Suitable Sheer Bolts

[aesmith]

Suitable Shear Bolts

So I've just collected our new (used, actually) snow blower and I've been checking it out and greasing it etc. I want to lay on some spare shear bolts, but I'm not sure what grade would be suitable. I would prefer to just use commodity parts rather than have to get branded spares, and the PTO shaft is a generic one anyway not from the same manufacturer. So I'm looking for suggestions for suitable grades or material. We're all metric over here but if anyone can suggest in US terms I'm sure I can find equivalents.

The PTO shaft has a conventional knuckle at the implement end, with a shear bolt through two flanges which would then be free to rotate when the bolt shears or is removed. The bolt is 8mm diameter, and it has a 10.9 fitted. That's equivalent to US Grade 8 and that strikes me as much too strong, but then I don't have a good feel for how much force is being transmitted by 25HP at 540rpm.

The augur is chain driven, and there's a shear bolt of the same layout on the shaft which drives the chains, but the fitted part here is an unmarked 6mm fully threaded screw. Clearly that's much weaker than the PTO shear bolt, and potentially could have more load since it's running slower.

I think the impeller is directly driven with no gears between PTO shaft and impeller, so effectively the PTO shear bolt is the protection if something jams the impeller. I can't see a specific impeller shear bolt or pin.

Any thoughts? My idea was to fit the softest bolts I can find in both places with a view to going up in grades if they turn out to be too weak. I'm also having a bit of trouble thinking through how the protection actually works, is the part of the machine downstream of the shear bolt protected, or the part on the tractor side? Or doesn't it matter?

Thanks, Tony S

 

[LouNY]

The shear strength of a bolt is not strictly related to it's tensile strength, many shear bolts are higher grade so that when it shears it breaks cleanly, a softer bolt will distort and in many cases smear in the holes and require a punch to remove the pieces were the harder bolt will shear clean and the pieces will just about fall out.
Now let the disagreements begine.

 

[Amvcane]

My Schulte 3pt snowblower has a grade 8 bolt on the pto drive shaft and a grade 2 bolt on the auger drive shaft. The impeller spins at the same rpm as the pto. Shaft goes straight through the gearbox and is protected by the shear bolt on the pto drive shaft. Pretty much the same as what you have.
I would contact the manufacturer, get a pdf manual, and install the bolts they spec.

 

[TractorNH]

I disagree with LouNY. All of the softer bolts I've ever used shear clean enough that most of the time you can't even find them after since they fall out of the holes. A grade 8 bolt isn't a shear pin and is certainly too strong for a 25hp tractor. The PTO bolt is intended to protect both the implement and the tractor. If the bolt can't break then something else will. I would change it to a grade 5 at most but would probably start with a grade 2 and if it shears too easy then go to grade 5.

I agree with Amvcane, try to contact the manufacturer and/or find a manual online. Some manufacturers will even send you a manual for free since they have safety info in them.

There's my suggestion but it's your tractor so do as you will.

 

[RickB]

Find out the spec for the original shear bolt and extrapolate that info. 8MM is a pretty small diameter bolt, it is entirely possible that 10.9 hardness will be just fine.

 

[Captain Dirty]

I would contact the manufacturer, get a pdf manual, and install the bolts they spec.

The manufacturer's engineers have figured out what is required. My Italian Goldoni snowblower uses M6 grade 12.9 (176,900psi) shear bolts; my American Lorenz uses 1/4-20 grade 5 (120,000psi) shear bolts. (Grade 8 is 150,000psi.) Could be anything, and on a used machine, the present bolts may not be the specified bolts.

 

[aesmith]

Thanks all. As I say the PTO shaft isn't particular to that snowblower, it's just a standard shaft that the previous owner used. However looking at a generic parts place, if I selected a PTO shaft to suit my tractor's output, it's right enough that it would have an 8mm shear bolt. I have some concern about the bolt rating. I still think I might just try it with 8.8 (your Grade 5), it's not a big deal changing it if that's too weak.

For the augur the manufacturer's diagram and parts list just describes it as "M6 x 30 bolt". I don't think it's ever been changed as it was still painted over when I removed it to grease up the joint. Again all I think I can do is try some standard bolts. That one's a bit more fiddly to get at, but not as bad as on our walk behind.

 

[Captain Dirty]

For the augur . . . That one's a bit more fiddly to get at, but not as bad as on our walk behind.

I do not have a clear understanding of the location of the shear bolt for the augur. Is it between the shaft and the drive sprocket or between the driven sprocket and the end of the auger shaft?
On my Lorenz the shear bolts I have broken are the auger shear bolts that connect the driven chain sprocket to a "tee" plate on the end of the auger near the side housing. Holes in the housing above and below the flange that holds the auger bearing allow access (with a socket on an extension) to the head of the shear bolt. On a cold, winter's night it is fiddly enough to change a shear bolt with that system. If you bought the Kelifri you referenced in an earlier thread it looks like the bolt spacing and pattern on the plate driven by the driven sprocket is the same size as the bolt spacing and pattern on the bearing flange, but it may be possible to grind a shallow notch in the bearing flange to access the inner bolts..

 

[oosik]

When I had a 3-point - PTO driven - snow blower the "approved factory shear bolts" were around $7 each - that was in '83. That was a pile of cow crap - I replaced the factor shear bolts with 3/8" x 2 1/2" grade two regular old hardware store bolts . Worked great - would always shear a couple at the beginning of the snow season on a big rock or a limb.

My Wallenstein BX62S factory shear bolts are grade 8. They are around $7 each - right now. I have very successfully replaced the OEM shear bolts with hardware store grade 8 bolts for about $1 each. Works just the same as factory shear bolts.

 

[CalG]

The manufacturers spec on my Meteor 60 inch blower matches the OP comments to a tee.

Grade 8 at the pto knuckle. That's single shear.

And an ungraded bolt at the chain drive sprocket.

I wonder if those shear element flange holes don't suffer when hard bolts are used.