One thing to consider is that the standard Acme thread pattern is less
likely to freeze up than a standard NC thread after it has been in the
weather for a few months and rusted.
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at:
http://www.rutuonline.com
Shameless Commercial Division:
http://www.spade-anchor-us.com
wrote in message
oups.com...
Steve Baker wrote:
wrote:
snip-de-dip)
I'll also suggest that there's no advantage to using acme thread in
this application. For a given size you get a coarser thread, which
means more movement per turn, but it's more costly and actually has
less strength since the threads are deeper.
Tim
(who's just designed a cradle for a 34' boat.)
But, but, but.....
In this application, the "bolt" is in compression, which puts the thread
in shear, which means that the broader thread base of the Acme is
WAAAAAYYYY stronger than a UNC thread.
Hence the fact that it is used in this application ;-)
Steve
Hi Steve,
I'm going to argue the other way. The threads are in shear when the
"bolt" is in tension, as well. Just that the shear is going in the
other direction.
If you look at the cross section of a UNC thread you have a 60 degree
pyramid. The base of the pyramid for each adjacent thread comes pretty
close to meeting at the thread root. The root space is defined as 1/8
thread pitch wide, so 87.5% of the thread pitch is metal and bears this
shear load.
Acme threads are trapezoid sections with the base about 63% of the
thread pitch, so there's less area to bear the shear load. If they're
made from the same steel, the acme threaded rod will strip threads
under lower load than a UNC threaded rod.
The other problem in compression is buckling. For a given outside
diameter screw thread, the acme is cut deeper, so the acme screw acts
like a skinnier cylinder and buckles first.
Acme threads were designed for power transmission, e.g., vise screws,
screw jacks, machine tool feed screws, etc. Less of the torque you
apply is lost to friction so they're more efficient at transmitting
power.
I suspect they're used more frequently in boat stands because they're
off-the-shelf items over 1" in diameter while big UNC threaded rod is
harder to find. And since the space between threads is larger, they use
less steel, which make them cheaper in high volumes.
Tim