Rotary brush cutter loud squeal when cutting

After my drive shaft broke in two early this year, requiring a new PTO shaft in my MF65 and bearings/seals on the Sidewinder 5x5 rotary cutter, I finally got things patched back up and took it out in the pasture tackling some 6' tall grass and weeds. It's been a wet summer so the grass is about as thick as I've ever seen, even more so given I had to skip the late spring cut due to the equipment issues.

After about one hour of normal operation, the cutter started really fighting the engine with the blades engaged. It wasn't sudden or anything, but started intermittently and quickly began getting worse. A glance behind me showed that with each blade rotation, there was a big clump of grass and a simultaneous extra-loud squeal. Risking further damage, I limped the tractor back to the garage without running the blades and noted no issues with the engine, isolating the problem to the spinning blades under some load.

I took a look under the cutter and noted NO visual issues, and that the bolts on the blades were tight. The stump jumper/blade assembly rotated FREELY by hand, so it's not a problem with the gear box. The blades are slightly worn but still sharp, if not a little banged up from years of use. Finding no obvious issue, I fired up the tractor and took a quick pass through thinner grass and the issue promptly returned. When the blades are spinning and not cutting grass there are no symptoms. But that the issue returns when cutting thin grass has me, no pun intended, stumped.

For you veteran rotary cutting folks, what would cause a cutter to bog down the engine and yield a loud squeal when under load? What can I check/adjust/replace?

Does cutter have a slip clutch? If so it's probably no tight enough.

It sounds like you have a more serious problem than a slipping clutch, if you have one. When a slip clutch slips they will get
hot, that's one way to check, also mark it with a felt pen and see if the mark moves.

I think my brain must not have comprehended that the engine is bogging down.

It is a shear pin input shaft, not a slip clutch. Which makes this even more odd.

My initial guess, after confirming the blades weren't loose, is maybe the entire cutter deck started shifting for some reason, causing the
blades to dig too deep toward the ground? But I have no evidence to suggest the blades are scouring the ground.

Bearings?

Bearings squeal when they get hot... they also get tighter and tighter when they get hot.

Until they don't...

(quoted from post at 13:32:45 09/03/19) Bearings?

Bearings squeal when they get hot... they also get tighter and tighter when they get hot.

Until they don't...

Click to expand...

I am not sure, unless it's the output shaft bearing. The input shaft bearings are brand spanking new. Leveled out gear oil level when I replaced them just a month ago, as well, so it's not lubrication. And with the blades spinning freely by hand as soon as I parked, it seems like the gearbox is running up to par. Unless new bearings wear out in only an hour... Maybe?

(quoted from post at 12:51:34 09/03/19)

(quoted from post at 13:32:45 09/03/19) Bearings?

Bearings squeal when they get hot... they also get tighter and tighter when they get hot.

Until they don't...

Click to expand...

I am not sure, unless it's the output shaft bearing. The input shaft bearings are brand spanking new. Leveled out gear oil level when I replaced them just a month ago, as well, so it's not lubrication. And with the blades spinning freely by hand as soon as I parked, it seems like the gearbox is running up to par. Unless new bearings wear out in only an hour... Maybe?

Click to expand...

In six foot tall stuff, there is an opportunity for something to get wrapped maybe?

The only analogous situation that I've been in is actually with one of those push model string trimmers. The ones that look like a push mower, but they use a belt to drive a hub in front of it that spins really thick trimmer string.

I have one that started acting up. It would run for maybe fifteen minutes, then start squealing and load down the motor. If you shut it off, you wouldn't be able to start it; because it would pull too hard.

Let it sit and cool down for twenty minutes, and you could start it again.

When I took it apart, I found that tall grass had wrapped inside the trimmer hub and burned up the bearing. When the bearing got hot, the shaft would expand and run hard, loading down the motor and squealing. After cooling, it would run again... for a while.

Which would be similar to yours being able to move freely after you brought it back and let it cool down.

The thing is, if your output bearing was shot, it would seem like the seal would go with it and you would be leaking gear oil as well.

Definitely a confounding situation.

Oil was good a month ago. After 1 hr in 6' grass, are you sure it is still good?

(quoted from post at 06:55:56 09/04/19) Oil was good a month ago. After 1 hr in 6' grass, are you sure it is still good?

Click to expand...

I'll pop the oil plugs and take a look. Do you think adding a tube of grease (as others have suggested on other threads) to the gear oil would help?

I did look at the output shaft as best I could above the stump jumper and didn't find any tall grass wrapped up- but that thought did cross my mind when I was out in the field.

Also, I sat down yesterday to go over what changed from last year to this year on the cutter, and I might be losing my mind but I feel part of the deck height looks slightly lower now than earlier in the year. I don't know how that could have happened given the only adjustment to the two leveling turnbuckles was on the pitch, not the height, before I took it out afield. But I did notice the pitch turnbuckle handle was not locked down when I returned to the garage- is it possible that the turnbuckle(s) vibrated itself(s) to a lower deck height after a while of cutting? I didn't see evidence of blades scouring the ground but it might validate the fighting of the engine and loud squeal with each rotation. If that's the case, what's the proper height and pitch setting?

Are you keeping the RPM's up so you are turning the PTO at 540? 6 foot green stuff is tough! Keep the
RPM's up and the ground speed down. Is the mower deck an inch lower in front, so you aren't cutting the
grass twice. May have to run over it twice. If it is a gear box, it has got to be HOT.

Does the engine lose power? Does the engine lose rpms? This would indicate engine problem. When the squeal happens, does engine rpms increase? Does the pto shaft seem to slow down, but the engine does not? The last 2 would indicate something(pto clutch?) is slipping. Does the pto shaft connect directly from tractor to gear box mechanically? Are you sure the noise is from the bush hog, and not the tractor? Mark.

ALso : could your stump jumper be slipping on the output shaft of the gearbox-broken keyway? Mark.

My money is on a bearing(s) that are heating up and intending to fail.

The totally useless, smart mouthed response that comes to mind is- If it's really squealing, it sounds like you've run over a pig! :shock: