Bush hog breaking shear pins

[Jason280]

I'm trying to troubleshoot an odd problem with a Land Pride bush hog, which keeps breaking shear pins. The bush hog has clearly been abused, and based on some of the dents on the front, has definitely hit a few stumps, rocks, etc. Now, I've never used this bush hog before, so I have no idea how well it functioned prior to me working on it.

I pulled the gear box cover, and the pinion gear looks good...oil is clean, no debris, and no chipped teeth. I can rotate it fully by hand, and rotation is smooth with no binding. The blades swing out correctly under centrifugal force, with no wobbling I can tell. I replaced the shear pin with a grade 2 bolt, and tried behind a Ford 3930. The bush hog made a lot of racket, didn't seem balanced, and promptly sheared the pin. Back to the drawing board.

I went through everything I could think of again, and replaced the pin with another grade 2 bolt. Rotated everything by hand, inspected the blades to ensure one didn't appear unbalanced, and tried again. Made the same racket initially, but smoothed out with no excess vibration. I tried it cutting some grass, bottomed it out a little on a hill of dirt, and again sheared the pin. Now, I have two different bush hogs that I have used for years behind a couple Kubota tractors, and have never sheared a pin...and that's occasionally hitting stuff that would shake the whole tractor. Basically, I am not sure what's going on with this one.

One thing I am not familiar with, is the slip clutch assembly ahead of the gear box. How do these things work, is there any adjustment, and are they able to be rebuilt. Any suggestions on how to proceed? The difficult part is rotating everything by hand in no way translates to what happens once you engage the PTO, at least to find issues.

Thanks!

 

[Jason280]

 

[RickB]

Service the slip clutch so you know it will slip as designed. Then put a grade 5 or 8 bolt in place of the grade 2 you are using. You don't need shearbolt protection behind the slip clutch, certainly not one that fails that easily.
Unfortunately LandPride takes the cheap way out and uses the same gearbox input shaft for both shearbolt and slip clutch models. Bush Hog and other outfits will use splined or keyed input shafts on their slip clutch machines so there is a more positive drive between the clutch and blades.

 

[Jason280]

What all is involved with servicing the slip clutch?

 

[Lineman North Florida]

What all is involved with servicing the slip clutch?

Take a measurement on the distance that the springs are compressed , loosen them up several turns and then put the PTO in gear and let the slip clutch break loose , that will knock all the rusty buildup off of it, do it several times then tighten the slip clutch bolts until the springs are compressed to the proper measurement and you should be good to go, this should be done yearly at the beginning of mowing season.

 

[RickB]

Scribe a line across all of the plates that make up the slip clutch so you can see for sure the discs can slip independently. You can probably download an op manual from the Landpride website, that should give you a baseline spring length dimension. Start out just a bit longer (1/16" or so) and monitor the clutch for slippage. Tighten as necessary for your hp and conditions. Don't get carried away, tightening each spring a little bit will make a substantial difference.

 

[Jason280]

It looks like these are all tightened just about as much as possible already...

 

[Schrodinger]

Do the above, but first mark the slip clutch plates with chalk or a grease pencil or something so that you can tell they have slipped with the loosened bolts.

Edited to add- No, LOOSEN the spring bolts to get the clutch to slip freely. then re-tighten them to the previous tension (distance between plates)

 

[RickB]

It looks like these are all tightened just about as much as possible already...

Which is exactly why I'm suggesting A) servicing the clutch. And B) identifying the correct spring length spec to use.

 

[RickB]

Do the above, but first mark the slip clutch plates with chalk or a grease pencil or something so that you can tell they have slipped with the loosened bolts.

Edited to add- No, LOOSEN the spring bolts to get the clutch to slip freely. then re-tighten them to the previous tension (distance between plates)

Pretty much what "scribe a line" means.

 

[Texasmark]

I would keep the weather off it also. Agree on the annual service as Lineman said. If you are wondering what's the right dimension on spring length, undo it like he said, get it slipping...shining up the rust, and tighten it somewhat....all springs same dimension. If you get into moderate vegetation and it slips tighten them all till it quits. If you get into heavy vegetation you have to make the decision to tighten till it quits, or don't take as big a bite or slow your ground speed in heavy stuff.

 

[Schrodinger]

Pretty much what "scribe a line" means.

Yea, just a helpful hint on easy things to use that can be wiped off. I suck at scribing.

 

[Menagerie-Manor]

Since as you say it's been beat on you might want to check and see if all the blades are reasonably matched.

 

[ericm979]

Shear bolt AND slip clutch? Usually it's one or the other. Is that how it came from the factory?

 

[DonRybak]

As ericm979 mentions, it's unusual to have both a shear bolt and a slip clutch. The shear bolt acts as protection for the gear box if you strike something with enough force to damage the gear box. The bolt shears rather that damage the gear box. The slip clutch serves the same purpose. The disks are supposed to slip when the designated force is applied. I would never replace a shear bolt with a higher grade bolt. What if the slip clutch is adjusted incorrectly to a much higher slip rating? If the clutch doesn't slip and the shear bolt doesn't shear then something in the gesrbox faces destruction. There are numerous youtube videos on how to properly adjust the slip clutch and it's possible that the disks are worn out and have been adjusted to depend on the shear bolt to protect the gear box. Your call but I would go the safe route and make things the way they should be.

 

[RickB]

As ericm979 mentions, it's unusual to have both a shear bolt and a slip clutch. The shear bolt acts as protection for the gear box if you strike something with enough force to damage the gear box. The bolt shears rather that damage the gear box. The slip clutch serves the same purpose. The disks are supposed to slip when the designated force is applied. I would never replace a shear bolt with a higher grade bolt. What if the slip clutch is adjusted incorrectly to a much higher slip rating? If the clutch doesn't slip and the shear bolt doesn't shear then something in the gesrbox faces destruction. There are numerous youtube videos on how to properly adjust the slip clutch and it's possible that the disks are worn out and have been adjusted to depend on the shear bolt to protect the gear box. Your call but I would go the safe route and make things the way they should be.

As described in post #3.

 

[CoyPatton]

Which is exactly why I'm suggesting A) servicing the clutch. And B) identifying the correct spring length spec to use.

It is also very important that servicing the slip clutch allows for light pressure on the plates and the clutch. You want to dress all contact surfaces.
Next important issue is proper tension on the springs when servicing. Too tight and you risk damage, too loose and the cutter is useless, as the clutch constantly slips. The marks are very good suggestion as you want to be sure it is slipping during service. And it should be done at least yearly.
Your description almost sounds as though someone wanted the slip clutch defeated and to use a shear pin. Unfortunately it generally works about like your experience.

 

[Cougsfan]

I know of a case where the slip clutch was adjusted too loose and was constantly slipping, overheated drastically enough to catch some flying grass on fire and burn a field down. I agree with the grade 8 bolt and checking out the slip clutch as needed. When done and you start using it, stop after a while to check that the clutch isn't getting hot.

 

[Jason280]

Thanks for the suggestions, I'll pull the slip clutch apart this weekend and see if it makes a difference.