Author Topic: Guy wire tension (Read 13180 times)

frackers · « **on:** February 27, 2009, 09:46:00 PM »

I've got the new 12m tower up and flying the 3m mill again after a gap of 5 months while I put in the fencing and gates to split a 1 hectare paddock into 4. The tower sits in the middle of the fence runs with the guys along the fence lines so they are out of the way.

My experience is only of aerial towers where the windage of what is on top is usually pretty low and in those cases the guys have been very slack but I'm unsure what I should do with 50kg sitting on the top getting blown around!

I've just tried putting my weight onto the guys at the anchor points - I can tread then down for about 150mm before they go tight - is this too slack? Another measure that might make sense is that if I 'twang' the wire it oscillates up/down about once every 3 seconds (more to do with the tower swaying I think than a subsonic musical note!!).

Obviously I put extra compressive force onto the tower the tighter the wires are and I also remove some of the capability to resist wind forces but there must be some understandable and consistantly reproducable rule of thumb that will result in the right setup.

Thanks for reading guys (and gals)

bob golding · « **Reply #1 on:** February 27, 2009, 03:09:30 PM »

hi,

i have a 10 metre tower with a similar sized turbine. i use nylon rope and keep it fairy tight. it does stretch over time and starts to wave about a bit,but so far no problems at all in 3 years. i live in very windy area next the the sea and often see 70 mph winds. i just tighten it as tight as i can by hand and tie it off.. hope this helps. if you have it too tight you will put an extra strain on everything. it will also depend on the diameter of your tower as well. i use 8 guys with separate anchors just to make sure.

cheers

bob golding

Ungrounded Lightning Rod · « **Reply #2 on:** February 27, 2009, 05:02:54 PM »

Caution with nylon rope guys: It degrades with sunlight. (That's why it's used in boats for the running rigging and dock lines, which can be allowed to break, but not the standing rigging, which will wreck things if it fails.)

If you must use it: Get the kind with a cloth covering, use extra runs so one can snap without dropping or folding up your tower, and replace it every few years.

And don't let anything, ever, be in the area that can be hit by the tower or turbine if the guys go and the tower falls.

bob golding · « **Reply #3 on:** February 27, 2009, 06:29:27 PM »

yep i know it degrades. just changed most of the guys after 2 years. one of them did snap when i was dropping the tower down the other day so i am changing all of them. no danger of any one walking under it. it is in the middle of a 5 acre field. thanks for the warning.

frackers · « **Reply #4 on:** February 28, 2009, 03:37:36 AM »

I'm using 6mm 19 strand high tensile cables so they don't have a lot of give in them compared to nylon!! Rated at 21,000 Newtons maximum load they should be good for a category 2 hurricane - especially now that I have put a reinforced concrete 450kg anchor in for each one.

Does anyone know if heating and cooling (summer/winter range is about 35C to -10C) will affect the 13m length of the guys very much and hence change the tension? I would assume the 115mm high pressure steam pipe I'm using (it has about an 8mm wall) will be able to support a few tonnes without buckling but I really don't want to have the system other than in a 'relaxed' condition.

Cheers

jmk · « **Reply #5 on:** February 28, 2009, 05:57:27 PM »

If your ground is wet it will move. If your footings go below the frost line then you probably wont have any problems, but if one of them isn't like the tower base they might all get to tight. You don't want to have your guy wires to tight anyway, it just will add unneeded forces to the tower. They should only be tight enough to keep the tower from swaying.

cardamon · « **Reply #6 on:** February 28, 2009, 06:57:48 PM »

Rohn specs 400 pounds of tension for the 3/16 ehs cables on their G-series towers. So how to measure it....Wire rope tension meters are a grand or so but you could make one with a spring and a dial indicator. You would just have to calibrate it on a known weight such as 5 80 pound bags of something like sand or concrete. I have not built one yet but it is on my list. My intuition tells me that guy tension becomes more important the higher you go since twisting, oscillation, and resonance become more of a factor, but for us wind people with relatively short towers its probably not as critical.

frackers · « **Reply #7 on:** March 01, 2009, 03:30:33 AM »

A bit of feedback to help some more suggestions along...

the ground is dry
the frost line is above ground level - I've never seen the ground freeze here
tight guy wires - well thats what I'm trying to find out about. What is too tight?

I'm sure there is some rule of thumb that says that says something like "if the guy is at 45 degrees and you hang a 10lb rock half way up then expect it to deflect by 2% of the length".

