Convert Degrees to Foot Pounds?

Metal fatigue plays a factor once the bolt is stretched. I know that there are a number of engines that use this method in tightening head bolts. And they are not to be re-used under any circumstances.

Lane

Reply to
Lane
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I think it's because they get hot while under tension and while getting hammered by the engine. That gives 'em a small amount of work hardening that won't like a restretch later on. I read it somewhere on the internet, but then again, I read it somewhere on the internet.

Reply to
B.B.

Recently I borrowed some material from a friend on bolts and read a lot of it. And as usual the answer is " it depends ". Torqueing to a low value and then tightning a number of degrees was stated as being more accurate than just using a torque wrench.

But without more information, I can't tell you if these bolts are being tightned until they yield or not. You can design bolted joints so that the bolts are tightened unitl they yield slightly, or you can design joints so they do not tighten the bolts to yield.

But if the original poster has the book that says how to tighten the bolts, I am pretty sure it would say if the bolts should not be reused. My SWAG would be that tightening them 45 degrees, probably is not into yield. On the other hand, using new bolts ( of the correct grade )will never get you into trouble.

Dan

Reply to
Dan Caster

Ideally, this is true. i.e. Yield point is stress level that separates elastic (returns to original dimention when the stress is relieved) from plastic (returns to original dimention when the stress is relieved) deformation. Regretably most materials do not behave ideally so you will often see yield point defined as the point where residual deformation is some small percentage of original dimension.

Ted

Reply to
Ted Edwards

How can you possibly correlate degrees to Ft lbs ? If I torque a 1/4 20..

45 degrees that will be much more torque then torque in a 1/4 28... 45 degrees. If the manual says torque then turn ...it's already calculated on a fresh bolt ... however if you reuse a standard bolt it has already been stretched once ,I would not want to stretch it again.
Reply to
invntrr

On Sun, 28 Nov 2004 20:03:17 GMT, Ted Edwards vaguely proposed a theory ......and in reply I say!:

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I am sorry. I am a bit confused by the above lines.

Which is to say that you use a bolt that only _just_ doesn't really do the job.

Reply to
Old Nick

The concept behind stress to yield bolts is to get a very consistent bolt to bolt torque value when tightened according to the listed procedure. Many modern head bolts have a reduced diameter section on the shank for this purpose. Torque variance can be as small as

1% using this method. Ordinary torque wrench methods give a bolt to bolt consistency variance that is often as large as 20%. Consistent torque reduces the tendency of aluminum heads to warp.

Gary

Reply to
Gary Coffman

I don't think I said anything about correlating degrees to ft lbs. But if you have say a 4 inch long 1/4 20 bolt and you tighten it so it is snug and then tighten it 45 degrees, you will have stretched the bolt 0.0125 inches. So for a 4 inch long bolt you would have stretched it .0031 inches per inch of length. Now assuming this is less than yield, you can loosen it and retighten it over and over. No new bolt needed. You don't worry about flexing the springs in your car, do you. Stretching less than yield means it returns to the original length when the stress is removed.

If you use a different length bolt, or a bolt with different threads per inch, you have to calculate how many degrees to tighten it for the stain you want.

Dan

Reply to
Dan Caster

If anyone is interested in knowing more, the author of most of the articles and books that I borrowed is John H. Bickford. Good books, just not cheap.

Dan

Reply to
Dan Caster

*You* didn't -- but it is in the "Subject: " header from the original question.

And to calculate the equivalent foot-pounds figure, you would need to know the diameter of the section being stretched, the length of the reduced diameter or section being stretched, and the tensile strength at the current temper of the material. (And, as has already been discussed, trying to do it all with a torque wrench is less accurate.

*Or* -- a different reduced diameter length, if present. And that would be more likely to be a torque to yield bolt application.

Enjoy, DoN.

Reply to
DoN. Nichols

The name of the thread is convert degrees to Foot Pounds so I assumed that's what the subject was ... happens all the time where a thread starts out one thing and turns into something else. I guess you have to go back to the original post to figure out where the subjects at ... I don't because some of the threads are really long.

Reply to
invntrr

The turn-of-nut method sets a fixed stretch in the bolt, which gives a fixed clamping pressure on the parts.

