Posted by Alan Bowes on February 26, 1998 at 18:10:52:
In Reply to: Re: Military nuts and bolts posted by Alan Bowes on February 26, 1998 at 16:53:40:
Just wanted to add a comment or two to avoid confusion:
Even though a grade 8 bolt will not permanently elongate as far as a softer bolt before breaking, this does not mean that it will break more easily. On the contrary. A grade 8 bolts have higher tensile, yield, and shear strength ratings than grade 5 bolts.
All bolts are designed to stretch a certain amount when properly torqued. This elasticity is what keeps them tight. However, if you stretch any bolt beyond its "elastic" limit, you will damage it. A grade 5 bolt can be stretched beyond its elastic limit at a lower torque than a grade 8 bolt. The difference is that if you stretch a grade 5 bolt too far, it will permanently elongate farther than will a grade 8 bolt before it breaks. Also, keep in mind that once you have stretched a bolt beyond its elastic limit, you have seriously weakened it and it could fail at any time; plus it would be loose, since it would no longer have its elastic effect on the parts that it is holding together.
I think I'd like to expand just a bit on something I said earlier:
When torquing say, a grade 8 bolt/washer/nut combination (all parts grade 8) you should normally use the torque specs recommended for that particular bolt size, grade, and thread type.
When torquing say, a grade 8 bolt into a forging or casting (where the female threads are in the forging/casting), I'd probably stick pretty close to the original torque specs in the technical manual for the vehicle, since the female threads in the forging/casting may be softer and weaker than the grade 8 bolt.
There are some important exceptions. For example, cylinder head studs are usually grade 8 or somewhere thereabouts, and they are often bolted into cast-iron blocks, or even aluminum. So why doesn't it strip the threads when they are properly torqued? Well, a lot of this depends on the number of threads relative to the stud or bolt size. If you have more threads in contact with the softer metal, you can generally apply more torque without overtaxing the threads. This is also a good example of why high-strength bolts or studs are better in this instance. If you use a softer, more easily stretchable bolt or stud, the upper threads will tend to carry more than their share of the load, but if you use a high-strength bolt or stud that does not stretch as easily or as far, it transfers the load more evenly across more threads. To complicate things, some forgings can be quite strong, and are not always weaker than the bolt.
Grade 5 bolts are typically made of a medium-carbon steel that has been quenched and tempered.
Grade 8 bolts are also made of medium-carbon, quenched and tempered steel, but are alloyed with different metals to improve their tensile, yield, and shear strength.
In my opinion (and please note that I am NOT a metallurgist or structural engineer, so don't quote me on any of this) in the majority of cases, I don't think there would be a problem from going to a higher-strength bolt, but there can be some situations where it may not be advisable. For example, if you are bolting two soft materials together, by the time you apply the proper amount of torque to say, a grade 8 bolt/washer/nut to put the proper amount of stretch on it, you may end up crushing or damaging the parts it is holding together. I suppose that there could be other situations as well.
Here's what I think is a good general rule of thumb: A fastener should be torqued somewhere within its recommended torque range to assure the proper amount of stretch. IF this torque is excessive for the application, such as possibly crushing or damaging the parts secured by the fastener (or stripping threads out of a softer metal part, etc.), then you should probably switch to a lower-grade fastener so that you can torque it within its recommended torque range. This rule of thumb may be superceded by the manufacturer's recommendations regarding fastener type, grade, thread, or torque specifications.
What it all boils down to is: be conservative, read the official books, and use your best judgement.
Hope that isn't too confusing.