Any volunteers to go out (yes I know its still winter for a lot of you) and see if they can measure their guy wire tension in some repeatable way?

Cheers

wooferhound · « **Reply #8 on:** March 01, 2009, 06:07:30 AM »

I have done tower maintenance for TV and radio stations, plus tower or mast assembly for many Ham or home TV antennas. I even got to see the Blueprints for a 1000 foot TV station tower. All of the smaller towers that I worked on (up to 125 feet) were snug but not tight. The best way I know to describe the tightness is the swag or droopiness of the wire. It should not be so tight as to straighten the wire out, but have a gentle arching curve from the anchor up to the tower, and all the wires should have the same swag. Remember, the tension on the wires is trying to pull down. The weight at the base of can get quite high with the weight of the turbine and the tower plus the tension of all the guy wires added together, then when the wind is blowing this weight is multiplied again.

When I was looking at the blueprints for the 1000 ft TV tower. They were showing a way to set the tension of the wire by tying one end of a 3 foot (1m) rod to the guy line. The bottom of the rod is the attachment point and the top of the rod hangs parallel to the wire. There will be some distance between the top of the rod and the guy cable. Measure this distance and duplicate it on all of the guys at each height around the tower. If this is unclear let me know and I'll rephrase it.

The wires on the 1000 foot tower had a tensions between 5000 pounds for the lower wires and 10000 pounds for the top wires, according to the blueprints.

Here are some pictures of some towers that I've climbed . . .

http://www.anotherpower.com/gallery/album85

frackers · « **Reply #9 on:** March 01, 2009, 05:37:36 PM »

I had a look at the pictures and my toes are still curled up!! Brings me out in a cold sweat just thinking about being 100 foot up never mind a 1000.

I think I know what you mean about the swag of the line - difficult to put into a generalised description because it will vary according to the weight of the wire, its angle and its length.

The feeling I have is that I'm not far off but I need to go a bit tighter - since the turnbuckle shackles have run out adjustment I'll have to fiddle with the grips and pull a bit of wire through the thimbles. Only one side is really bad, the side I winch up from - it needs a bit of slack to get the shackle onto the anchor so I must remember to adjust the turnbuckle to give me that slack and then tighten afterwards.

Cheers

neilho · « **Reply #10 on:** March 01, 2009, 07:58:09 PM »

Rohn recommends that 1x7 EHS wire be tensioned to 10% of its breaking strength. There's a more complicated formula to determine this, but it simplifies to x=12500/length where x= cycles per minute and length is expressed in feet. So to use this method, get the guy wire going at its first natural frequency by moving it up and down. Once into a rhythm, count the cycles per minute. Tighten or loosen to suit.

I know that this works for EHS wire and it may very well work for other steel cable. Synthetic rope? Dunno.

Neil

neilho · « **Reply #11 on:** March 01, 2009, 08:04:53 PM »

After reading Frackers' original post again, once every three seconds is very slack, indeed. That's 20 cycles per minute when typical rates are 80-140 cycles per minute. All dependent on length, of course...

Neil

Ungrounded Lightning Rod · « **Reply #12 on:** March 01, 2009, 11:42:24 PM »

Does anyone know if heating and cooling (summer/winter range is about 35C to -10C) will affect the 13m length of the guys very much and hence change the tension?

The guys and tower are both steel so they have coefficients of expansion that are virtually identical. They're not parallel and the ground doesn't also expand at the same temperature curve (or much at all, since it's sitting on the continental plate which is at lava temperature down there and doesn't vary much on even an annual time scale), so there is some tension change. But much of the expansion tends to cancel out.

(Or at least that's what I think the case is. I trust others will correct me if I'm off enough to be hazardous on this.)

SparWeb · « **Reply #13 on:** March 02, 2009, 03:09:28 PM »

Hi Robin,

Do you have turnbuckles installed in the guy wires? How about some over-center tighteners? Either way, you can make an estimate of the tension in the wire by the torque required to tighten each of them. Set up a test in your garage/basement or wherever you can hang heavy stuff from the ceiling.

For example, hang a dead weight of about 500 pounds from a turnbuckle. Crank it tight with a wrench, lifting the weight off the floor. Pull on the end of the wrench with a spring-scale. A box-end wrench is perfect for hooking on spring scales. You now know the force, the arm, hence the torque on the wrench you used to tighten enough to raise the 500 pounds load. You can repeat that in the field as you tighten the cables on the tower. Needless to say, if you use the same wrench every time, you'll get a feel for it.

