JD 790 Sprockets?

[Rodney R]

My U/C is wore out. I've thrown the tracks on my 790 a few too many times. The last time I noticed the holes in most all of the bushings, so I pulled the other one, coiled them both up, and got them ready to move. Then I went to pull a sprocket off...... Looks like the final drive has to come apart so the sprocket can be changed. I called a couple U/C shops to get some prices on new chains and sprockets, and then I find out that sprockets are REAL hard to get. JD won't sell them anymore, and another machine with 22 teeth on the sprocket is real hard to find, so re-rimming doesn't sound too promising. Is there anywhere that has sprockets?

Rodney

 

[lantraxco]

The old inside out drive, what a pain.

I'll do some checking with my contacts, refer you if I find anything.

Cheers,
Lanway

 

[Rodney R]

I talked with a local track shop, and they suggested re-rimming the sprockets - going from 22 teeth to 21 teeth - they were concerned about keeping the same pitch - is there any real reason to do so? They also mentioned that many folks will only do sprockets say every other chain on an excavator because they don't wear as much?

I think mine are shot.View attachment 142101

 

[lantraxco]

Your sprocket pitch has to match the chain or the chain won't fit on it, the 790 I believe is 190mm pin center to pin center, a popular chain. Also the cups between the sprocket teeth have to properly fit the OD of the bushings. Your sprockets are shot not just from running wear, but also because excavator chains are grease filled and when they go dry the pins wear on the inside of the bushings and the pitch gets longer and longer. This is why the idler keeps getting further out as you adjust the tracks also. Worn sprockets will wear out your chains much faster on the bushings.

Worst case you could get some U clips and weld them in, it's an ugly job but they do work. If you decide you might have to go that route I may have a contact that can hook you up on the inserts. I'm waiting for responses from a couple outfits overseas that may have aftermarket.

The other thing you can do, it's a long shot, look on Machinery Trader and Rock and Dirt websites for dismantled 790's, call around and see who might have a set of sprockets or even track groups in decent shape?

 

[Rodney R]

I've talked to a few folks and they recommend not going the weld in clip route, on account of that changing the pitch. They say it would be better to weld on the sides of the teeth on the sprocket, and not in the valley.

When I talked about changing the pitch, I was thinking that we would get sprockets to match the pitch of that chain.

I'm at a loss of what to do, because this machine doesn't see many hours of use, but I don't want to cobble it together to last only a year or two. But I really don't want to drop a boatload of money into it either

Rodney

 

[lantraxco]

Have you looked into the dismantled machine route? Might pick up a complete decent undercarriage somewhere for less than new rails would cost, considering you have to swap what, a hundred pads? That's 800 bolts in/out, and oh yeah, have I been there, lol.

 

[mjt]

Have a look at the sprocket buildup procedure at https://www.heavyequipmentforums.com/showthread.php?53342-Kubota-KD-15-CTL-Backhoe
mjt

 

[excavator]

Is this still a production machine? How many hours a year do you put on it? There's a number of things you can do that will last years if you are not using it that much. Personally I would go with the inserts or even spending some time welding/building up the sprockets.

 

[Rodney R]

It is no longer a production machine - hours are variable, given my amount of free time, and projects at hand - could be just a few, or a hundred or so. I have found new sprockets, they should be here this week. I will keep my old ones, and possibly build them up with weld. I am sure that the next time this happens I will not find new ones. I talked with 3 track shops and 2 of them could even get the inserts anymore. They favored the welding approach, since they figured that the bottom of each root(?) would not be worn, so a guy would do most of the welding on the sides. I tried a few junk yards, but every 790 I found had the sprockets robbed off already.

Any tips on changing pads - is a 3/4 air impact enough muscle? Can the nuts and bolt be reused if they can be taken apart? Or does a guy just cut the heads off? It's going to run about $450-500 for new nuts and bolts, so I can afford to buy a tool, if it will help.

Rodney

 

[lantraxco]

In my humble opinion it's a waste of time and energy to try to reuse track pad bolts. If properly tightened using the torque turn method they will all have been stretched somewhat, stretching them a second time causes them to take on an hourglass shape and they will fail at the narrowed spot. If it was me I would carefully wash the heads of with a scarfing tip, then pop the pad loose with one good shot of the BFH. Might try a few first with the air gun, see if they'll spin out easy. sometimes they surprise you. Mostly they bind up and you end up cutting them anyway.

 

[repowerguy]

If your time is worth anything to you at all, hit them with the gas axe. When I cut them off I cut from the top, but when I had one of the guys do it, they cut from the bottom at the link because it is hard to cut at the pad and not gouge into it. Make sure you hit the bottom of the pad with a wire wheel before you reinstall it, and also grind off all paint from the top of the new links. The pads will not stay tight if it has dirt or paint in the contact area.