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dan-l

7790 Rear Axle: Will this work?

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dan-l
HI All, (This is kinda long and technical, but I hope some of you are intrigued by what I am asking, and want to think about it with me...) I posted a want ad a couple weeks ago about my 7790 rear axle breaking, and got several responses answering my question as to whether it is unique to the 7790. Yes, it apparently is - and NLA, to boot. A few of you suggested that instead of looking for a real hard to find used part, I find a transaxle from a 7000 or 7100 series tractor and replace mine with it, pointing out that it would fit my frame and all, and then I could get busy with my fall cleanup. Good advice, but I'm hesitating while I make sure I don't have any other alternative. And today I may have thought of one. I've been told that what's different about the axle in a 7790 is that it's longer - by 2-1/2 inches or so. OK, but I have noticed in the numerous parts diagrams I've been studying that the right side hub, and diff assembly parts all seem to have the same part numbers in the 70xx, 71xx, and 7790 tractors. So, that tells me they would all have the same retaining ring groove and keyway and they're probably the same distance in from the right end of the axle. My groove is 5-3/16 in, and the keyway begins another 3/16 further. Now, I also noticed that on the left end of my axle, there's over 2 inches of "extra" axle beyond the collar, between it and the hub (or plate as simplicity calls it). The collar itself is about a half inch thick, making the total distance from the inside edge of the hub to the inside edge of the collar 2-11/16 inches - "extra" inches I'm starting to think. If you're still with me, my thought is to take the axle from a 70xx or 71xx and shove it all the way in (without a collar) until the hub meets the transaxle tube (or shim it back a little until it's exactly where it need to be) and then I'd have a groove and keyway in the right place, and the end should stick out the correct distance on the right side to put the collar on after the wheel, etc. goes on. What's left is to make a 2+ inch spacer to bolt between the left wheel and the hub, which is too far in towards the center of the tractor, so the left wheel is spaced out enough to clear the oil filter and PTO, etc. Pretty easy, me thinks. So, my question is: Will that work? and... Just what *is* the difference between a 7790 axle and all the other 7xxx tractor axles? Just the length (and just towards the left of the groove and keyway)? Thanx to all who have helped so far, and in advance to anybody willing to think this thru with me. --Dan

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Tiny
Geez, I'm not certain but I think the transaxle is the same between the 7000 and 7100 series along with your tractor (7790). I'm thinking the front of the tractor is extended out some to fit that diesel engine is all. Somebody will come along here and straighten us out sometime I'm sure. BTW, Welcome to the club!!

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CarlH
dan-l is correct, the axle and axle tube on the Sovereign diesels are longer than on the other 7100/900 hydro tractors. All the other components of the transaxle (gears, case, etc) are the same. The other significant different items for the diesel, besides the engine and mounting frame, are the front frame is widened, the fuel tank has a fitting for a fuel return, and the console is wider for a larger battery.

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dan-l
Well, a lot of interest, but not many comments (well, other than to get the pictures I've been slaving over UPLOADED!) 8) I figured getting the dimensions right was most important. I thought this kind of thing might be "old hat" to a lot of you guys - who needs pictures? well, thanx to Mr. Duckman, my camera's got axle grease on it and I spent the majority of the day making 8x10 glossy photos with circles and arrows, and a paragraph on the back of each one. (apologies to Arlo Guthrie) :p OK, here is what I'm talking about:












I guess I need to know the following, if somebody knows or can make measurements: 1. how long is a 7000- or 7100-series axle? 2. how far from the right end is the groove for the gear that is keyed to the shaft? 3. how far out do the transaxle tube ends extend from the side of the case on 70xx and 71xx tractors? This all assumes I'm right about the other parts for the differential assembly and right hub, etc. being the same. If so, then the axle diameter must be the same, and the details of the whole right side must have the same dimensions. Depending on the length difference, and the amount the two tubes extend from the transaxle case, the "extra" length the 7790 axle has just might be compensated for by a spacer on the left side like I'm trying to describe. Thanx to all who are following this! :D --dan

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dan-l
Done! Sorry - I read the FAQs for posting pix (about 4 times) and strictly stuck to the size limit, etc., but didn't see there's a separate comment in the sticky message at the forum top 'til I went looking for why the request to reduce.

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2burning
I haven't ever dug this far into an axle before, but with no set collar on the left side and just using washers, etc., do you think you will be able to get most of the slop out and get the axle keys installed?

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dan-l
Hi all I have possible access to repairs via my job where the guys in the machine shop might be able to help me out on private time or something, but i'm a bit leary of trying to pin or weld this right at its weak spot (the keyway). This just happens to be where the drive force is also concentrated, since the gear keyed onto it is what makes it turn! Hey duckman - those pictures good enuf for ya! ^ i have no idea why the axle broke when it did. i was mowing a flat yard. the *only* thing was i remember my hand slipping and rather abruptly switching from reverse to forward like a half hour earlier, but c'mon - thru a hydro - that can't be what did it. this things been plowing and tilling and subject to a lot more than that! i suspect the crack's been slowly propagating over the years, and just ended up all the way thru that afternoon. latest news is a local dealer has managed to get dimensions off of blueprints for my axle and the sovereign or 7000/7100 axles by talking to the factory. i was busy at work this afternoon (still here - geez it after 9:30!) so i need to call him back tomorrow and see what he found out. also, my girlfriend has taken this as a challenge, and has been calling around herself. she seems to have found a 7790 axle from a parted out unit. 60 miles away even. quite a gal! ( i better praise her because she has been referring to my dimension by checking in here now and then) :p so, if *that* works out i may not even have to follow this short axle + spacer route at all. but i will let folks know if it looks like it will work, for reference anyway. that's it for me right now - got to head home... --dan

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2burning
Maybe that quick change in directions was the last straw for your axle?:(! Others may disagree, but I think that the abrupt change in directions and the instantaneous? forces generated by any slop built up in the keys and gears, is harder on the parts than a good, hard, smooth pull like the load of a plow. Good to hear you have a line on a good used part though! You have quite a GF there, keep her happy :)

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dan-l
i bought the axle. haven't put it in yet. parted out by dealer in polo, il - dambman lawnmower hospital. on the simplicity dealer locator page. if the starter is "weak" or wiring burnt, but still good otherwise, consider having it rebuilt at any place that does these for cars. ask at your local auto parts store if you're not sure where to find somebody.

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dan-l
@ 2burning: i don't see what i thought i sent to reply to your comments - maybe i didn't hit "post". anyway, i totally agree that shocks are much tougher on parts than steady loads - even high ones. i still can't fathom how an abrupt change in direction of a hydro drive would snap an axle, tho. i "shocked" my tractor (as well as myself) plowing snow and hitting a big tree root a few years ago. tractor stopped right away, but that unstoppable lombardini kept right on going at full throttle. drive shaft took up the difference. i think those kinds of things would be more likely to break and axle than changing directions, but who knows??? on your other question, i'm certain that the right amount of washers could shim the shorter axle correctly - especially now that i got exact dimensions for the short one to compare to mine. when i install the one i got, i'll look for any unforeseen 'gotchas' in case someone else needs to do something like this...

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