Metalworking: Shaking my head in disbelief!

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Something else going on there.....urban legend perhaps?

The "spring rate" / "axial stiff" of two geometrically identical bolts is the same....only the yield loads are different.

The elastic moduli for alloy steels are all pretty much the same which explains why the bolts perform the same until one yields.

Are your trying to tell me that a Grade 8 bolt will go "plastic" at a lower load than a Grade 5 bolt?

......I don't think so, show me the data.

better yet, here's the data

If the bolts are short, there isn't going to be much stretching going on. If want to get some stretch, you have to reduce the bolt's "spring rate" (aka "axial stiffness")

this can be done by:

going to a smaller diameter bolt, a longer bolt, adding some bolting compliance, like a disc spring (aka cupped washer, Belleville washer) using a different bolt material (copper based, Ti)

not by using a lower strength steel alloy

cheers Bob

Excellent question.

Gunner

GUNNER'S PRAYER: "God grant me the serenity to accept the people that don't need to get shot, the courage to shoot the people that need shooting and the wisdom to know the difference. And if need be, the skill to get it done before I have to reload."

0

Something else going on there.....urban legend perhaps?

The "spring rate" / "axial stiff" of two geometrically identical bolts is the same....only the yield loads are different.

The elastic moduli for alloy steels are all pretty much the same which explains why the bolts perform the same until one yields.

Are your trying to tell me that a Grade 8 bolt will go "plastic" at a lower load than a Grade 5 bolt?

......I don't think so, show me the data.

better yet, here's the data

If the bolts are short, there isn't going to be much stretching going on. If want to get some stretch, you have to reduce the bolt's "spring rate" (aka "axial stiffness")

this can be done by:

going to a smaller diameter bolt, a longer bolt, adding some bolting compliance, like a disc spring (aka cupped washer, Belleville washer) using a different bolt material (copper based, Ti)

not by using a lower strength steel alloy

cheers Bob

No, I am saying the Grade 8 will not stretch as much in the elastic range before getting into the plastic range. The Grade 8 will hold a lot more force for the same diameter. And if you are going in to internal theads, threads are not normally grade 8 and will pull out before you can get enough preload on the bolt to stretch it the length needed.

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For identical bolt / assembly geometry, a Grade 8 & a Grade 5 bolt, when preloaded to the same load, will stretch the same amount.

Since the yield point (either measured in load or stress) of the Grade

8 bolt is higher than that of the Grade 5 bolt....the Grade 8 bolt will actually stretch (elastically) more before yielding than the Grade 5 bolt. Of course, at the same load (as long as the load is below the yield of the Grade 5), both will stretch the same amount.

for example; consider a "mystical" bolt that has a cross-sectional area of .1 sq inch.

uisng the material properties data from the "bolt chart"

the Grade 8 bolt with yield at ~ 13,000 lbs the Grade 5 bolt with yield at ~ 9300 lbs

so if each bolt is taken to a load just their respective yield points; the Grade 8 bolt will actually strecth 1.4x as much as the Grade 5 bolt. The actual bolt stretch will be driven by the active length of a bolt.....that's why longer bolts perform better where one needs to maintain a pre-load.

wrt base material threads not being "Grade 8", just increase the thread engagement length. Also that's what thread inserts (aka Helicoils) are for.

Cheers Bob

For identical bolt / assembly geometry, a Grade 8 & a Grade 5 bolt, when preloaded to the same load, will stretch the same amount.

Since the yield point (either measured in load or stress) of the Grade

8 bolt is higher than that of the Grade 5 bolt....the Grade 8 bolt will actually stretch (elastically) more before yielding than the Grade 5 bolt. Of course, at the same load (as long as the load is below the yield of the Grade 5), both will stretch the same amount.

for example; consider a "mystical" bolt that has a cross-sectional area of .1 sq inch.

uisng the material properties data from the "bolt chart"

the Grade 8 bolt with yield at ~ 13,000 lbs the Grade 5 bolt with yield at ~ 9300 lbs

so if each bolt is taken to a load just their respective yield points; the Grade 8 bolt will actually strecth 1.4x as much as the Grade 5 bolt. The actual bolt stretch will be driven by the active length of a bolt.....that's why longer bolts perform better where one needs to maintain a pre-load.

wrt base material threads not being "Grade 8", just increase the thread engagement length. Also that's what thread inserts (aka Helicoils) are for.

Cheers Bob

Why would you say it stretches 40% more? Because there yield points are 40% different. Load both bolts to 9000# and how much will it stretch? Do the same for Grade 8. How much will it stretch? A lot less.

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om...

different.Load both bolts to 9000# and how much will it stretch? Do the same for Grade 8. How much will it stretch? A lot less.

Actually the all bolts , grade 2, grade 5 and grade 8 will all stretch essentially the same as long as the bolts are not stressed above their yield load. There are slight differences between the values of Youngs modulus for various steels, but for engineering work one can use

30,000,000 psi for all steels.

