The foregoing info ignores the following: If the load on the bolt is in
shear (as it should be) and the load is on the un-threaded part of the bolt
(as it should be) then there is no difference in the strength of the two
thread types. Also grade 8s can be bent a long way before they fail and a
good design will limit the amount of bending that can take place anyway. I
would not weld the chain. Welding will anneal the link that you weld on and
substantially decrease it's strength. Using S hooks is a disaster also.
Thru' bolt the chain onto the trailer and shackle onto the vehicle!

wrote in message
...
On Thu, 17 Jun 2004 16:31:27 -0400, "Gary Warner"
wrote:



But of course. Why else would there be so many different threads?

You want fine thread. The engagement, or amount of material 'gripping'
between bolt & stud, will be approx twice as much. Stronger and less
tendency to vibrate loose.

BTW hardened steel may not have been the best choice either. High
hardness usually corresponds to more brittle. It may just be surface
hardened which wouldn't make a huge difference.

Fresh Breezes- Doug King


That makes sense, but just brings up more questions....

I understand the fine-threads would give more surface area. But
the thickness - and presumably the holding power - of each
thread would be less, no?

Would there ever be a time that course thread
would be better (other than for cost reasons) ?


Bear in mind that a machine screw thread is basically an inclined plane
that is
wrapped around a cylinder. Coarse threads represent a steeper incline. You
get
to the top faster, but it takes more grunt power.

If maximum holding power isn't a grave concern, and it's a bolt that will
be put
on and taken off frequently, the coarse threads are "faster", as in less
turns
to remove or install.

BB