Wrecker Swing Gearcase Rebuild Notes

[KN6KXR]

Posting this to document some information that may come in handy to others. Item rebuilt was on a M936A2 but should be the same for other platforms as the bed and crane are similar.

Synopsis: New to me M936A2 rebuilt by RSMS in 2012. Sat until 2016 when bought by previous owner. Stored inside an aircraft hangar until I bought it in late 2021. Odometer showed 2,090 miles since reset. Certain that crane was used for demonstration only after reset. Noticed problems with sluggishness on the turntable straight away. Saw it was shy about 20 gallons of hydraulic fluid topped off seemed OK with no load. Went to try and pick up a load (3k pounds) and could not swing smoothly. It would jerk and hit the pressure relief. Tuned the pressure relief on the system to 2200psi same result (it was about that when I started). Took a sample of hydraulic fluid for analysis came back fine. Lubed the heck out of the turntable, cleaned the gears of crud, lubed all again. After each one of these I would pick up the 3k test load and it would not swing smooth. Crane and boom worked amazing I went to see if I could pull a stump one day and got the front tires 2 feet off the ground without trying too hard. So hydraulic pump and winch motor seem fine. The last test I did left a big spot under the wrecker. Smelled like gear oil. So I called it and decided to take it apart.

Removal: Don't think you are going to get the gear case out without draining the system. The suction line from the resevoir is in the way it has to go. You'll have to work with the boom vertical to get to the suction line end under the crane. So first elevate the boom all the way. I'm lucky my boom is rock solid if yours sags you may need to crib/strap/block it. After draining the system you'll need a 2 1/2" and 2 5/8" box wrench. Even then pipe wrenches and cheater bars may be necessary I used a bandsaw and pieces of pipe then cut to length to get in there. The motor unbolts and can swing to the side. Hold down bolts (2 each on the side feet, 2 interior) are obvious. You'll need typical blocks and prybars to now elevate the case (since the suction line is out of the way) and pull it out. Have a couple strong friends or another wrecker handy that case is pretty heavy. I can lift it complete (without motor) but only dead lift and not even chest height. My little Kubota with the backhoe did the trick. Needless to say this is the time to clean that suction filter, add that extra valve in the plugged bung on the tank, or anything else you can think of.

Disassembly (begins): The top cover of the gearcase has provision for 3 jack screw. I didn't see this at first as the holes were covered and painted over. Just take 3 of the 6 bolts that hold the cover and insert in the holes. This will pry the cover off. Turn the case over on something like a set of sawhorses (to clear the pinion/spur gear assembly) and use the "bigger hammer method" to drive the assembly out. I used an engineers hammer (4lb hand sledge) and a block of wood to cushion the blow.

Attached pictures are what I found. As you can see the failure was catastrophic.

 

[KN6KXR]

Disassembly of case is fairly straightforward. I used a cheap Harbor Freight 12 ton stand up press I inherited with my place. If one needed they could build an adequate tool with lumber or unistrut; but the cheap press works great. A bearing splitter like the one in the picture was handy for the drive shaft end bearings. Standard pullers for drive spindle end and double race bearing. Nothing crazy here all the bearing are press fit but not by much. It was apart in an afternoon.

 

[KN6KXR]

Now it was time to look for parts (see attached from TM -P). Obviously I needed all new bearings, gasket, seal and drive gear. Note that there are only two types of roller bearings item 6 (qty 3) and item 10 (qty 1). Some quick looking you'll find that item 6 is an SKF 309 and item 10 is a 5212. This is where eBay is a real win. I wound up with a 5212C which cross referencing shows a higher tensile grade than a standard 5212 but still a C3 spec and at least the same radial and side load characteristics. I paid maybe 30% of retail on my bearings by using eBay. Highly recommend.

