Bill Babcock wrote:
>
> Hard versus strong is easy to explain, but I'm not sure what physical
> parameters denote it, certainly not grade 5, grade 8. A strong bolt can
> I probably should read a couple of the metals books in my library. It's
> interesting stuff.
The engineers can probably ignore this, or correct it, as required.
<smile>
I, too, got into some metallurgy when making knives. I think the
essential quality required of the bolts mentioned by Kas is "toughness,"
which is a combination of the qualities of tensile strength, to a great
degree, and to a smaller degree, resistance to deformation and cracking.
The relationship between tensile strength and elasticity is determined
by Young's modulus, which is, effectively, the multiplication of stress
and strain (stress being the applied force, and strain the amount of
stretch in the material for that applied force). Virtually all alloys of
steel are very close to one figure -- 2.9 x 10^7 psi. So, the lower the
tensile strength at the yield point, the more the bolt will stretch
before yield without permanent deformation (known as elongation). The
higher the tensile strength, because of Young's modulus, the less
elongation before the yield point. A grade 2 bolt will, under load,
stretch by 15-20% before yielding, and still come back to its original
dimensions when the load is removed. A grade 8 bolt will take a much
higher load, but will elongate maybe 4% before the yield point.
Bolt hardness comes into play particularly in situations like flywheel
bolts. Surface hardening a bolt does make that small amount of the
surface material more brittle, but also helps prevent very small nicks,
such as can happen when the edge of a bolt hole bangs into the bolt
under big loads. Under large pulsating loads, nicks grow into cracks,
causing the bolt to fail.
The relationship of pulsating or cycling loads (good examples are
flywheel mountings and cylinder heads) to the fasteners used is that the
tensile strength before yield of the fastener must not only be greater
than the peak load, the fastener must also be able to be torqued to a
high enough value to pre-stretch the bolt so that the load applied on
the bolt when torqued exceeds the peak load by a comfortable margin.
That way, in the case of a flywheel, the bolt absolutely restrains the
mounting area of the flywheel from moving at all. And, because the shear
loads at the flywheel are very high, the bolt also has to have
sufficient strength to resist shear.
The problem is complicated in the areas mentioned above by the fact that
the block and the crankshaft are cast items, and the tensile strength of
those castings is not equal to that of the bolts used. That means there
has to be sufficient receiving thread area to spread the torqued load so
that the threads don't yield, but are still put in tension enough to
resist movement under peak load.
Getting back to metallurgy, bolt cracking is always of concern with any
steel alloy, because of the cubic geometry of iron's crystal lattice
structure. That makes for lots of flat planes to slide against each
other if there's sufficient force to create a displacement (the starting
point of a crack). By contrast, titanium alloyed with very small amounts
of iron creates a hexagonal crystal structure, and those hexagons lock
into each other (something like cells in a honeycomb), making
displacement in all planes much more difficult.
Unfortunately for us, commonly available titanium alloys do not always
have the ultimate tensile strength of, say, a grade 8 bolt, so a larger
bolt must often be used, and they are quite expensive and require some
careful handling and protection. Certain alloys of titanium are very
susceptible to environmental damage. When the first versions of the
SR-71 spy plane (the airframe of which is virtually all titanium) were
being assembled with titanium bolts, workers would assemble a portion of
the frame, and come back the next day to find all the bolt heads laying
on the floor. After some investigation, it was discovered that they were
using cadmium-plated sockets to install the bolts, and the cadmium
transferred to the bolt heads during installation was enough to cause a
chemical change in the alloy which reduced its strength to next to
nothing. (!) Thereafter, every tool kit had only sockets with a black
oxide finish.
Cheers, all.
--
Michael D. Porter
Roswell, NM (yes, _that_ Roswell)
[mailto:mporter@zianet.com]
The gulf between content and substance continues to widen....
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