bolt strength, fine verses coarse

[opeldean]

grade 8 bolts are gold tinted with 6 marks on the head. in the hardware store,the fine threaded were about double the price of coarse threaded.

Q: Given all things equal, which is stronger?and why?


a) coarse thread
b) fine thread
c) no difference

Hint: don't go to great lengths researching this answer.

[soybean]

The answer is A. You can torque them down more. More threads/inch. Jarrell

[hrcollinsjr]

More thread contact between bolt and nut on fine thread.

Coarse thread into soft metal like AL casting is usually coarse thread.

Harold

[jeff denton]

My answer is neither. The difference is in how tight you can get the bolt. Bolts are categorized several different ways, they can do different things for different applications. Shear strength is one, but has nothing to do with threads.
When you need a part clamped to another real tight, use a fine thread. Notice with a fine thread bolt when you try to get it tight against a lock washer, it will spit the lock washer out, it is clamping so hard!
Fine threads should never be used in aluminum threaded parts, they will pull the threads. Like Mr. Ging's drywall screws in wood experiment...
Also, fine threaded bolts/nuts may be impossible to get apart again, on the big trucks I prefer to impact them tighter to twist off, it's a lot easier to deal with... Fine thread bolts are never re-used. Especially the big U-bolts that hold the truck spring to the axle, You'll never get 'em tight enough again to stay put!!!
As the fine threaded bolts are used for maximum clamping force, it is imperative to use good thick hard flat washers under the head and the nut. You do not want more than three "extra" threads sticking out of the nut.
Also the use of Loctite is recommended, and it is common to find "castellated" nuts used with fine thread bolts, these are the nuts with slots to allow a cotter pin to pass thru into the bolt.
There, I hope I didn't get technical, just simple guidelines about when and where and how to use bolts... From the heavy equipment and trucking aspect which I have some experience with, anyway

[ging]

Daniel just had to do a 'stress test' at work last week testing types of fasteners in wood products. Not exactly the same thing but still... . He found that the coarse threaded drywall screws were the toughest. Again, not metal/bolts. He performed a 90 degree and a 45 degree test and the coarse thread held more weight than the fine. There were no shears - just yanked the wood out. 45 degree coarse thread held up to 255 lbs and the 90 held thru approximately 350+. (Tests were with particle board and MDF - number results were on the MDF (medium density fiberboard)).
Just some trivia to add to the mix.

[namba209]

In the aircraft industry, almost all fasteners are fine thread, excepting of course the dzus, cam-loc, etc., 1/4 turn type panel fasteners. Now that being said, right behind using the right screw/bolt is the torque setting of the specific application. Also, if warranted in the application the use of anti-sieze compound. And to expand on what Jeff said about threads extending through the nut or nut-plate, the hardware bible for aircraft, USAF Technical Order 1-1A-8, states no LESS than 1-1 1/2 threads shall extend through the nut/nut-plate.

A story about that during my time with the Blackbird. There are louver panels on the aircraft where the screws can be seen protruding through the nut-plates. Quality Control would continually write us up if the screws were too short, a legitimate discrepancy. We found some extremely long, fully threaded titanium screws and used them in the louver panels where the Quality Control folks would see them and would write us up for the screws being too long. So we said why, where is the length of the screw through the nut/nut-plate stated as being too long. They couldn't find it so we did a gotcha on them, for once. Of course they were a PIA to remove after flight, but a gotcha, is a gotcha.

[BQS4]

The threaded portion is not what makes a bolt strong, it's the "grade" of the actual steel. With Grade 2 being the weakest, Grade 5 being a good general bolt, and Grade 8 being the strongest

[hrcollinsjr]

Originally Posted by namba209 Quality Control folks would see them and would write us up for the screws being too long.

I think I can answer this one. They didn't know if you had used the correct length or not. Assuming you had used the correct length and they were showing too many threads then you had definitely overtorqued them.

When bolts are tightened a certain amount of stretch is expected. The amount depends on the diameter and the grade of the bolt. In critical situations or high torque applications once the bolts have been stretched they are not supposed to be reused.

