re: the strength of the attach point on tilting up a 1400 lb tower:
Art/Clint,
Saying 1000 lbs is max force is very wrong, right?
That all said, a 5/16" bolt can be fine. I would use grade 5 or 8. I'd
be more concerned that the welding etc, is fine. Ideally you get one so
the thimble is not across threads. I'm assuming that only 3/8" of length
of bolt or so, is exposed between the two metal plates the bolt goes
through, so there isn't a big bending moment?
For typical lifts of crankups, the pull at the tower attach is always
greater than the weight of the tower, not less. (it's a statics analysis).
You have to work out the analysis, but the pull on an attach point can
easily exceed 2 to 3 times the tower's weight. The key variables are
length of the tower, it's weight, any other stuff on the tower, the
height of the gin pole, and the attach point to the tower. The angles of
the cable doing the pull are what's key, and the attach point (how high
up the tower).
To see the problem mentally: imagine the pull if the gin pole or lifting
device is 5 ft tall vs 10 ft tall. The angle of the pull matters.
Next imagine if you attach at the mid point of the tower, vs the end of
the tower. That matters.
I use two sewn nylon webbing loops (climbing/mountaineering), then a big
shackle, to avoid damaging the galvaning. I wouldn't trust it to one
webbing loop, unless they were brand new.
Remember how people complain about the force required with lifting
winches? and compound pulleys? Well, all that pulling force is applied
to that single point on the tower.
Design for something that can handle 5000 lb pull, and you should be
okay. If you use a tall gin pole at the base of the tower (like 10'),
you can reduce the attach point force to maybe 2x the tower weight.
Provide more details on the lifting strategy and maybe you can design
for less, but then you'll have to be pretty confident of your analysis
and no shock loading if the tower bounces a little while lifting. If you
have antenna, mast at the end of the tower at the start of the pull, the
force is higher.
You can reduce the force by lifting the end of the tower with an
additional device (shop crane or ??) before you start pulling on the cable.
The first feet off the ground cause the greatest force.
Art said
"plan is to use 1/4 inch 7x19, same stuff the tower uses, on a thimble
around a bolt through the pad ordinarily used for a pulley with the
erection fixture. "
I don't understand that sentence, in terms of what the pull will look like.
-kevin
AD6Z
W5CPT wrote:
> I would use a 1/4" shackle (which has a 5/16" pin) to attach what ever you
> were going to use to get the tower upright. A 1/4" shackle has a working
> load of 1000 lbs, which should be more than enough since you are not dead
> lifting the tower.
>
> see this link:
>
> http://turnerseymour.thomasnet.com/viewitems/hardware/screw-pin-anchor-shackles?&plpver=1001&forward=1
>
> Clint - W5CPT
>
>
> ----- Original Message -----
> From: Art
> To: towertalk@contesting.com
> Sent: 03 December, 2009 7:58 PM
> Subject: [TowerTalk] Old HWS671
>
>
> I'm about to pull a HWS671 vertical. There's a pad welded on the outer
> section. For those with an erection fixture it was used to install a
> pulley wheel to winch up the tower. I don't have a fixture, but I do
> have a tractor that I intend to pull it up with. The plan is to use 1/4
> inch 7x19, same stuff the tower uses, on a thimble around a bolt through
> the pad ordinarily used for a pulley with the erection fixture.
>
> The question is, what grade of bolt do I buy that will pull it up and
> not break? I see it as a fight between ductility and brittleness, grade
> 8 being brittle for shear loads like this. The hole is for a 5/16 --
> which seems a bit small to yank up this tower. I'm sure it used to work
> when these towers were new 30-40 years ago. The alternatives are, drill
> out the hole for a bigger bolt or use a chain, either one putting the
> galvanizing at risk.
>
> HWS671 is a 4 section 71 ft middling strong crankup weighing about 1400 lb.
>
> 73 Art
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