Anyone with a birf available for a quick measurement? (1 Viewer)

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IdahoDoug

IdahoDoug

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I'm planning on a birf repack and while it's apart am considering having the outer shaft drilled lengthwise so I can pump grease from the axle tip directly into the birfield. Before I head off to the local machine shops to see what this will cost, I need to know the total length of the assembly, so I'm hoping someone can secure the following measurements:

Total length of the outer axle and steel "tulip".
Estimated length of the outer axle alone for drilling purposes. By this, do your best to estimate the length of the axle itself (w/o steel tulip).

Thanks!

DougM
 
doug

birf overall 243mm
outer axle to top of abs ring 167 mm
abs ring depth 11mm
depth to centre of inner bell from bellend 64mm

do the math and it seems like the inner bell terminates almost at the abs ring.

damn now my hands smell like birf grease
 
Now that is a great idea !! Will drilling the core decrease its overall strength ?
 
Semlin,

Thanks - exactly what I am looking for. So, if I'm visualizing this right, the actual length of the drilling operation will be 243-64 or 179mm's. I wouldn't think this is rocket science, but I'll see what the local shops can do. Wish I were in Holland, MI where Scott could do this in his sleep!

As for strength loss, I've heard that it will not materially reduce torque capacity of the outer axle. A measurable amount, yes, but the outer diameter is where the strength comes from. One engineer even posited that the slightly more elastic outer axle might reduce peak loads a bit and help save a birfield from grenading. Pure speculation without a graph, IMHO.

I plan to play with it by greasing it a couple times on the bench to get an idea of how many tubes it takes to nearly flush the birfield, etc. On the truck, I plan to remove the fill plug while the actual greasing takes place (with my already determined quantity going in) so the old grease has somewhere to go. It will be interesting to see if I can come up with some way to get the grease to exit deeper into the birf, such as using a copper tube to place the actual grease exit in a better spot to maximize old grease evacuation. I cannot recall if there's any clearance for something like that, and don't want to add complexity that could lead to further complications.

Overall, I look at this as something that would let me increase the repack interval by, say 30%. It's not a substitute as that inner axle seal's gonna go at some point, and the knuckle bearings also will need a repack. But if I've successfully realigned the axle by my last repack shimming so the inner axle seal is perfectly centered, PLUS do this, then I may have something that can last even longer. Time will tell.

It was a wierd grease day. I have two hand grease guns and needed to repack both 80 drive shafts with red Mobil 1. Then switch to conventional moly for the 97's knuckle fill via square plug (half tube each). Then switch to Amsoil moly for the 93's knuckle fill. Then switch to blue aluminum based grease for the Mercruiser Bravo III's outdrive fittings for the gimbal bearing and main shaft splines. Got all done and managed to waste only minimal grease - saved $300 at least in doing this myself vs paying a marina and car dealer X2, eh?

DougM
 
By trying to change all the grease you will overfill the housing without a way to remove the excess? A better plan maybe to just add a few pumps every oil change to put some fresh grease right where it's needed?
 
I wouldn't do it. You will turn it into a giant banjo bolt. Ever twist one of those in half?
 
Good point on the overfill potential. I guess the downside of that is thermal expansion of the grease may force it past the inner axle seal vs having it weep out the birf sweep triple seal. Hmmm.

Dan - I was thinking of an orifice on the order of an eighth inch - obviously at the axle's center. Do you think it's going to affect strengh that much? Anyone else have an informed engineer's opinion on the strength impact?

DougM
 
I've read somewhere (could be fairy tale though) that hollow axles are stronger (from a twisting perspective anyway). yeah that's it....... not a fairy tale it was great basin rovers.....


http://www.greatbasinrovers.com/catset.html

from their extreme duty axle section.............

EXTREME DUTY REAR AXLE KITS (30 Spline)
This kit includes larger axles than stock (1.315") and a larger spline count (30). These axles are manufactured from the finest aviation grade steel available, are fully waisted and bead blasted. This kit requires a conversion kit to replace the side gears in the differential housing and also require the drive spindles to be bored slightly oversize (+.020"). We can supply the side gear conversion kits for both ARB and Detroit Locker differentials. In addition we can provide spindles bored to the correct size on an exchange basis. This kit also comes with our special drive flanges. We can also supply these axles "gun bored"**. This is a process where a hole is bored down the center of the axle. There are two advantages to this, first the axle is lighter reducing the unsprung weight of the axle components, which is not particularly an advantage in a four wheel drive and secondly the axle is significantly stronger. This may seem like a contradiction, removing some of the axle mass and ending up with a stronger axle! The concept is that the axle now has 2 surface areas that need to break for a failure to occur. The best way to illustrate this is by an example that everyone is familiar with - a hollow tube drive shaft is stronger than a solid one!
 
