Brush mower shear pin

[johninmn]

Long story kinda short.....
I cobbled together my old brush mower with a new gear box after the old one shelled out. The new gear box is rated at 30 hp. The new gear box shaft has a 1/2 inch shear pin hole. The old drive shaft has a 3/8 shear pin. I put a 3/8 grade 5 bolt in and it held up just fine so far but ive only used it about an hour in heavy grass no brush.
Question 1: am i doing damage using a 3/8 bolt in a 1/2 inch hole?
Question 2: does a 1/2 inch shear pin seem like overkill for a 30 hp gearbox?
Question 3: i don't have a slip clutch so should i be using a grade 2 bolt?
Its a 5 foot mower and i pull it behind my H so im sure im not surpassing the rating on the new box.
Thanks all

 

[MN Mike]

I used a 5/16 grade 8 in my 5 footer and blew the bolt about once a year during normal use so 3/8 seems about right. might see some wear in that 1/2 inch hole though over time.

I'd be concerned about a 1/2 inch bolt not shearing before damage was done but maybe they expect a grade 2 bolt on that size hole???

 

[MN Mike]

correction. I use a grade 5 also.

 

[johninmn]

That was my thought exactly! But i didnt really want to drill out the old shaft before seeing what others thought

 

[old]

I would figure out some sort of bushing so you have a tight fit instead of a sloppy fit where the 3/8 bolt is in a 1/2 inch hole

 

[Russ from MN]

King Kutter recommends a 1/2 gr 2, but I know JD uses a smaller and harder bolt. I would drill it out to 1/2 and use a gr 2. And while you are drilling put a grease zerk in it so when it shears it doesn't weld itself to the shaft.

 

[Janicholson]

Good Idea, The bushing cannot go through both components, or it becomes an added shear component. Maybe drilling a new shear hole next to the 1/2" would be reasonable. Jim

 

[Tom Dowling]

Question #1 you will mess up the 1/2 inch hole
Question #2 1/2 inch # 2 no 8's or 5's unless you want to break something worse than a shear bolt If you hit something solid
Question #3 softest you can find, buy several

 

[jeffcat]

I would stay with a soft bolt but.......if you want to go with a 1/2 then I would make my own shear bolt. Copy a snow blower shear pin design. Take that 1/2 bolt and make a couple of little groves in the bolt on the grinder wheel. You now have a tight fitting "3/8" shear pin in a 1/2 hole so it will not wear. I like the grease fitting idea a lot!

 

[rustred]

well yes, you cant run a 3/8 bolt in a 1/2 hole. the hole will just wear oblong and the bolt will wear also. just not the thing to do. you need a grade 2, 1/2 bolt, or just use a 1/2 " roll pin. and the other thing you can do is get a bushing made for a 3/8 bolt for the new gear box shaft and pound it in ... that way both shafts are 3/8 and good to go. you got a new geaqr box so do it right , otherwise that hole will wear and break the shaft!

 

[Russ from MN]

I did that when we used to run a 5 foot King Kutter on a JD M. I put the bolts in a lathe and relieved them in the shear points. I was worried about damaging the little M gearbox, I heard they were weak.

 

[Jon Hagen]

We have a near new 6 ft TSC mower that uses a 1/2 inch grade 2 bolt for a shear pin. We were using it in tough stuff and broke a few shear bolts. We accidently got a grade 5 bolt mixed in with the grade 2 shear pins. We got into some tough stuff and totally twisted /wrecked the 2 week old PTO shaft because of that tough grade 5 bolt. Lucky it did not shell out the mower gear box or tractor PTO.
That was a hard $200 lesson, we now carefully check the grade marks on any new shear pin.
You may damage the 1/2 inch shaft hole with that hard 3/8 bolt.

 

[K-Mo]

Something to think about.
Is there anything to prevent the shaft from disconnecting from the gearbox when the pin shears. Some setups have a snap ring to keep the shaft on.

 

[johninmn]

Yes there is a snap ring to keep tbe shaft on