SHEAR BOLTS

lakeside · Jul 15, 2015

I have a B26 with a 42" Pronovost snow blower. The blower is properly sized to the tractor but I'm having problems with my PTO shear bolts breaking. The PTO came with 6mm shear bolts which kept breaking even under light snow conditions. I drilled out the 6mm shear bolt hole hole to 1/4" as it was easier to get 1/4" Grade 5 or 8 bolts rather than a metric size. The increase of .010" in diameter wouldn't have been enough to over stress the tractor so I was good to go. Although the frequency of breaking the bolts went down, I'd still shearing bolts under conditions where bolts shouldn't break.

I checked the blower for binding and everything seems to turning smoothly.

My question is could I re-drill the hole from 1/4" to 7mm and use a metric grade 8.8 bolt instead of the 1/4" SAE grade five bolt (Metric grade 8.8 is roughly equivalent to SAE grade 5)? I'd go back to metric as I really don't want to up one SAE size.

The increase in diameter between 1/4" and 7mm is .010" or about 12%. I know shear bolts are meant to, well, shear to protect the tractor but replacing a shear bolt at -20c every time I blow snow isn't fun.

SPYDERLK · Jul 15, 2015

lakeside said:
I have a B26 with a 42" Pronovost snow blower. The blower is properly sized to the tractor but I'm having problems with my PTO shear bolts breaking. The PTO came with 6mm shear bolts which kept breaking even under light snow conditions. I drilled out the 6mm shear bolt hole hole to 1/4" as it was easier to get 1/4" Grade 5 or 8 bolts rather than a metric size. The increase of .010" in diameter wouldn't have been enough to over stress the tractor so I was good to go. Although the frequency of breaking the bolts went down, I'd still shearing bolts under conditions where bolts shouldn't break.

I checked the blower for binding and everything seems to turning smoothly.

My question is could I re-drill the hole from 1/4" to 7mm and use a metric grade 8.8 bolt instead of the 1/4" SAE grade five bolt (Metric grade 8.8 is roughly equivalent to SAE grade 5)?

The increase in diameter between 1/4" and 7mm is .010" or about 12%. I know shear bolts are meant to, well, shear to protect the tractor but replacing a shear bolt at -20c every time I blow snow isn't fun.

Be sure there are no threads in the shear zone. -- The shank should overlap the zone comfortably. ... I am surprised you could drill the shear area because it should be extremely hard. Being able to make a good cylindrical well dimensioned hole in the hard steel is paramount.

I would not go any larger - nor would I upgrade to Gr8 before investigating and talking with mfg tech support.
,,,larry

LD1 · Jul 15, 2015

Have you looked into putting a slip clutch on it

lakeside · Jul 15, 2015

I looked a few years ago but didn't find a suitable clutch. I'd be grateful if anyone had the name of a possible vendor

Gem99ultra · Jul 15, 2015

All I've ever read, or used, are Grade 5 bolts. And NEVER use Grade 8 bolts. Bolts are cheaper than broken PTO shafts.

JerryK · Jul 15, 2015

I don't think going up just .010" is going to matter much. What you should mainly be looking at first is if you can find a bolt that only has threads in the ' nut ' area, no threads internal to the slip area. Threads in this area just chew up the hole. Soon after you install a new shear bolt it will stretch a little and the bolt will become loose in the shear area. If there are threads there, it helps to elongate the original slip fit hole. We have a great bolt place here in Gaylord, but I ended up going online and obtaining the right shank length bolt from a bigger supplier. Put a nut on the new bolts, cut off the excess thread, then cleaned up the new end with a bench grinder.
Another problem you are having is breaking shear bolts. Most new blowers are a little tight, but soon loosen up and spin easily. The biggest problem after that breaking in period is if you are engaging the PTO at RPM, and not down by idle. It puts a big strain on the bolt. With 'normal' blower operation, you should just about never be breaking shear pins. Unless, you are picking up the occasional big chunk of ice... stone... neighbors lost tire chain... etc. Runing more than about a 15 degree angle is bad on the PTO shaft, and I guess you could break pins if you went way past this angle.....

Agvg · Jul 15, 2015

7mm is not an std metric bolt dimension and will be very hard to find, 8mm is the next size.

lakeside · Jul 15, 2015

Thanks for the reminder about engaging the PTO at lower RPM. I finally clued in that I was breaking bolts just by engaging the PTO at higher speed after a few -20c dark snowy bolt replacements. Why can't snow blower bolts break when it's sunny and +25c

I now engage the PTO at idle and then up the RPMs. I did some searching and 7 mm grade 8.8 bolts are available where I live but I'm going to hold off to make sure there are no threads internal to the actual PTO.

Jay4200 · Jul 15, 2015

lakeside said:
I have a B26 with a 42" Pronovost snow blower. The blower is properly sized to the tractor but I'm having problems with my PTO shear bolts breaking. The PTO came with 6mm shear bolts which kept breaking even under light snow conditions. I drilled out the 6mm shear bolt hole hole to 1/4" as it was easier to get 1/4" Grade 5 or 8 bolts rather than a metric size. The increase of .010" in diameter wouldn't have been enough to over stress the tractor so I was good to go. Although the frequency of breaking the bolts went down, I'd still shearing bolts under conditions where bolts shouldn't break.

I checked the blower for binding and everything seems to turning smoothly.

My question is could I re-drill the hole from 1/4" to 7mm and use a metric grade 8.8 bolt instead of the 1/4" SAE grade five bolt (Metric grade 8.8 is roughly equivalent to SAE grade 5)? I'd go back to metric as I really don't want to up one SAE size.

The increase in diameter between 1/4" and 7mm is .010" or about 12%. I know shear bolts are meant to, well, shear to protect the tractor but replacing a shear bolt at -20c every time I blow snow isn't fun.

"Re-engineering" shear bolts is generally not a good idea. I think you are over-torquing the bolts when you are putting them in. They should just be snug. If you stretch the bolt by over torquing, it will shear prematurely.

johnrex62 · Jul 15, 2015

My experience with shear bolts has been very confusing. The inaugral outing with my shredder resulted in a sheared bolt within 20 minutes. I backed over a small pile of cut grass and voila! sheared bolt. I replaced it with one of the two spares that came with the shredder and finished mowing, albeit carefully. The next day I went out and bought a dozen spares considering how easily the first one sheared.

That was in 2010, and that was the last sheared bolt, even after I got brave and cleared 4 acres of brush and a few 4" saplings hidden in the small stuff. I guess I just got a bad shear bolt on the shredder.