Rotary Cutter Need weaker shear bolt

[Snowman9000]

I have a 4' cutter on my JD 755. For a shear bolt where the PTO drive shaft connects to the gearbox, it came with a 1/2" bolt. The bolt head has no markings on it to indicate that it is high strength. It does have SJ stamped on it.

Anyway, the bolt is too stout. I have hit things that have stopped the motor, RIGHT NOW. Maybe it's because the tractor is on the small side HP wise. But whatever the reason, I need a weaker bolt.

Assuming it's just a plain steel bolt, what other ideas do you have? I can't say that I've seen brass or aluminum in half inch.

My best idea is to grind grooves in the bolt at the two spots where it needs to shear. Probably not a bad idea, but I'm open for anything else that has worked for you.

 

[Mousefield]

I have been looking for a ½ inch shear bolt for my auger, stores seem to carry grade 5 and up. My understanding, depending on the implement, grade 2 would be a good shear bolt.

 

[wrenchturner]

The SJ would be the manufacturer of the bolt. My guess is the pto shaft came off of a much larger bushhog it being 1/2 in diameter. Is it possible to drill another hole for a 1/4 bolt and place a grade 0 bolt in it. If too small then drill for 5/16.

 

[fredhargis]

How about a bronze bolt? In industry (at least in the one I participated when I had a day job) our shear bolts were all 1/2", but then we machined a groove into it that our engineers spec'd to give the right shear force for the machine. You could do this your self (I think) on a drill press using a small file.....though unless you do the calculations it would be a trail and error effort.

 

[Slacker]

Replace it with the same size bolt from Lowes or Home depot. They shear VERY easily on my 5ft Howse bushhog run by a 36PTO HP machine. I finally went and bought some grade 2 at Fastenall. I think the Lowes bolts are pot metal, but that may be a good thing for you're situation. Before you drill holes or buy expensive bolts, try the chinese crap that the box stores sell.

 

[kday64]

Have you verified that you have the correct bolt in place not just a bolt that fits.

The wrong type and strength bolt can result in injury to others.

What does your manual say?

 

[Snaker]

A 1/2" shear bolt on a 4' mower? Thats pretty big. Are you talking a 1/2" bolt diameter or 1/2" wrench size?

Are you sure that it is actually intended to be a shear bolt or could it be a mounting bolt.
If it is indeed a shear bolt, then you should see some sort of retaining setup (retaining clip?) to keep the PTO shaft from falling off.

 

[Transit]

Home D has a wide selection of bolts. The grade markings are a sires of lines on the bolt head. no lines indicates a grade 2 bolt.

 

[wrangler6831]

Tractor Supply carries grade 2 bolts. Replace it with a grade 2 and try it before you do anything else. We bought a brand new HOWSE and twisted the PTO shaft in too and also broke the u-joint. took the whole assembly back to the dealer and he accused us of replacing it with a grade 8. I suggest he look at his HOWSE on inventory and sure enough about 5 of them had grade 8 bolts. Howse stood behind it and gave us a brand new drive shaft with the correct bolts.and it shears them correctly now.

 

[unbidden]

I agree with using a grade 2 bolt. If that is still too rigid, just a thought, try using a fully threaded bolt. That at least in theory should break eaiser.

 

[jonyyuma]

All thread rod is softer too. I vote for the grade two or unmarked bolt.

 

[johnrex62]

If you are running an undersized tractor for the cutter size and must reduce the shear level below cutter spec in order to protect the tractor, I would look to cutting the shear bolts at the shear point. A small cut will greatly reduce the strength, so don't cut too deep or you will have insufficient strength to run the cutter. Also be sure that you cut where the bolt will naturally shear to make the shearing more predictable.

I would go this direction before drilling new holes for different sized bolts.

 

[MarkV]

I guess you have gotten good advice. A 1/2 bolt does sound big for that size mower. If you are talking the shaft size of that bolt I have never seen one that big. Before I would switch sizes or guess on grade I would try and find out what the manufacture recommends. A bit more info about the mower might lead us towards what it was designed to use.

MarkV

 

[nickel plate]

My Bush Hog 480 Squealer calls for a 1/2"x3" grade 2 bolt shear pin. When I did find them at one of the local hardware stores, I bought several for spares but only have had to replace it once when the original pin sheared after several years of use. It happened at my dear neighbor's place when I ran over (in tall weeds) a fairly fresh CROWBAR that one of his kids was playing with and left out in the field.

 

[Barry 1]

Tractor supply sells a PTO slip clutch that you can put on your cutter.Should fit right on it,but you might have to shorten the PTO shaft.

 

[mdelmarcelle]

I recently purchased some new shear bolts for a cutter I have. I went to a local store the sells King Kutter stuff. It's not the brand that I own, but those bolts were grade 2, marked on the package. My machine (m7040sud) likes to shear them. When they shear, they appear to be made of clay. I like the idea of cutting a grove in the bolt to reduce the strength. If you're really concerned, consider using a 1/2 inch wood dowel. It may provide the strength needed to get the mower going and if it breaks, just saw off another piece and put it in.