If you want to experiment with the strength of fasteners here is a simple test. Buy a common grade low quality bolt and over tighten it, remove it and check the threads with a thread gauge. Don't have a thread gauge use another bolt of the same diameter and thread pitch and use the second bolt as a thread gauge.

Harold

[hrcollinsjr]

Originally Posted by BQS4 ..., and Grade 8 being the strongest

Nitpicky correction here. Grade 8 being the strongest mentioned, but there are higher grades than this. I believe the 12.9 grade used in our engines is a little higher grade than 8. Now of course I've introduced another scale used for metric nuts and bolts to help confuse things a bit.

Harold

[bq97]

I know the answer to this one! My fatherwas a mechanicial engineer and when i was a teenager I asked him what the difference was one day.After a long disertation he told me that within the same grade, fine thread bolts are roughly 1.15 times as strong as course thread bolts when properly torqued due to the thread contact area and bolt stretch factor, both aparently affect breaking strength. He also told me that for a given grade and bolt diameter there is a minimum and maximum torque value for maximun strength. For a comparision I pulled this out of one of his books.

1/4 bolt
----Grade 2---------Grade 5-------Grade 8
Coarse--- Fine---Coarse--Fine---Coarse--Fine
2350-----2700---3800---4350---4750---5450 (in pounds)

hth
Brian

[heimue]

The fine thread bolt has a little more of cross section area (core diameter) than the coarse bolt, because the outer dia is the same, and the course threads take away more of the cross section than the fine threads. Considered that the tensile strength of the material is identical, the fine thread bolt can handle more "strength" (=force in axial direction) than the coarse thread bolt.
I just don't know what that "hint" picture meant in the first post. Was it a trick question after all?

Dieter

[opeldean]

Originally Posted by heimue I just don't know what that "hint" picture meant in the first post. Was it a trick question after all? Dieter

when Craig and Chad were working on my suspension, i brought both fine and and coarse grade bolts to the shop for them to use.I asked them which they preferred.. Craig said" Fine is stronger, 1-
take a piece of thread( dental floss for example) and wrap it in the thread patttern, 2-now do the same with the coarse thread
3- now compare the lengths."

so the the bolts in my spring eye bushings are fine thread 4 inch,,,NOt coarse thread 4 in

[opeldean]

the longer thread pattern( longer line when the floss is straightened out) allows you engage more length of the nut to the bolt.results in less applied stress to the threads because the force is dispersed through a longer area..
just like bqs4 post, which i enjoyed, along with all the others...

[opeldean]

Originally Posted by opeldean Hint: don't go to great lengths researching this answer.

I was trying to be clever. this was a fun post from my perspective, with real world application, i guess you do get what you pay for. Now i know why the harware store charges more and keeps them stored in a special box.

[Paul]

If one studies the lenght of thread engagement, one would find that only about 3 to 4 threads are actually carring the load. Doesn't matter coarse or fine. Just look at the lenght of a nut and the number of threads inside the nut.

What matters on a bolt is the cross sectional area. Looking at my machinery's handbook... example 3/8-16 UNC minor diameter is .037"
............................3/8-24 UNF minor diameter is .330"

If considering shear forces, the Fine thread bolt would be a little stronger due to a slight increase in area if the load is through the thread. But if the bolt has a long shank where a portion of the body is not threaded and the load is only present in the non-threaded area, then there is no difference.

[namba209]

This is getting real heady, but fun.

Harold, the idea for using the real long screws was to show the Quality Control folks that the screws were not too short, which was a relative opinion in this case. Think about looking through a louvered panel, seeing a line of nutplates and figuring if there is one or more threads of a screw showing through a specific nutplate. If you see one thread, on one side, is it the same on the other side or no thread showing on the other side. If you see 1 1/2 threads then there has to be a thread showing on the other side, maybe two. As far a torque stretching the screw, not really, we were using, in this application, 10-32 titanium screws torqued at 25-40 inch lbs, with about 2-3 inches of thread protruding through the nut plate.

