of the vehicle too.

 

[JeepinDoug]

Ryan has it correct, FF or full-floating axles detail the rear axle only. At the end of the axle housing a spindle is mounted to the bearing flange. This spindle holds a hub/drum/rotor assembly in which the wheel is mounted. The axle shaft itself has a small diameter flange on the end with 6 or 8 bolts that mate to the hub. The diameter of the flange is somewhere between 3"and 4. 5" depending on the ton rate.

A semi-float axle has a large diameter flange with the wheel mounting studs.

Here's a couple of pics of FF axle inners;

A GM 14 bolt corp FF axle shaft,

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Same axle SRW and DRW hubs,

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Dana 70 FF shaft removal,

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These are semi float.

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The first three pics were suppied by BillaVista at the PBB.

 

[Gary - K7GLD]

"I take that to mean that the COT and MOT for AAM axles is the same (they are rated for continuous maximum torque output). If that's true it would be VERY impressive given how much lower the COT ratings are for every other axle in the list. "





UMMmmm - I suspect you live in a dream world, or still believe in the tooth Fairy and Santa Claus!



It would take a REAL stretch to *assume* the COT for the AAM axles would be HIGHER than the competition, while their MOT is significantly LOWER...



I think I'd look for, or wait for additional FACTS before leaping to arbitrary conclusions...

 

[Tony T.]

Wow I was way off, oh well never did claim to be a mechanic :-laf I just need to learn to read an not post when not for sure :-laf .



Tony

 

[Joseph Donnelly]

An axle loses about 20% strength when used in a front application, because it runs backwards (except for the high pinion Ford and the AAM that were designed "backwards"). I wonder if this 20% was figured into the above numbers. I also don't know what they found to be the weak points, or if each mfg. gave their own numbers based on their own test methods--probably what happened. That procedure would penalize the more conservative companies. The ring gear on the AAM 11. 5 is bigger in diameter and the teeth are bigger than the D80, so that would not be an AAM weak point.



In practice, except for sled pulling, the failures have been primarily related to contaminated lube (wet or dirty, or decomposed from heat), or poor set up. Dana procedure from the mid 90s was to set up their diffs looser than before then--about . 004" preload which quickly went to zero after breakin. It was fairly common to see a diff with a few thousandths of an inch slop in the carrier, meaning it could slide sideways in the housing. In the old days they used at least . 010" preload. The other failure common to truck diffs comes from rust in the axle tubes. It gets washed into the carrier bearings because lube circulates through the axle tubes.



Neither of these failures is related to ultimate strength. At least we TDR members select high quality lubes and change them regularly. We also tend to use higher lube levels that help keep the axle tubes coated in oil so they won't rust, in addition to improving outer (axle hub) bearing lubrication.



Another failure issue comes from overloading the axle, bending the tubes slightly.

 

[NIsaacs]

In transmission and rear axle applications, sometimes the smaller the tooth, the more teeth make contact, which translates to more torque capacity.



In the big truck market, the trend is to use higher rear axle ratios and direct transmissions. This transfers the torque multiplication to the transmission more so than the rear axle. With the higher rear axle ratios, you get larger pinion diameter with smaller and more teeth for more torque capacity. The down side to this is bigger drive lines and u-joints to handle the increased torque input to the rear axle to get the same output torque of the lower ratio axles.



It is interesting to note the three 10. 5" axles are rated so much different. Makes you want to know where the difference is. Probably, the same guys made up this chart, as the guys that make up gvw's and gcw's





"NICK"

 

[rbattelle]

It would take a REAL stretch to *assume* the COT for the AAM axles would be HIGHER than the competition, while their MOT is significantly LOWER...

True. I also thought of something else. What exactly does "Maximum Operating Torque" mean? Is it the ultimate torque (axle breaks) or the yield point (plastic deformation but no breakage). There's a note at the top of the table in the magazine stating the data came direct from each manufacturer, so I assume there's little or no consistency in how the numbers were obtained.



-Ryan

 

[Yo Hoot]

It should be noted that although Dodge and GM share the same AAM 11. 5" rear axle, the Dodge is speced with a larger tube. Read that in a Four Wheeler that I don't know I still have laying around but the Dodge AAM has a higher GVW than the GM AAM.