How to rebuild a Bush Hog Rotary Cutter gearbox.

[beemerphile]

You shouldn’t have to file a part this expensive. Where is their quality control?

Elsewhere or non-existent, I'd say. I figure I'd better quit while I have something I can make work. I made a bit more progress today.

Castle nut and shim off of the pinion gear.



Removed the pinion gear



A couple of shims



Pressed out the shaft (gently, no tonnage)



Removed upper pinion bearing



Here is the broken shaft with half of the stump jumper boss still attached.



I decided to explode the broken shaft to do a parts count on it and maybe salvage the shims and protective washer. At about 20 tons there was a pretty healthy "BANG".



A clean gutted gearbox housing ready for (I hope) reassembly...

 

[HCJtractor]

Looking good

 

[beemerphile]

I'll document the assembly here in case this project ever comes up for someone else.

Thought I had all the parts...

New stump jumper and blade set...



Shaft and Misc small parts. Bearings and seals for both shafts. Shims, snap rings, and cotter pins. Looks like nuts and washers are not included in a "blade bolt set". Just two bolts, so more to order...



Take-off parts inventoried and cleaned for re-use where applicable...



First step is installing the new outer races for the pinion shaft bearings...





Took a needle file to the buggered pinion shaft threads until the nut would run down easily...



The pinion shaft inner races were tight on the shaft and I didn't want to force them, so I'll let the shaft spend the evening in the freezer...



Tomorrow morning I"ll heat the bearings to 250F or so and see if they will slip fit together...

 

[beemerphile]

Nice frosty shaft and a bit of Goodson PFL-200 press-fit lube and the bearing slid on easily...

 

[ovrszd]

Very nice writeup. Excellent picture support. Valuable addition to the thread archives here. Thank you very much for taking the time to do this.

 

[beemerphile]

Beginning the re-assembly today. Here are all the parts of the output (pinion) shaft...



The outer bearing and inner race was installed previously using heat/cold for the fit. The shaft is inserted from the outside in and held there while the upper bearing and inner race is dropped into place; followed by the two shims; then the pinion gear; then another smaller shim (AKA washer); and, then the castle nut. The 55mm nut is tightened until the rolling torque is 6 ft.-lbs (with no seals in place). After that, the nut is torqued until the next alignment of the cotter pin hole. The pic does not show the cotter pin in place, but that is the final step on the output shaft assembly.



The next step was to remove the old inner bearing and race from the PTO shaft end of the input shaft.



After dripping some Kroil into the clearance, I covered the shaft with a short length of 2 inch PVC pipe and capped it with a piece of scrap metal (which happened to be the broken stump jumper hub that started this whole mess!)...



A good hit with a 2 lb. engineer's hammer and the old bearing gave up the fight...



I dressed up the bearing journals with some Nevr-dull and a Scotch-Brite pad...



After testing the bearing ID fit, we will need heat/cold again to relieve the interference...



I use a roaster oven (my own, not the wife's) to heat the bearing inner race...



Back in a few hours. I have plenty else to do.

250F bearing installed on -10F shaft (drops easily into place without malice)...



More to come...

 

[HCJtractor]

Very interesting. Enjoying your labors.

 

[searcyfarms]

I just went through this entire process on a 3pt tiller - mine was a Chinese brand - parts were cheap for sure, chose to make my own gaskets and shims - mine went from straight/miter gears like yours to a spiral bevel gear setup - boy it sure is quieter and not near as sloppy. I think they had issues with the straight gear setup so that is why they changed to the spiral bevel.

mine also came with no shims so I had to make my own out of shim stock - worked like a charm!!!! I basically just preloaded mine until there was no play in the gear and it wasn't hard to turn after running it once/tilling, I added two more shims to get it tight after things settled in from the first tilling.

so far so good!!!! all to the tune of 146 bucks including, shaft/bearings/gears/seals + shim stock/oil

you will enjoy the hard work once u get it back together, mine is a cat2 tiller 60HP g/b with a tilling width of 6ft. IT will flat put a 60hp tractor on its knees or a face plant since my 2 point IH arms are in the non float setting it will dig!!!

