Best Guess for PTO Shear Bolt

[4Papa]

I've got a new Chinese made, heavy duty 86" articulating flail mower that sheared a shear pin when I hit a (hidden by tall weeds) deteriorated round bale. The hammers stopped dead and I saw billows of white smoke coming from the gear box/belts area. At first I thought I destroyed the gear box as neither the operating/parts manual says anything about using shear pins. I figured they figured the belts would slip if the 20 2.5 lb hammers hit an unmulchable object. After removing the PTO shaft I discovered the collar nearest the gear box was missing a bolt, presumably the shear pin. A call to the distributor from where I bought the unit is looking into it, but I figured I'd get faster, more accurate answers from this forum. So the question is, what specs should I look for in the replacement bolt? I know the hole will fit a 3/8" bolt at least 1-1/2" long, but I'm not sure they didn't have a 1/4" bolt in there. I assume shear bolts should snugly fit the holes they occupy. I also don't know if they used a Grade 2 or 5 bolt. My inclination is to go with the higher grade as the mower is designed for tractors up to 120 hp. Thoughts?

 

[MESSMAKER]

Most shear pins are grade 2, 1/2 inch bolts about 4 inches long.

 

[AWSubie]

I would go with a grade 2, 3/8, and if you start shearing bolts under normal load then go up to grade 5.

 

[oldnslo]

Being Chinese it could be a 10 mm bolt vs 3/8. I suspect 3/8 would be close enough. Did the old bolt shear clean or was the ends smeared over making the bolt hard to remove? Normally softer bolts grade 2 will smear more than harder grade 5.

Like AW, I would start with grade 2 since shear pins are cheaper than most other components

 

[CalG]

I've got a new Chinese made, heavy duty 86" articulating flail mower that sheared a shear pin when I hit a (hidden by tall weeds) deteriorated round bale. The hammers stopped dead and I saw billows of white smoke coming from the gear box/belts area. At first I thought I destroyed the gear box as neither the operating/parts manual says anything about using shear pins. I figured they figured the belts would slip if the 20 2.5 lb hammers hit an unmulchable object. After removing the PTO shaft I discovered the collar nearest the gear box was missing a bolt, presumably the shear pin. A call to the distributor from where I bought the unit is looking into it, but I figured I'd get faster, more accurate answers from this forum. So the question is, what specs should I look for in the replacement bolt? I know the hole will fit a 3/8" bolt at least 1-1/2" long, but I'm not sure they didn't have a 1/4" bolt in there. I assume shear bolts should snugly fit the holes they occupy. I also don't know if they used a Grade 2 or 5 bolt. My inclination is to go with the higher grade as the mower is designed for tractors up to 120 hp. Thoughts?

PTO shaft shear bolts are often grade 8

You could do the math, but a 10mm grade 8.8, or 3/8 " in grade 8 (if that is a good snug fit in the holes) is where I would start.
100 hp is gonna take some toll on an ungraded piece of steel wire.

Note : grade 8.8 is 120ksi material while Grade 8 is 150ksi. so not that much to say one over the other as far as "strength" goes.
shear being about 75% of tensile.

Anecdote: Meteor Snow blowers use grade 8 or 8.8 shear bolts on the PTO. ungraded (G2) for the fan and auger.

 

[4Papa]

Being Chinese it could be a 10 mm bolt vs 3/8. I suspect 3/8 would be close enough. Did the old bolt shear clean or was the ends smeared over making the bolt hard to remove? Normally softer bolts grade 2 will smear more than harder grade 5.

Like AW, I would start with grade 2 since shear pins are cheaper than most other components

There was no bolt to be found, so I assume a clean shear.

 

[4Papa]

PTO shaft shear bolts are often grade 8

You could do the math, but a 10mm grade 8.8, or 3/8 " in grade 8 (if that is a good snug fit in the holes) is where I would start.
100 hp is gonna take some toll on an ungraded piece of steel wire.

Note : grade 8.8 is 120ksi material while Grade 8 is 150ksi. so not that much to say one over the other as far as "strength" goes.
shear being about 75% of tensile.

Anecdote: Meteor Snow blowers use grade 8 or 8.8 shear bolts on the PTO. ungraded (G2) for the fan and auger.

Depending what I find, I may buy a few pins of each grade (2 and 8) and experiment.

 

[CalG]

Pick up several grade 5, they might be "just right". ;-)

My money is on the higher grade. However, I don't know the particulars of the layout.

 

[WranglerX]

I would go with grade 2, as this what I see most recommended..... If you still have manual that came with mower I would see what it recommends...

Dale

 

[oosik]

Well - - IMHO - what does your Chinese Op manual have to say about the shear bolt. I've had three implements with shear bolts - 3-point snow blower- Grade 2 Wallenstein Bx42S- Grade 5
Walenstein BX62S- Grade 8. Sooooo.........what do I know.