balancing blades-Is this an option

[Reno]

Ok so the balancing the blades by suspending them at their center with a wire didn't or doesn't work for me. So sitting there and thinking, ouch, I started pondering ceiling fans. To balance one you mount it turn it on and then look for wobble and weight where required. So using this method I mounted my blades on a DC PM motor sitting on its end on a bench bolted to a 2X6 clamped standing on end. Connected 12V to the motor and checked for wobble. It seems stable, there was a slight vibration but it was barely noticeable. I am going to wire for 24V to give the blades a faster spin if they pass at 24V I am going to assume the balance is good.

Anyone one out there see something I am missing I really don't want these things 35 ft in the air based on flawed thinking.

thanks for any comments.

[DanB]

What sort of alternator are you hooking them to reno?

I prefer to balance them on the machine - often times my alternators are not balanced, so I balance the whole deal in one shot and it seems to workout fine most of the time (except when we try to do it on a windy day).  It's easy that way...  you let it go, the heavy side goes down.  Then...  pick the heavy side up 90 deg, and put weight on the opposite side till it's balanced.  I keep repeating that untill it has no preferance.  

This only works well on a very free spinning alternator, any drag in the bearings - or cogging, and I think there would be problems.

[Reno]

It is going on an Ac conv.

Your method sounds good even though with an AC conv there should be decent balance other that the magnets glued in. I will keep this in mid as i wan to build an alt from scratch. Does the method I tried make sense I wll also do what you do when I mount it to the alt.

thanks

[DanB]

a conversion might cog too much to get things really well balanced on the shaft.  in that case - Id probably try to either hang the blade by the center with a string (Ive done that and it should work) or balance the blade on a 'point' at the center.  I think your right, a motor conversion should be reasonably well balanced.  But if it's got any issue with cogging I doubt youll be able to balance the blades well by doing it on the shaft.

[Flux]

Reno

Your method ought to work as long as your mount on the temporary motor is exactly at the same point as the final position on the alternator, and Dan has a good point about the alternator being perfectly balanced.

Hanging it on a bit of string is not very sensitive unless the string is fixed near the centre of the blade, not at the front edge. Also it is only as good as the accuracy with which you can repeat the positioning on the alternator, 1/8" error in positioning will put you miles out.

If your alternator is free, use Dan's method as a check , but if you are sure the alternator is balanced and you position your test motor accurately, the rotating method should be ideal if the windage will let you take it up to a reasonable speed.

[Flux]

Reno

With the motor conversion the alternator balance should be ok, but I doubt if it will be free enough to use it for balancing the prop.

Here is another way that you may like to try as a check. Find a piece of bright bar that fits the hub properly about 2ft long.  Support the end near the prop on a piece of angle or similar, support the outside end with a similar angle clamped over the bar on blocks so that the thing can roll freely on the top of one support and on the bottom of the other.

Check that your support angles are level with a spirit level and you should have a mount that is about as sensitive as you can get.

If you have a hole through the prop it is even better as you can support both ends of the bar without having to invert one of the supports.

Also check that the tips of your blades track properly as they rotate past a fixed pointer.

good luck

Flux

[BrianK]

You might try finding one of those old bubble balencers for car tires

   

[Reno]

Thanks guys

I took the DC motor and blades and hung them over the deck and did the spin and balance trick that Dan suggests. After some testing I got the blades so they never end up in the same position. What I did was I put duct tape around the other two blades backwards so the sticky side was out. I then stuck 1/4 bolts to the tape and retested. the funny thing is the heaviest part was actually between two blades. I ended up with 3 bolts on one blade and 1 bolt on the other. the original blade that ended up at the bottom on the origial test got no bolts. I then weighed the bolts and tape now off to find some thin lead to make some weights. I am going to counter weight the tips since this will require far less material. the couter weights were

11 grams and 4.7 grams if anyone is interested.

thanks again

[windstuffnow]

   I tend to go the long route when balancing although Dan's method works fairly well.

