Shear strength of a bolt

Can someone tell me the shear strength of a 1/2" bolt in grade 2 and grade 5.

Reason I ask.
I bought a Howse 5 foot economy brush cutter. Has been working great so far.
It has a cutting capacity of 1.5 inches
I am under the impression that grade 2 bolts should be used for shear bolts but the manufacture web site says to use 1/2" X 3 1/2" BOLT NC GR5 HHCS ZINC for the shear bolt.
The dealer gave me several bolts when I bought the cutter but I do not know what grade they are.
They only have the letters SBY on the head of the bolt.

Thanks for your help.
 
grade 2 is 44,400 psi shear strength

grade 5 is 72000 psi shear strength

1/2 inch bolt area course thread is 0.141 in squared
 
Think I would discard the mystery bolts and go with what the mfg recommends. My guess on the SBY bolts would be they are grade 2.
 
The amount of lines or ridges on the bolt head denote grade. Grade five bolts have three lines or ridges. This should help identify the misc bolts the dealer gave you.
 
I would use the ones they gave you, they sound like grade 2, and then that's all I would use. Grade 5 and you are going to break something expensive!
 
If it calls for a grade 5, use a grade 5. Grade 2 will work as well and you shouldn't ever not shear a bolt when you should've, but if the book says grade 5, go grade 5.

Donovan from Wisconsin
 
John watch putting a soft bolt in for a shear bolt. IF the manual calls for a grade 5 bolt then USE a grade five bolt. Why worry about a grade 2 bolt??? I have seen yokes ruined buy grade two bolts. What??? That is right ruined by too soft a bolt. The reason is the softer bolt did not shear clean, it smeared around the shaft. This locking the yoke onto the shaft. Almost like friction welding it. A slightly harder bolt with shear cleaner.

So when all else fails go with what the manual says. LOL

I have found that using stainless bolts work well for shear bolts and they are more brittle. I had problems with the break away markets on a planter. The regular shear bolts would shear jaggedly and were hard to replace. I happened to only have a stainless bolt one day and used it. It was the same grade two bolt but sheared clean and fell out. So I switched to all stainless bolts on that market setup and it has worked out much better.
 
Grade 2 fasteners (3/4" diameter or less) have a tensile strength of 74,000 psi, while Grade 5 fasteners have a tensile strength of 120,000 psi. So the shear strength of a Grade 2 bolt will be roughly 60 percent of the shear strength of Grade 5 bolt of the same diameter.

It sounds like your dealer gave you some Grade 2 bolts. The fact he gave you extras indicates he assumes you're going to break some. A lot of people are of the opinion that you should use soft shear bolts. I'm not one of them, but it wouldn't surprise me if your dealer substitutes Grade 2 bolts for the original Grade 5s when assembling machines.

I would go with Grade 5 as the manufacturer recommends. The quality of Grade 2 is spotty, and in my experience they are soft and will fail under normal use. Break enough pins and your yolk will get loose. Grade 5 will break when they need to.
 
I totally agree. Tried a bolt from my bolt bin on my baler. Bolt sorta pulled apart and a piece got stuck between flywheel and drive.
 
Bolts and PTO parts on the hog are a whole lot cheaper and easier to replace than
PTO parts on the tractor.However if I'm using a cutter where its going to shear any kind
of bolt on a regular basis I'm going to replace that set up with a slip clutch on the cutter to protect the drive line on the tractor.
 
Funny you mention this. I've been trying to get used to the engagement process of a Ford independent pto on my new to me Ford/New Holland 4630. I made sure to read the operators manual and its clear on how its supposed to work, still a bit confusing to as to how to "feather" one of these, (their language), if you engage the lever too quickly, it just shears the bolt cleanly and immediately.

It seems to me you are supposed to move the lever to the stop,(which can be adjusted to find the "sweet spot") Stop briefly at that point and then slowly move the lever forward to engage. It seems to make a ratchet noise, thought it was grinding, hope like heck its not, I have feathered it in one time like my 850 ford's trans driven pto, nice and quiet, but that seems to escape me to duplicate, but I can engage the pto without shearing grade (2) 1/2" bolts at least. They do tolerate quite a bit, but I finally sheared one on a stump, so it performs just as it should. Mine also has shear type shoulder blade bolts, left hand thread, those will shear off if the blades get jammed up with a large object.

If it calls for grade 5, then the manufacturer must have it designed that way and I would use what is called for, but I would also question how it still works the same to shear properly before any damaging shock load is transferred.

In comparison, to the above scenario, there's no way I'd want a grade 5 on this mower or any pto powered implement and this tractor, given the shock loads. I don't mind changing them and they do shear perfectly every time, and I can at least know that it works like it should to protect both the tractor and implement. The ones I am using are from TSC 1/2" x 3 1/2" grade 2. The supplier of the bolts is Hillman.
 
So John--what are your thoughts on the relative difference on the shear bolts strength of grade 2 vs grade 5? do you have any info on the maximum torque of the brush hog or your pto unit?
 
I was more interested in the ratio between the two bolts because I do not have any info on what the PTO shaft will handle. I own a Ford 861 and have seen plenty others asking about twisted PTO shafts on this board so I do not want to push it to hard.

I do not know what kind of bolt was in it from the factory but it sheared the first time I let out the clutch. I installed one of the bolts the dealer gave me and they have been working great. I feel they are grade 2 bolts as they have no lines on the head of the bolt. I think the SBY is just a manufacture logo on the head.
I have sheared 2 bolts so far. 1 when I hit a stump in the weeds and 1 when I tried to cut a tree a little bigger than I should have been cutting.
The cutter is rated at 1.5 inches. I have been clearing about 5 acres that has not been cut in several years. Plenty of trees in the 3/4 to 1.25 inch range and it has been cutting them with no problem and not shearing the bolt.

So basically if these are grade 2 bolts they are shearing at what the manufacture says the cutter is rated at. I am kinda fearful of using the grade 5 bolts after hearing how much stronger they are. I would rather replace a few bolts they break something expensive. Once I get the area clear I should never shear another bolt as I will be able to keep it cut now with this brush cutter. I do understand using to soft a bolt; and breaking bolt after bolt will mess up the yolk to gearbox shaft fit.

I think my next step is gona be talking to the dealer and asking him if these bolts are grade 5 from the factory or are they just a box of grade 2 bolts he bought at the hardware store.
I think the latter as the manufacture web sight shows nylon insert lock nuts and the ones he gave me are regular nuts wit lock washers.

Thanks for your insight guys.
 
I just did a random search of various manufacturers' manuals, and it seems most recommend grade 2 bolts. That includes King Kutter, Woods and Behlen. That said, I'll continue to use grade 5 for my brush hog, as I would be replacing shear pins every time I use it with grade 2. Of course, the manufacturer doesn't know what tractor you have, so the call for grade 2 may be as much to protect tractors with weak PTO shafts as to protect the cutter.
 
I have that same brush cutter and I use a grade 2 carriage bolt as a shear pin. I do have to change it once or twice a year. But at least I know I am not damaging the tractor when I hit something solid. Just be careful not to engage your PTO when you are throttled up and you will be fine.
 

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