Shear bolts

Heyseed

Member
Is a grade 2 bolt harder or softer than a grade 3?
a268747.jpg
 
Hmm...there really "is" a Grade 3 bolt! Appears to only be in metric sizes though, as near as I can find.

Only seeing them for sale on eBay and from China & India.

There's some info on Grade 3 bolts here, but so far I can't find info directly comparing them to Grade 2. However, as Grade 5 is harder than Grade 2, and Grade 8 is harder than Grade 5, it stands to reason that Grade 3 is somewhat harder than Grade 2.
http://www.pmmetal.com/special-stee...l-fasteners/astm-a194-grade-3-fasteners/#info
 
The higher the number, the stronger the bolt.

Doubt you will find a grade 3. A bolt with no marks is a grade 2, 3 marks is grade 5, 6 marks is an 8.

But using a higher grade bolt for a shear bolt can give varying results.

If the holes are still in good condition, not wallowed or elongated, using a harder bolt will defeat the purpose of having a shear bolt and could cause damage to expensive components.

But if the holes are damaged from previous failures, going to a higher grade bolt could cause the harder bolt to snap at an even lower torque than the soft bolt. Then the broken remains spinning inside the shear coupling will cause more damage and be difficult to remove.
 
just a reminder. a shear bolt is the bolt itself, not the threads. thread area is softer. make sure the shear area is in the meat part of the bolt!
 
Thanks guys, I can't find the package now that they came in, but it said grade 3. They have three marks, the zinc ones at Rural King have no marks and are labeled as grade 2. This is for a Bush-Hog rotary cutter and the holes are clean and sharp. I'm going to use the grade 2 and keep on mowing.
a268755.jpg
 
I suspect the grade 3 label was just that, a label.

Probably whoever had the packaging printed saw 3 marks and assumed "grade 3".

As for metric grade 3, that may be possible, not that familiar with metric grading, but no metric bolt I've seen uses marks for grade, it's always a number.
 
Thanks Steve, I wondered about that too. Maybe it was a typo on a grade 5??
they are 5/16 so definitely not metric.
These birds like the tall grass so much I kinda feel bad about mowing.
a268763.jpg
 
Beautiful pics!

Since we are discussing shear bolts... I picked up a subsoiler recently, and it has a pin (like this at steiners in place for the shear bolt. I'm pretty sure this isn't a proper shear bolt to use. I read on the forums that you can put a groove in a bolt to help it shear cleanly. Would that be good to use with say a grade2 bolt?
 
I always thought you should use grade 5 for shear bolts because they break cleanly. Grade 2 will stretch, bend, and jam in the hole.
 
Good idea. We bought a new 6 ft 3 pt rotary mower from TSC a couple years ago. Brother was mowing tough stuff, so broke shear pins quits often. He grabbed a dozen grade 2 bolts out of the TSC bolt bin and got to work. He sheared several bolts and kept replacing them from the ones he bought at TSC. He got into tough stuff and twisted the week old PTO shaft into a pretzel. The shear pin that did not shear was a grade 5 bolt someone had looked at and tossed back into the grade 2 bin. Back to the TSC store for a new $110 PTO shaft and another dozen grade 2 shear bolts. I convinced brother to inspect the head marking of every bolt he picked from the grade 2 bin, then inspect them again whenever he replaced one in the mower. Expensive lesson learned. Just lucky we did no damage to the tractor or mower gearbox.
 
(quoted from post at 15:09:50 05/26/18) I always thought you should use grade 5 for shear bolts because they break cleanly. Grade 2 will stretch, bend, and jam in the hole.

Tom, shear bolts come in all different sizes and hardness grades depending on the machine. When buying used equipment one wants to check the manual and make sure that the right grade is in there. One thing for sure it is better to have a grade that is too low than one that is too high. If the hole is wallowed out the softer bolt can and will deform a little but it will still shear when it needs to saving you down time and major repair costs.
 
I've got an old Rhino SE-6 and it is shear bolt protected. For years I did not shear the same grade 2 bolt, but when I got a much newer and more powerful tractor, it will shear them easily. You might scalp a few times before it goes, or just hit something more substantial like brush that is too thick, a tussock along the edge of the marsh or anything of the sort, will shear like it is supposed to. I think a slip clutch would be better but other than the above, grade 2 works like it should and you know it will shear reliably if too much of a shock load is applied. I have seen the damage caused by using grade 5 or 8 in these, goes right back into the tractor and the most recent one was a JD 2020, broke the pto sliding coupler, requiring a full disassembly to repair.
 
Everybody in our neighborhood had Brillion 6 ft pull-type rotary mowers, Bush-Hog's. We pulled ours with my Super H, mowed hay with it, clipped pasture, chopped corn stalks, hundreds of acres a year. Dad would buy me the cheapest 5/16" grade 2 all threaded bolts for shear bolts, and eventually I would shear them, have to replace them. If I dropped the mower just a bit too much and scalped the ground they would shear almost instantly. Just before the land auction, owners of our home place selling the buildings & ground, Dad was Not interested in buying the place, He wanted to mow some weeds, trim up around the place, used the 4010 with live pto, couldn't even engage the pto at idle using the 4010 with the cheap soft shear bolts he got me, so he went to town, got a handful of grade 8's.

Yep, it was good fun for him to mow waist high grass in 5th gear for a while, till the output shaft from the gearbox twisted in two flush with the oilseal. Took him a while to realize he wasn't mowing anymore. Expensive repair. He fixed the gearbox, and the next day that moneypit POS 4010 tried to shift from 540 to 1000 pto while mowing, stub shaft loosened up. Tractor left the farm to get repaired, guy at the repair shop knew somebody looking for a nice 4010/4020, new paint, 4 new tires, 50-60 hours on a fresh M&W engine rebuild, new engine and pto clutch, fairly low hours. I never saw it again. I even hauled concrete to the farm the kid lived on. Less than a year later something in the transmission or rearend failed. Tractor was Cursed!
 
Related question: How tight should the shear bolts be? I have an old International trail behind cutter that measures about six feet across. I pull it with an 8N. It's about all that tractor wants but seems to do OK as long as I take it easy. And yes, I do have an over run clutch on it.

I have never broken a shear bolt with this combination. I used to pull this cutter behind an H Farmall. I was always breaking bolts with it. Got the 8N about five years ago and tried it and did fine. I did notice the shear bolt is really tight now. I can see how it would take away from the effectiveness if the shafts can't move any.

So, again, how tight should the bolts be?
 

Separate from assembly instructions for structures or pieces of equipment, or engines etc. fasteners have their own torque values. Try googling torque values and you should find them for all different grades and sizes. I have a booklet in a drawer in my shop from fifteen-20 years ago so it should be on the 'net now.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top