   I start out by weighing each blade to find the heavy one.  Then weigh each end of the blade ( root and tip ) and record it on paper.  This gives you an amount of weight difference as well as were the mass is located.  If there isn't much weight difference between them such as less than an ounce then you can locate small lead weights and put them in the wood where the blade balances ( root and tip weight are the same ).  Once all is in proportion you can assemble the blades on the hub.  If your using a wood hub then it might be necessary to balance the assembly but it shouldn't be off enough to cause any real problems but its good to check.

   I know, it seems a bit tedious and redundant but I hate to find out its going to vibrate badly when its up on the tower in a 40mph wind when its basically to late to do anything about it except watch it fall appart...

Have Fun

Windstuff Ed

[DanB]

I used to do that Ed... weighing each blade, and finding its center of gravity.

Being the sort of person who does precision machine work on my metal lathe with a tape measure... it seemes a bit tedious ;-)

I often now balance blades on the end of the tower right before we tilt it up.  It usually goes fine... unless its breezy that day - then there's a bit more luck involved.  Seems to me though - if its still outside, the whole operation should take about 10 min.

[cevonk]

Probably pointing out the obvious.....(attempt at humor follows)

Use dynamic balancing.  Just pop the thing up there and let run up to speed in about a 70 mile an hour wind.  It'll balance itself dynamically.  Probably end up with three very SHORT blades.....  (end of attempt at humor)

A co worker used to fly copters in the early days of their use by the US Army.  Those blades had little aluminum trim-tabs on them that had to be bent up or down to properly balance the lift on the blades.  They would go out there with the chopper running and hold up a broom (bristle end up) until it just touched the lower running blade at that trim tab to adjust it.  This always sounded to me like it would have the opposite effect of what they wanted, but he swore that's how they did it.

Perhaps such tabs could be used if the lift on your blades does not match.  It wouldn't help with the weight issue, but it might help if blades had differing lift characteristics.

Eric von Kleist

Spartanburg, South Carolina

[cevonk]

No, wait, now I remember how he said they did it.  They didn't adjust the trim tabs with the broom.  They dipped the tip of the broom in white paint and held it up beneath the rotor to find out WHICH blade was dipping lower...THEN they adjusted the trim tabs manually.

[Ungrounded Lightning Rod]

Here's another approach, if you have a garage (or other sheltered structure) bigger than the circumference of the blade.

• Get a couple of light padded clamps suitable for grabbing onto the blades.
  
• hang the alternator/blade combo by a foot or so of rope facing down like a ceiling fan.
  
• Point a fan up at it to power it.
  
• Give it a push to get it started.  (The fan should keep it going.)
  
• If it isn't balanced it will wobble.  Then:
  
• Put the clip on the trailing edge of one of each blade in turn, near and at the same distance from the hub.  Note which blade it's on when you get the least vibration.
  
• Put the clip on the blade where the vibration was the least.  Try various positions in and out until the vibration is minimized.
  
• If the vibration is still non-zero, leave the clip on and repeat the process with the other clip:
  
• If there are a multiple of four blades try it just on the two at right angles to the blade with the first clip.  (Its in-and-out adjustment should not interact with the first clip and should bring the system to balance.)
  
• If there are some other number try it on the two just a little more than a right angle from the one with the clip.  (Its in-and-out adjustment will also require a smaller compensating adjustment of the first clip to maintain the first dimension of balance.)
  
• (If it still doesn't completely balance you also have a dynamic balance problem.  You can null this out by adjusting the tilt of the blade with respect to the axis, or by a more complicated procedure involving pairs of weights on opposite sides of the blades.)
  
  

Once you have it balanced you can replace the clips with weights at a convenient distance from the hub.  The weights should be at the same angle from the center of the hub as the ciips were, and weigh the weight of the clip times the ratio of the radius of the clip location to the radius of the weight location.