wood blade balance

[Scotth]

 My genni seems to be doing what it's supposed to up on the test tower,however,I felt there was a little too much vibration when it gets into the faster windspeeds and starts making good power.So,I pulled the prop off yesterday and re-checked the weight of each blade and got them nearly identical.Double checked the distance from the center to tips,which was within 1/16".Tried to balance each blade on a straight edge mid way along the blade and found one had a lot of the weight towards it's end even though they weigh the same.I then lagged the rotor to a big beam so I could spin it up and check for balance that way,only to find the blade with the heavy end wanted to stop low nearly every time despite the blades weighing the same.I don't see how I can even this out short of making a new blade and even then,you never know how the weight distribution will be until it's done.Is this anything to worry about?Having nothing to compare to I may just be overly cautious and my percieved "excessive vibration"may be nothing to worry about.Should I start loading more weight near the center to try and eliminate the heavy side?I want to do everything possible before going back up that tower!

[DanB]

Perhaps I make it all too simple.  I ignore the weight of the individual blades all together.  I assemble the blades, bolt it on the alternator - and balance the whole deal together, usually on the end of the tower right before I raise it.  It seems easiest and works out fine in most cases.  When you bolt that blade on, even if your alternator is perfectly balanced, i expect that even 1/16" slop in the holes would throw you out enough for things to shake.  So I tighten everything down and... if the alternator turns nicely, then the heavy side will come down to the 6 Oclock position.  Simply put that heavy end up to 3 Oclock (or 9) and add weight to the opposite side untill the blade seems balanced.  It usually takes 10min - when done, the blade should have no preferred 'landing position'.

For weights I oftain use lead 'decoy weights' (get them at sporting goods stores) and screw them down with wood screws.

[Scotth]

Hi Dan,

I know that balancing the whole thing as it sits on the tower is best,but my test tower is fixed and it seems every time I have time,it's windy as all @#$% and when I'm too busy to work on things it's dead calm,hence my attempt to get things straightened out on the ground.So,I'll strap on the tool belt and attempt it on the tower if the wind ever stops blowing!

Scott

[Flux]

Scott

I tend to agree with Dan. Weighing each blade is not much use, if you have to do it at all it would be better to support the root on a block and weigh the tips, it is the weight in the outer part that has most effect on balance.

If the prop fits nicely without slop on the alternator and you can find the exact centre you can fix a small plate on with a hole at the exact centre and hang the prop on a piece of strong string or fishing line .

It should hang perfectly level and if not you can add balance weights until it does.

For best sensitivity the string should be attached just above the centre of gravity but unless you have a hole through the centre you will have to be content with it on the front or back surface..

This should get you very close, but as Dan said, an error of 1/16" in the position of the centre will make a lot of difference

[Experimental]

        Hello Scott,

      Just finished balancing my blades a couple of days ago, and finally, yesterday -- we had enough wind to turn the mill !!

     SUCCESS, it spun, with no noise or vibration !!!

     The first time I did the balance, it was hung horizontally by a string, in the center hole -- then used a small bubble level on the hub...

 I have used this method many times before, but it didn,t work on this prop -- So, I built a plate, that bolted to the wall, with two very small precission bearings in it, and an adapter, that fit the prop hub -- all a pain and very time consuming!

     I used the flat weights with the sticky backs, they use to balance mag rims on cars -- they come in a strip about 1/2" wide, and you can nip them off in 1/4 ounce sections -- I tape them in place ,to test for balance -- when that is accomplished, I mark the spot on the back side of the blade, drill a small hole in the weight, strip off the tape from the stickey back of the weight, and stick it to the blade -- then a 1/2" long "tiny" screw, thru the hole, into the blade -- this is just extra security, should the sticky tape give up !!

     The screw I use, is brass, and countersunk -- it is very nearly, the same weight as the lead removed for the hole and countersink..

     In your case, if your mill turns freely, you can do this "in Place", but my mill, "cogs" badly, so I could not do it in place ...

     Knowing how high your tower is, it looks dangerous, and scary -- trying to do it up there !!  But, if you pre drill the weight, and have a couple extra screws (to drop!) It could be done...

     I put that screw,  toward the leading edge, with the weights trailing behind ,on the "back" side and about half way out toward the tips -- the further out, the less weight required, but the tip is to thin to be running screws into and more affected by aerodynamics !!

    I hope this helps you, be carefull and best of luck,  Bill H.....