I wish I had known the hot/cold trick, I wrestled mine thoroughly but with my neighbors 20ton press we got it all done accordingly. Thanks for the great info/write up - GREAT JOB!!!

 

[beemerphile]

I wish I had known the hot/cold trick, I wrestled mine thoroughly but with my neighbors 20ton press we got it all done accordingly. Thanks for the great info/write up - GREAT JOB!!!

I learned this working with old BMW motorcycles. It is a factory technique for assembly of many parts on the bike's engine, transmission, and shaft final drive assembly. The cooled parts were brought to the assembly line in a cooler with dry ice. I have an assortment of hammers and a 40 ton press, but it stands to reason for replacement part longevity that less force is better. When you are working with interference fits in tenths, a little heat and cold can make a huge difference. Some times I use CRC freeze spray for localized cooling when a freezer doesn't make sense and an industrial hot air gun for heat when the toaster oven doesn't make sense. The PFL-200 press-fit lube is also handy for tight fits and is made for this purpose. These bearings didn't need it and were easily placed (almost "dropped") into position by hand after heating and cooling the parts.

 

[Turbys_1700]

Very nice writeup. Excellent picture support. Valuable addition to the thread archives here. Thank you very much for taking the time to do this.

Agreed...

 

[beemerphile]

A couple of observations before I get started with today's installment...

First, if I had it to do again I would have bought SKF or NTN bearings from a local bearing house instead of Bush Hog parts by part number from an ag supply place. The bearings turned out to be common metric series 30209 for the input shaft and 30210 for the output shaft. The originally installed bearings were HL brand which is a Chinese product of Gansu Hailin Zhongke. The replacement bearings ordered by Bush Hog part number were Dura-Roll which is a Chinese product imported by A&L Bearings and Components and is used it lots of ag equipment. I don't know whether the Dura-Roll bearings are as good, better, or worse than the original HL bearings; but, I am diggety-dang sure that they would not be as good as name brand bearings from one of the known market leaders. Further, in pricing them, I could get SKF bearings for 1/3 less than the Chinese bearings cost me through Messick's. Lesson learned.

Secondly, it occurred to me that normal gear oil changes in this gearbox never exchange the gear oil that the output shaft bearings ride in. This is an issue because every time I have changed the gearbox oil, it was a water/oil emulsion because of condensation cycles. Since there is no drain plug, the oil is removed by suctioning it out through the vent plug. That only empties it down to the top of the inner output shaft bearing. There is a cavity formed between the two output shaft bearings where the oil is never changed. Water being heavier than oil, I imagine that there is more water down there than there is in the upper housing. Here is a picture that shows the area between the two bearings that does not get its oil changed...



It is clear from this picture that draining the gearbox reservoir does not even touch the lube in the lower cavity.



Hence forth, my plan is to place the cutter on its side with the fill plug facing down and drain it overnight through the fill plug. This should allow the lower cavity to drain through the upper bearing. After I refill the box, I will recheck the level the next day to replace the amount that went down into the lower cavity.

 

[searcyfarms]

the Chinese tiller I have has fafnir bearings in it lol

 

[beemerphile]

Back on this project for the input shaft fit-up. Getting ready for assembly, the input shaft and the outer races for both 30209 bearings went into the freezer. The inner races and bearings for both 30209's went into the oven at 250F along with the blind side bearing shim and the crown gear.

First install the input shaft snap ring and two shims



Next install the input bearing outer race from the inside with the taper facing in. With the race frozen, it drops into place with an easy tap.



Set the pre-heated 30209 input shaft bearing and inner race into the outer race.



Set the pre-heated crown gear into position aligning the inner bore with the bearing.



Install the external snap ring that fixes the position of the crown gear onto the input shaft and insert it from the blind bearing side until it is seated in both the input shaft bearing and the splines of the crown gear. The snap ring should be resting against the crown gear if it is sufficiently inserted.



It is not shown in this picture, but now the thick blind bearing shim is installed on the input shaft.



Next the blind 30209 bearing is installed with the rollers facing to the outside.



