Believe nothing, no matter where you read it, or who said it – no matter if I have said it! – except it agree with your own reason and your own common sense.” – Siddhartha Gautama, a.k.a. the Buddha

Wind, revisited.

A few of my readers have contacted me for further details about my alt-energy setup. One of those points is something I felt rather needed sharing.  The blades.

I have looked around at several sites for aftermarket blades and almost every one of the dealers has stated the same thing or similar.  I won’t go into too much detail but the statement is this, “perfectly balanced “.

Gotta say, Caveat Emptor, let the buyer beware.  Only one set I bought was even close to balanced correctly. But, this article isn’t to slam any dealer/manufacturer, it’s to instruct you in how to balance your blade set, bought or made. 

Moat small turbine systems keep the swept area under 6’diameter.  There is a reason to this, and that reason is speed.  Larger blade sets turn slower even though they generate more torque.  The smaller gen sets don’t need oodles of torque, but do need the speed.   Higher speeds tend to show balance issues in much more dramatic ways: think unbalanced tire at greater than highway speeds.

You are going to need some tools right off the bat

  • A scale, accurate to 100th ounce or in grains,, and it needs to be able to handle the weight of one of your blades.  Mine can handle 1Kg or 2.2#.
  • A lawnmower blade balancer.  I found mine for $4 at a Do-it-best hardware store.
  • Wheel weights .  A strip of 1/4 Oz stick on type cost me $2.50 at Napa.

Now, weigh each blade and note it down.   I write it on each blade as well as note it on paper. If they are within a 1/4oz of each other, you are almost done.  If not, work is about to get under your skin.  

Blade construction at this point can save you or grind you.  If the blades are polymer or ABS, you can “trim” them to match each other.  I use a deburring tool and scrape LIGHTLY along the trailing edge to trim down weight.  Go slow, you can’t put back on!  A couple of passes, weigh it, note difference and repeat as needed.  Using this, I as able to get my raptor blades within 1/100th of an ounce of each other. A problem  you may hit is the scraper/deburring tool gets some chatter in it, and the cut ends up ragged.  Stop, get a fine flat file, and smooth that area out. It may not hurt anything, but then, it may make a butt-ton of noise.  Why chance it.

Now, if your blades are aluminum, you are going to go about it a similar fashion, but you really need to focus your material removal to the trailing edge as far out from the hub end as possible. You don’t want to take too much either so only do this if they are pretty close. If they are wildly different, contact the seller and raise hell.

Now, balancing things. First word od caution: do this indoors with no air movement.  The balancer is VERY sensitive and a minor breeze will screw you up a lot.  With the raptor style blades, it was pretty straight forward.  Mount the blades to hub, set hub on balancer, watch for any tilt.  If you see it, lay a 1/4oz weight opposing the tilt (uphill side) but do not attach it,yet.  You are going to have to shift things around until your are satisfied.

The aluminum blades are a bit different.  The way they are bent, they are top heavy and will skew all attempts at proper balance.  Here’s the trick, on paper, lay out where you are going to put what blade on the hub. Subtract the weights on adjacent blades and note it down.  You will cut a weight to that difference for each pair. Three blades, three weights,  etc.   Now, with the blades facing up (wind side up) put it on the balancer.  It’s going to try and tilt no matter what you do, so be patient.  Now place each of the three weights on the blade near the hub where the blade and hub meet; the weight will go on the lighter  of the pair noted. (IE:if blade 1+2 have a 1/4oz difference,  the 1/4oz weight will go on the lightest blade, next is the 2-3 pair, then the 3-1 pair.  ) .  Observe any changes.  If it steadies down without wobbling crazy, mark your position with the sharpie.  If not, move the weights around a bit at a time, one at a time and observe.  You nearly need the patience of Job for this part.

Ok, so you finally get the thing to settle down and you marked where the weights were at that time.  Take the weights off, flip the hub wind side down (you may need to use a can or bucket to raise the balancer up) and watch.  Now it is likely leaning all tilted so place your weights on the blades oposite your marks. It should go dead flat at this point.if it does not, no worries, shift your weights one blade counter clockwise. Observe. Repeat as needed until satisfied. You may even find removing one weight will improve things.  A little trial and error is in order here.

Now, it may sound like I am jerking your leg on this, but when you mount the weights, DO NOT place them where you made your marks, on either side.  They will be subjected to far too much centrepital force and will fly off. You will mount them on the blade nearest the hub and preferably between a set of bolts as reinforcment.  The difference in the adjustment is so minor, it won’t hurt a thing. 

The reason you want to balance your gear should be obvious, as in longevity of your investment, but also in performance.  An unbalanced blade set will actually self brake before it really starts to work.  It may not be seen at first, but sit observing an unbalanced unit while metering it’s output, and the problem will stand out glaringly.  You’ll see the voltage climbing and then just stop, if not reverse. At the same time, you may even see the guy lines shaking if not the whole tower. Due to the fact that the genny is being powered by the wind, it will never climb past that state, unlike in a wheel on a car that shimmies at one speed,  but settles down above or below it.  The genny just can’t accelerate past that point.  And an unbalanced unit WILL fail at some point, and usually by sending the blades flying off willy nilly.

Hope this helps someone else in solving an issue, or better yet, in avoiding one.  If you buy your blades, check ’em anyway, no matter what claims are made by the dealer. If you make your own, you should be doing this as a routine anyway.

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