[harrie]

Hi Scotth.  I think like Dan said, the best way, is balance the works after tightening the hub bolts as you would for operation. With the three Phase, it is easy to do, cause there is no restriction, or magnet drag as there would be on a single phase due to the attraction to the laminats. I pour lead into a peice of 3/4 inch copper pipe with a cap on one end. Than I drill into the thick part of the blade, near the mounting plate, using a 3/4 inch drill bit. The depth of the hole, is determined by the amount of weight that is needed to bring the light blades to the same weight as the heavy one. I usually just drill a 1/2 inch hole, and stick the lead cylinder into it and see what happens, if its too much, I cut some off the lead cylinder intill it is right. Sometimes it takes more than one hole in each of the two lightest blades to get it all the same. I forgot to mention that once you pour the molten lead into the copper cylinder, it will slide out quite easy. I make them about 3 inches long. When you have it the way you want it, you can measure the lenth of the lead cylinder that is needed, than drill that depth into the blades, and drive it in flush. Of course you than should seal the drilled area, so that it will not let water soak into the area of the hole. The problem with the different weight, is not always the way they were made, it is because the densidy of the tree it came out of may be different, or may have come from a different area of the tree. This is one reason to try and take all three blades from the same part of the same tree if you can. It is also very important to make sure all of each blade is sealed to prevent water from soaking into it, includeing the mounting holes.

Great fun Harrie

[Scotth]

OK guys,I went up the tower and re-balanced the whole works up there.It will not stop in the same place twice so I figure it's bang on.I checked and double checked the runout at the hub which is zero.the tips of the blades track nearly identical It still shakes so bad at high speeds that I would give it 2 days max before it would self-distruct.I'm going to make another slightly smaller set of blades and try that.I still can't help but think that one blade that's 1/2 lb heavier with it's mass farther away from the center than the others is the culprit.I'm at a loss of what else to try.

[nothing to lose]

Can you take the whole set off as a unit so that the rotor is not changed? Maybe then bubble balance it and see how it sits. It would seem that if one blade is heavy that side should lean down and raise the other two blades, right. Now either shave some off the heavier blade or add weight to both of the others, or a combination of both, till the entire rotor sits level on the balancer. Then I would not think it would be a matter of one blade being heavier.

Also while rotor is off I would sit it on the ballancer with something like a small post just barely clearing the blades, almost touching, slowly rotate the rotor and watch the blades and post, does any blade hit the post or are any farther away than the others? If so then that's probably the problem, a long or short blade?

If not the above, then I would go for mount holes. Could the entire rotor be offset 1/16" off center? Perfectly balanced in all ways, but sits low on the hub? This would probably normally cause one blade to always be down, but then you ballance it out so it always randomly stops thinking balance is right, but it still moves up and down off center at high speeds and shakes. Also if you ballance out an offset like this, then it may apear to be ballanced stopping randomly at low speeds, but in fact you proably just made one or 2 blades too heavy which causes out of ballance at highspeeds.

Having played around with tons of blades, mostly plastic junk, I have had all kinds of the above problems. The tiny offset with rotor sitting low on the mount got me pretty good with a 4 blade set. All 4 apeared fine in every way but always stopped in one spot and hard to start up, had the shakes at high speeds. Got blades to stop in any position randomly as if balanced but it nearly shook off the mounting. Drove me buggy, then I happened to see a slight oval look to the center when it was spinning, yep offset to one side a tiny bit.

Sometimes the simple things get us.

[witapple]

I just installed a set of blades that had slightly over 1/2 lb difference between the heaviest and the lightest blades and after placing the center point of the hub on a pointed rod sticking straight up i added weight to the center disk opposite of the direction they want to fall till they fall no certain direction. The result was almost no detectable out of balance at any speed. It doesn't matter where the heavy spot is as long as you get it balanced at the centerpoint

[Scotth]

I'm going to take the whole thing down tomorrow and check it again.I really hate to give up on that set of blades as the mill is turning out a good 30-35 amps in a fairly tame wind.

[nothing to lose]

I geuss that's partly my thinking on using a bubble balancer also. Kinda a pointy stick with a level

Fits most wheels so if using a trailer hub or larger type spindle/hub it should work well I think. Might not be good for rotors that mount to a small shaft though, ballancer would be to large for the center hole.

[Scotth]

You are right,everything is off center a tad.I'm going to start over and try to be a little more precise when assembling the blades.