Slip clutch vs shear pin for bush hog

[ME83]

Lets get technical. ... Quick calcs indicate shear of a 3/8 Gr5, 120,000 psi, bolt oin a 2" shear joint takes about 100HP @ 540 rpm. ... Check me on this..

Actually it starts getting very technical. A 3/8" grade 2 bolt shears at 4900 lbs, double that since it is a double shear so 9800 lbs. On a 2" shaft, or 1" from centerline, that would require 117,600 ft*lb of torque. At 540 rpm that is 12,091 hp.

A shear bolt on a spinning shaft like this breaks due to impact, not straight weight or power. The calculations for impact are completely time dependent, which is why such a large force can be exerted. To calculate it, it is not just power being supplied but also the inertia of the brush hog spinning.

The easiest answer is that a grade 5 bolt is roughly 60% stronger than a grade 2. If there is a small gap, a grade 5 will still bend just as much but will require more force. The shear pin is designed to be the sacrificial weak link in the drivetrain, do not change that. A $1.00 bolt is much easier and cheaper to replace than possibly a $200 driveshaft.

 

[SPYDERLK]

Actually it starts getting very technical. A 3/8" grade 2 bolt shears at 4900 lbs, double that since it is a double shear so 9800 lbs. On a 2" shaft, or 1" from centerline, that would require 117,600 ft*lb of torque. At 540 rpm that is 12,091 hp.
......... I think you multiplied by 12 instead of divided. ..........

A shear bolt on a spinning shaft like this breaks due to impact, not straight weight or power. The calculations for impact are completely time dependent, which is why such a large force can be exerted. To calculate it, it is not just power being supplied but also the inertia of the brush hog spinning.

The easiest answer is that a grade 5 bolt is roughly 60% stronger than a grade 2. If there is a small gap, a grade 5 will still bend just as much but will require more force. The shear pin is designed to be the sacrificial weak link in the drivetrain, do not change that. A $1.00 bolt is much easier and cheaper to replace than possibly a $200 driveshaft.

That would give about 1000ft-lb torque @ 540. That would give 540 x 2pi x 1000 = 3,391,200 ft lb energy per min. Then divide by 33,000 for HP. ...That gives about 102HP, BUT in shear I understand the fail force is about 2/3 tensile. Did you find the shear in a table, or is 4900 a tensile value assumed valid in shear?

Impact is virtually no different in its effect than force applied more slowly. Impact just allows the use of less energy to create a very hi force.
larry

 

[ME83]

Oops, you are correct. That is what happens when my 6 mo old daughter wakes me up well before daylight on a Saturday morning. I looked up a table for shear strength, but the general rule is about 2/3 tensile. So a grade 2 bolt is about 84 hp.

I mentioned the impact since bolt hole are typically slightly oversized to aid in assembly. This can cause a slight gap but also does not perfectly load the bolt due to a non uniform contact area. These are minor things but they do add up.

 

[k0ua]

It might come down to how often you break shearbolts -- if seldom, a clutch, as alternate, would be a danger because it wouldnt be getting the shocks and little slips that self maintain it. OTOH, If youre breaking shear bolts fairly regularly a slip clutch adjusted fairly tight is what you need.

I think this pretty well answers it for me. I have NEVER broken a shear bolt on my little "hog". But then again I don't use it all that much and I never cut much brush over 1 inch. Mostly just grass and weeds, and the ubiquitous rocks!:laughing:

I don't think a slip clutch would work out for me, as I would not shock it enough to keep it freed up.

 

[LazyHRanch]

So if a grade 2 bolt will take 84hp before it shears then how can I know keep breaking grade 8 bolts like they are made of butter? I am running 30 PTO HP and a 5 foot cutter.

 

[newbury]

Doesn't it depend on the diameter of the bolt?

 

[SPYDERLK]

Lets get technical. ... Quick calcs indicate shear of a 3/8 Gr5, 120,000 psi, bolt oin a 2" shear joint takes about 100HP @ 540 rpm. ... Check me on this..

So if a grade 2 bolt will take 84hp before it shears then how can I know keep breaking grade 8 bolts like they are made of butter? I am running 30 PTO HP and a 5 foot cutter.

Doesn't it depend on the diameter of the bolt?

Yes. ... It started with 3/8" subject size.

-- LazyHRanch, A point was made earlier about shearings gradually damaging the shear joint. Soft bolts do it by smearing and galling. Hard stong bolts do it by deforming the metal parts little by little. Gr 5 is probably the best shear choice.
>> To address breaking 100+HP bolts with a 30HP PTO - its drive train inertia. The strength of the drive parts is apparently sufficient to supply that hi hp drive pulse for a split second as the bolt shears. Id think a 30HP drive would be very near its catastrophic limit. Youre being "lucky". ... And also showing that theres a safety factor abve 3X in your drive rating. I would be apprehensive to push it that far for fear of incremental damage to bearings, gears, casings, etc.

Is your shear joint still tight or is it wobbly? It sounds like your application would really benefit from a slip clutch. You could cut much better, hassle free, with repeated incremental slips at ~ 90HP. Your clutch would be just warm ordinarily.
larry

 

[Marveltone]

Sounds like I'm the lightweight of the bunch. My mowing rig consists of a 54" RC pulled by ~ 19-20 PTO HP. I ordered my RC with the slip clutch because I originally didn't want the bother of replacing shear bolts. I've since learned I can adjust the slip clutch to work with my meager PTO HP. I doubt I could do this with shear bolts.

The seasonal maintenance of the clutch is not a big deal at all.

I certainly don't know it all, but I know what's been working for me.

Joe

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