[TowerTalk] crank-up tower safety

[Dave Hachadorian]

On Thu, 14 Dec 2000 09:30:37 -0800 "Al Williams" <alwilliams@olywa.net>
writes:

> I suspect that the winch on the Triex may be overloaded and also 
> feel that
> the pulley system should be modified.  It has been many years since 
> my
> physics courses.  Are my assumptions and calculations valid?
> 
> 1. The ~1200 lb tower is resting on the pivot bolts at the bottom 
> end and a
> log at the
> other end.  The tilt-raising cable is approximately 8' from the 
> bottom end
> and the other
> end is 16' from the cable.  For simplification I assume 600 lb of 
> the tower
> at each end although I know the bottom end is heavier.  Is it 
> correct that
> the force required at the
> cable (8' from bottom end) is 600x24/16=900 pounds?

This is a sum of the moments problem.

The worst case moment on the tower is when it is horizontal.
moment = (1200# dead weight) x (11' approx distance CG to hinge)
moment = 13,200 ft-lbs. In other words, the tower is trying to
rotate into the ground with a moment of 13,200 ft-lbs.

To raise the tower, you need to exert an upward moment greater
than 13,200 ft-lbs to get the tower to rotate about the hinge.

If the SHORTEST (right angle) distance from the winch cable to the
hinge is 5 feet (which I think is about right), then the cable needs
to exert a force of 13,200/5 = 2640 pounds. Since this is divided
between two cables, the tension in the cable is 1320 pounds.

That's the way you do the arithmetic on this problem. The critical
measurements though are the distance from the hinge to the CG, and the
hinge to the cable, both of which I had to estimate.

As the tower starts to go up, the horizontal distance from the
CG to the hinge gets shorter, and the distance from the cable
to the hinge approaches 6', so the cable tension becomes lower.

It will be a lot easier when you flip the tower over the other
way for installation of rotor/mast/antennas, since the hinge will
be 8' above ground and the whole arrangement will be more balanced. 

Dave Hachadorian, K6LL
Yuma, AZ
K6LL@juno.com


--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com