Steve's (k7lxc) prime directive not withstanding (do what the mfr says), I
think that Garry at U.S. Tower might be correct in stating "...the
winch...should not be a problem"; However the real concern is the
tilt-raising fixture assembly strong enough to safely raise the tower.
Remember that the safe load is usually several time less than the working
strength load. I hope that some experts will
correct and/or further explain the following:
1. It is typical that metal structures bend/bow until they suddenly collapse
with a little increased load. Does this bending also progressively weaken
the structure?
2. What does the winch rating of 1500lbs really mean? One mfr states that
winches are often rated by their pull capabiliity. The pull capability is
determined by how much pull can a typical man exert on the crank handle
multiplied by the leverage of the winch handle and the gear ratio of the
winch. This has nothing to do with the safe rating. Instead the safe
rating of the winch concerns how strong is the raising ratcheting mechanism
and the clutch mechanism of the brake part of the winch when lowering, and
how do they deteriate with usage?
3. I read the response of the winches 1500lb rating with only a 900lb +
200lb load as leaving a safety margin. However there are two leverages not
being considered. The first leverage is a linear one caused by not lifting
the load at the center of gravity. For simplicity assume that the tower,
antenna, and mast weight are approximately evenly distributed. With the
winch cable attached to the tower at approximately 7' from one end of the
tower and approximately 24' from the other end (21 + 10 -7), the leverage is
24/7. Of course the actual leverage is less because the weight is not evenly
distributed but the leverage is still considerable.
4. The second leverage is an angular one caused by lifting the load at an
angle away from vertical. In this case the height of the raising fixture is
about the same as the distance from the bottom of the tower that the cable
is attached resulting in and angle of about 45 degrees. This means that the
only .707 of the pull of the cable is applied to lifting the tower. Also
.707 of the pull of the cable is applied to trying to pull the tower towards
the concrete base. I need some help here. Does this mean that the leverage
is 1/.707 or 1/(.707 + .707)?
Now multiplying the 1100lb tower and antenna load by the two leverages, say
3:1 for the linear leverage and 1.414 for the angular leverage shows that
the winch and raising fixture is being stressed to around 4000 pounds!
Again, will the real experts please correct and explain better my analysis?
k7puc
--- Original Message -----
From: "KT2Q" <dxdx@optonline.net>
To: "TowerTalk" <towertalk@contesting.com>
Sent: Tuesday, August 16, 2005 2:48 PM
Subject: [TowerTalk] Thanks: Rasing fixture strength replies.
> All:
>
> Thanks for all the informaitve replies on the strength of the TRX80
> raising
> fixture. The only "complaint" I recevied is that the structure tends to
> bow
> a little with the heavier masts and antennas and that the standard winch
> should be replaced with one that has more leverage.
>
> I called Garry at U.S. Tower and he said that the winch is rated at
> 1500lbs
> and should not be a problem with my 900lb tower and 200lbs or so of mast,
> antennas and rotor.
>
> For saftey reasons, the consensus is to support the tower to take some of
> the load off the cable when working on the anetennas; makes sense to me.
>
> Thanks again...
>
> 73, Tony KT2Q
>
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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