Best Guess for PTO Shear Bolt

[Rock knocker]

I meant grade 8. You were infering that Class 8.8 and Grade 8 were broadly equivalent.

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.

I would never use a Grade 8 as a shear, and would be hesitant to use a Grade 5.

 

[PILOON]

My observation is that the softer bolts will stretch B4 sheering and be hard to remove. They will also cause the mating surfaces to get damaged while A higher grade bolt will sheer clean and simply fall out.
Same goes for snug fitting, use the correct diameter for best results. From personal experience I'd be using grade 5 especially due to the HP this implement was designed for.

OK, my experience has been mainly with lower HP and small snow blowers but I think relevant just the same.
On a 5 ft blower driven by a 20 hp engine 1/4" grade 5 gave me the best performance and I later learned that it was what the factory actually specified.
When I used soft bolts I discovered that the shear faces were mangled and bolts were stretching.

 

[Texasmark]

I use grade 2 that fit correctly in the slots and don't tighten the nut all the way so it can move in and out a little. This has allowed me to get serious whacks numerous times before the bolt fully shears and has prevented them from shearing fully as frequently as when I used Grade 8 (as recommended by JD on my baler for one instance) which snaps when hit.

Other thing is I can find the hole in the gearbox shaft much easier with the Grade2 as it smears when it shears and the smear is easier to detect on the shaft than a snap that is just the size of the hole.

Also I use SAE bolts because they are cheaper and Metric, where applicable, are usually slightly larger than SAE so a SAE bolt of comparable size easily fits a Metric slot.

 

[CalG]

I meant grade 8. You were infering that Class 8.8 and Grade 8 were broadly equivalent.


I would never use a Grade 8 as a shear, and would be hesitant to use a Grade 5.

No, I was not inferring that. I gave the tensile values for both. If you infer that 150ksi is broadly equivalent to 120ksi material, that is your choice. I find the two values sufficiently different to not interchange if one or the other value matters.
For bolting the handle on a sidewalk wagon, either will serve just fine.

 

[CalG]

4Papa

Is that tool running 540 or 1000 rpm input?

'makes a difference.

 

[CalG]

Just because I find this "shear bolt protection topic interesting.

100 HP is about 1000 pound feet of torque at 540 rpm. That is , with a lever arm of one foot, it would take a 1000 pound resistance to stall a spinning PTO.

With a lever arm of only an inch, (2 inch diameter) the load would be 12,000 pounds to stall the PTO

A 3/8ths grade 5 bolt in single shear is good for 8280 pounds. In double shear, that number goes to 16,560#

So a grade 5 bolt used as a torque limiter exceeds the 100 hp input by the ratio of 3:4 No allowance for impact etc.


With impact loads and the 120 HP reference in the op. It would seem a metric bolt of grade 8.8, or a domestic grade 5 bolt would be a safe start.

This assumes a 2 inch (50 mm) diameter at the shear interface.

Please correct the logic where it strays.

 

[Rich D in Saugerties]

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?

 

[Rich D in Saugerties]

I also got a new Banking 61" flail a couple of days ago. Got it set uo and it ran well for about 50' but then the hammers stopped rotating. The PTO to the gearbox is fine, but no rotation out of the gearbox to the the belts. Very poor documentation - in a drawing, I found a key near the gearbox on the horizontal shaft to the pulleys that could be the problem, but I see no shear pin - where should I look?

 

[LouNY]

I find all this chatter about which shear bolt to use is amusing at times.
There are many variables in shearbolt design and use. This is using a pto shaft shearbolt as an example.
There are several different shear bolt yokes available they can differ immensely in design and torque through put, they can and will have differing distances from the centerline of the shaft. The further out from the shaft centerline the more torque will be allowed to pass through for a given bolt size and hardness.
The same as a harder bolt will allow more in the same size.
From what I have seen on most larger ag equipment almost never is an ungraded bolt used in any shear application either 5's or 8's.

 

[Williy]

Grade 5
A 5/16 in x 2 1/4 in grade 5 PTO shaft shear bolt set is available from WoodMaxx.
Grade 8
A 3/8 in x 1 in long grade 8 hex head bolt can be used as a PTO driveline shear bolt. Grade 8 bolts have greater tensile, yield, and shear strength, as well as greater fatigue resistance and torque specs. However, using a higher grade bolt than the original can damage the unit.
Grade 2
A 1/2 in x 3 in grade 2 shear bolt can protect the gearbox and drive-line from damage on a Bush Hog rotary cutter. Grade 2 bolts are hard enough to maintain rotation, but soft enough to shear if rotation is prevented.

willy