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TIG
10-04-2009, 08:46 PM
I saw a thread earlier asking about bolt strength, so I thought I'd post an article I wrote awhile back about bolt science. Some of you may find it of use, some of you may find it more than you wished to know.

"Bolts, bolts, bolts….they are an integral component to an engine build. With high power engines torquing the bolts improperly or using the wrong type of bolt (size/material) you can wind up with a blown headgasket, or worse, a destroyed motor. There’s a lot more to bolts than most people realize, and we’ll take a look at that.

Let’s start with some the science behind a bolt – the deformation modulus. I’m sure some of you have heard the term “modulus elasticity” before, and that is what we’ll be talking about. Modulus Elasticity is defined as normal stress over strain. But what exactly does that mean and what does that have to do with bolts?

All materials deform in two ways, elastically, and plastically. Elasticity is the ability for a material to stretch under load, and then return back to its original shape when the load is removed. Plasticity is the deformation in which the material is strained beyond its yield point and the material becomes permanently deformed. When a material is strained past its elastic limit it has exceeded it yield stress and will deform uniformly across the bolt until its ultimate tensile strength is exceeded (the failure point).

Now let’s apply this to our engine. Head studs, main bolts, rod bolts, etc are designed to be torqued past the yield point. The logic behind this is to achieve the maximum clamping force, in kPa, while resisting fatigue. A bolt is strained when it is tightened, however a bolt is further strained when it becomes loose, then tight, and the process repeats.

One example would be a headstud on a high boost engine. Naturally the head would like to lift off the block due to the tremendous pressure exerted upon it. For the head to lift the bolt would need to stretch. If the head stud is not torqued beyond its elastic limit (yield point) it will continue to stretch further. Then when the pressures are lessened (lifting throttle) the head will lower back down, causing the stud to reform its previous shape. This cycle of straining causes fatigue in the material and causes it to eventually break. Think of twisting a paper clip back and forth, eventually it will break. However, if the stud were to be torqued beyond its yield point the ultimate tensile strength has been achieved without further elastic deformation. I will note at this point the amount of force required to strain (stretch) the bolt further becomes exponentially greater (because the material becomes harder the further it is strained, as well as the deformation curve observed in Hooke’s Law/Young’s Modulus), so head lift should not be a worry.

That gives you the basic’s, but lets cover a couple more bases real quick; first various materials, stud sizes, and lastly the all important – torque.

Many fastener companies have developed various materials for different studs and bolts. The most commonly known would be ARP’s special ARP2000 material. ARP2000 is extremely common for head studs, main bolts, etc. It is design extremely well and can yield great strength if torqued correctly. Another high performance material would be L19, though less common on the street it can be found in extremely high performance race engines. L19 and yield greater strengths per the stress area over clamping force than other materials. However, it does have its draw backs. It would be for race applications only, as it rusts and corrodes and needs to be replaced frequently. And it is expensive!

Let’s say you feel you need a bigger stud for more clamping force. Some people will bump up to a stud that is 20-30% larger than stock to achieve a greater clamping force. Many times the results can be less than ideal. As mentioned previously the material needs to be strained beyond its yield point (elastic limit) to achieve the ultimate tensile strength without fatiguing. When a 10mm head stud stud may require 85ft/lbs in a 15-90-90 angle torque pattern, a 14mm head stud may require 135ft/lbs in a 15-90-90 angle torque to achieve a strain beyond the yield point. I will guarantee you there are many people that will do one of two things: not torque it enough, or torque it to spec without accounting for block deformation.

When the larger head stud requires a greater force to surpass the yield point (elastic limit) the block also becomes strained. This results in deformation of the cylinders. However, this can be avoided if the block is machined with a torque plate using the proper head studs torqued to the recommended specs in the correct torque pattern. Using a torque plate in this manner will deform the block the same way the head will deform the block and thus the machinist can hone the cylinders of their oval shape.

The opposite holds true as well, when the stud is under torqued it cycles the stud through strains and creates fatigue which eventually causes material failure. Another factor of under torquing is that you do not achieve the same clamping load as you would with a smaller stud. Thus the paradox that bigger isn’t always better, in fact it can be weaker.

And we’ll close this chapter with torque. Don’t ever reuse bolts that were angle torqued. They’re no good anymore. They have deformed plastically and will never achieve a uniform clamping load. Angle torque is often used to torque bolts because it is more consistent when using lubricants. Torque values can actually vary based on lubrication (and even the threads).

Now you know more about bolts than you ever cared to know.:)"

duffman1278
10-04-2009, 09:51 PM
Only read half of it but I like what I read, and it seems on par. Great report.

S14_Kouki
10-04-2009, 10:10 PM
Great write up. I did not read anything thing about the tensile strength or shear point in the write up tho.

roco
10-05-2009, 04:42 PM
thats a good read thanks, and very well explained, i remember learning similar stuff to that in phisics in collage, forget most of it now though, :duh:.

93nismo
10-05-2009, 04:51 PM
i just learned this last year in school. i feel like im reading a textbook again lol nice writeup:]

MELLO*SOS
10-05-2009, 04:52 PM
Great article so far!

Could you please explain common grades and acceptable uses for each grade.. Pros/Cons, etc..

Question, from some other stuff I've read it makes it seem that stud + nut combo provides more clamping force than a regular bolt. With this said, is it always better to use a stud and nut instead of a bolt? Just curious, would help me design some of my projects better...