expansion problem for the engineers

[shankle]

Working on a mesh buttonhook dish.
Had to put guy wires on it to center the feedhorn.
This morning I installed the guys wires and got them pretty even in 70 degree heat. Maybe 75ds.
Later today I went to check what I did and the temperature was from 90 to 100 degrees.
The steel wire I used had expanded to the point it was not doing any good.
Can't tighten them in the heat of the day as when they cool down they could warp the dish.

What about a cord in the plastics? Say a heavy fishing line. I would venture a guess that the force
on each turnbuckle is no more than 20 lbs. More like 10 lbs.

[dedndave]

hard to believe such short pieces of steel expand/contract that much
i have considerable experience with steel guys on towers, but they are very long guys
also - when we set up a tower, we expect it to sway in the wind and account for it
in copper-clad wire, they use copper around a steel core to reduce expansion/contraction
(the steel is for expansion - the copper is for RF conductivity - RF travels on the skin of a wire)
i am willing to bet it is the structure that is expanding and contracting - not the steel wires

anyways, your situation is a little different
good heavy fishing line sounds like a good idea - although a bit expensive
i might try some good nylon string - a little bigger than kite string - much stronger
it won't expand/contract much - and it has a little give to it
of course, you could use solid copper or aluminum wire - those materials have some give also
you don't really care - as long as all the guys expand the same amount

another approach....
get an early start
get the lines taught
get the hardware set
all before the tempurature has a chance to change much
a few less beer breaks, Jack - lol

[shankle]

Thanks Dave for the help.
I will try to get the nylon string. I guess it will have to be replaced every year or so
but that's better than a feedhorn that's not centered.

[MichaelW]

i am willing to bet it is the structure that is expanding and contracting - not the steel wires

I agree, the linear coefficient of expansion for steel is something like .000006 per degree F, so for example the change in length for a 30 inch wire over a range of 30 degrees F would be something like .0055 inches. If the structure is black and in direct sunlight, its temperature variation could be much more than 30 degrees F. I seem to recall seeing coil (extension) springs used to reduce the stiffness of guy wires, to minimize the change in forces on the structure. Plastics generally have a much lower stiffness than metals, but a much higher linear coefficient of expansion, and they tend to not stand up well to UV radiation.

[dedndave]

sorry Jack
i was under the impression these were temprorary guys while you set the final support structure
not being on site makes it difficult to make recommendations
i would have thought there might be some type of bracket to hold the feed horn in place

[shankle]

Now I don't know what to do.
The dish is an aluminum dish with a button hook in the center.
The guy wires are woven steel and are permanent.
The guy wires were tight in the morning when I installed them at about 70d.
Loose in the afternoon at about 95 to 100d.
Could the heat of the sun cause the turnbuckles to move? I don't think so.
If plastics don't hold up in the sun what are my alternatives???????

[dedndave]

i would use steel and get them snug when it is warm (not too tight)
the aluminum frame should give enough to handle the stress
if it is a big difference, get some springs - 1 in each guy
make sure the springs can handle the difference in length

[MichaelW]

Perhaps you could shift the anchor points on the dish closer to the center, where there would be less deflection with temperature changes.

[shankle]

to Dedndave,
That means the guy wires would be fine during the day and loose at night when I watch tv
letting the feedhorn be out of adjustment.
Right now I am at a loss of how I could get springs to work.

To MichaelW,
Since the feedhorn is only out of alignment 1/2" or less it might work.
Angle would be more severe requiring more force to align the feedhorn IMO.
Problem is how to secure the wires to the 1 1/2" pipe that holds the feedhorn.
It would be more of a downward force and not perpendicular as it is now.
There are wires running through the middle of the pipe to the LNBs.
A bolt through the pipe would interfere with running the cable to the feedhorn.
Guess I need to investigate some kind of clamp. Not one they use around
automobile hoses.
Thanks guys.

[dedndave]

The guy wires were tight in the morning when I installed them at about 70d.
Loose in the afternoon at about 95 to 100d.

well - tighten them when they are loose   :bg

[hutch--]

I would agree with using draw springs, trick being to install them at the coldest and make sure the springs are a long stretch type that will handle the heating during the day. RE temperature for bits that expand and contract, a cheap spray can of silver paint will go a long way to keeping the temperature down so you minimise expansion due to heat differential.

Most springs will rust over time and nylon fails in extended sunlight, you could trick the springs by taping loose plastic around them so they can stretch but are protected from the rain.

[shankle]

Thanks guys for replying.

I have gone the shorter guy wire route and still testing it.
Five' from edge to center has been divided in half.
So I have only 30 " of expansion to worry about.
I like the idea of the silver paint. White would be better
but I've got neighbors. :toothy