Grade 8 'shear-bolt' on cutter

[hsvhobbit]

Got home friday and my step-son told me he broke the shear bolt on my Rhino SE6 cutter. HUMMM thinks I, the cutter's got a slipclutch on it and it's not supposed to HAVE a shear bolt. He shows me the bolt head and darned if it didn't have those 5 star points on it, yup, grade 8...

Now, I haven't been a good boy and loosened up the slipclutch and let it spin like I'm supposed to so maybe it froze up on me. While replacing the bolt we loosened it up and it spun under hand pressure, sure didn't seem to be rusted or frozen.

Not sure just what's going on there. Any hints? (oh yeah, the springs were compressed pretty much just like the spec said they should be)

 

[deereguy]

How did it shear? Did he hit anything?
It takes a heck of an object to shear a grade 8 bolt.

 

[hsvhobbit]

Says that all he hit was a clump of grass. I figure it had to be a cumulative thing on the bolt because the area of the property he was cutting has no big rocks, no stumps, nothing to even give pause to a riding mower, let alone a cutter.

 

[JerryG]

Normally I go by the manuals on equipment, but this is one instance where I don't. Instead of doing it the way the manual says to, I tighten the bolts until the springs are compressed a small amount while keeping the amount the same between all bolts. Then I try it out in thick grass. If it slips, then I will tighten them one turn each. Usually I have to stop twice to get the adjustment that I want.

 

[heymack]

My cutter calls for a Grade 2 bolt. Well when I sheared the first bolt, I went down to the dealer looking for a Grade 2 replacement. All they had were 5's and the parts guys said that's all anybody uses. I installed the Grade 5, but kept an eye out for some 2's. When I found the correct bolts, I replaced the 5 with a 2 (month or so later). Much to my surprise the 5 was bent to heck but not broken. It was evident that the bolt took some pretty hard knocks when hitting rocks and stumps. I don't believe I did any damage to the unit, but will only be running 2's from now on.

 

[Birdhunter1]

How big of a bolt is it?

I used to have an old Bushhog that called for a grade 8 bolt as the shear pin, but it was a 5/16" bolt. My current bushhog takes a 1/2" grade 2 bolt.

 

[heymack]

How big of a bolt is it?

I used to have an old Bushhog that called for a grade 8 bolt as the shear pin, but it was a 5/16" bolt. My current bushhog takes a 1/2" grade 2 bolt.

Mine takes 1/2" x 3" bolts.

 

[Farmwithjunk]

Slip clutches will "freeze" (rust) to a point where they won't slip even if adjusted properly. They need to be forced to slip and slipped enough to "shine" the mating surfaces. A mower equipped with a slip clutch that is stored outside will most likely need the treatment more often than one kept out of the weather, but none-the-less, I do it with my mowers AT LEAST twice per season.

On the shear bolt... Is (was) the bolt that sheared of proper size? (Still, it SHOULDN'T have been grade 8, even with a slip clutch) Too small of a diameter bolt will shear with quite a bit less resistance than a properly fit bolt.

Heavy grass can offer up quite a bit loading on a mower. I've waded into a tall stand of bermuda grass before that stopped a 60+hp tractor with a 6' cutter, almost dead in its tracks. The only thing that kept it from killing the motor was when the slip clutch went up in smoke.

 

[MarkV]

Is it possible that the shear bolt was replaced by the factory or dealer with a grade 8 because the mower was being shipped with a slip clutch? Of course that would not explain why it sheared unless the slip clutch was frozen.

MarkV

 

[Dieselmh]

How many hours are on that mower? It may have failed due to age and abuse over time. Kind of like "the straw that broke the camel's back" kind of thing. Hours and hours of stress on a bolt will eventually break it, slip clutch or not.

 

[hsvhobbit]

It's a 1/2x3.5 bolt. I suspect when they put the slip clutch on it's standard to use the grade 8. The rig has maybe 40 hours on it. I figure since it's hit it's share of med sized rocks the bolt just got hammered.

I'm going to revisit how tight to keep the springs and do some experimenting with tension over the next few weeks.

 

[zzvyb6]

Lets git technical here. There are reasons to use either a soft or a hard shear bolt. When you have two parts with a surface area between them, then a soft shear bolt is to be used. The bolt keeps the surfaces together and torque is transmitted by friction contact. This type of safety is used to prevent overloading the device from powerful motors. This is a horsepower issue. So, if you used a 100 hp tractor on my baler, the friction safety would stop you from twisting the drive shaft if a slug of wet hay was encountered. The soft bolt lets the surfaces slide relative to each other, no impact load involved. Thats what the flywheel energy is for.