The blind side bearing outer race is then tapped in using a mandrel, followed by two shims. If you use the heat/cool process, a light tap is all that is needed. For a mandrel, I used a 2-1/2" PVC conduit bushing. The OD of the bushings vary, so I selected one that was a bit under the 85mm (3.3") of the bore.



Next the snap ring is installed. Here is another advantage of a frozen shaft. Since the shaft has shrunk in all directions, there is little interference caused by the preload of the bearings. Set the snap ring in the bore and insert the conduit bushing and give it a light tap with a mallet. It will fall into the groove easily. Once the shaft is at room temperature, preload will magically appear.



I'll seal it up in the next post.

 

[beemerphile]

A new blind seal was tapped into place. I use either dish soap and water or tire bead lubricant to ease the installation. I don't like to use petroleum lubricants because I want it to dry once the seal is in place.



For the input shaft seal, I grease the land area where the seal will run and the ID of the seal itself. The OD of the seal, is again lubricated with soapy lube.





Same process for installation of the output shaft seal



The output shaft seal is pressed into a recess depth of 1/8" to allow fitment of the protective washer.



The protective washer is then tapped into place in the recess.



The cover was cleaned of old gasket material and the vent was pressure cleaned.



A light bead of Permatex Ultra Black was applied to the top of the gearcase.



FINALLY the job is finished!

 

[sandman2234]

Well, thanks for the posting of the rusty and the damaged thread shafts. I recently replaced a gearbox on a 3008 mower and thought about buying the shaft and putting it together myself. I would probably not have been quite as nice when the second one came in damaged, but like Messick's said, it is the way they are coming in from Bush Hog. No way to ever know for sure, but your post makes me glad I purchased an entire gearbox and spent the extra $55 dollars to get the entire gearbox. I couldn't justify my time for the amount of work you just did. I appreciate your time to post the whole rebuild, and would hope I never need it.
David from jax

 

[3Lfarms]

Very nicely done!
I overhaul aircraft gearboxes for a living and you did a great job.

I’m curious if you checked the backlash before installing the seals, or if they even gave you a specification for backlash.

 

[HCJtractor]

I’m impressed! That’s quite a project.

 

[beemerphile]

Very nicely done!
I overhaul aircraft gearboxes for a living and you did a great job.

I’m curious if you checked the backlash before installing the seals, or if they even gave you a specification for backlash.

Wow. I am just an old BMW motorcycle hobby mechanic and a retired chemical engineer. I finally spoke with a tech at the Bush Hog factory, and although they do not publish any guidance, he told me what they used for in-house overhauls. He said that so long as the original housing is re-used, that no shim changes should be required. To obtain pre-load, the output shaft nut is tightened to achieve 6 ft.-lb. of rolling torque with no seals installed and then bumped up to the next alignment of the cotter pin hole. There is an .005" clearance spec for the crown/pinion gear. The input shaft preload is entirely by shimming. I took his word that no shim changes would be required, however, after the internals were at room temperature I detected no movement of the input shaft with 50 lbs of thrust load in both directions using a dial indicator. So, I don't know what the actual preload is, but there is some preload there. I do not know how they arrive that the factory shimming.

To @sandman2234 the price I was quoted for the gearbox was over $1,700 so it was no small change difference in my case. I considered used replacements but figured I might buy one just about to need the same work. This way I expected that I would be as good as new and better than used.

 

[ovrszd]

As in automotive drive axles, shimming is done to correct casting error. Putting the shims back where they were originally puts the backlash where it belongs. I've only saw this fail once. That was the time I bought a cheap aftermarket gear set.

Not saying you did that at all. Just explaining the philosophy of shimming a rebuild.

 

[beemerphile]

As in automotive drive axles, shimming is done to correct casting error. Putting the shims back where they were originally puts the backlash where it belongs. I've only saw this fail once. That was the time I bought a cheap aftermarket gear set.

Not saying you did that at all. Just explaining the philosophy of shimming a rebuild.

Thanks. That matches what I was told by the Bush Hog tech and it matches what I did. The replacement output shaft was OEM and it was preloaded by a torque spec. The input shaft which is preloaded by shimming was not changed - only the bearings and seals were renewed.