Rotary Cutter Brush Hog Blades COMING OFF

[Shaneard]

I think you need some left handed bolts. That way they tighten themselves instead of loosen themselves like they are doing now. I don't know, just a guess.

 

[Shaneard]

Grade 8 bolts should never be used in that application. Grade 5 are made for towing, suspension, steering and other life safety applications. Grade 8 should NEVER be used in safety applications unless specified by the engineer. Grade 8 break suddenly and without warning.

According to this page you couldn't be more wrong.
Are Grade 8 bolts more brittle than Grade 5? (8.8 vs 1.9 Metric) | Helping you work smarter not harder

 

[MechanicalGuy]

According to this page you couldn't be more wrong.
Are Grade 8 bolts more brittle than Grade 5? (8.8 vs 1.9 Metric) | Helping you work smarter not harder

That page doesn't refute the fact that grade 5 bolts are used in suspension, braking, towing, and have better elongation characteristics than grade 8.

 

[Shaneard]

That page doesn't refute the fact that grade 5 bolts are used in suspension, braking, towing, and have better elongation characteristics than grade 8.

Grade 5 bolts are used by manufactures where they can because it saves them a ton of money over time.

Do a test yourself, get a grade 5 bolt and a grade 8 bolt, clamp them in your vice and hammer them to a 90 degree angle. See which one breaks first, if either.

 

[Ford850]

Here's another link, to a bog regarding the myth about grade 5 being used for suspension etc. Killing the myth of the Grade 5 bolt. Industrial Chassis Inc.
And that page has a link to some great bolt science resources Information related to bolted joints

From some of that reading, the reason for the OPs bolts loosening could be from the mismatched bolt causing a sloppy fit on the blade http://www.boltscience.com/pages/Why_nuts_and_bolts_can_self-loosen.pdf
The solution is probably to get the correct OEM part.

 

[MechanicalGuy]

Grade 5 bolts are used by manufactures where they can because it saves them a ton of money over time.

Do a test yourself, get a grade 5 bolt and a grade 8 bolt, clamp them in your vice and hammer them to a 90 degree angle. See which one breaks first, if either.

No. That is not correct. Instead of being combative and argumentative, you might take a minute to learn something from the link that you posted yourself. Grade 5 is specified for brakes, towing, suspension, and other applications for the toughness of the material. The grade 8, as you can see in the graph you posted yourself, breaks suddenly, and without deformation. The grade 5 has a long yield curve. Now it's not only important to be able to read and understand that graph, but it's important to understand why manufacturers who are subject to liability lawsuits for the products that they make, would specify the grade 5. The grade 8 only retains it's superior strength if the bolt in question is torqued properly, for the life of the fastener. That is key to understand. Through the life of a fastener, use cycles, heat cycles and corrosion, bolts lose their torque. When this happens, they fail. Grade 8 fail in a catastrophic manner, whereas grade 5 provide warning. Your brake bracket will still be attached to your axle, or your trailer hitch will still be attached to your frame.

Go ahead. Look at these components and see if they are not put together with grade 5 bolts. I'm not just making this **** up. I have a degree in Mechanical Engineering and I actually read manuals when I install aftermarket brakes on my trailer, or my trailer hitch on my vehicles. Grade 5 are used for their ability to yield slowly and not catastrophically fail.

 

[MechanicalGuy]

In engineering, the area under the curve is known as the toughness. Here it can be seen that the grade 5 has it in spades over the grade 8

Sorry if that doesn't comport with a blog on the internet.

There are many tutorials but this one should suffice;

Strength, Resilience, Ductility, Brittleness, Toughness, Rigidity in materials - YouTube

 

[countrybumpkin]

You two need to start another thread, your way off topic to be helping the original poster now.

 

[countrybumpkin]

The bolts have a shoulder that bottom out on the blade carrier they don’t tighten up against the blade, the blade has no effect on the nut turning.

 

[radios1]

View attachment 617886

In engineering, the area under the curve is known as the toughness. Here it can be seen that the grade 5 has it in spades over the grade 8

Sorry if that doesn't comport with a blog on the internet.

There are many tutorials but this one should suffice;

Strength, Resilience, Ductility, Brittleness, Toughness, Rigidity in materials - YouTube

if they are heat treated to RC33-39, that makes them more tough. as a matter of fact, milling machine clamp sets are RC50, and never broke, they bent instead..

