[TowerTalk] More friction needed

[JVarney]

Dave AB7E wrote>> Personally, I think the idea of using multiple U-bolts is
the best way to go. <<

I agree.  And instead of shooting from the hip and guessing, with U-bolts
you can calculate and
engineer the number and size of U-bolts needed to resist the mast torque.
This is a simplified
version of the procedure in TIA-222-H

Nominal torsional strength of U-bolt assembly = Tr = 0.075 (D)(Tp)
Tp = assumed U-bolt leg tension = (20 ksi)(Ag of leg)

where D = mast diameter, Ag = gross area of U-bolt leg.  English units.

U-bolts should not be tightened past yield and should be limited to 0.85 Fy
Ag.

===

Example:  3 3/8 U-bolts and a 2-in mast.

Ag leg = 0.19 sq in
Tp = 20 ksi x 0.19 = 3800 lbs
Tr = 0.075 ( 2 in )(3800) = 570 in-lbs per leg

System Tr = (3 U-bolts)(2 legs each)(570 in-lbs) = 3,420 in-lbs torque
resistance

===

Considering a Yaesu G-800 has a brake resistance 0f 4,000 in-lbs, the 3
U-bolts is
in the ballpark for a medium tribander. If you ignore the 20k yield
assumption and use
42k yield for stainless and tighten to 0.85FyAg, that gets you 1,000 in-lbs
per leg,
equal to 6,000 in-lbs total torque resistance.

This simplified estimate ignores vertical slippage forces due to the weight
of the
mast and antenna acting on the U-bolts, which reduces the available torque
resistance somewhat.

73 Jim K6OK