790 blows gears in front end

[tallyho8]

My neighbor has a JD 790 that has just blown the front end gears for the 4th time in 4 years. He tears up the gears labeled #8 and #10 on the diagram.

I have told him he needs to load his rear tires so the rear end will work harder and take some pressure off the front end but I don't know if it is possible to set the backlash on these gears because maybe they aren't meshing properly making them easier to strip.

Any ideas to help him stop tearing up these gears?

 

[zzvyb6]

What tires and wheels are on the machine, and what pressures have been checked on it? If the loaded front and rear wheel circumferences don't match, then there will be strain on the drivetrain.

 

[pmsmechanic]

The front wheels on a front wheel assist tractor are always pulling a little more than the rears so that the front end can stay ahead of the rear around a corner. So if your tires are factory size and inflated to the proper pressures then I think the problem is in the gear box. The parts book doesn't show any but I would buy some shims and use a gear marking compound to check the wear pattern and I would shim the gears to about .008-.0012" backlash at the tightest spot. I would also check the fit of the gears on the splines of the shafts and make sure the shafts aren't excessively worn. Is there any play in the shafts or bearings when they are assembled? Is the housing thoroughly washed before reassembly? Are the bearings tight in the housings? Are they the right bearings? Are you using synthetic gear oil?

 

[kennyd]

Does he have a loader? If so does he use it often? How about REAR ballast? Filled tires are great for stability and traction, but do little to unload the front axle when a loader is used.

 

[tallyho8]

It's a 4 year old tractor with the original tires and he keeps the pressure at recommended levels. He has the stock loader and the only time he strips gears is when the loader is loaded and usually in reverse.

 

[kennyd]

Ballast?

 

[tomd999]

Hiya,

It might be a long shot but since the spindle gears are located higher than the axle shaft, is the oil level checked regularly throughout the year? I have found that after a refill, it takes several days of running to stabilize the level. Since Deere specs hy-guard for the front, I would substitute a full syn 75-90 GL-5 gear oil. It's not that much more viscous and the high pressure additives may assist.

The point about having proper ballast to unload the front is correct, consult your loader manual and remember the suggested weight is considered minimum, add a little more if you can.

Tom

 

[dirt ditch]

I would not use Hy-Guard in the front axle of a 790. The operator's manual for my 2007 790 lists John Deer GL-5 SAE 80W-90 as the MFWD front axle oil. Synthetic 75-90 GL-5 should work fine.

 

[Dutch445]

It's a 4 year old tractor with the original tires and he keeps the pressure at recommended levels. He has the stock loader and the only time he strips gears is when the loader is loaded and usually in reverse.

definately a call to get some rear ballast, and not just in the wheels , but on the 3ph. a good heavy ballast box would reduce a lot of
the extra forces on that front axle. and it sounds like it's happening with a full bucket, going in reverse, all of that weight of the tractor
and now the loader is concentrating on the front end, and maybe he does this often?
also, those tractors were discontinued after 2008, so it's a little more than 4 yrs old, what condition are the tires in? are they
the R-4 , turf or R-1 tread?

 

[pmsmechanic]

I'll disagree on the ballast and I'll tell you why. The gentleman has problems with his gears not the wheel bearings. While adding ballast to the rear definitely takes weight off the front axle all you are really doing is adding stress to the gears. Once the rear is ballasted then all you have is four tires that can't slip to release gear pressure when you turn corners etc. The best suggestion yet is to not use front wheel assist when you don't need it.

 

[Dutch445]

I'll disagree on the ballast and I'll tell you why. The gentleman has problems with his gears not the wheel bearings. While adding ballast to the rear definitely takes weight off the front axle all you are really doing is adding stress to the gears. Once the rear is ballasted then all you have is four tires that can't slip to release gear pressure when you turn corners etc. The best suggestion yet is to not use front wheel assist when you don't need it.

then i guess all this propaganda from deere in our sales information is all a bunch of malarchy
per the JD Ballast information:

Why is ballasting necessary?
Proper ballast maximizes a tractor's overall stability and productivity by keeping all four wheels on the ground. For instance, when doing loader work, rear ballast provides a counterweight for material in the loader bucket. This counterweight keeps the most powerful part of the tractor's drive system, the rear wheels, planted firmly on the ground.

In addition to improving the tractor's performance, proper rear ballast will also maximize the life of the front-wheel-drive axle as well as other drivetrain components. For tractors equipped with loaders, signs of improper rear ballast may include premature tread wear on the rear tires, leaking front axle or pinion seals, and premature failure of drive line and front axle components.

Similarly, when using heavy 3-point hitch implements, it is important to have adequate ballast on the front of the tractor to optimize steering and increase traction of the front axle (for tractors equipped with four-wheel drive).