Bear in mind that as you tighten one cable, you also tighten the opposite on the other side at the same time. They are, roughly, always going to have the same tension. What can go wrong is shortening one cable as you tighten it, and then the tower is crooked. Looks funny, and over-stresses the tower.

Your tower is about the same size as mine. Hope it's got 1/4" guy wires with 7x19 construction on it. I wouldn't trust any less. A cable pre-tension of 500 pounds (200 kg) is enough for an 8 foot windmill (2.4m) diameter. Scale up or down as appropriate.

SparWeb · « **Reply #14 on:** March 02, 2009, 03:10:38 PM »

Forgot to mention that that process is "approximate" only, and varies with temperature, moisture, wear, lubricants, etcetera etc. etc.

frackers · « **Reply #15 on:** March 03, 2009, 02:29:23 PM »

Thats certain worth looking at. It occurs to me that I could make a jig that deforms the wire a small amount and measure the force required.

What I have in mind is a length of timber with a block at each end that keeps the guy wire away from the timber. Use a spring balance to pull the wire in at the centre.

| spring balance

V

============================================================== wire

XXX XXX block

XXX XXX

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX timber

I hope that makes sense - there doesn't seem to be a

 html command on this board (and even on the scoop website I can't find a user guide!!).
Cheers

>

SparWeb · « **Reply #16 on:** March 03, 2009, 02:44:51 PM »

That's not a bad idea, but blocks of wood would be hard to handle. You're starting to re-invent the wheel. Try one of these:

http://www.buysail.com/accessories.htm

This is a simple device that you can get from anyone who assembles boat rigging.

wooferhound · « **Reply #17 on:** March 03, 2009, 02:57:30 PM »

""on the scoop website I can't find a user guide!!""

It's in the top righthand box on this page labeled "Menu" then "How to use this board"

http://www.fieldlines.com/special/faq

frackers · « **Reply #18 on:** March 03, 2009, 03:06:59 PM »

What I meant was the info on what the html tags do - since some of them aren't html (for example 'code' is a non-standard extension understood by scoop but nobody else as it isn't documented anywhere!!). I was hoping to use the 'pre' preformat tag for the ASCII drawing but scoop doesn't parse it...

wooferhound · « **Reply #19 on:** March 03, 2009, 04:39:37 PM »

actually , , , you are right, those tags have confused me too.

jmk · « **Reply #20 on:** March 03, 2009, 07:28:30 PM »

I have never measured my line tension, but 500 lb. seems to be a lot of pressure to me. I just tighten mine up till the top doesn't sway. It might move an inch in 54 foot but you can't tell. I bet I have about one hundred pounds at the most, but maybe it is deceiving? I would guess that I have about six inches of an arch in my upper guys. I guess how hard would you have to pull to get a 1/4" guy that long on a 45 deg angle to get it that straight? Maybe it's more than 100 lb.? I also just use my hand to tighten the turnbuckle and I can tell when the guy gets tight by the big difference in the way the turnbuckle turns. I built Hugh's 12' machine and the tail is really heavy. The side that the tail is on has a slightly bigger bow in the guy and the other is almost straight. I could crank down the turn buckles but I don't feel it's a necessary load to ad.

imsmooth · « **Reply #21 on:** March 17, 2009, 07:22:44 AM »

Do you all use turnbuckles for your guy cables? Right now I tighten my cables with the clips to what seems like an adequate tension when most of the sag is taken out of the cable and it feels taught.

frackers · « **Reply #22 on:** March 17, 2009, 02:47:41 PM »

Yup - I've got turnbuckles that I got the wire rope supplier to crimp into each guy rope. They are a bit undersized (I should have spec'ed them better) for the rope I'm using so I have a loop of 5mm wire rope looped through the thimbles of the rope ends in case I get a breakage.

Three of the guys are fixed directly to the anchors with the thimbles clamped directly round the 13mm rebar I have embedded in the concrete anchor blocks, the last one is used to lift the mast and is fixed with a pair of 10mm shackles to the anchor.

I should be good for a cat 1 hurricane now - have already had gusts over 100km/hr without any issues (with the mill running!!).

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Author Topic: Guy wire tension (Read 13180 times)