Broken rear axle !

1941 - 1945, MB, GPW Technical questions and discussions, regarding anything related to the WWII jeep.
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JAB
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Post by JAB » Tue Oct 31, 2006 1:15 am

JAB wrote: I have to disagree. There is no "drive gear" on conventional differentials. I think this is one of the most often perpetuated myths concerning differentials. The reason the right rear usually spins (looses traction) is related more to the fact that the driver is seated on the left side (more weight) or due to road surface conditions or that the vehicle is turning a tight right (right side "lifts" allowing traction to break) more than any other internal friction factor. Look at how the carrier assembly is built, (not including posi, etc. units) & you'll see that no side is favored over the other. They each have their own side-gear which meshes with other gears that are nestled in the carrier. When the carrier rotates the side gears rotate with it but at varying speeds comensurate with the load. If you play with it on the bench it will look like one side is going backwards; while it's possible to do that, in-use it's usually that both sides are actually rotating forward but that one side is rotating at a much slower rate than the carrier itself, which gives the appearance that it's going backwards. Which wheel slips on an "open" rear-end is determined by the load/traction condition, not which side it's on. It is a length thing as all other factors are essentially equal from the mechanical standpoint.
Larry E Long wrote: I never mentioned a "drive gear" and have no idea where that came from.
No, you mentioned "main drive wheel" (your quote below). In the context of this discussion I used that term as meaning the same thing as I was trying to move the discussion from the wheel to the differential & axleshaft.
Larry E Long wrote: Due to the direction the driveshaft rotates the differential gears, the right rear will always be the main drive wheel going forward, unless you have a powr-lok installed. If you've ever seen anyone pop the clutch on a standard rear wheel drive car it's always the right rear tire that spins (posi-traction etc. excepted). Same thing if you're stuck in snow or mud. The RR axle shaft takes the majority of the torque therefore it becomes the one prone to breakage. The length of the shaft is a non-factor.

Larry


Larry E Long wrote:

The driver sitting on the left is irrelevant. Playing with it on a bench has no relation to actual road use but what you're describing is the function of the spider gears and that is relative to driving around corners. It's a matter of physics and how the turning of the gears applies torque to the differential and rear suspension. As the clutch is engaged torque is applied to the driveshaft which in turn applies it to the pinion shaft. When the pinion gear transfers that torque to the ring gear, however, there is a secondary effect which is torque twist. The differential and suspension are twisted. It's similar in effect to what happens on the front axle of an MB when the brakes are applied hard which twists the axle which causes a steering problem. That's why the front torque spring was added. As the rear differential suspension is twisted there is an increase in downward pressure on the left rear tire and a decrease in downward pressure on the right rear tire. The power being transferred from the motor to the driving surface will follow the path of least resistance which is to the right rear tire. That is why, on open differential (no powr-lok etc.), leaf sprung, rear wheel drive vehicles the right rear tire spins first............Also, remember that a spring leaf suspension is entirely different than coil spring, trailing arm or any type of independent suspension and reacts entirely different to applied torque.

I agree with much of what you say as far as physics & the effects that a leaf spring type suspension has on traction, especially in abusive situations as you describe, but to me most of your statement can be condensed into the words "load" & "traction". Irregardless of the reason, (effects of the suspention type, direction you are turning, weight bias in the vehicle, etc.) load (static or dynamic) & available traction combined with applied power determine which tire, if any, will slip. Obviously in a driving situation this is constantly changing & a drag race scenario is one excellent, but extreme, way to observe it in action. Although I didn't go into the degree of detail that you did & I completely ommitted the effects of the type of suspension could have I essentially stated that the tire with the least traction relative to power applied would break free & spin first. Pretty much what you & JP have stated although in much simpler terms.

Larry E Long wrote:The reason the axle breaks is because each time the tire spins loose, even a little, the torque to the splines is increased significantly at the moment the axle regains traction. After a while the splines get twisted, cracks occur, and eventually it breaks, and there is no predicting when or under what load that will happen. That does not mean that a left rear axle can't break but it's far less likely. There are situations, however, when a left rear axle can become stressed. If you are stuck in snow and you rock the car back and forth by shifting from forward to reverse and back again you can alternately stress one side then the other. If you stop at a red light on a hill and go to start again the vehicle may roll backward just a bit before you engage the clutch. That changing from one direction to another can stress the left rear axle as the afore-mentioned differential twist plants the left rear tire. I've also seen axles break because there was a defect in the metal.



Larry
I agree 100% with that statement.


Where we seem to disagree is to what degree (if any) the effects of the axle twisting under load has on it's life expectancy & how it's length relative to the other axle apply & how Articifer & I try to explain it. Are you saying that the axle is so stiff that if it twists any measurable amount it will break? I don't think you are, but neither am I saying it twists like a rubber band tied to a toy airplane propeller. Common sense was mentioned a few times here, as was the term "all things being equal". In my explanation what I meant by "all things being equal" was the load & traction situation. If the load & traction are equal on both sides of the jeep then the only variable would be the length of the axle shafts. If the axle does indeed twist within a pre-engineered acceptable range for a certain number of cycles....the shorter one would fail first as there would be less length to "absorb" that given amount of twist. Your drag racing example & the video clip JP supplied is a good way to describe how it all works under hi-horsepower situations in leaf sprung vehicles but would it really apply to a low horsepower vehicle such as our jeeps as far as the effects of so much torque "the left wheel lifts off the ground"? I'm not so sure that it really applies here to the degree that you imply. I still think the twisting throughout the length of the shaft is a bigger factor in this situation but am always open to other ideas if convincing evidence is presented.

Are there other vehicles with substantially different axle lengths besides jeeps & Landrovers? What are their failure rates relative to axle length & which side the long (or short) one is on?

I guess we're at the point where it's time to search the web for more evidence to support our positions or just let it go :lol:
-Jeff

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REG
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Re:

Post by REG » Thu Nov 07, 2019 2:25 am

artificer wrote:
Mon Oct 23, 2006 1:34 am
See the nice shiny seal surface near the axle splines on Mark's pics. And just $31!!!
John, I have a set of long and short rear axles as emergency spares.
They are pitted but the splines are clean and sharp.

However the 'nice shiny seal' you mention isn't and ideally I'd like to get them polished or, if need be, sleeved.

a. Is it worth it (the pair were a good price, for obvious reasons)?
b. What are the specs of the 'seal' part, please?

Many thanks
REG
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Chuck Lutz
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Re: Broken rear axle !

Post by Chuck Lutz » Thu Nov 07, 2019 11:02 am

Artificer is on "vacation" so an email might be a better way to ask his opinion on this.
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REG
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Re: Broken rear axle !

Post by REG » Mon Nov 11, 2019 10:01 am

Chuck Lutz wrote:
Thu Nov 07, 2019 11:02 am
Artificer is on "vacation" so an email might be a better way to ask his opinion on this.
Oh dear, ta Chuck :roll:
REG
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Re: Broken rear axle !

Post by Joe Gopan » Mon Nov 11, 2019 10:36 am

Other than a sudden shock casing the skaft to break, metal fatigue via exceeded elastic and plastic limit takes place. It just gets tired.
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