Depending on the design, the bolt is either acting like a spring, and: modulus*(cross section)*stretch/(unstrained length of the stretched section) gives the clamping pressure

or it it being stretched to yeild and: (yeild strength)*(cross section of yeild zone) gives clamping pressure

In either case, the bolt should not be reused. If it is TTY, the next time it is pulled in, the previously yeilded portion will have been reduced in cross section and yeild at a lower tension. If not TTY, the original tension is likely calculated to be 80% or more of yeild, and there are likely high stress points that will yeild either as the bolt is pulled in or during service.

The bolts are also quite likely to break, as the closer to the yeild they are brought, the fewer fatigue cycles they can take, the obvious limit being one cycle when they are stressed to the ultimate strength the first pull (

Reply to
e

On 30 Nov 2004 19:29:53 -0800, snipped-for-privacy@krl.org (Dan Caster) vaguely proposed a theory ......and in reply I say!:

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This was my argument. But it would seem that in the real world, that stretched bolt has been "damaged", or is assumed to be damaged. Minimalism in engineering, heat, vibration etc.

Reply to
Old Nick

On 30 Nov 2004 20:57:09 -0800, enl snipped-for-privacy@yahoo.com (e) vaguely proposed a theory ......and in reply I say!:

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But surely if it's stretched to Yield (_and beyond_ if it's deformed), then the clamping pressure will sart to drop straight away. Unless you got _exactly_, and I mean _exactly_, Yield, you would not get (yeild strength)*(cross section of yeild zone) but some lesser figure.

But I do have to admit,,,snip of very useful stuff after that.

Reply to
Old Nick

Amazingly, you just about do get exact clamping pressure. In fact, if you don't go far into yeild, the pressure goes UP slightly as yeild goes on a) because the yeild isn't an exact point, as the material begins to stretch and neck, the stress goes up slightly, compensating to a good extent for the reduced cross sectional area, and b) as the material yeilds, it work hardens slightly, increasing the yeild stress slightly. Ya, the effects are closely related. In practice, if it is dead critical, a tool is used to detect yeild and stop there.

Reply to
e

There's no really practical way other than doing it as specified. "Angle-to-torque" conversion would involve too many things that could be variable - What's the pitch of the thread on the bolt? What's the coefficient of friction for the material the bolt is made from? How about for the material that the bolt is going into? What's the shape/area of the bolt-head to fastened surface contact point, as well as the CoF for that interface? How many tuyrns of thread are going to be in the hole and meshed with the threads that are in the hole? Etc, etc, etc... LOTS of "etc"s that add up to "You're better off doing it as the instructions say."

Add in the fact that, as others have said, these are probably "stretch bolts", intended to be cranked to a specific torque as a "reference point", then cranked an extra 1/8th of a turn in order to put a specified amount of stretch on them, and you're still ending up back at "do it the way the instructions say."

Besides - 45 degrees = 1/8 of a turn - It's really not that difficult to "eyeball" a well-within-reasonably-accurate approximation.

Reply to
Don Bruder

On 1 Dec 2004 08:34:06 -0800, enl snipped-for-privacy@yahoo.com (e) vaguely proposed a theory ......and in reply I say!:

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hmmm...OK. If that is the way it's worked, then I can see why you would not use the bolt again.

I couldn't see why you would want to go to, and beyond Yield, when you would get an elastic stress there to clamp the joint, without needing to wreck the bolt. But if there is a point when you can actually maximise the clamp even aftger Yield then I can see the reason. As a RAG (Rough As Guts) "engineer" I just use a bigger bolt!

Thanks.

Reply to
Old Nick

Even standard bolts generally shouldn't be reused in high stress applications like an engine head because the properties change when they are first torqued.

Typical designs for load-bearing bolts loaded in the elastic range are

50 to 80 percent of the yeild stress. All you get for using a bigger bolt and not loading it to this high a stress is a bigger hole and a more expensive bolt. Greater than 80 percent gets into the range where unknowns in the installation process may bring the bolt into yeild. Often not a good thing, as bolts not designed for this usually don't have good yeild characteristics (they tend to yeild at the thread root, where the minimum diameter is. bold designed for TTY usually are reduced shank diameter)

news:...

Reply to
e

replying to Dan Caster, Tom wrote: How about if I'm using my old head bolt it's a nissan altima 2.5 liter it only give me a 35 foot pounds then add 75 to 80 degrees

Reply to
Tom

Called torque to yield - AKA big expense as they are not to be re-used - and they are not cheap. Also called "stretch bolts"

Reply to
clare

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