 

[ArlyA]

I agree, they need to start a new thread about bolt grades.... Nonetheless, the OP blade bolts shouldn't be unscrewing themselves weather they are grade 5 or 8. Left or right threads either. Why then?? Could we see photos of this setup?

You two need to start another thread, your way off topic to be helping the original poster now.

 

[Shaneard]

No. That is not correct. Instead of being combative and argumentative, you might take a minute to learn something from the link that you posted yourself. Grade 5 is specified for brakes, towing, suspension, and other applications for the toughness of the material. The grade 8, as you can see in the graph you posted yourself, breaks suddenly, and without deformation. The grade 5 has a long yield curve. Now it's not only important to be able to read and understand that graph, but it's important to understand why manufacturers who are subject to liability lawsuits for the products that they make, would specify the grade 5. The grade 8 only retains it's superior strength if the bolt in question is torqued properly, for the life of the fastener. That is key to understand. Through the life of a fastener, use cycles, heat cycles and corrosion, bolts lose their torque. When this happens, they fail. Grade 8 fail in a catastrophic manner, whereas grade 5 provide warning. Your brake bracket will still be attached to your axle, or your trailer hitch will still be attached to your frame.

Go ahead. Look at these components and see if they are not put together with grade 5 bolts. I'm not just making this **** up. I have a degree in Mechanical Engineering and I actually read manuals when I install aftermarket brakes on my trailer, or my trailer hitch on my vehicles. Grade 5 are used for their ability to yield slowly and not catastrophically fail.

I never went to college. I grew up on a farm. I know if you replace a grade 5 bolt that keeps breaking with a grade 8 the bolt don’t usually break.

There’s charts, graphs and theory and there’s application.

 

[Shaneard]

I agree, they need to start a new thread about bolt grades.... Nonetheless, the OP blade bolts shouldn't be unscrewing themselves weather they are grade 5 or 8. Left or right threads either. Why then?? Could we see photos of this setup?

Show me one thread on this forum that has stayed on topic, any forum for that matter.

Left vs right hand threaded bolts on something that spins does matter. Ever change your lawn mower blades? They have left or right handed bolts depending on which way they spin so they self tighten.

 

[Jstpssng]

I don't claim to be as mechanically adept as many (most?) here, but is seems like he gives the root of his problem in post #5.

From what I can tell the holes in the original stump jumper got stretched out so there is a 3/8" plate welded on the bottom and a 3/8" plate welded on the top of the stump jumper that holds the bolt. These each have a 1 1/8" hole drilled in them and the bottom one has a small part machined out for the crimped "tab" on the bolt that keeps it from rotating. It's very heavy duty.

Some repair jobs work; but given the stresses which the blades are going through, I prefer to trust the company which designed the machine.

Actually upon rereading his description I think that he has it wrong; if memory serves me there is a splined flange which the gearbox shaft goes through, and that the blades are bolted to. Correct me if I'm wrong but the stump jumper slides over that, and is there solely for the purpose it's name implies. The blade actually should spin on the bolt; so that it gives if you hit a rock or other solid object.

 

[countrybumpkin]

Show me one thread on this forum that has stayed on topic, any forum for that matter.

Left vs right hand threaded bolts on something that spins does matter. Ever change your lawn mower blades? They have left or right handed bolts depending on which way they spin so they self tighten.

And that’s thanks to people like you.

As for the lawn mower blades your not complete correct there either.

 

[Shaneard]

And that’s thanks to people like you.

As for the lawn mower blades your not complete correct there either.

Thanks for the compliment.

Why are some mower blades left hand thread? Why are some crankshaft threads left handed?

 

[Gordon Gould]

This is all pretty interesting. Since there are 2 extra 3/8" plates the geometry of the connection is way different with the 3/4" longer bolt passing thru a sandwich of 3 layers. Is it possible that during an impact the sandwich compresses and/o r the bolt stretches enough to allow the nut to become loose enough to back off a bit. Eventually becoming loose. ??????????????

gg

 

[oosik]

I would tighten the nuts as tight as possible and weld the bolt/nut together on the top. The idea of a blade flying off at PTO rpms isn't appealing.

 

[Fuddy1952]

Another thought is over the years is using two nuts. Install properly (Bush hog blade needs to be able to swivel) then tighten second nut onto that one.

 

[radios1]

if a castle nut, and a drilled bolt were used, this would not be happening!.