On Wed, 21 Dec 2011 19:59:35 -0500, Ned Simmons
wrote:

On Wed, 21 Dec 2011 13:28:50 -0500, Ed Huntress
wrote:

On Wed, 21 Dec 2011 13:12:41 -0500, Joseph Gwinn
wrote:

In article ,
"Wild_Bill" wrote:

I believe there would be reasons for using fine threads from an engineering
standpoint.. but if I did know, it's been forgotten.

In pondering my own experiences with a lot of mechanical assemblies, I'm
certain that fine threads are most often chosen for fasteners associated
with moving parts in power transmission components.

In automotive applications, items bolted to the engine block are often
coarse threads.. but wheel studs, connecting rod caps, crankshaft end parts,
etc.. have generally always had fine threads IME.

In many applications the thread choice is related to the metal/material
types which have threaded holes.

My understanding is that coarse threads were intended for steel screws
in cast iron assemblies, and the fine threads came about for steel into
steel.

Joe Gwinn

Yes, and the reason for the move to finer threads in steel is twofold:
Fine threaded *screws and bolts* are stronger in both tension and
shear (counterintuitive, perhaps, but check it out) and they are
better at self-locking: they're less likely to loosen from vibration.

In favor of coarse threads, aside from some installation issues, such
as less likelihood of cross-threading, they are less likely to strip
out of a soft or fragile material. And it is easier to make a strong
threaded hole in brittle material (cast iron) with a coarser pitch.

As for total strength, then, it's a matter of the relationship between
bolt strength and thread strength in the hole. In hard, strong
materials, fine is better. In softer, weak, or brittle materials,
coarse is better. In the latter cases, bolt strength is much greater
than the hole strength either way.

I can see how a coarse thread may be less likely to strip in a very
brittle material, or perhaps in a coarse grained material where the
size of the grains is large relative to the threads. But a properly
cut (or formed) fine thread should be somewhat more resistant to
stripping than a coarse thread in a reasonably ductile material,
regardless of the strength of the material. I'm using "stripping" here
to mean shearing of the threads as a result of tension on the screw.

Apparently this is generally true. The older theories that I learned
are based on the fact that the stress on the first couple of threads
results in failure of those threads before there is significant stress
on subsequent threads, and shear strength was dependent on some
product of thread contact area and the pyramidal strength of the
thread form -- coarser threads producing stronger pyramids.

I haven't looked at any of this for 30 years, except in passing. From
what I've seen, the current thinking is that shear strength is almost
exclusively a product of shear area (divided by a factor for helix
angle) and that this says finer threads are always stronger in shear,
except, perhaps, in brittle materials.

Somewhere there must be some good engineering test results to clear
this up, but I haven't looked for them.

--
Ed Huntress