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Forum Index : Windmills : Balancing Turbines?
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| RossW Guru  Joined: 25/02/2006 Location: AustraliaPosts: 495 |
OK, dumb questions time :) I'm in a location where the quality of wind is a complete unknown. I've been monitoring wind run, direction, gust etc, and in the end decided to just bite the bullet and get a cheap turbine and see what happens. I have the thing up and running. The hub and blades weigh about 30Kg, and the initial check I did on the ground indicated it was "close". I just presumed (*bzzzt, wrong*) that being a commercial unit, it'd be ok. Once I got it up and waited the obligatory 3-4 days for some wind, it was painfully obvious that it's out of balance, somewhere, because it shakes like a bitch. Worse at some speeds than others. It's one of the cheap imports - made to a price - but for a trial it'll do the job. 8m mast of reasonably heavy 5" pipe, 4 guys. The top guy is about level with the bottom of swing of the blades (it's a 3m dia swept area with a 1000/1500W 3-phase machine). Anyhow, yesterday arvo I got a dozen 125mm U-bolts and cut a dozen 350mm "rungs" of 40x40 RHS, drilled 2 holes through them and have put a set of steps up the mast so I can get to it quickly. I brought it to a halt this morning and climbed up, and it appears that I have one light blade (or two heavy ones!). For testing purposes, I've estimated the imbalance and using some wide "hundred-mile-an-hour" tape, stuck a 2" x 1" lump of 1.6mm thick gal on the outer end and back side of the "light" blade, just as a test. It's better, but still not right. Is there an easy way to balance a turbine? I considered getting an old harmonic balancer from a car and bolting that on the front, but haven't tried it yet. Any guidance from the "old hands" would be appreciated. |
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| peter Newbie  Joined: 15/01/2006 Location: AustraliaPosts: 25 |
Hi I make blades with each having the same weight and the same CoG it works for me. peterc
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
I'm sure it'd work for me too, if I were making it from scratch. But I'm not - I have the thing already. Assembled, in the air, working. |
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Bryan1 Guru Joined: 22/02/2006 Location: AustraliaPosts: 1344 |
On the limited amount of blades and turbines I made so far the way I balance a blades is clamp some bar stock the same diameter of the bore on the blade holder boss so it's a neat sliding fit. Then simply turn the blades and mark where it stops, repeat a few times recording the stop position and the 6 o'clock position is the heavy end so apply weight to the top. Repeat until the blade stops in any position and you know it's balanced. It's easy and once you've done it kiss goodbye to those vibrations.
Cheers Bryan |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Yeah, that's what I'm trying to do - but in-situ. I don't mind working in the air, but it's hard to find times when it's dead calm. Once upon a time, the tyre shops used to have the old static balancing (bullseye-bubble) where the wheel sat horizontal on a cone and was balanced that way. Great for larger wheels like tractors. Alas, I've called every local company and nobody still has one in working condition. My next thought was a 2" steel ball, sit the hub over one on the workshop floor (which is level to within 2mm) and balance it that way, but alas, I've just moved house and can't find my large ball-bearing. Ideally, I'd like to dynamically balance it because I'm not certain the blades are perfectly in line either. Looking at the movement of the mast and guys, it's not *totally* in the plane of the blades, and if I very carefully touch the back of the blades while it's spinning, I can hear "tap, tap, thump, tap, tap, thump" as the blades go past - so one of them is at least a few millimeters off-plane. Am I being too fussy? How much vibration does one expect from a 3m dia prop? (I calculate a little over 200 KMH tip speed at nominal full RPM) RossW |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hi Ross. Nah, your not too fussy, blade balance is important. You just need to find the point that is good enough without tearing your hair out. My main windmill has a set of timber blades, with a diameter of about 2.1 meters. I set it up in the workshop and balanced it the same way as Bryan suggested. Its close, but not perfect. In low winds, just before cut in, the tower gets a wobble. Its not a bad wobble, and would not cause any structual problems, and once it speeds up the wobble is gone, so I'm not going to worry about it. Your idea of using a large steel ball is another proven method. You could also match each blade individually before mounting on the hub. When I used to play around with Datsuns ( Had a Datsun 1600 with a L18 motor and TO4 turbo, what a weapon ), I would balance each con rod by matching the ends. I set up a pivot on the little end of each rod, then weigh the big end on a set of electronic scales, and grind them down so all the big ends were within a gram of each other. Then go through the same process for the little ends. You could try this method on your blades, then mount them on the hub and do a normal balance. It would take some time, but you'll get close to a perfect balance. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548 |
Hi Ross, I just balanced my set of new fibre injected blades yesterday- pic attached- Important point --remove the rotor as the magnets will stop the mill from turning freely and you'll never balance it ...IMHO. I added small nuts to the light arms til I got it stopping in "any" position . Takes a while ,but see if you can get it down onto a wall of your house or workshop on a dummy shaft , that makes it easier,or do as I did in the pic -lower the tower and prop the shaft so it's horizontal
But remember, you want dead still conditions--this mill is so light and so well balanced ,just a very light breeze got it spinning . Bruce
Bushboy |
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| adam666 Newbie Joined: 01/02/2006 Location: New ZealandPosts: 14 |
*noobie advice here* I was planning on balancing using 'mag wheel' weights, the stick-on type. come in std 5,10,15,20.25g sizes, and a VERY sticky :) will test and advise cheers Adam "to capitalize on windy wgtn" -excuse the pun :p |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Word of caution Adam (from one who used to calibrate wheel balancers etc for a living, and has balanced rather a lot of rims and tyres!) Those stick-on weights are designed to be used on the rim, where centrifugal force pushes them out *ONTO* the rim. The adhesive is only there to stop them moving or falling off while stopped. Sticking them on the turbine will have the weight trying to fly off sideways - ie, the adhesive pad will be in shear force, not compression as it was designed for. I have no doubt they will last a little while, but they'll eventually fail for sure. (Meanwhile, I'm still playing around - I used some N45 magnets to prove the point and am now "refining" how much weight and exactly where I need it. Seems a static balance got the blades in perfect balance but I can now see fore-and-aft forces at speed, so suspect there's an imbalance in a different plane as well - and that might be fairly difficult to find/fix!) |
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| adam666 Newbie Joined: 01/02/2006 Location: New ZealandPosts: 14 |
oh cool! thanks for that! I have space to add bolt on weights so will maybe take that route then Cheers Adam "to capitalize on windy wgtn" -excuse the pun :p |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
If anyone is interested in this still.... I finally have a day with reasonable weather, where I'm at home, and not already over-committed with other projects! I dropped the mast and with a tape measure, spun the prop and measured from blade tip to ground. One blade was nearly 30mm off-axis! I've removed all blades, cleaned and re-seated them in the hub, re-tightened and re-checked. Had to put a small shim under one of them but they're all within a few mm now. The rule of thumb that says no wind for 4 days seems to have taken a leave of absence - I'd barely even got the mast back up before the wind started - I was still tensioning up the guy wires! It still has a little bit of a shake, but hopefully a static balance will fix that. (I pulled off the weights I had on before since the operating parameters are all different now). When I have half an hour of calm, I'll climb up and try it, but already the shake is noticably less. |
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