Author Topic: Fatigue... (Read 4147 times)

rossw · « **on:** December 17, 2009, 01:53:21 PM »

I wasn't going to post this, but have been encouraged to by the guys on IRC.
Metal fatigue is a sneeky bugger, you can't tell from looking at something that it's in imminent danger of failure - it just lets go (at the most inopportune time, of course!)
Here's my story.
Yesterday started out nicely. Calm, sunny and warm. By lunch time we had broken cloud, temperature was up around 38 deg C (almost 100F) but the wind was kicking up a bit. About 13:30 I was outside watching the turbine - the wind was gusting to 54kmh and the turbine was right on the edge of furling. Every time the wind swung towards the south, the turbine furled, tracked the wind and then took off again. Output was about 850 watts.
While I was watching, the top guy on the windward side of the tower suddenly dropped to the ground!
The tower lurched to the east and I BOLTED inside and applied the (electric) brake (shorts all 3 phases).
With the turbine going at these speeds, it slowed the mill a bit but didn't really do much good.
I grabbed a long rope and ran up top, planning to try to secure the thing somehow. But how to (safely) get a rope up there? I couldn't throw a rope that far up in this wind - and even if I did, it would be a fair bet I'd just get it caught in the prop and do untold damage.
I quickly took one end of the rope and went around (over the top of) the remaining 3 guys and took the two ends back to the anchor of the (missing) guy and "walked" the rope up the other guys until it was at the top attachment point. Truckies knot and I was able to tension the rope enough to pull the tower back to straight and tie off the rope.
Good thing, because shortly after, the storm kicked up a bit more - I only saw 81 kmh, but the official wind gust was 107kmh. The place opposite me lost its roof - it was ripped clean off at the top of the walls.
There was a brief lull after that, when I ran up, attached the winch and lowered the tower. Upon inspection, it became obvious that it wasn't a guy failure at all, it was the half-inch steel rod the manufacturer had originally welded to the mast, which had let go.
Here are some pictures (click thumnails for larger photos):

[img width= height=]http://house.albury.net.au/17dec2009/thumb.100_4039.JPG[/img]

[img width= height=]http://house.albury.net.au/17dec2009/thumb.100_4041.JPG[/img]

[img width= height=]http://house.albury.net.au/17dec2009/thumb.100_4042.JPG[/img]

I'm pondering (unless anyone can give a good reason NOT to do this) - to re-string the guys, so each line comes up to the tower, in through the rebar loop, AROUND the tower, back out the loop - and then use the cable clamps to tie the two ends. This way, each guy will go completely around the mast.

HaroldCR · « **Reply #1 on:** December 17, 2009, 02:19:21 PM »

Looks to me like the welds failed on your tower

Never have seen this method posted on here, sooo,, When we were building telescoping towers, for Wind generators, we made up brackets that consisted of 3 pieces of ¼" X 1¼" flat bar. We bent the ends out away from the flat of the tower faces (3), and bolted them around the tower. The bars were about 4" longer then needed, and, we drilled holes in each piece, to put a grade 8 bolt on the outer ends of the braces. We put the guys on SS Thimbles, that the bolts went through, and that was our guy system.

One every 25 feet. As the tower was cranked up, after winching it vertical, all the guys could be strung through the anchors and held by one person, to steady the upper section, while the 2nd person cranked the winch for raising the second section.

We did over 30 of these towers at 100' high. We nearly always had the generator mounted on the tower, before raising. Never lost a one, even in 100 MPH winds, that I know of.

We sold Winchargers, Whirlwinds, and Bergeys, + the 4KW I built. Largest was 4KW, 4 different locations.

Hilltopgrange · « **Reply #2 on:** December 17, 2009, 02:57:46 PM »

"I'm pondering (unless anyone can give a good reason NOT to do this) - to re-string the guys, so each line comes up to the tower, in through the rebar loop, AROUND the tower, back out the loop - and then use the cable clamps to tie the two ends. This way, each guy will go completely around the mast."

My only concern with your proposal would be the guy wires could possibly saw through your tower given enough time and movement! maybe you could sleeve that part of the tower to prevent rubbing or weld new guy anchor points and secure the guys as before but add a separate cable threaded through each of the anchors and around the tower to catch any that break. Sort of belt and braces approach.

I hope that makes sense

Hilltopgrange

halfcrazy · « **Reply #3 on:** December 17, 2009, 03:03:51 PM »

what we do is run a scrap piece of cable through all 4 thimbles and around the tower at the same time then clamp the ends together effectively making a solid loop around the tower through the thimbles

frackers · « **Reply #4 on:** December 17, 2009, 04:10:55 PM »

Exactly what I've done...

5mm HT cable looped round the tower a couple of times, through all the thimbles and a good quality cable clamp to hold it all together.

Note the locking wire on the shackles :-)

rossw · « **Reply #5 on:** December 17, 2009, 04:40:07 PM »

Ummm... isn't that wire clip on backwards?

