PTO shear bolt: tractor end or implement end?

[ericm979]

I lost drive to my Woodmaxx chipper today because the shear bolt went missing. I think it fell out; there was only small stuff in the chipper at that moment and the chipper spun down rather than stopping. I turned it by hand to check that it wasn't jammed.

Re-reading the manual I see that the shear bolt end of the PTO shaft is supposed to go on the tractor. For some reason I've been doing it the other way all along. Does it matter? This PTO shaft doesn't come apart when the shear bolt is missing, it just spins.

Would it be a good idea to loctite the shear bolt's nut? It's a nylock nut but those don't always hold.

Last the unthreaded part of the spare shear bolt from Woodmaxx measures .306 inches, which doesn't match the 5/16 the manual says and would be 7.7mm. Any idea what size I should use?

 

[CalG]

Shear at the tractor end
Nylocks are "good enough" just snug the nut. Not too tight.
Stick in a 5/16 bolt of the correct grade. It's not rocket science, Bolts are bolts. Exact shear loads and forces are never certain.

 

[Aquamoose]

There's different types of hardness. Mine broke because the PTO is switch activated. Replaced it with a low grade 5/16 bolt I just throttle down, engage, and ramp up to speed.

 

[CincyFlyer]

By putting the shear bolt in the distal end you are also protecting the two u-joints in the drive shaft.

 

[ericm979]

I suspect the shear bolt fell out. The chipper was working on < 1" brush. It spun down slowly after the PTO let go. I'll look for the bolt. The chipper has 20 hours use. The spec is for a 5/16 grade 2, which is what I will use in the future when I run out of the spares that came with the chipper. I've chipped some big stuff and even stalled the tractor once without breaking the shear bolt so I think it's strong enough.

So far I have one tractor end with no explanation why, and one implement end with an explanation. Anyone else want to chime in?

 

[bigtiller]

By putting the shear bolt in the distal end you are also protecting the two u-joints in the drive shaft.

The shear bolt is the weak link so I don't think it matters if it is distal or proximal orientated.

 

[Hardway]

Are shear bolts just that, a standard grade 2 bolt?

 

[RickB]

Are shear bolts just that, a standard grade 2 bolt?

Shear bolts are whatever may be specified for a particular application. SAE grade 2, 5 or 8; Metric 8.8 and 10.9 are all used as shear bolts in applications I am familiar with. Some shear bolts have special relieved shanks that shear easily at a given point in the shank.

 

[rd_macgregor]

My Wallenstein chipper specifies that the shear bolt goes on the chipper (outboard) end of the PTO shaft.

 

[ericm979]

I'm thinking it doesn't matter if the shaft's constructed so it stays together when the shear bolt shears. If it comes apart you'd want the shear bolt at the tractor end so the broken shaft isn't flopping around and damaging things or people.

These shear bolts are regular 5/8 grade 2. I've seen ones for snow blowers that have grooves cut into them to make them shear easier.