Shear bolts and overpowering equipment

ScottNC

Member
I have had repeated trouble with failed shear bolts on a brush hog. Pretty sure the problem is/was with me. I'm new to two-stage clutches and have moved from a 23hp gas to 47hp diesel tractor. I figured once I got the hang of the two-stage and used the correct diameter shear bolt, I would spend more time mowing, less replacing sheared bolts. Bought four shear bolts and ended using all of them yesterday. Just letting the clutch out too quickly will fail a shear bolt, mowing along and hitting a fire-ant mound will fail one, mowing into heavy grass or a one inch sapling - snap! Having the blades go to ground shears them as well. Over six years, the gas tractor rarely snapped a bolt no matter the sapling size or ant mound size, so it must be me and this tractor.

My question is this; while it is common for a piece of equipment to require a certain level of horsepower tractor, is it conversely true that you can over power a piece of equipment... like a simple brush hog?

It's an older, unknown mfg. brush hog, spent it's life behind a couple of Ford N's and a 1900 before being gifted to me. I am considering going up to grade 5 bolt.
 
Short answer yes. It takes the same amount of energy to spin the mower up as it does to power it down. The more "hp" (and torque) you have the quicker you speed it up and the
more "load" the shear bolt sees. Also if when you hit the dirt, mound, or sapling, when previously, you would bog the motor down b/c it didn't develop enough torque to shear the
bolt, now you have ample reserve torque and the "weak link" is the shear bolt (as it was designed).

While it is possible the original design may have been a higher grade bolt, if the shear bolt becomes too strong, then your weakest link might be in the shaft or the gear box,
which will take more time to fix than putting a new shear bolt.

The shear bolt needs to fit snugly (ie not loose) in the hole otherwise the shaft will "hammer" on it and it will shear more quickly. Increasing the diameter will also get you a
higher capacity, but probably not as much as going up a grade.

Finally, you should not have any threads in the shear plane, as this will also reduce the "capacity" of the shear bolt.
 
My brother had similar trouble, not so much troubled by the shearing, but the hassle of replacing the bolt. He installed a slip disc in the PTO driveline,
adjusted it properly and never gets off the tractor for shear bolt issues - Money well spent.
 
Are you engaging the cutter at full throttle? I believe in engaging equipment with the tractor idling and then power up, just
makes more sense to me. And like bjb said, the shear bolt needs to fit, snug! I had a friend that was using 3/8 bolts in a 1/2
hole because they went in easier, and then he wondered why they were shearing. Also, if the yolk is worn and sloppy on the
shaft it will shear easier. A 47 hp tractor should not shear bolts that easily.
 
Ha-Ha... Dad had a 6 ft pull type Brillion rotary cutter/mower/Bush hog. I pulled it over thousands of
acres with a 33 PTO hp Farmall Super H. It used two 5/16"x 3" bolts as shear bolts. Dad got me the
softest fully threaded (different name for half broken) bolts available. I broke them occasionally, no
big deal. I was working another job one summer, Dad wanted to mow some grass waterways and some tall
weeds around the building, wanted to mow in reverse and stop or inch backwards. So used the 4010 Deere,
fresh rebuild with M&W 4020 kit, couldn't feather the PTO easy enough to not shear the shear bolts. Ran
to town and bought a dozen grade 8 bolts, and he was in business. Then about the second day he's sailing
down a grass waterway in 5th gear cutting weeds 3 ft tall and then he looks behind him, he was just
knocking the grass down, he turns around and heads up-stream. There lays the entire blade assembly and
half of the output shaft from the gearbox, twisted off flush with the oil seal. Not sure how much that
repair cost him but more than the hard bolts by large margin. He got the mower repaired and ran it for
another hour or two and the 4010 tried to shift from 540 to 1000 rpm PTO which shut him down again.
Tractor went to the repair shop that did the engine rebuild and was split and repaired, new engine clutch
too. Tractor was sold and new owner had M&W turbo added. New owner came back to visit about a year later,
wanted to know how much Dad was going to pay of the transmission and final drive rebuild. A quick
explanation of "As is Where Is, no warranty expressed or implied" and the new buyer was on his way.

I really hope that tractor was parted out, scrapped, sent to China or something. Every year it cost about
what Dad paid for it to keep it running. A relatively low hour 5 year old tractor should not have a
serious break-down every year.
 
An N had what 17 hp,going to a harder shear bolt will just break the gear box. Lint the mower and
idle down before putting it in gear. What you are going to need is a bigger mower.
 

OK, you can install a Friction (Slip) Clutch and It can cure your problems, BUT.. you MUSt have it adjusted to SLIP when it needs to...

That means knowint that you MUST loosen the clamping springs every spring and slipping it a good bit, then setting the tension ONLY tight enough to handle the load the mower can handle...
 
Sounds like someone should have got a new operator just try replacing of a shear bolt to get it to hold more horsepower shows how bright the guys running that operation must be but can?t figure out why nothing will stay running hmm 🤔 I wonder why
 
Huh, my post of yesterday is gone. Lets try again.

Easing the clutch out with the deck raised at low engine RPM was lesson number one. Number two was to NOT push the clutch pedal all the way down every change of direction or gear selection.

The yoke turns freely on the gearbox input, looser than I'd think prudent but the there is no wear visible. Like it has always been that way. Bolt fit is a slight drag but can be done with the palm of the hand, no hammer required.

Had an epiphany; I am beginning to suspect the slop in the telescoping, square drive section of the PTO shaft might be a large part of the problem, it's pretty loose there. I can see varying loads (dense, high grass then a near bald patch then high grass again) allowing the shaft to load, unload, then load again, much like a jack hammer. Plus, the reach from gearbox to the tractors PTO splined shaft is longer on this tractor - which would magnify the slop. What do you think, sound reasonable?
 
I use Grade 2 bolts because they take several whacks before they shear and when they do they smear so you have something to find when looking for the part
stuck in the implement shaft slot. I use self locking nuts and do not tighten all the way down...leave slop between the bolt head-nut and the yoke. Yes on idle
rpm to start mower....yes on getting used to half way (or thereabouts with your machine) for ground movement changes with the cutter rotating. If you forget,
cut the throttle back down to idle before you let the clutch back up from all the way down. Should improve your performance even with sloppy fit of PTO shaft
yoke to input shaft on implement.
 

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