shear bolts B2782 snowblower

[NIdaho]

Hey Guys,

Let me chime in on the use of Grade 8 bolts as shear pins. Yes, Grade 8 have a much higher tensile strength than Grade 2 or 5, BUT THEY ARE BRITTLE. I was told that for shear pins you could use a Grade 2 (they are soft) or Grade 8 (hard/brittle) but never use a Grade 5 (you will damage something else as they are not soft like the grade 2 and they and not brittle like the grade 8).

My brother (who happens to be a Lead Millwright that keeps a LARGE sawmill running) explained to me about the Grade 8 being brittle and NOT to use a Grade 5. He said that there are applications where the steady state forces are enough that Grade 2 will shear off but that the Grade 8 will shear if there is an impact.

 

[oldballs]

I'm wondering if the zero and below temperature affects the shear properties. I just broke one today on my walk behind blower, and don't think that I hit anything except maybe a chunk of ice. It's zero here and have deep drifts from storms over the past 10 days.

 

[lakeside]

The spare shear pins I got with my Pronovost were black 1/4" bolts with no head markings. I'm assuming these were grade 2 bolts, as grade 5 and 8 have head markings on them.

I Googled the specs on Grades 2 through 8 bolts and the 8s are quite a bit stronger. I guess we need a mechanical engineer to wade in here.

I don't think the temperatures some of us see in North America would have any real impact on the brittleness of bolts. Now, if you're talking -40c or less then yes you would start seeing metal parts break. The -20c some of us experience likely wouldn't be a factor.

 

[Chilly807]

What size is the Pronovost? If it has 1/4 inch shear bolts, I'm surprised at that... I wonder if they provide higher strength shear pins knowing that if the owner installs aftermarket pins they'll shear prematurely? I'd get pretty frustrated in a hurry and end up going back to the dealer for more pins... My walk behinds, over the years have varied from 1/4 inch grade 2 bolts in an 8 hp unit , up to 5/16 grade 2 in a 10 hp machine.

That may be the difference we're seeing, the grade 2 bolts may be larger diameter than a smaller grade 8 bolt, but with the same shear strength?

Just a guess on my part. I'm mechanically inclined, but not an engineer !

Chilly

 

[irvingj]

On the M-F front-mount blower (identical to Kubota's SCUT blower, I believe), OEM parts are a single-grooved grade 8 for the fan and a double-grooved grade 5 for the auger. Even "color-coded." I've also been told that a plain old grade 2 is acceptable for the auger....

I think I've also discovered that over-torquing will result in more sheared bolts-- "9 ft-lbs" isn't a lot of force, but that's what they call for.

I was just cranking them right in there, I suspect quite a bit more than 9 ft-lbs. They were snapping right & left. Since reducing torque to just enough to compress the lock washer, I haven't broken one yet. Does this make sense, or am I just imagining things here? Anybody care to comment on this?

 

[lakeside]

My snowblower is a Pronovost 52" Puma. The only shear bolt is on the PTO shaft at the end nearest the PTO take off. I've looked high and low and that"s the only one I found. I bought the blower in late August '09 and there was a clear plastic bag with five spare shear bolts. The PTO shaft comes from another manufaturer but I can't remember which one.

I'd assume if the shear bolts were Grade 5 or 8 they would have markings on the bolt head but mine are black with no markings or groves.

My shear bolts didn't come with lock washers but with nuts with plastic inserts. I didn't know about the critical torque values, great information.

 

[Gomer]

ok here are the pics with Kubota part numbers.

 

[Chilly807]

Possibly the reason for the torque spec is so you don't collapse the hollow auger tube onto the cross-shaft of the blower. It makes the auger tube **** to get on or off if you have to, and changes the diameter of the tube, which I think is the same as the grooves on the bolts.

That's the way it is on most walk behind blowers, anyway.

I've never seen a Pronovost blower, according to their website the chute control, auger, and driveline are all shear bolt protected. Whether it's one bolt, or three they don't say.

Normally there's one bolt on each section of auger (through the outer tube) one on the fan tube, and something on the driveline to protect the tractor, whether it's a slip clutch or a shear bolt.

Chilly

 

[irvingj]

Gomer-- Shhh- don't tell anyone, but a couple of the shear bolts for my M-F blower actually came in an orange parts bag!

(Used to be, the Kubota dlr was a lot closer.)

 

[smfcpacfp]

I just went to the dealer to buy some additional shear bolts. The bigger bolts for the auger are 5/16 by 2 1/4" and are #5. The owner's manual says these are special bolts. Presumably what makes them special is that they have a little grove about 1/4" down from the head.

The bolts for the impeller are #8 which is what the owner's manual states, nothing special apparently.

For those of you who say they are too hard for this job, I can only say that I have had 3 auger bolts shear and one impeller bolt shear and they did their job perfectly. They are very easy to replace, now that I have had some practice.

8 auger bolts, washer, nuts and 4 impeller bolts, washers and nuts cost - $19.97