3rd pozi

[DGamelin]

While changing the diff fulid I noticed 3 hair line cracks in the pozi housing. Looking at the end that has the helix gears exposed you can see a cracks between each set of gears. Now the truck is getting its third pozi. I made sure to get the new updated versoin, I'm not sure of when they updated it but I wanted the one the 2012 towmax had. I told myself after the first one went bad I would put an open in it. After looking at it for awhile I didn't think it would last as long. The helix type just has a lot more gear surface contact compared to a set of spider gears.

Anyways I just thought I would show you so you can make sure to check when doing a service.

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93 250 2wd xc 354 auto 307k

04. 5 3500 4wd cc dully 373 6speed 173k

 

[Bob4x4]

How are you using your truck to be going though these?

 

[sag2]

Wow, that diff fits MY 03-13 trucks. Average weekly demand is 4 for the entire country. You must have or be doing something way different than the average bear.

 

[DGamelin]

The truck pulls a goosenecks I run from MI to TX a lot. It has has a trailer on at least 80% of it's life. If it isn't working it's not running. It does make continues long runs 800 miles a day. I try to keep the load at 18k or less, it has had a few heavy ones. When the first one went it had 55k on it so it was done under warranty. I did some research and found a grear lube that helped alot. These call for an lube change every 15k if you are working them hard, they need it. Because of the way the helix gears are forced against the housing they make the lube look like metallic paint. I dont see this in the average grocery getter or something that tows a 9k boat to the lake twice a month. I do believe it could use a bigger axle. But we have had big trucks in the shop with this type of pozi and seen the same kind of metallic like logging trucks. Heres a picture of what came out of the axle tubes from the first pozi the first one looked realy bad and a shot of a load from TX.

 

[mfurrh55]

They claim these rear ends are bulletproof, at 80,000 miles my rear end housing cracked.

 

[Bob4x4]

Lol nothing is bullet proof if you use the right caliber

 

[DGamelin]

... ... Amen... ... .

 

[p-Bar]

If it moves it's going to break sooner or later

 

[mfurrh55]

Man you got that right,Hope this one lasts longer than the original

 

[RVTRKN]

DGamelin, is that a 3:73 rear I see on your signature, if your working your truck like you describe, I would go with a 4:10, if in fact it's a 3:73 ratio.

 

[DGamelin]

The sceond one went over a 100k more then the first one. I think the lube helped this. The first one wore out the pockets that the helix gears ran in. The sencond one housing cracked. The new one has more housing. They must have knowing it needed more meat for the higher tow ratings.



I think it should have 4:10's also. I bought the truck used with 18k on it. Just haven't been able to spend the money on a gears change front and rear. But another concern is if I went to 4:10's it would create more torque - twisting force on the pozi unit. (do to mechanical advantage) More gear helps to relieve stress on every thing forward from the rear u-joint but increses stress on the pozi and axle shafts. If the truck is already eating pozi units would more gear increase it's appatite?

 

[brods]

With that gunk in the tubes, how are the wheel bearings surviving?

 

[AH64ID]

What lube are you using?

Is the max tow version still a helical gear style?

 

[DGamelin]

I haven't taken the hubs off to look at bearings, I know shame on me. They are the original, I replaced a seal a on one side a long time ago. They were fine then. I suppose it is time?

Castrol Syngear SYN80-140 I did a lot of research on gear oils and found an article that was published form an independent lab that was hired to find out what gear lube had the best shear rate. They built a machine just for there test. They listed all the lubes they tested and castrol was 20% better then the rest, the numbers were very close on all the rest. They didn't say why or how it works, just it what the numbers where. So I got some, it is not sold in my area so I have to order it in 5 gallon pails from some place in ohio. I could see the difference in the amount of metallic immediately. I check the drain plug magnet after every long run and the amount of metallic dropped by more then half. But to be fair I didn't start this until after the first dif went bad. Because of the amount of metal that was gone from inside the first one it makes me wonder if it was soft housing or maybe not casted right? On the second one I had a smarty set on level 2 (that took out a bearing in the trans) maybe that and the big loads might have did it in. The loads did exceed the diff ratings. The truck is set back to stock now and the new diff covers the load ratings.

Yes they are still helical the new one I have picuterd on top is for a tow max, I have the tow max rear cover also. With that pozi I now have the tow ratings for the size loads I tow. At least as far as the rear axle is concerned.

 

[sag2]

I think I posted photos of a diff unbolted so you could see the helical gears. Without going back to look, I'm thinking you could remove the helicals and run it like an open diff. That is if you don't need the posi.

 

[DGamelin]

I am not sure but I believe If you were to remove the helical gears you would have no drive at all. The gears that the axle shafts slide into get thier drive from the helical gears. The helical gears are pulled buy the housing, they ride in pockets. Because of how helical gears react with each other they will only let one spin so much faster than the other. That's why one tire can spin at a differnt speed in a trun. But if one starts to spin a lot faster then the other it will create drag and start working like a pozi. The pockets that the helical gears ride in is where most of the metallic you see in the lube comes from. The larger the loades the more they wear. There is a great deal of force on them pushing against the housing, I think this force helps to creat some of the drag that slows down the helical gears to help make the pozi affect. Now I might have this wrong the last time I had one of these apart was in the 90's it was a trutrac. We ran them in the front axle of our mud trucks they didn't hold up well on a set of 40" tires.



I Think!!!

 

[sag2]

Went back and looked at my photos. You're correct the diff will just spin around the axle shaft gears. The helical gears act as the spider gears in a standard open diff. Seemed like a good solution at the time!

 

[cerberusiam]

If the truck is already eating pozi units would more gear increase it's appatite?

The deeper gears will increase the leverage which will decrease the TQ applied to the diff and shafts. The engine will not have to generate the TQ peaks to move the load.



The pockets in the diff the gear presses on should not be throwing that much metal if the brake shoes are working correctly. I wonder if the fluid or heavy is causing the brake shoes to deteriorate ending up with metal on metal. Possibly the load coupled with the ratio is hammering the LSD so bad it is failing early.

 

[DGamelin]

I agree with your torque statement. Especially if the loads were only 5000 lbs and the roads were flat, but considering the size of my loads and the amount of time I spend with the throttle wide open going through the Ozark Mountains the engine is already at max torque so added gear is going to mean added leverage, and that leverage would be after the gears. ?? But I would still like to have 410s.

There is no brake issue that I have noticed. Considering I have only put one set of brake pads on and the original rotors are still in good shape. When I leave MI I run until AR for the night, each time I stop I make it a point to walk around and put a hand on every tire, truck and trailer. If something was dragging I would at least smell them, I would think.

 

[cerberusiam]

added gear is going to mean added leverage, and that leverage would be after the gears. ?? But I would still like to have 410s.

Think of the gears as a fulcrum. When you go to a deeper gear you lengthen the lever to the side you are pushing on. It takes less force to move the load when the fulcrum is further towards the load. Less force equals less input TQ to move it so the TQ rise and peak goes down. Not that you can't put the same or more TQ on the axles and diff but that requires more throttle. All things equal, it will take less power to move the load with deeper gears. The strain from th eload is the same no matter what gearing is there, all thta changes is the inout power to do the work.