Hardest Bolt you can buy

I am looking for the hardest bolt I can buy. As far as I know, grade 8 is the hardest -strongest -. Do they make a bolt harder than a grade 8? Have a 7/16 bolt on a sweep in a grain bin that keeps sheering. Not enough shaft to drill it out for a size larger. Does anyone know if they make a stronger than a grade 8?
 
(quoted from post at 18:57:34 08/16/16) I am looking for the hardest bolt I can buy. As far as I know, grade 8 is the hardest -strongest -. Do they make a bolt harder than a grade 8? Have a 7/16 bolt on a sweep in a grain bin that keeps sheering. Not enough shaft to drill it out for a size larger. Does anyone know if they make a stronger than a grade 8?

Find out what is wrong and fix it. If you can find something that won't sheer you will tare something up
 
They make a grade 10. We use them mainly on cutting edges for backhoes and dozer but it does sound like you have a problem somewhere. Just don't rule out something simple
 
if it's shearing in the threads; use a longer bolt. get the shear point off of the threads. this is the most common problem.
 
I had a rotary cutter that kept breaking them till I quit tightening the bolt all the way. Agree that you want the bolt longer than the hole to keep the threads out of the shear zone. I used a locking nut or 2 nuts jammed together to keep the nut on.

Other thing I learned from someone else was that you use a grade 2 because it won't snap, will slowly shear and in doing so you can find the hole to punch out the dud. I never tried it but I did have problems finding the dud to punch out having used grade 5 or better. My JD round baler specifies grade 8 so I don't know what to believe, except for what I said initially.
 

McMaster-Carr, (www.mcmaster.com) lists a grade 9 bolt @ 180 Ksi and RC38 for hardness. I have used grade 9 bolts without success, they tend to be brittle. For comparison, grade 8 bolts are listed at 150 Ksi and grade 5 bolts are 120Ksi.

If you have been shearing grade 8 bolts I suspect the shaft is damaged/worn resulting in more of a bending failure than shearing. A hard bolt will not like bending loads. If there is room you might consider drilling a new hole in a different location and trying a grade 2 or at most a 5 bolt. The new hole will get the shearing edge back to the outer most diameter of shaft providing a lower shear force for a given torque.
 
Get an Aircraft Spruce and Specialty catalog and order an aircraft bolt . BUT you gotta figure how you keep breaking grade 8 bolts. That's not normal.
 
Harder is likely going to be brittle, what's causing it to break now, worn hole? Overloaded, there must be something wrong
 
I see all these myths about grade 8 being brittle . I don't think brittle is a term used in any hardware engineering . Fact is due to shear and tensile strength being superior in a grade 8 the grade 5 will fail way before a grade 8. I am willing to learn if you can show me a Machinerys Handbook that list brittle as a spec.
 
(quoted from post at 13:41:28 08/17/16) I see all these myths about grade 8 being brittle . I don't think brittle is a term used in any hardware engineering . Fact is due to shear and tensile strength being superior in a grade 8 the grade 5 will fail way before a grade 8. I am willing to learn if you can show me a Machinerys Handbook that list brittle as a spec.

I'm no engineer and brittle may not be defined in any book. For example take peanut brittle and peanut fudge. The peanut brittle will brake and the fudge will bend. I know it isn't relevant, just a thought.
 
Dr. Sportster: you're not usually going to see "brittle" as a spec, in large part because most bolts are designed to be used in tension, not shear. However, there's still indications that brittleness has been taken into account. One is the note you'll typically see on Grade 8 bolts that they're Q&T'd, or Quenched and Tempered. One reason for the tempering is to reduce brittleness. It's a bit of reading, and I'm not saying it's the be-all-end-all on the topic (as many, many volumes have been written on the subject, and knowledge continues to evolve and expand) but if you want a bit more information read through the PDF I've linked, which is from Fastenal and gives some more in-depth information on this and other bolt-related subjects.
Fastenal technical reference guide
 
That's the other reason for using soft steel.....thanks for reminding me.

Tempered steel has a high tensile strength but in a bending moment, what makes it high in tensile strength makes it brittle and it doesn't bend, it
breaks. I never had a 1/4" Grade 8, that the operator's manual specifies, shear on my JD 375 round baler so I guess I haven't stressed it
adequately to test it's shear strength.
 
Was assembling a project recently and had a bit of misalignment on the holes. As it was all from old, repurposed scrap steel anyway and it wasn't going to affect the finished project, I didn't bother correcting it. Grabbed a handful of bolts out of the 1/4" drawer and when I tightened it down, between the misalignment and the fact the pieces I was using were warped to begin with, there was enough of a side load that one of the bolts snapped off as soon as it started to come tight. Looked at the head, and it was a Grade 8--the rest I'd grabbed were either 5's or 2's and drew up fine. Ran an oversize reamer down through, grabbed another bolt (making sure it wasn't an 8 this time!), drew it up and it was fine. Hardly a scientific test (and there's an interesting note in the Fastenal article I linked saying that there's been quite a spread of values when dealing with shear loads on bolts that's traceable to different testing setups, so apparently even scientific tests have some difficulties in this area!) but it was interesting that the Grade 8 bolt was the only one to snap in the whole assembly.
 