 

[kennyd]

I'll disagree on the ballast and I'll tell you why. The gentleman has problems with his gears not the wheel bearings. While adding ballast to the rear definitely takes weight off the front axle all you are really doing is adding stress to the gears. Once the rear is ballasted then all you have is four tires that can't slip to release gear pressure when you turn corners etc. The best suggestion yet is to not use front wheel assist when you don't need it.

So if the majority of the tractors weight, including what's in the bucket is on the front axle, and it's in MWFD then the small gears in the front diff won't see extra stress? Only the front wheel bearings will?

 

[pmsmechanic]

Oh boy I guess I've started up some controversy again. Dutch 445 first. If you read the article it doesn't talk about slippage at all it talks about using ballast for stability. And I agree that you need a certain amount of ballast for stability. Now think about it for a bit. If the tractor is so ballasted down that the tires can't slip to make up the difference in the distance traveled between the front and rear tires something has to give somewhere. I don't know off the top of my head what the lead is but every mechanical front wheel assist tractor has a certain amount of lead built into the drive train. The front wheels travel slightly faster than the rears for two reasons. One is that the tractor handles better that way and the other is so that the front wheels will pull the tractor around a corner when turning with FWA on. The front tires travel a longer distance when turning a corner.

At this point I read the John Deere article again and realized your point. I could very well be wrong on my theory. This has been my experience. I at one point in time had 1000 lbs of ballast hanging about a foot behind my 4410 three point hitch. I could stick the nose of the tractor down into a ditch and back out with a full bucket of mud but how often do I do that? With that much weight on the back I couldn't dig anything. A 1 inch stone was enough to lift the bucket while digging. That was a pain so I removed 400 lbs and have left it that way ever since.


kennyd I had to read your questions several times before I could see what your asking. You make a good point and I never thought of that when I wrote what I did. I'm coming to the conclusion that both of you were right in calling me on what I stated. After reading the John Deere article and thinking things through a bit I come to the conclusion that you need some ballast on the rear tires to let the strongest part of the drive train do the majority of the work.

I have issues with over ballasting though and I will let my statements stand in that kind of a situation. If I'm driving my ballasted tractor on a dry surface for any distance the fwa gets disconnected. It's the only way I know how to save my front end gears. One of my issues is that so many people don't seem to understand that in a fwa tractor the front tires are geared to travel a different speed than the rears.

Thanks for making me explain myself guys.

 

[grsthegreat]

holy cow...4 times. I had the front gears in my 1990's era JD870 blow out after the tractor was about 12 years old. I found out later that this was a common issue with the series of tractors. But after 1400 hours and one failure i didnt think anything of it. It was a great tractor.


Now if that happened 4 times.... id be soooo pissed off.

now, i have a question to those that talk about rear ballast. Can adding too much rear ballast allow for the operator to put too much weight in front loader. and if so, could this help cause these issues??

 

[pmsmechanic]

I don't know. What's to much weight? I always figure if the loader can lift it then it's not too much weight. I could be wrong. I've been wrong before.

 

[kennyd]

The PRV "should" kick in if to much is picked up in the loader, or tried to anyway, unless however it was adjusted above spec so the operator could pick up more than intended by the OEM.

I suspect the root cause here is twofold, operator error and lack of rear ballast.

 

[kennyd]

Oh boy I guess I've started up some controversy again. ........

Thanks for making me explain myself guys.

Just friendly discussion that's all...it's all good stuff.

 

[Dutch445]

a lot depends on conditions. if this all happened in hard dry conditions where no slippage occurs, then i guess your theory of less
weight might be true, although in those conditions we wouldn't recommend using MFWD. and if in fact the OP was using
MFWD in conditions that didn't warrant it, then yes, i can understand excess forces being applied to the MFWD and
possible failures. and in that case, maybe more weight could be a problem.

lift capacity is designed into the tractor and these tractors weren't all that powerful in that respect, so i doubt there was
any over-loading of a bucket going on, but, more due to either lack of rear ballast, and/or using MFWD when it wasn't needed.

maybe this discussion will help somebody else so they don't experience a similar breakdown.

 

[dirt ditch]

Just to add information to this thread...

First photo is of my 300 loader operator manual's minimum required ballast for the 790. Fluid-filled rear tires + 617 lbs 3pt ballast.

Next 2 are screen shots of an excel ballast calculator I downloaded form the JD website sometime ago. It's for the 3005 but it's the same as the 790. First with no ballast. Second with minimum required ballast.

Notice minimum rear ballast takes 414 lbs off the front axle and at the bottom look at the percentage of weight on the front and rear axles. Ballast makes a big difference.

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