bolt shear question for the brain trust

Reading another post about a grade 8 bolt shearing got me to think about a problem I have. The machine is a rotary cuter six feet in cutting diameter.Brand is unknown, condition is old and well rusted. Cost was $75.00 five years ago. Use is cutting brush on and close to fence rows. Its 3 point mounted on modified DC Case with steel wheels to avoid tire problems. The underside has an approx two foot diameter metal hub for attaching 2 blades. The hub is splined , siding up onto a splined shaft protruding vertically from the gearbox. This 2 foot diameter splined wheel is held up with one 5/8 course thread bolt 3 inches long. From years of wear the hub splines and gearbox shaft splines are wore to the point there is wiggle when brought up tight. I have sheered 4 bolts dropping the large wheel with blades from the machine. I have drilled and removed both grade *8 bolts as well as grade five. I am wondering if grade 2 would be a better choice as the softer bolt might nor shear as quickly as harder bolts? Seems the bolts usually shear when hitting a mound of dirt or small tree of maybe 3 inch diameter. Possibly some company makes a bolt of spring steel lol Replacing either the gear box shaft or the wheel is not foreseeable in my cheap nature!! Any advice is requested I realize the explanation is long, however most request never totally state the problem.

our local farm store has replacement weld in hubs for the stump jumper or blade holder .take it to a machinist and have him cut old 1 out and weld new 1 in.sounds like its so loose it jumps when a hard load hits it

Thanks had no idea replacement hubs might be offered.

Yeah Bill, we know you are cheap. LOL

This is a very non-expert opinion, but I think until you can tighten up the hub to shaft tolerances, you will always have problems, the hub cannot turn on the shaft because it is splined so the only reason the bolt would break would appear to be from movement of the hub caused by hitting a rather solid object. A #2 might actually stretch initially instead of breaking which would allow the hub to be very loose before the bolt actually failed and let the hub fall off. I would imagine it is a very tense moment when the hub actually falls off at speed.

Hey tom at least I do admit it!!! I just found several weld in hubs, had no idea. Now I have to drop the wheel and count splines.Weld in hubs only cost about 15.00 dollars. Still have cost of mower under $100.00!!! Neighbor has milling machine is his garage and needs a place to cut dead trees for heat.Yep sometimes a blind squirrel finds an acorn!!!

Sure the softer bolt will work great much more [sarcasm ] shear strength so of coarse it won't shear faster.

In most bolted joints it is the clamping force and resulting friction that holds the load, not the shear strength of the bolt. Unless you're talking about a true shear bolt application the bolt should not be in shear loading. If it gets into this condition it means the joint is loose, the clamp load is lost, and regardless of the grade of bolt it is bound to fail. The benefit of a higher grade bolt is it allows for increased torque which translates into increased clamp load. If you have space to do it try putting several hardened plain washers under the head of the bolt to increase the shank length that is available to stretch - this is also a common way of increasing the integrity of a bolted joint.

I can be as cheap as anybody, but there are times when it is more economical to spend the money to fix it once and for all.

Might try a smaller bolt like a 1/2 inch would give the room for the slope in the splines without the bolt taking the hit. This would be a till end of season use then get new shaft and hub this winter.