Rotary Cutter Need weaker shear bolt

[boggen]

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?

i have sheared bolts in half. that were not meant to be shear bolts! (frowns) just because they got a little loose the vibrations alone from mowing acted like a saw zaw and cut the bolts in half.

i have seen a few tractors limping back to the shed. with only half of the PTO shaft. do to the shear pin sheared away! and without them realizing it. the back half of PTO shaft fell off as well and went missing some place. retaining ring to hold shaft on was gone as well.

i am hesitant. to say cutting a grove in a bolt. you allow more play within the hole the sheer bolt goes through. that can cause oval of the holes and long term could result in not needing a grove a bolt. due to bolts just being sheared in half or i should say cut in half by pure vibrations alone.

i am hard on equipment. i run things to there limits at times. and at times keep pushing the limits till something does break or the job gets done. also not a real big fan. of possible parts flying out like a bullet when they do break. have had enough rocks shoot out from under a finishing mower deck.

being able to know after a couple trail and errors with a slip clutch. and then going back to that setting each year during maintance. and knowing and being more able to count on when things do slip. to me is a more reliable piece of equipment. vs relying on luck of non stop trail and error. and trying to find the shear pins after a year or two some place in the grage. (ya i labeled the box) but which box among all the other boxes of bolts, nails, etc... or did i place it....

picture is from www.kingkutter.com rotory kutter manual

 

[MNBobcat]

I posed the question to friend who has been a machinist for over 40 years. He said use a grade 5 bolt.

 

[johnrex62]

i have sheared bolts in half. that were not meant to be shear bolts! (frowns) just because they got a little loose the vibrations alone from mowing acted like a saw zaw and cut the bolts in half.

i have seen a few tractors limping back to the shed. with only half of the PTO shaft. do to the shear pin sheared away! and without them realizing it. the back half of PTO shaft fell off as well and went missing some place. retaining ring to hold shaft on was gone as well.

i am hesitant. to say cutting a grove in a bolt. you allow more play within the hole the sheer bolt goes through. that can cause oval of the holes and long term could result in not needing a grove a bolt. due to bolts just being sheared in half or i should say cut in half by pure vibrations alone.

Boggen, If you take a bolt, lay it on the table, and make a small cut perpendicular to the bolt length; how does that allow more play within the hole? The bolt is inserted in the shaft and is the same diameter as before. The cut is aligned to the inside diameter of the PTO yoke and the outer diameter of the gearbox shaft so that it reduces the strength in the same line as the intended shear-line.

I understand that bolt fail and sometime an operator will not notice a sheared pin, but we are not talking about a loose or poorly maintained bolt. Regardless of the operator's attention to the job, the shearing of a pin is the intended result when the workload exceeds the capacity of the equipment. In this case the problem is that the mower is more capable than the tractor. When he hits a rock or stump it stalls the tractor rather than shear the pin. That is a problem. His pin needs to break before the tractor does.

Btw, I just got confused about what you intended with the manual page. I know where the retaining clip and shear pin go. That has not been an issue in the thread thus far. Did you have another intent?

I posed the question to friend who has been a machinist for over 40 years. He said use a grade 5 bolt.

Bobcat, Are we reading the same thread? If his grade 2 bolt stays intact when he hits something solid enough to stall his tractor, why would it be better to upgrade to a stronger bolt? That would make sense if he is blowing through shear bolts, but that is not what I read.

 

[nickel plate]

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.

mdelmarcelle,
Have you actually tried what you're suggesting? "It may provide" connotes that you have not. So to the test-I cut a 3"x1/2" hardwood dowel, inserted it into the shear hole on my Bush Hog 480 Squealer, engaged the PTO and that fat wooden toothpick sheared before one full revolution. Hey marcelle! Are you listening to me?

 

[Carykong]

I have used fully threaded bolts on my 5' Howse with moderate success. They break too easy. I broke them sometines just engaging the PTO at idle.

 

[eastexan]

I have used fully threaded bolts on my 5' Howse with moderate success. They break too easy. I broke them sometines just engaging the PTO at idle.

Not grade 5 though, I'll bet.

 

[DarkBlack]

For instantanious loads, It's not the HP of the tractor that decides it. It's the inertia of the rotating mass. A bolt shears in milliseconds when you hit a rock. for example: The govenor on a 100hp tractor could be at your 25hp when the bolt sheers, it happens too quick for the govenor to direct the injectors to increase the fuel to increae HP. So you 1) add mass to your PTO shaft ( weighted collar, etc..). 2) use a fully threaded ungraded bolt, or un-hardened threaded rod, or 3) go down to a 7/16 ungraded bolt. Home depot's clear head bolts are nice soft junk. Cutting 1/2" bolts can wrk but it's more trouble and less reliable because the needed inital stress riser points can be where you didn't plan for them with cuts