A hard shear device like a pin is used when a true impact load could be encountered. This typically means a shaft within a shaft type safety. The hard bolt is strong but has low shear strength, like diamond: its more brittle. An impact causes the pin to be sliced by the cutting edges. In this case, the softer bolt might flow (called strain) a bit, be distorted and yet tranmits the impact torque to your gearbox. Thats' often why the gearbox takes it and the shear pin is intact (but hard to remove.

That's my 2 cents. I use both types of bolts depending on which type of surfaces are between the power source and the receiver. BTW: in the case of the surface type device, if you are shearing the pins, use a fine thread bolt and tighten it more to increase the contact friction. This bolt has a spring rate that is effective in keeping a uniform contact pressure. The hard bolt's spring rate is much higher so the rate of change of load must be higher to fracture it. Also in this case, a shear bolt that's too small will fracture just from clutch engagement because the shear rate is almost infinite when the contact begins. Also, you don't want to tighten the hard shear bolt very tight because the clamping force will support the torque and you want tonly the bolt to do this, not the two shaft contact points.

 

[RollTideRam]

I just picked up my dealers last JD LX5. It did not come with a slip clutch and it has a metric shear bolt, Grade 8.8. I did not know it was metric and picked up some 3/8x3 inch grade 2 bolts at tractor supply. Are grade 8 and 8.8 about the same? JC

 

[greg_g]

. Are grade 8 and 8.8 about the same? JC

Nope. M8.8 is a Grade 5 equivalent

//greg//

 

[RollTideRam]

For some reason, I'm thinking that a grade 5 has more shear strength than a grade 8. Is this right or wrong? It sure would be cheaper to buy 3/8 bolts instead of metric, Is that okay? I know they are a hair smaller. JC

 

[Theowegian]

Got home friday and my step-son told me he broke the shear bolt on my Rhino SE6 cutter. HUMMM thinks I, the cutter's got a slipclutch on it and it's not supposed to HAVE a shear bolt. He shows me the bolt head and darned if it didn't have those 5 star points on it, yup, grade 8...

Now, I haven't been a good boy and loosened up the slipclutch and let it spin like I'm supposed to so maybe it froze up on me. While replacing the bolt we loosened it up and it spun under hand pressure, sure didn't seem to be rusted or frozen.

Not sure just what's going on there. Any hints? (oh yeah, the springs were compressed pretty much just like the spec said they should be)

I'd check to make sure the PTO shaft is the proper length. If the shaft is too long, then it is possible that raising or lowering the cutter could stress that bolt enough to shear it or weakening it.

Is your shaft splined or a solid shaft with a bolt thru it? My slip clutch has splines...Is yours similar to this?

dwight

 

[hsvhobbit]

No splines on either the slipclutch or the gearbox shaft. Smooth only.

The PTO shaft seems about the right length, I guess. Rig's been raised fully and lowered quite a bit in the time it's been run, I'm assuming the bolt would've snapped pretty quick if it was being stressed from an overlong PTO shaft. Or is that a poor assumption?

 

[Theowegian]

No splines on either the slipclutch or the gearbox shaft. Smooth only.

The PTO shaft seems about the right length, I guess. Rig's been raised fully and lowered quite a bit in the time it's been run, I'm assuming the bolt would've snapped pretty quick if it was being stressed from an overlong PTO shaft. Or is that a poor assumption?

I don't think there are any poor assumptions yet. You're still in the discovery stage on this problem.

But I think you can figure it out.

At this point, perhaps, you could read that Rhino cutter owners manual carefully, and set the mower up as the mfg recommends.

Personally, I wouldn't concern myself too much with this condition. Maybe just put a new gr 8 bolt in there and run it like you stole it!

If the bolt shears again, then you have a repeat problem and a basis for even deeper troubleshooting. Maybe even a phone call to the mower mfg technical group...if thre is one...

dwight

 

[rcaretti]

I also have a Rhino SE6, and was surprised when I broke a shear-bolt on it last year. Since it has a slip clutch, until it broke, I didn't think it even had one.

After replacement, it hasn't broke again. My guess is normal wear and tear, and I now have a few extra bolts.