Now for clamping forces, think this way, what is the application? Holding up an engine, as in the crossmember for our Opels or the adjustment on an alternator bracket. The size of the bolt is critical, you wouldn't use a small bolt to hold up and engine, there's not enuff strength in the bolt, but it could be used to keep an alternator from sliding down the adjustment slot. The application is critical, shear downward force on the head of the bolt, or clamping action to keep sideways movement from occurring. Now, going back to the original question, which is stronger, simple, the fine thread has more load bearing surface than a coarse thread, so the fine thread bolt is stronger, in the threaded area.

[heimue]

Originally Posted by Paul and the load is only present in the non-threaded area

Sorry, but I strongly disagree. The load is the same all the way from the head to the nut or from the head to where it is screwed in.

Dieter

[Paul]

Dieter,

I believe you misread my post. My statement was under the condition that the load on the bolt was shear, Not Tensile.

The tensile forces are the result of tightening the bolt... applying torque to the nut in such a quanity to cause the bolt to stretch within its plastic limits. And again, the bolt with fine threads has a bit larger minor diameter thus more strenght in tensile. Once the clamp load is applied, the bolt can carry a great deal of shear load that is never seen at the theads.

In any case (fine or coarse), when discussing a bolt and a nut, failure will occur in the bolt not the nut. If discussing a screw. The opposite usually occurs.

[Paul]

A bit of trivia....

What is the real difference between a bolt and a screw. The same fastener can be both... The answer can be found in the definition....

GL

[namba209]

Why not trivia? Going through the USAF aircraft maintenance school way too many years ago, we were told the type of screw head denotes relative strength of the screw. Basically a common head (slot type) screw is the least rated in strength, with the cross point, (phillips, reed & prince) being next in strength. Next is the coin slot or Hi-Torque, predominatly used on the F-4, and a real PIA to get out or loosen. They caused many a scratch and scars on aircraft and maintenance folks. Next and last was the Torque-Set or commonly called swatiska, originally only titanium screws had this type of head, it was easy for us "nose-pickers" to see if the screw was correct to use on the Blackbird. Unfortunately, that type of head is now commonly used on steel screws because of the ease of torquing. Taking them out was a different story, I have matching scars in each eyebrow, where the apex bit snapped while trying to remove screws with a speed handle, and the needle point made when the bit broke smacked me before I could stop the rotation of the speed handle. Oh the good ole days.

[Paul]

Wow, no one willing to venture a guess?

Its really quite simple when you think about it......

[Old Hippie]

My opinion is this:
All depends on application as far as threads are concerned. Fine threads are preferred for 'controlled' tightening apps. Corse threads are a general app use. There are 'generally' 3 sets of threads in use:fine, medium and corse. As stated previouslly by Jeff and Ron, the grade of bolt and its design are what makes the determination in use. Shear-grade are used in apps where you want the bolt to break before the implement is damaged, such as in mower equipment. Stretch bolts are common in head use as the will "stretch" as they hit the required tourqe limit and you DON"T want to snap one in the block.
Barry

[Paul]

Bolt trivia revealed

Screw: A fastener is a screw if the tightening torque is applied to the head of the fastener.

Bolt: a fastener is a bolt when the tightening torque is applied to the nut.

Be careful when torqueing a bolt to a specification, not to turn the head of the bolt. The torque limit of the bolt is based upon the stretch of the bolt in pure tension only. Twisting the bolt can cause it to fail before the proper torque value.

Notice the above was discussing a bolt not a screw. A lower torque value for the same fastener will be specified when its application is a screw.

Bolt Torque is only an approximation of the required number of turns of the nut (or screw head) to stretch the shank of the fastener to 90% of the yield of the material. Its only an approximation because the friction between the threads and also the friction between the underside of the head and the surface it is turn against is estimated.

A higher strength bolt requires more force to stretch it to its yield limit.

[Dmcbrass]

I read the responses to this post rather fast and may have missed something. The advantage of fine thread bolts are that they are much less likely to vibrate loose. As far as I know that is the only advantage.

[namba209]

Doggone it Paul, that was too easy, and I should have known that. Actually I did know that, but didn't think of it. Being old enuff to be retarded has its' drawbacks. I was just out torquing the rear suspension too. Had to use a little bit of ingenuity to get the torque wrench on the nut of the panhard rod just because I knew not to torque from the bolt head.