[TowerTalk] Re: Gin pole loading

[Kurt Andress]

Wow! A variety of ideas on this one.

How about another one?

1) If the load being lifted is 200#, and the lifting line is vertical, the load on
the line is 200#

2) If the tension in the line is 200# on the load side of the gin pole sheave, the
line tension is also 200# on the lifting side. This neglects friction in the
sheave assembly.

3) If we make a little sketch of the sheave on top of the gin pole we see two
loads acting on it. On the load side is 200#, on the lift side another 200#. So,
we add them up and get 400# of load on the top of the gin pole.

I can't see anyway that adding a mechanical advantage to the load side or lift
side of the system will result in a reduction in the load applied to the top of
the gin pole.
It can make the job easier for the fellows pulling on the rope, but the gin pole
doesn't get to enjoy the benefit. Gotta have a net load of 200# on each side, no
matter how you achieve it! The load at the top of the gin pole is determined by
the line tensions exiting either side of the sheave and their angles.

The reason gin poles bend over and give us heartburn is because we cannot
practically raise the load in a position that keeps the load side parallel with
the pole. We usually keep the load out away from the tower with a tag line to
prevent it from getting fetched up in the tower. As a result, the gin pole rarely
sees balanced loads. The lift side line is always running nearly vertical, down
the inside of the pole, and the load side line is always running of at an angle.

The resulting force applied to the top of the gin pole is always exactly halfway
between the axes of the load side and lift side lines. So, we produce a side load
on the gin pole that causes it to bend.

Beyond the clamping strength of the unit, the sheave pin capability, and
beam/column buckling properties of the pole, rating gin pole strength is largely a
collection of anecdotal evidence.

If we were to define the actual limits of load side line angular displacement from
vertical, we could then determine a safe rating for the pole.

The rating would appear something like this:

This product can safely lift XXX Lbs when the load side line is XX Degrees from
vertical.
It is XXX Lbs at XX degrees.....etc.

Of course, the above mentioned features would have been adjusted to suit the worst
load case.

Until then, keep asking others what they have had success with.
Use good judgement and keep the load as close as possible to the tower and as
close as possible to the tower leg that the gin pole is attached to!

Happy heaving....

--
73, Kurt

K7NV "That's K7 "Nevada" (ex - NI6W)

YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/



--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm