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.