Surely the red taped end would be the "dead" cable - yet it's against the saddle.

Or doesn't it count if it's just a short loop like that? <grin>

Ungrounded Lightning Rod · « **Reply #6 on:** December 17, 2009, 04:58:45 PM »

Both ends are live on that setup.

Dave B · « **Reply #7 on:** December 17, 2009, 05:04:51 PM »

Never saddle a dead horse, good observation on the cable clamp. Dave B.

Ungrounded Lightning Rod · « **Reply #8 on:** December 17, 2009, 05:07:47 PM »

Fatigue, all right. Look at the white crystalline structure at the break.

Looks to me that the problem is the way the bar was arranged. Instead of facing the load, it stands out horizontally. Every time the tower leaned away from a guy that guy's attachment flexed downward and was horribly stressed in bending near the weld.

Replacing the horizontal bars with vertical half-rings welded to the tower should do the job for you.

97fishmt · « **Reply #9 on:** December 17, 2009, 05:55:38 PM »

Around the tower is a very good for your belief in being secure. And through welded vertical half sections of pipe. Hugh's book suggests this method, it's the the one I followed. I also used a safety cable around for my winch cable, but you need more than one cable clamp. It is always recommended to use at least three, and used correctly.

I liked your quick thinking with a rope around the other guys and walking it up to secure the tower. Thanks for posting.

rossw · « **Reply #10 on:** December 17, 2009, 05:57:25 PM »

Yeah... 20/20 hindsight.

This was a commercial tower, and thats the way it was supplied.

It didn't even come with thimbles (I added those myself), and only one cable clip per guy per end (again, I added extras).

An extra loop of wire through the end of each guy sounds like an excellent tip, one I'd not thought of, but will certainly do when I put it back up.

bzrqmy · « **Reply #11 on:** December 17, 2009, 07:42:17 PM »

I have chain links welded to my tower, cable thimbles through these links down to the anchors. I run a safety cable through all 4 cable thimbles, around the mast, just in case the welds break, the safety cable will keep the guy from falling to the ground. I do something similar through the turnbuckles and anchors at ground level. So basically, all my connections are backed up with a safety cable. Hope to never use them, but they they are there.

dlenox · « **Reply #12 on:** December 18, 2009, 04:39:20 AM »

It looks to me like this was the butt end of the steel rod as it was 'wrapped' around the post, and one end barely had a weld on it.

Dan Lenox

Ungrounded Lightning Rod · « **Reply #13 on:** December 18, 2009, 12:52:44 PM »

Even if not, the right side failed a while back. You can see rust along nearly all the surface there and only a tiny bit of clean, recently broken, metal.

dlenox · « **Reply #14 on:** December 18, 2009, 02:36:15 PM »

ULR,

Yea - it's the right side that I was referring to.

Dan

ghurd · « **Reply #15 on:** December 18, 2009, 03:52:29 PM »

In addition to connecting all the thimbles together,

What if the 'dead horse' ran 360 through the other guy's thimble loops, nearly tight, and then back to fasten to itself?

How I am thinking about it now, seems like the vector sum of the faulty guy would be spread out more evenly.

Seems like if the factory loops were tilted downward ~30 degrees, the welded assembly would have been better.

G-

SparWeb · « **Reply #16 on:** December 18, 2009, 04:24:56 PM »

Great job rescuing your turbine! I love a little drama like that (except when it happens to me!)

There's a better way to describe that failure than a catch-all word like "fatigue".

The term is "stress-corrosion-cracking" and it decribes your situation a bit better than fatigue. SCC is the failure at low loads of a structure that is constantly under stress, has minor cracks to begin with, and then corrosion sets in, prying the cracks open until the stress breaks the part.

The stress doesn't have to be very high - much less than needed to bend those pieces of rod. The crack doesn't have to start very big at all, either - just enough for water to get in (capillary action). I don't know where you live, so I don't know have quickly rust forms where you live. Dust, salt, or cold water freeze/thaw can all move the SCC process along.

The broken end of rod is rusted all the way through, and the bent/broken end of rod seems to have the same thing going on. There's no paint under the rod where it's welded, it's a kind of area that is typically hard to get paint into. Somebody's given it a spray-paint touch-up, but the place that needed it didn't get hit by the spay either.

Since the rod would have been bend into a square to make this ring, where do the ends of the rod joint with respect to the failure?

neilho · « **Reply #17 on:** December 20, 2009, 09:29:12 AM »

It's nice to hear about failures, too. Thanks for posting your pix.

The little bit of recently broken metal was what was remaining. If you want, the stress it took to break the bracket can be calculated by measuring the area of that little bit (not always so easy) and applying the right formula.

Welds are especially subject to this kind of failure. The codes vary in their degree, but all regard a weld without stress relief to be cracked from the beginning. The American Bridge code, IIRC, allows a maximum of only 16% of the calculated strength of the welded connection. Pretty grim. It's a good reason to avoid welds if possible and oversize and stress relieve them if they're absolutely necessary.

