Wobbling Towers

[adaml]

Quick question for the tower guru's.

I have a 12ft tower, 6ft 4x4 6ft 2" scaffold bolted onto the timber.  8ft dia blades on my converted B-H motor.  Tower is guyed on three sides.  (If I had sussed out how to post picture on new board I would - sorry.)

Question is this;  I know the balancing is not great, tracking is OK in my opinion, within quarter of inch.  However, tower wobble is apparent between 60 - 90 rpm, once above this it settles and tower is rock solid.

I have been attributing this to bad balancing or motor reaching cut-in which is 120rpm.

Has anybody else had similar experiences with initial wobble then settling and what is the considered conclusion?

[TomW]

I vote harmonic imbalance.

In other words the tower has a harmonic resonance at the lower speed due to an imbalance. I have seen similar symptoms on my 2 meter conversion.

Don't mistake me for an expert but I figure it is no immediate big deal if it is not violent or continuous and goes away when it gets up to running speeds. It stresses everything tho so be aware of that!

You can try to fine tune the balance and this is probably the real solution. Or you could try to damp the harmonic in the mast.  Possibly as simple as shortening or lengthening the upper stub and snugging up all connections. You might be able to prove / disprove the harmonics theory by clamping a length of angle iron to the top stub above the board or maybe the board itself?

I would fix it myself before it shakes all the fasteners loose and comes crashing down. Because it eventually will.

Tom

[Boss]

Our ten footer wobbled a bit at low RPMs before we installed the blades we cut using a CNC router. This is not to say the hand-carved blades we took off  were unbalanced, it actually took less duck weights (as Dan called the little lead bits) to perfectly balance the first set than the newer set. What I believe the difference was, is the precision in which the blades were centered on the hub bolts. I think the first set of blades were off-center very slightly, I know the wooden disk on the outside was off a bit, we could see it move in an elliptical pattern. I don't think it was a coincidence that the tail wobbled slightly in a related motion to the center disk

On the other hand what Tom says is totally possible as well. When I was an apprentice mechanic one of the tortures I was made to suffer was learning everything there was to know about balancing tires and wheels. Wheels absolutely wobble more at lower RPMs if there is an imbalance. Most imbalances would appear to "go away" at high rpms, this was back in the days when we had a giant electric motor we would slide  under each slightly raised wheel and spin and balance it on the car
Hope this helps

[poco dinero]

(If I had sussed out how to post picture on new board I would - sorry.)

 

Directions for posting pictures seems to be the best kept secret on this forum.  Everybody has to re-invent the wheel over and over again.  Some of us have the necessary smarts but not the time.  Please help us out.

poco

[Boss]

Posting pics: fist thing is have the image on another tab or window: Put your mouse pointer over the image you want, anything besides a Mac, right button select, choose "copy image location" Lastly come back to the "reply" window select "Insert Image" which adds (img][/img) with the cursor in the center already, just do Ctrl & V simultaneously. That is it.
You can type before  and after  the image, absolutely do a "preview' before post to make sure the image displays

[97fishmt]

adaml,
I agree I bet the center is off a touch.

I made a real nice set of blades 8' diameter,
blades and got the center off slightly in the hub
and pretty much the answer was to redo the
blades to a new set of plywood disks.

Or maybe just fine tune the balance?

8' might be too much for your tower?

[taylorp035]

My tower wobbled a little bit at a few mph before cut in, but at high speeds it went away.  Another possibility is the difference of air pressure as the blades go by, but there isn't much you can do to fix that.

[imsmooth]

I have a 10.5' diameter turbine and the tower is rock stable at all speeds.  We studied this in engineering, which as Tom said is an imbalance.  At the resonant frequency of the system you will get large oscillations.  This is likened to the Tacoma Narrows bridge collapse.  You can also think of it as a washing machine that jumps off of the floor during the spin cycle because the load is not centered.  It reduces to a point mass spinning around the center.

If you don't balance your blades you could have some issues; it is possible, though, that the resonant point is at only one frequency.  If it was mine I would rebalance.  I perfectly balanced my blades by.

1. getting each blade within a few grams of each other
2. doing a static balance of the prop

[Ballyk]

Hello,
My mill was shaking so much it eventually snapped the tail arm clean off. I posted the same question as you and got some really great help a week or so ago. Have a look at the "blade balancing act" post and there you will surely find the answer you need. I am still applying all the advice I got in steps and the machine will be up again next weekend and hopefully without and shaking and wobble.
Regards, Ballyk

[TomW]

Hello,
My mill was shaking so much it eventually snapped the tail arm clean off. I posted the same question as you and got some really great help a week or so ago. Have a look at the "blade balancing act" post and there you will surely find the answer you need. I am still applying all the advice I got in steps and the machine will be up again next weekend and hopefully without and shaking and wobble.
Regards, Ballyk

For the search impaired here is that thread:

http://fieldlines.com/board/index.php/topic,143273.0.html

The new search functions actually work pretty well on this new software!

Tom

[poco dinero]

Insert Quote
Posting pics: fist thing is have the image on another tab or window: Put your mouse pointer over the image you want, anything besides a Mac, right button select, choose "copy image location" Lastly come back to the "reply" window select "Insert Image" which adds (img][/img) with the cursor in the center already, just do Ctrl & V simultaneously. That is it.
You can type before  and after  the image, absolutely do a "preview' before post to make sure the image displays
 

So, uh, Boss---

With the new software, do I no longer have to worry about how many pixels the photo has?  Or file size, or any of that stuff?  With the old software people used to get yelled at for posting pictures that had too many pixels, or bits, or bytes, or whatever.

poco

[DamonHD]

It's still unkind to link to a *huge* image in the middle of your piece since it may take ages to load (and cost real money, circa 10p/MB for me now if I hit my wireless limit for example) and distort the page.

Rgds

Damon

[Boss]

Like DamonHD said, it is always a good idea to respect the users with dialup or poorly implemented broadband.
I keep all my images to less than 800pixels wide just so that users with old CRTs can see the text posted with the pics without scrolling from side to side. I use Linux's the Gimp http://www.gimp.org/ tp modify both the viewable image size as well all the file size. As far as SMF it has settable limits to image size. It is up to the admins to set those. I always try to keep the image to ~ 100kb  and images in signatures to way less. Since I hurt my back and have been posting here and at http://biodiesel.infopop.cc/eve/forums three and four times per day, people may get tired of seeing my sig-image, but I assure you it is only 73k, hmm, now that I checked my "facts" I really ought to shrink that file size even more. I  hoped it was 25k. I will shrink it more. Anyway the Gimp is free and it is easy to use, oh and it works in Windows too.

[picmacmillan]

hi adam..one thing i picked up on is that you have it guyed on 3 sides...but only once?...might help to guy it on 3 sides again...the moment(distance) from the turbine to the gound may be large..so i am guessing it hits resonant frequency, or wobbles below where you have it guyed...my 10 foot turbine had to be guyed twice.(tower is 25' up).once just below where the blades might hit it, and once down lower...remember that in a triangle the mass moment of inertia?? or the center point is 1/3rd from the biggest side..not like a square where it is 1/2...so if youre tower is what they call 100% guyed, the cables holding it would be the same distance away from the turbine, as the turbine is high..and then of course like some others have stated, youre blades could be off balance, or youre plates could be drilled a hair off...not too mention the offset of the tail...just a matter of sorting out what it is ....hope this helped a bit...pickster