 

[jake98]

How about a 3/8" bolt in the 1/2" hole?

 

[Slacker]

I recently purchased some new shear bolts for a cutter I have. I went to a local store the sells King Kutter stuff. It's not the brand that I own, but those bolts were grade 2, marked on the package. My machine (m7040sud) likes to shear them. When they shear, they appear to be made of clay. I like the idea of cutting a grove in the bolt to reduce the strength. If you're really concerned, consider using a 1/2 inch wood dowel. It may provide the strength needed to get the mower going and if it breaks, just saw off another piece and put it in.

Seriously? Wood dowel? No disrespect intended but do you own a bushhog?

 

[boggen]

with "Barry 1" a slip clutch could work for you. there is another thread going i just posted too today/yesterday. that deals with slip clutch on a bush hog. and how to set them. well at least slip clutch that is on my bushhog. granted mine is most like 50 plus years old. ((honestly dont know age))

new slip clutchs out on the market most likely you adjust differently than this old beat slip clutch i have. but you could set it. and not worry about a shear bolt. granted slip clutchs that fit between tractor and pto shaft or pto saft and gear box on bush hog. are a costly little item. but if shear bolts are not working for you. it beats repairing possibly much more costlier stuff on the tractor.

though everyone elses comments about getting a grade 2 shear bolt. would most likely be a good thing to try first. (it would be cheaper) to test. but *shrugs*

just remember shear bolts and slip clutchs are meant to break, or allow slippage. for say when you hit a very large rock or hit a stump and stop the blade dead cold in there spot.

if you are stalling out your tractor by mowing very thick tall weeds and most likely very wet. then you are trying to cut to much at one time and a shear bolt or slip clutch is not going to help you. but you will need to take "half passes" and set the bush hog up as high as it will go. and make it so front of bush hog seats lower than rear of bush hog. so it only needs to cuts the weeds once vs mulching the grass and cutting it a couple times before getting out from under the tractor. and go with lowest possible ground speed. and higher engine / throttle speed. if you are making first pass around a given area before you can start doing half passes.

if you have a FEL (front end loader) with general duty bucket on the front of the tractor. lay the bucket down flat on the ground then tilt / curl the bucket up some. then raise it a couple inches off the ground. and raise as needed to say go up a hill. but try to keep it a cuople inches off ground and mow forwards. this will act like a ""bumper"" so if you hit a big branch of a tree that fell down or a stump your bucket will hit it first causing the bucket to hopefully to run up and over it. causing you to come up and out of your seat or hopefully cause enough vibration for you to notice there something there, and you can slam brakes and push in the clutch ASAP. and get to a stop and check things out of what is there.

if you do not have a FEL on the tractor. and this bush hog is a 3pt hitch version and not a pull behind. then i would suggest you have some sort of suit case weights or something on front of tractor. for counter balance if not you risk rolling tractor over. them bush hogs can weigh rather heavy and the extend far from the rear end of tractor that causes even more teter totoring effect. and can cause your front tires to come off the ground and you loosing control of the tractor. trust me it is no fun.

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i really do not suggest cutting into a bolt some to weaken it. when it shears if it does shear your next bolt you may not take enough out of it or take to much out of it. and you will be all over the place doing guess work. at least if you went with a say a grade 2 shear bolt you have a known working specs of force you can rely on.

a slip clutch and if adjusted on some trial and error things. would give you a better setup possibly. but from understanding. requires you to readjust and maintain every year before you first use it for the mowing season. so you know it works correctly and is not rust frozen in place or what not.

 

[johnrex62]

i really do not suggest cutting into a bolt some to weaken it. when it shears if it does shear your next bolt you may not take enough out of it or take to much out of it. and you will be all over the place doing guess work. at least if you went with a say a grade 2 shear bolt you have a known working specs of force you can rely on.

Boggen, I think I understood Snowman say that he is using grade 2 bolts and his tractor is marginally underpowered for the size of the mower he has. His problem is that the bolts are not shearing before his tractor stalls when he hits obstacles.

Given that, sticking with the grade 2 bolt will not protect his tractor since it is already proven to shear at a higher level than necessary.

I concede that cutting a bolt does not provide accurate shear specifications, but then is that necessary or is getting in the ballpark good enough? Sure it might shear too soon from time to time, but that is better than not shearing at all as is the case with the unmodified bolt. Without an engineering analysis of his load and tractor capabilities it is impossible to know the exact shear strength required, so best guess is the best approach I can think of to keep working and dial in an adequate protection comfort level. It will take a bit of trial and error, but each trial will be less risky than continuing on as he has with the existing grade 2 bolts that have not sheared. Don't you think?