Another comment, if I may. Using cable clamps to terminate the guy wires reduces the effective strength of the guy wire by about 50%. Guy grips, Big Grips or preforms, as they're sometimes called, allow the full strength of the guy wire to be developed. They're only slightly more expensive than cable clamps and much easier to apply and remove, especially when using EHS cable.

Neil

Dave B · « **Reply #18 on:** December 21, 2009, 10:08:08 AM »

Big Grips for terminating the cable ends are one of the greatest things invented, right up there with wire ties and duct tape. I promise that anyone using these will say the something very similar. They will save you hours of time, well, well worth the money and as mentioned a stronger arrangement. Dave B.

rossw · « **Reply #19 on:** December 21, 2009, 12:29:17 PM »

Update (cross-posted from TheBackShed).

Saturday I ducked into town and grabbed a length of the heaviest chain I could get. Pity the links were made of only 10mm dia material. Anyhow, extracted 4 intact links and ground a flat on the "joined" side. (The idea being that if I affix this side to the tower, even if the link was poorly welded, it'd effectively be reinforced by having both "loose" ends affixed to the tower.

Then I took an anglegrinder and cleaned a section of the tower back to bare steel big enough to work in:

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4060.JPG[/img]

The plan is to add 4 links around the tower, like this:

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4061.JPG[/img]

It's just as well I'm a good system admin, because I'm a lousy welder. (The little dings all over it were me making sure no more sl*g was comming off and that the weld was sound):

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4063.JPG[/img]

Welding upside down really sucks!

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4067.JPG[/img]

Finally done, you can see the "easy" 3 here:

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4068.JPG[/img]

A lick of "cold-gal" and they look acceptable. I doubt they will fall off.

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4071.JPG[/img]

After I moved the guys from their old mounting points to the new ones, I added another wire through all the thimbles. I ran out of clips (I did the same on the lower guy attachment point), I'll put another clip on this next weekend.

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4075.JPG[/img]

Someone was asking what the "truckies slip knot" was - here's another one I used to hold the gin pole while I moved its guy. (This provides a 2:1 mechanical advantage for tightening up ropes):

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4076.JPG[/img]

And a couple of shots - one down the tower from the generator head end, and one sideways, before I raised it again. It's back up and flying now.

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4077.JPG[/img]

[img width= height=]http://house.albury.net.au/20dec2009/thumb.100_4078.JPG[/img]

I re-raised the tower and adjusted the guys since the attachment point had moved, releasd the brake and in a most uncharacteristic display of optomism, the turbine took off. OK, only 250 watts, but it seemed happy to be back in the air.

Hilltopgrange · « **Reply #20 on:** December 21, 2009, 12:40:54 PM »

Personally I would grind those new anchors off and either re-weld them or have someone else weld them on. Those welds wont hold much by the look of them, they look weaker than the original anchor points.

Think of what would happen if or rather when they fail? Try hitting them from the side with a hammer and see if they stay attached?

Just a word to the wise for safety sake

Russell

rossw · « **Reply #21 on:** December 21, 2009, 02:21:25 PM »

I did bash them with a 4lb hammer and they sound and feel solid.

SparWeb · « **Reply #22 on:** December 21, 2009, 02:57:39 PM »

It took a moment to realize why the horizontal pieces of tube are there... holy smokes they are ladder rungs that you climb up.

There is pitting visible in the photos of the chain link welds, so the same process that allowed the failure of the rods will progress until the chain links fail. You will get a few years out of them for sure... but later you'll see rust streaks running down the pole and it's the beginning of the end. Geez I really hate criticizing. I wish you hadn't wacked them with the hammer, too. Especially now that I know you climb it.

Hilltopgrange · « **Reply #23 on:** December 21, 2009, 04:29:01 PM »

Many years ago when I was an apprentice if I asked my journey man to check a critical weld on a job he would reply "does it pass the sleep test"

The welds on these anchors are perhaps the most critical of the whole build. If they are not perfectly in line with the ground anchors then they will be getting twisted as well as pulled. Personally I would have used 2x¼ flat bar about 6 inches long with a hole bored to take a shackle this would spread the load far better. Same as in this recent story by halfcrazy .http://www.fieldlines.com/comments/2009/12/20/104644/84/6#6

It would be a real shame to see a turbine crash to the ground for the sake of a few inches of good clean weld

Not to mention the risk to life or limb!

So with that in mind will YOU sleep well during the next storm?

I didn't mean to get on your back about it, I just see a real danger and an accident waiting to happen!

Enough said.

I hope you have a nice Christmas

Russell

halfcrazy · « **Reply #24 on:** December 24, 2009, 04:47:56 PM »

Yeah the Pad Eye's make a nice attachment point. they are cheap at our local steel supply and make for a easier attachment of cabling.

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Author Topic: Fatigue... (Read 4147 times)