853 gear reduction mayhem

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Old_853

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Feb 28, 2024
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I had this weird popping noise under heavy load, but only when turning one direction. Figured it was the chains, which are worn. Turns out the input shaft bearings came apart in the gear reduction, taking out both the small and big gear along the way. It still worked fine. Big gear is about a grand on bobcat. Little gear input shaft part number 6558271 is no longer available. I have scoured the depths of ebay, every spare part site I can think of looking for that or the whole gear reduction case part number 6708266 as well. Lots of them for the smaller machines. Zero for the 853. Any ideas? I've considered building the teeth up with the welder and grinding, but I know it wouldn't be as strong.
 

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wateka

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May 25, 2022
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I found this guy on Craigslist because he's near me in Texas. He says he has one - give him a call
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Old_853

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Feb 28, 2024
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I found this guy on Craigslist because he's near me in Texas. He says he has one - give him a call View attachment 6108
That's awesome, thank you! I just sent him a message. Kind of early to call on a Sunday. Hopefully he's willing to ship it to Wyoming.

Edit: He's shipping it out in the morning. Thanks again, I was ready to weld and grind every tooth.
 
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Old_853

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Feb 28, 2024
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Got both rebuilt and back in. I had to pull the drive gear on the left side to get it back in there. I don't remember how I got it out, but it's pretty tight even with the gear removed. Both left and right sides had gnarfed gears, as well as the craigslist find. Must be why bobcat got away from the design and parts are unobtanium. I cobbled together the least borked sets out of three, and might try to weld and recut the spare set if/when the current ones grenade. I tried ordering the primary shaft from a 743. Close, but no cigar. Everything is the same except the drive motor input. It may be possible to swap drive guts with a 743 motor into an 853, but the shaft is much smaller. The large secondary gear is the same part number between the two. The 743 takes 80 series chain, and the 853 takes 100HS chain, so you can't swap the entire gear box easily. Eh. I think the easiest thing would be to delete the entire gear box and fit a more modern drive unit, although you would lose brakes without some hillbilly engineering. Also did axle bearings and seals, and rebuilt the drive motors while I was in there. Next is bushings all around for the bobtach/rams. The neverending project.
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Old_853

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Feb 28, 2024
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Hydraulic fluid leaking into chain case when running. Why? Greased seal lips, no nicks or burrs. Chaincase drain hose is clear, blew air through both side to be sure. Chain case drain ports in motors clear. Can blow air through the two pin holes in the drive shaft. No obvious issues. Order new seals again and say a prayer? I don't know what else to do.
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Old_853

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Feb 28, 2024
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Just put everything back together and put in 4 gallons of fluid and let it sit awhile. Without it running. The chaincase level increased. That means it has to be Oil Seal 6513588. Ordered two. I'm going to try to find a seal driver. I installed these ones with a brass hammer and thought I was careful, but that was on the bench. Installing them in the hull will be a good time.
 

Mitch500

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Feb 4, 2024
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Just put everything back together and put in 4 gallons of fluid and let it sit awhile. Without it running. The chaincase level increased. That means it has to be Oil Seal 6513588. Ordered two. I'm going to try to find a seal driver. I installed these ones with a brass hammer and thought I was careful, but that was on the bench. Installing them in the hull will be a good time.
That seal head me worried when re installing the drive motor. I should check my hydraulic reservoir and also chain case level as well.
 
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Old_853

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Feb 28, 2024
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New seals carefully installed. The old ones looked perfect. No nicks, gouges, or bends. I used a length of 3.5" exhaust tubing and an old seal to make a driver. Used loctite 518 around the outside of the seal just in case it was getting past there. Used silicone grease on the contact surface of the seal and shaft. Let it dry overnight. Installed motors and put just two gallons of hydraulic fluid in and let it sit. No change in the chaincase level. Topped it off and let it idle for 30 minutes. All looked well. Went for a test drive for 300 yards or so, and went back to the shop. No hydraulic fluid in reservoir. Crap. Pull chaincase drainplug again and there is at least 5 gallons extra fluid in the chaincase.

I test fit the seals on a spare shaft I have. They fit very tightly on the shaft. Nominal OD of the shaft is 2.5". Granted these have several thousand hours on them and the shafts are slightly worn where the seal rides. Less than the thickness of a fingernail. Maybe a seal saver sleeve thing? New shafts would be very difficult to make even for a seasoned machinist. It ain't worth making new ones.

EDIT: Looking closer at the manual I noticed I may have installed them backwards. Or the manual guys did. Either way it would ride on a "new" area of the shaft, because they were installed rubber in when I took them apart. Hmm.



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brdgbldr

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Jun 1, 2012
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1,239
Maybe something in the return line is clogged or one of the relief valves isn't working. This could build up pressure that maybe would be just enough to overcome the seal but not destroy it.

This is just a guess but should be checked.
 
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Old_853

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Feb 28, 2024
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34
Maybe something in the return line is clogged or one of the relief valves isn't working. This could build up pressure that maybe would be just enough to overcome the seal but not destroy it.

This is just a guess but should be checked.
Can blow air through the return and hydraulic pressure is normal. Motors have fresh seals and will run on air, and hold air pressure even. (I was gentle, low pressure only). I thought maybe there was an internal leak in the motors.
 

Mitch500

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Feb 4, 2024
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I'm trying to understand these machines more. When I had my drive motor out I was wondering what these holes are for in your pictures.

Does the gear reduction get hydraulic oil or chain case oil
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How is it lubricated (via chain case oil or by being supplied oil from the drive motors). I've circled the holes I'm talking about in you pictures.
 
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Old_853

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Feb 28, 2024
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34
The chaincase has an open hole to the gear reduction and is lubricated with chain case oil. I use old hydraulic fluid in my chaincase, which I have in surplus due to these shenanigans. The two holes in the coupler shaft go to the interior of the gear coupler, they are drilled at an inward angle. The drive motor shaft doesn't run true, it kind of wobbles in an orbit. Those holes drain the fluid that gets squished by the end of the drive motor gear's eccentric orbit. In a properly operating system, the outward most face hole takes the excess fluid that leaks through the motor, and returns it to the reservoir through the case drain hose. I have an early model that doesn't have case drain filters, so there are no restrictions to the returning fluid. I think the seals are backwards combined with worn shafts. We'll see when parts get here.
 
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Old_853

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Feb 28, 2024
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34
Also couldn't find a muffler, and adapted 7350311 to fit. The right hole and exhaust tip lined up perfectly. Had to clearance the door and radiator cover a bit. It's larger pipe, so I welded up a coupler to the factory flex pipe. It's a lot quieter than the rotten one with no tailpipe I was running.

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