For what its worth, I wouldn't do it either though
 
With the outer diameter remaining fixed, drilling the center does not increase the strength of the shaft. It reduces its strength. I don't imagine a small hole will be significant as far as overall birfield strength. Has anybody broken the outer shaft without first breaking the other end of the birf?

It is correct that pumping grease in to the birfield can have the same effect as pumping in through the existing fill plug - the knuckle cavity can be overfilled. Certainly wouldn't be able to do a "grease exchange" without first filling the cavity.

I do expect that pumping grease through the shaft would better lube the joint than pumping in through the knuckle plug.
 
Last edited:
It's just too damn much fun taking everything apart......twice. What's the life expectancy of an inner axle seal ? They look a bit flimsy and I wonder if they need to be changed out at the same interval as the birf/knuckle service. So maybe one would need to get in there anyway to change out that inner axle seal ?
 
Beast,


Anyone doing a birf/knuckle service without changing the inner axle seal is simply wasting time. IMHO, with today's modern synthetic birf lubes the inner seal is THE reason to do the entire operation. It has a limited life and when it's over, there's cross contamination between the thin gear oil in the diff and the thick grease in the birf.

Any other thoughts on drilling the outer shaft? You guys still haven't talked me out of it.


DougM
 
Beast,


Anyone doing a birf/knuckle service without changing the inner axle seal is simply wasting time. IMHO, with today's modern synthetic birf lubes the inner seal is THE reason to do the entire operation. It has a limited life and when it's over, there's cross contamination between the thin gear oil in the diff and the thick grease in the birf.

Any other thoughts on drilling the outer shaft? You guys still haven't talked me out of it.


DougM
 
Rich said:
With the outer diameter remaining fixed, drilling the center does not increase the strength of the shaft. It reduces its strength. I don't imagine a small hole will be significant as far as overall birfield strength. Has anybody broken the outer shaft without first breaking the other end of the birf?

It is correct that pumping grease in to the birfield can have the same effect as pumping in through the existing fill plug - the knuckle cavity can be overfilled. Certainly wouldn't be able to do a "grease exchange" without first filling the cavity.

I do expect that pumping grease through the shaft would better lube the joint than pumping in through the knuckle plug.
Click to expand...


Why do you think the drilled axle would be weaker?


from great basin rovers:
We can also supply these axles "gun bored"**. This is a process where a hole is bored down the center of the axle. There are two advantages to this, first the axle is lighter reducing the unsprung weight of the axle components, which is not particularly an advantage in a four wheel drive and secondly the axle is significantly stronger. This may seem like a contradiction, removing some of the axle mass and ending up with a stronger axle! The concept is that the axle now has 2 surface areas that need to break for a failure to occur. The best way to illustrate this is by an example that everyone is familiar with - a hollow tube drive shaft is stronger than a solid one!
 
The drilled shaft will be weaker because there will be less material. It is the material that provides the strength. The quote you posted "The concept is that the axle now has 2 surface areas that need to break for a failure to occur" is totally bogus. It is an irrational statement. The empty space inside a shaft does not provide strength. Drilling of course provides a weight savings.

Also, the quote "The best way to illustrate this is by an example that everyone is familiar with - a hollow tube drive shaft is stronger than a solid one!" is also totally incorrect. Take two shafts made of SAME MATERIAL and SAME OUTSIDE DIAMETER. The hollow one is weaker than the solid one in terms of torsional, tensional, and compressive strength.

Don't take engineering lessons from great basin rovers!
 
i've never heard of anyone breaking the shaft and doug is not an extreme rockcrawler so this all seems to be much ado about nothing. The fact it works on the mine trucks is enough for me stength wise.

the real question is how much maintenance does it avoid. We repack birfs for three reasons: (a) we think the grease is worn out (b) the inner axle seal has gone and is leaking or (c) we need to service something else in there.

With this system (a) is prolonged forever, and (b) is prolonged until you actually see the diff oil on the knuckle ball (anywhere from 100-160k) and even then you could battle it for a while by adding grease. That leaves (c). I would say the only reason to go in there would be to replace the knuckle bearings. A 120k interval for that seems reasonable to me.




the mine truck experience is pretty useless for that.
 
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