tompatjr said:
You are also posting bad information! I would like to see your source!
A punch is not a bolt, and a nylon strap is not a bolt...bad examples.
I am in no way a expert about bolts or really anything else for that matter. But I can read and have done so, a little, about bolts.
Basically from what I have read about bolts, is this. You can use a higher grade bolt in an application that calls for a lower grade bolt, but not the opposite.
A example of (tension) a bolt being pulled apart, like you are saying a hitch would do, would be a connecting rod. They use a very high grade bolt for those, most likely better, much better, than a grade 8.
The problem here is that you're both largely right.
While a "stronger" bolt (higher SAE number or ISO property class) will handle more loads, and can be preloaded more, it should also be borne in mind that as steel gets harder, it gets more brittle. Don't forget, also, that shear strength is determined by the MINOR diameter of the bolt (at the thread root,) rather than the major diameter or the nominal diameter, unless the shear load is borne by the unthreaded shank of the bolt (in which case, the shear cross-section area is determined by the actual shank measurement, ultimately. Use ~75% of the UTS of the bolt to determine shear strength - the only way to get a more accurate number is to test to failure.)
However, the point on ductility/brittleness can and should be taken into consideration. Even when preloaded below the elastic limit, a bolt that is going to take more shear loads than tensile should not, in fact, be hardened to the point of becoming brittle - like on a trailer hitch. Most of the loads applied, after installation, are going to be in shear - and are going to be 'shock loads' as well. An SAE 8 or ISO 10.9 bolt is going to fail under such loads before an SAE 5 or ISO 8.8 will, because "hard" ~= "brittle."
Given a choice for trailer hitch bolts (I usually am...) I'll select 300-series CRES ("stainless") hardware wherever possible. CRES is heat-treated, generally, to what would be SAE 6 specifications (or ISO 9.8,) so it retains useful ductility - but also has the advantage of being resistant to corrosion. Stress failures - whether tensile, compressive, or shear - will begin at a corrosion pit - it's like having a dotted line that says "tear here." In some cases, silicon bronze and some of the more exotic bronzes can be stress hardened to an equivalent of SAE 5/6 ISO 8.8/9.8, and are actually more resistant to corrosion than CRES. However, CRES is usually easier to find.
Connecting rod bolts are a different animal entirely. They are under a constant cyclic tension load (there is no load on the bolt when the piston is going up, and very little when the piston is on the power stroke,) and the greatest load on the conrod bolt is when the piston is going downward on the intake stroke (due to pulling on the big end cap, and suction overtop the piston.) You won't find specs useful for conrod bolts anywhere on the SAE grade chart - I believe they carry an ASTM number. However, careful attention has been paid to the alloys used and the heat-treatment processes that have been designed for these bolts, and you just can't compare them to anything else. Ditto the nuts used on them - that's got a lot to do with why so many conrod bolts use a non-standard thread (11/32"-32, I think.)
If you'd like to know more on the subject, I highly suggest you go to your local bookshop and look for Carroll Smith's
Engineer to Win - considering it goes into the metallurgy of fasteners and properties of materials in relative depth, but sticks largely to layman's terms. It would also be instructive to get hold of a copy of
Machinery's Handbook[/I] - anything after the 20th Edition should be fairly detailed on both SAE Grades and ISO Property Classes.
5-90
