Simple question, not so simple to find answer?

[biggeorgetechnical]

Question: I am wondering why the bases of the large wind turbines I toured recently had something like 64 large, "left hand" thread bolts in the base. The tour guide did not know, but he did say they were torqued to something like 2000 lbs. Why the '"left hand threads?
Thanks,
George

 

[JStephen]

Well, you made me curious, but I didn't find an answer either.

I find some threaded rebars are available in left/right threads so you can match two end-to-end and thread a coupler on them without rotating the bar. But if they're available in both orientations, I can't think why you'd use left-handed for anchors.

I found one link that mentioned certain bars were threaded left-hand so they could be screwed into soil or rock and were apparently threaded left-handed to match other equipment used, "together with a lefthand thread for standard rotary percussive drilling".

 

[biggeorgetechnical]

Interesting. But, I thought it might have something to do with the yaw controls or something twisting the tower and they were this way to resist it.

Maybe its just as simple as you suggested.

"When you go looking for problems, you will find them... They may not be the problems you went looking for!"

 

[JStephen]

I find a Dayton Superior product brochure of wind-turbine products, and in it, they have all-threaded rebar for use as anchor bolts, and they're all left-hand thread, but it doesn't say why:
"Dayton Superior offers anchor bolts in Grade 75 (W6), Grade 95 (W9) and 150 ksi (W3) strengths ...Bolts are available in #6 to #24 sizes...All bars have left-hand thread. Bars are fully threaded and available in any length up to 60 feet."

The threads in question are not a conventional bolt-thread pattern.

I just now sent an inquiry in to them to ask, see if we get a response!

 

[biggeorgetechnical]

Thanks!

"When you go looking for problems, you will find them... They may not be the problems you went looking for!"

 

[bigTomHanks]

Very interesting topic. I found this on wikipedia and think it may be the reason why:

In a situation where neither threaded pipe end can be rotated to tighten/loosen the joint, e.g. in traditional heating pipes running through multiple rooms in a building. In such a case, the coupling will have one right-handed and one left-handed thread

http://en.wikipedia.org/wiki/Screw_thread

I just don't know exactly how they make the rest of that connection.

 

[biggeorgetechnical]

Maybe BigTomHanks is on to something. I looked up a couple wind turbine base patents, and the ~64 large left hand bolts pass through a massive base of concrete, to a ring on the bottom side of the base. Can't tell if the other end of the re-bar is right hand thread. But, one of the stated purposes of the patents is to provide compression. Maybe its also helpful during assembly, and during service to resist the wind forces blowing the tower from all directions better? The link BigTomHanks gave also has another link with an interesting animation:

http://en.wikipedia.org/wiki/Precession_(mechanical)

"When you go looking for problems, you will find them... They may not be the problems you went looking for!"

 

[TheTick]

Vandalism resistiant?

 

[rb1957]

it'll be interesting to find out why ...

i can't see it myself ... why not RH threads with a locking feature (if there is some loosening effect from loads)

but it is a tricky design ... how to make sure they don't strip the buried thread ...

Quando Omni Flunkus Moritati

 

[GregLocock]

My guess is idiot proofing a non standard nut.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[RossABQ]

I assume these are post-tensioned anchors? From a search on "left hand threads in post-tensioned anchors" I find many references to LH threads used because of the grout pumped into the center of the tendons. LH threads allow disconnecting the grout connection without loosening other components. I'm not a civil, so take a look for youselves...

https://www.google.com/search?q=pos...42,d.aWc&fp=aec810c57fc5a1bf&biw=1344&bih=691

 

[Compositepro]

As Greg says, my experience is that when strange non-standard designs are used where "standard" items could be used, it is to prevent the use of "standard" items. This could be for safety reasons so someone cannot install the wrong grade of nut, for example. Sometimes it is just to force customers to buy parts from the OEM at inflated prices. The left-hand thread is a very obvious signal that "this is not standard".

 

[chicopee]

I think that Compositepro's statement "...This could be for safety reasons so someone cannot install the wrong grade of nut,..." may have some merit.

 

[biggeorgetechnical]

These are post tensioned anchors. But after a bit more research myself, I think there is little doubt RossABQ has provided the answer.
Never having been present at the building of such a base, I cvisualize how the process is actually performed.

"When you go looking for problems, you will find them... But they may not be the problems you went looking for!"

 

[JStephen]

I'm still not seeing a reason why they would be left-handed. In doing the search for "left hand threads in post-tensioned anchors", I'm finding left-handed bars available, but they seem to be equally or more available in right-hand. One of the references is talking about a coupling, not a rod, that is left-hand-threaded. Of course, regular nuts and bolts are available to some extent in left-handed-thread, but you don't normally use them without some specific reason.

Note that none of these bars are furnished in left-right stud-type configuration, where one end is threaded one way, the other end the other way. It's one direction or the other the full length. The threads are already a non-standard configuration, it would seem, so little issue of a nut mix-up.

 

[biggeorgetechnical]

Like I said, I cant visualize how its actually done, maybe its "just the way it was patented" or something obscure like that?

"When you go looking for problems, you will find them... But they may not be the problems you went looking for!"

 

[apieron]

As a few others have stated, I believe it is just for safety reasons. Prevents other nuts/couplers/rods on the construction site from being used.

 

[FACS]

Compisiepro had it right.

ASTM F1554
I believe there are three grades, and the washers and nuts also have to meet the grades... thus .....

"ASTM F1554 is the newest construction fastener specification developed by the American Society for Testing and Materials and covers steel anchor bolts intended for anchoring structural supports to concrete foundations. Chemical requirements, mechanical properties, and dimensional tolerances are all covered under the F1554 specification"


Charlie

http://www.facsco.com

 

[JStephen]

ASTM F1554 uses Unified Coarse Thread series, Class 1A or 2A fit, or 8-thread series. This is per the -07 version. I don't see any mention of left or right hand in the standard. Note that it applies to anchor bolts, not to threaded rebar.

 

[apieron]

Also, RossABQ is right IF they are hollow self drilling anchors, however I am almost positive they wouldn't be.