How to rebuild a Bush Hog Rotary Cutter gearbox.

[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.