The lip seal was a tough one. The NSN pointed to a part that didn't seem to cross anywhere. Working off the NSN item spec, a bunch of checking with a caliper as well, I was able to find an almost identical part for item 9 in an SKF 26128. When I say almost identical it's about 0.7mm thinner. Same OD, same ID, just a bit thinner. Since this doesn't matter in the application it's good to go. This seal is just driven flush with the bottom there is no shoudered area and even if there was the other end is open.... Anyways it works just dandy and is a boatload less than what the "real" one would be. Once again eBay to the rescue think it was $17,

 

[KN6KXR]

Right. Now the money stuff. I needed gears..... Once again I recommend eBay because why not? As luck would have it I found item 7 for $122. No kidding. Wish I could have found the others that cheap! I hadn't actually thought I could afford to replace it but at that price absolutely. Picture of old and new for comparison. Super happy I could do this when i had it out.

 

[KN6KXR]

So there were two gears left that I needed and this is the bulk of the cost of the repair. I needed item 12 and item 19.

Probably the biggest "win" in the parts department was on item 12 which is NSN 3020-00-262-7572. I got quotes on this from $2,200.00 up to over $7,000.00 from the NSN folks. Could not find it anywhere. By googling the part by manufacturer number I stumbled on a post that indicated this same part, a Garwood 7538688, is the same gear used in the front winch of an M35 (the 10k PTO winch). I found the TM -P for the M35A2 and used that NSN (totally different number) to cross back and came to the same Garwood number. In addition to that the same number is physically stamped on the gear I had. A call to Boyce, who referred me to Memphis Equipment, had me talking to them and they had 17 on hand at $585.00 each. DEAL! I'm very glad I was able to find this if I hadn't my repair would have had to wait for quite some time. Apparently this is rather uncommon knowledgeas the folks at Boyce and at Memphis had no idea this even went anywhere on a M936. Then again the wrecker is not a common item so that makes sense.

The last gear I needed was the worm gear item 19 which is NSN 3020-00-972-2641. There is no cross I could find for this. It's a complicated piece, actually. Made of hardened 8620C, spriral cut on the outside and spline hobbed on the interior of the hollow center. This part cost me $730.00. If I had been more pateint I could have gotten this for maybe $100 less if I waited for quotes and played the vendors off each other. The NSN parts people.... what a game. Anyways I'm not much of a game player so that's what it cost me. Bummed I couldn't cross it but actually, given the complexity of the part, it's not a bad deal.

Gaskets? No fancy stuff needed. I can roll my own with the best of them in that department. Learned from a bunch of old scholl engineers in the Merchant Marine. Spec calls for 1/32" paper so that's what I used. Bought a roll of it (on fleabay once again). I tend to favor a light coat of Toyota FIPG on both surfaces but some Permatex #2 would work great as well. YMMV.

 

[KN6KXR]

Reassembly of the case is pretty simple. The bearings are a slight interference fit or "crush". In fact the double rollor will slide in to the case if the same temperature but if warmer will not. This concerned me a little so I used some Loctite 638 retaining compound on all the bearings (50ml bottle on Amazon for $14). A standard ball peen will drift them in and the "feet" of the case will press them in final position on assembly.

All in all I found the reassembly process straightforward and gratifying. No rocket science here just good work. Cleaned inside and out, scraped, primed and painted. Not too bad.

 

[KN6KXR]

So you might be wondering what the heck could cause such damage? What happened? Well the picture here tells the story. At some point in time, probably prior to RSMS in 2012, this thing was full of water and it sat there. And sat there..... and sat there.... With this amout of pitting on the worm drive gear it was only a matter of time. I didn't take a picture of how I found it this is what it looked like after under the wire wheel for a bit. So the pitting on the worm gear ate the copper alloy spur gear until it bacame so distorted and bound that finally it twisted so bad as to shatter the double roller 5212 bearing and cock the pinion shaft wearing that as well. Doesn't help with the hassle but nice to know what happened. Also for you fellow wrecker owners.... change the oil in your swing gear case and save yourself the drama. This thing was quite the project....

 

[M35A2-AZ]

Nice job with the rebuild.