Dan

Actually the all bolts , grade 2, grade 5 and grade 8 will all stretch essentially the same as long as the bolts are not stressed above their yield load. There are slight differences between the values of Youngs modulus for various steels, but for engineering work one can use

30,000,000 psi for all steels.

Dan

30,000,000?

I'm no mathematician, nor am I an engineer---------but 30,000 seems more appropriate.

What am I missing?

Harold

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Dan

The elastic modulus (Young's modulus) is (for most steels ~30,000,000 psi)

You most likely are confusing yield stress for dead soft steel ........it is in the range of 30,000 psi

for alloy steels (depending on alloy & heat treat) yield stress can be like 70,000 psi to 200,000+ psi

the modulus (E) relates stress to strain via Hook's law stress =3D E * strain;

since strain is "dimensionless", both E & stress have the same units; psi (for example)

btw most, if not all, mathematicians have no clue about Young's modulus, Hook's law, stress or strain

cheers Bob

young's modulus is (in Imperial units)

Pounds force x inch length

----------------------------- sq inch area x inch extension

30 million is about right

Mark Rand RTFM

Heh! Wish you guys would speak English!

Thanks.. It's way over my head, but at least I understand that the

30,000,000 wasn't an error.

Thanks, too, to Bob for his excellent response.

Harold

Possibly more useful is to know that typical tungsten carbide composites have about three times the Young's modulus of steel. This makes solid carbide boring bars, drills and milling cutters more useful than their hardness might imply. One third the deflection of a steel boring bar is rather useful when boring a deep hole :-)

Mark Rand RTFM

Found on the InterToobs somewhere. Author unknown.

The designer sat at his drafting board A wealth of knowledge in his head was stored. Like "what can be done on a radial drill Or a turret lathe or a vertical mill?"

But above all things a knack he had Of driving gentle machinists mad. So he mused as he thoughtfully scratched his bean, "Just how can I make this thing hard to machine?

"If I make this perfect body straight The job had oughta come out first rate. But 'twould be be so easy to turn and bore That it would never make a machinist sore.

"So I'll put a compound taper there And a couple of angles to make them swear, And brass could work for this little gear But it's too damned easy to work I fear.

"So just to make the machinist squeal I'll make him mill it from tungsten steel. And I'll put these holes that hold the cap Down underneath where they can't be tapped.

"Now if they can make this it'll just be luck 'Cause it can't be held by dog or chuck And it can't be planed and it can't be ground So I feel my design is unusually sound."

And he shouted in glee, "Success at at last! This goddamn thing can't even be cast!".

Engineer is a professional . They screw up and get paid for it .

Grade II bolt is so bad , its not used in any critical stuff . Its softer and galls . ...Zinc plating does a sim stunt on bolts . But exclude Japaneese yellow Zinc treatment on car bolts . Its way better .

Very hi UTS aka Tensile strength .. makes steel competitive with other materials , next they want mucho Elongation to pass the stress on to other parts of the structure .

A Square steel bar 1" by 1" elongates about .0012" for about 30 kips , if its CRS aka Cold_Rolled_Steel.

Hot Rolled is worthless . Ornamental iron gates ...

NC bolts are better than NF . you get more thread strength . They do not loosen easier , even with lube .

Aircraft bolts take ONLY shear loads , look at the tiny head . The extra weight of more and bigger bolts as compensation is only a few pounds .

But we use both . Hardened washers increase the strength mucho .

The most accurate torque wrench is a beam type with a H'F' dial Indicator . You can move the D'I' around to get it to read rite . You can make one in 15 minutes . Use a 3/8 drive breaker bar . Add a long lever and the D'I' . Another thing ..... A MUST have tool is Harbor Freight $6 hand impact . hammer on one end , phillips #1 bit on other end .

All Japaneese motorcycles needed this in the 80's , then they switched to hex heads .

Sears has some deals . I cant find 12 point sockets , Sears has them , also couldnt resist some off brand socket sets at $6 ! .. Also the Micro-Water pump pliers / ignition pliers are $10 .

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=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Dan

On Wed, 14 Oct 2009 18:26:59 -0800, the infamous "Paul Hovnanian P.E." scrawled the following:

That's great, Paul. Thanks for sharing. I used to feel that all the automotive design engineermuthuhs in Detroit used his exact style back in another life, when I was wrenching for a living.

-- The blind are not good trailblazers.

-- federal judge Frank Easterbrook

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0= =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Dan

Too much mis-information in this post to refute all of it in detail

huh?

if true....basically true for all steel alloys, modulus is not a strong function of alloy

basically wrong; NC has its applications as does NF but NF is generally stronger

wrong again; designers often shoot for fasteners in shear but not always possible, "aircraft bolts", as you call them, come in a wide range of types and styles. Bolt heads & nuts sizes are typically sized to fully develop bolt shank.

cheers Bob