Kubota BX 2750D Snow Blower Shear Bolts

[smfcpacfp]

Why would they use a grade 8 bolt for a shear bolt? grade 8 is the strongest bolt there is.

...and the most brittle, you want them to snap off under side load

 

[dsmiley]

The first bolt I sheared with my BX2750D was within the first 30 feet of the first time I ever used it on the first storm of 2010. I was blowing on unfrozen gravel and picked up a rock. I shut the tractor down and checked out the blower. The fan was loose on the shaft so I cleaned out the fan shroud of snow and the offending rock, lined up the shear pin holes, pushed out the shank of the old bolt which dropped into the bottom of the shroud, and inserted a new bolt. I tightened the new bolt, got back on the tractor, started her up, and engaged the PTO. It sounded like it was going to go but then nothing. What the heck? Checked it again and now the fan wouldn't move, at all; not even backlash. The culprit was the old bolt shank; it was wedged between the the end of a fan blade and the shroud resulting in yet another sheared bolt. So now when I shear a bolt I make sure to get that old shank out of there.

 

[don white]

like2wheel
I also wanted a quantity of them, but my dealer was getting almost $2/bolt. So I bought a bunch of bolts each grade from Napa & grooved them myself. I have a small metal lathe, but I'm sure it could also be done with a drill press & Dremel or hacksaw. Duplicated the groove positions & depth with a caliper. the auger pin has 2 grooves, the fan pin (grade 8) has just 1.



I would be concerned that you are getting the same bolt that Kubota is supplying. Kubota may be using a custom heat treatment on the bolt to obtain the shear value they want.
I am betting they test the shear strength as they produce them.
I don't think this is worth messing with unless you can get Kubota's print.

 

[keyman1808]

My dealer has the auger bolts but said to get the fan bolt from a local hardware store. Just to use s reg grade 8 bolt

 

[like2wheel]

I would be concerned that you are getting the same bolt that Kubota is supplying. Kubota may be using a custom heat treatment on the bolt to obtain the shear value they want.
I am betting they test the shear strength as they produce them.
I don't think this is worth messing with unless you can get Kubota's print.

I seriously doubt they would be using a "custom heat treatment" on a shear bolt. It's not the space shuttle, so why would they go through the trouble to re-invent the wheel? All it needs to be is weaker than the parts upstream, if they wanted to change the shear strength, it would be much simpler (& cheaper) to alter the grooves. The blower is not even made by Kubota, & the owners manual even states that they are grade 5 & 8.

My dealer has the auger bolts but said to get the fan bolt from a local hardware store. Just to use s reg grade 8 bolt

I doubt it would hurt anything, the drive parts it protects are pretty sturdy. I'd be more worried if the auger bolts were wrong, since they protect the blower transmission. That said, I would probably try an ungrooved grade 5 for the fan first, & see if that holds under a load.

 

[dsmiley]

My dealer has the auger bolts but said to get the fan bolt from a local hardware store. Just to use s reg grade 8 bolt

That said, I would probably try an ungrooved grade 5 for the fan first, & see if that holds under a load.

The beauty of the OEM bolts is that the grooves prevent a ragged end shear which make it difficult to remove the old shank. The shanks of these sheared bolts nearly fall out of the shaft. We like this. I'm thinking, when it's cold and blowing and you're trying to punch out that mangled stock hardware bolt with the hammer and drift pin punch that you had to go get, you'll wish you had spent the extra dollar for the OEM bolt. I carry a spare bolt, 2 wrenches, and a nail when I blow and can change a sheared pin in just a couple of minutes. I think the OEM bolts were well engineered.

 

[PILOON]

Graded bolts will shear cleanly as compared to softer ones.
The grooves in factory shear bolts determine where the break will occur but also weaken the bolt somewhat.
In my blower (60" on a 20 hp) I do shear from time to time using grade 5, usually on chunks of hard ice or the occasional rock.
The breaks are always clean and my holes remain nice and round.
I once, in a pinch, used a softer hardware store ungraded bolt and boy was that hard to drive out. Also the aligning holes were slightly rounded out around the edges.
I now (5 yrs or so) always use grade 5 with NO threads near the shear areas as threads weaken the bolt.
I just tighten up snugly and use fiber lock nuts. I also keep the auger shaft well lubricated.
My only sheared fan bolts generally are caused by the city plow contractor having 2-3" stones in his sander that often end up at the entrance to my driveway.
I, for one, do not believe factory pre-weakened shear bolts are needed.
Most major blower manufacturers suggest standard grade 5 bolts. (Normand, Bervac etc)

 

[dsmiley]

Graded bolts will shear cleanly as compared to softer ones.

Yes. The harder a bolt is, the cleaner it will shear. However, hardness is but one of the variables involved. The tolerances play a large part in these results. The diameter of the bolt holes relative to the bolt shank diameter and the diameters of the driving and driven components relative to one another will certainly impact the results of a shear. But an inference that grades 5 and 8 bolts do not suffer from plastic deformation would be incorrect. You cannot count on there being no burrs on the pieces involved in a shear. If the burr is smaller than the hole it needs to pass through (as in the case when it's at the bottom of a pre-made groove), then no problem. If the shank diameter to bolt hole tolerance is .010" and a burr protrudes .015" on a bolt with no grooves, the old shank could be difficult to remove. I have the head of the last grade 8 bolt (w/grooves) I sheared and there is a burr, albeit, a small one, but it is there.

The grooves in factory shear bolts determine where the break will occur but also weaken the bolt somewhat.

The grooves do not determine where the break (shear) will occur. The shear will always occur where the bolt intersects the driving and driven pieces whether there is a groove there or not. Assuming normal tolerances, it cannot do otherwise. A groove can be 1/8" to one side and it will still shear at this intersection. If there were torsional stresses on the bolt I could agree with your statement, but that is not the case here. However, if the groove should be at this intersection you can control 2 aspects. One, if there is a burr (and there probably will be) it will not interfere with shank removal. And two, you can control the shear strength of the bolt with a uniformly reduced cross-sectional area.

Yes, you can use straight grade 5 or 8 stock bolts with no grooves for shear pins but if the tolerances are tight and you shear, you may need a hammer and punch to remove the remnants. Piloon, you may not be familiar with the BX2750D snow blower but access to the fan shear bolt is inside the fan shroud where you cannot see into the bolt hole without a mirror. So ease of bolt replacement is imperative. We want the old shank to literally fall out of the hole and RAD Technologies or Kubota or whomever designed these bolts
made it happen. Having sheared 6-8 bolts in 2 winters (so far), I for one appreciate this feature. It's cost, maybe $1 extra per bolt, is cheap enough in my opinion.

 

[Garpoonga]

well I sure don't know how you guys are getting your pins to simply fall out when they break. I go through about a dozen breaks a season and almost always have to punch them out which is a huge pain with the impeller.

Pins for me are either 5.07 or 4.01 from Kubota which is crazy. I know this is an old thread, but I am still looking for a solution to Kubotas insane prices on these bolts. I also live very remote and have to blow 1.5 km of road so I cant afford a major failure due to something so simple as a bolt. I have now been reading half the afternoon and still don't know if I can neck down my own bolts or just use grade 5 or 8 1/4 bolts.

 

[sandyc]

You might want to try ordering the shear bolts at parts@mbtractor.com. Prices are good for Kubota parts and they ship quickly. Just looked up shear bolts for my B 2781 A and they are as follows:
70060-01074 $.85
70060-01337 $1.95
I always keep a bunch on hand and buy them by the bag.

Good luck!