Flail mower shear bolt — 12.9?

S854 · Jan 21, 2023

As the title says… I found a 12.9 shear bolt on my brand spanking new flail mower PTO shaft… it’s roughly 5/16” X 2” …

I know, 12.9 is a metric rating but SAE bolts are easier to find and quite a bit less spendy… hence the std. measurement…regardless…

i pulled the bolt and underneath all that glorious goat-vomit yellow paint is a black (tempered?) bolt…

What’s a good SAE match for that 12.9?

I was thinking about starting with a grade 2… then moving up to grade 5 if the bolt shears too readily… good plan?

BnRidge · Jan 21, 2023

leonz · Jan 21, 2023

If the bolt is black in color it is a 12.9 metric/grade 8 bolt or higher.
You need a grade 8 bolt and nut equivalent for this PTO Shaft if you
do not want to invest in 12.9.

S854 · Jan 22, 2023

Thanks for the replies…

i‘m not quite sure I understand the various shear strength applications… I’ve read, here and elsewhere, “Don’t use anything other than a grade 2 bolt… you’ll destroy your tractor/implement gearbox”…

Then there’s my Danuser PHD… the users manual states, “Use only Danuser shear bolts as a replacement”… when I contacted Danuser to order a couple extras, they told me I could use a locally sourced, grade 5 bolt of the appropriate diameter…

The gearbox on my Danuser appears infinitely more robust than the box on my flail mower… and, yes, the bolt on the PHD is 1/2” rather than 5/16”…

I’ll be using a grade 8 (or higher) on my flail… I’m just trying to understand the recommendations of the engineers who designed these things…

LD1 · Jan 22, 2023

See what the manual calls out for it.

I assume its a round bore and a round shaft that the shear bolt is actually bearing the load??
Sometimes the gearbox input is splined as is the shaft and the bolt is nothing more than to hold it onto the splines. I have seen people mistake the retaining bolt for a shear bolt, completely overlooking the fact that its a splined coupling.

Flail mowers are usually belt driven....and dont strilke me as the kind of application that requires a shear bolt protection.

But if in doubt, you could always put a slip clutch on it and not have to worry

ovrszd · Jan 22, 2023

LD1 mentioned my concerns. Is that truly a shear bolt? Does the flail have a slip clutch?

ptsg · Jan 22, 2023

That's indeed a shear bolt. You can see the Shear bolt style yoke of the PTO shaft in the back.

Looks like this:

No idea how a 12.9 grade bolt ended up there but it's not correct at all unless it will be powered by tractor with a lot of HP. Should be a grade 4.8 or 5.8 at max.

LouNY · Jan 22, 2023

I have seen a lot of high tensile bolts used as shear bolts. It all depends on the implement and were and what it is protecting. The shear bolt hub pictured in ptsg's pic can have several torque ratings depending on the bolt used. Also, in just that style of shear pin protected devices there are many variations between the diameter of the bolts used the grades of the bolts used and then the hubs will vary in the bolt locations radius distance from the shaft centerline. When you start putting well over 100 hp or even 300 hp through the pto when a shear bolt is used it will be spec a bit different then implements built for 35 hp.

Jims1025R · Jan 22, 2023

The other thing to consider is that if you use a very hard bolt, let alone it maybe not shearing as soon as it should, if/when it does shear, it may do local damage to the holes in the coupling, which will be much more expensive to replace than some shear bolts. Use low strength bolts first, and only move to stronger as you find necessary.

diesel85 · Jan 22, 2023

I have a couple implements that use this. I always start with a Grade 2 then work my way up until I'm satisfied.

Also goes without saying, most people forget... you should remove that shear bolt and make sure both the coupling and shaft actually move and not rusted together.