Charpy impact test is used to determine how "brittle" a material is.

https://en.m.wikipedia.org/wiki/Charpy_impact_test

There is a relationship between hardness of steel and it's ultimate tensile strength. As strength and hardness goes up it also tends fail in a more brittle fashion and not have much plastic deformation before rupture. Read up on stress-strain curves.
 
(quoted from post at 02:35:49 08/18/16) Wing bolts on a Beech King Air are made out of Inconel.
You need two credit cards to buy one.

[i:fc5c8bd047]Everything[/i:fc5c8bd047] on an airplane costs cubic dollars; that is a rich mans toy. My brother spent 7 years building an RV-8, put 150 hours on it and sold it. He couldn't afford the $100 per hour fuel cost on top of the hanger rent, insurance, annual inspection, and I don't know what else. Dumb idea....

Socket head bolts, AKA allen bolts are about as hard and strong as I've seen.
 
Its hard to evaluate without a picture when you say it shears off to me that tells me there has to be a force movement that would act on the bolt in a side to side motion , If the fastener (bolt) is in tension it is literally pulled apart. Tension fasteners have a bigger head and nut diameter to resist this force. Its always best if you don't know to use a tension fastener if space allows it because the shear value is the same but you gain in tension strength. I have to agree with the others if you have threads in the hole bearing surface that is inside the hole your inviting structural failure of the part or the fastener. Use steel washers max one under the head 2 under the nut to adjust this. If you are installing the bolt on a shaft this is what it sounds like to me. The bolt needs to have a flat surface area for the head and the nut in addition accommodate any washer as well this can be accomplished a number of ways you can counter bore the shaft this is the easiest fastest way so the bolt head,nut and washer combination sets flat as can be or you can fabricate a piece of flat stock to fit the contour of the shaft and drill a hole in it . In addition if the hole is not straight through the shaft you may need to install a Self Aligning washer nut combination like that used on aircraft as well. If the part is in tension rather than shear on a round shaft only a small portion of the bolt head and the nut will be taking the load increasing your chance of a catastrophic failure. You can get aircraft Bolts that are avaiabale in 1st, 2nd and 3rd oversize so your bolt now is .437 diamater ist oversize is .452 it go up in icreaments of 1/64 so second oversized is .467 you say you cant drill it up to the next size you mean 1/2" .500 then the shaft is rather small the head size of a 7/16 bolt is .679-.690. My best advice to you is at a minimum use a tension bolt and nut with fine threads they are typically stronger in tension than the same bolt with course threads.
Best of luck Byron
 
Yes, Bowman Corp makes a Bowmally bolt with 12 marks on the head. Said to exceed 200,000 lbs tensile strength. I have several sizes. What length do you need?
 

Yea Tim tightening a bolt in a crooked hole is likely to shear it off but its the tension load on the fastener in your case not the shear load that is the issue additionally all bolts have a radius under the head so if you don't install a washer in a close tolerance hole or slightly countersink the hole on the head side this is called a fillet relief the fastener is only touching in an area slightly bigger than the bolt diameter particularly in a very hard metal application. By reaming the hole bigger you did 2 things you allowed for the fillet relief and you straightened the hole either one could have caused the failure. I do agree the tensile strength in a case like you mentioned is a factor because of the ability of the weaker grade 5 and grade 2 bolts to deform without breaking. I'm adding a partial definition of tensile strength and shear to clarify,

Ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength, is the capacity of a material or structure to withstand loads tending to elongate or stretch the bolt, as opposed to compressive strength, which withstands loads tending to reduce size. In other words, tensile strength resists tension (being pulled apart), whereas compressive strength resists compression (being pushed together). Ultimate tensile strength is measured by the maximum stress that a material can withstand while being stretched or pulled before breaking. In the study of strength of materials, tensile strength, compressive strength, and shear strength can be analyzed independently.
shear strength is the strength of a material or component against the type of yield or structural failure where the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force. When a paper is cut with scissors, the paper fails in shear.

Some materials break very sharply, without plastic deformation, in what is called a brittle failure. Others, which are more ductile, including most metals, experience some plastic deformation and possibly necking before fracture.

Hope you don't mind my rambling I rarely get to comment on something I know a bit about.
Byron
 
Likely, the hole in the shaft and tube is "wallowed out", putting stress on any new bolt you install, vs. a clean, straight hole as-original.

Might have to replace the shaft, could possibly drill a new hole 90? away in the tube, as well.
 
I agree with virtually every reply.
Back when I worked in a large industrial plant, large rolling mill bearings had to be installed with a large nut.
Nut was about 4 feet in diameter with holes to install a bolt with a steel cable loop inside.
Nut was tightened with an overhead crane.
For some reason, grade eight 3/4 allen head bolts would not shear as easly as hex head bolts? ? ?
Never knew the reason--that's just the way it was.
 

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