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Forum Index : Windmills : Prop Balancing - String Method
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Gill Senior Member  Joined: 11/11/2006 Location: AustraliaPosts: 669 |
I have found this method most satisfactory. It gives a high degree of accuracy for the static balancing of a prop. Here is an action shot with no movement evident. 
With this method the prop is a beam balance and the string entry to the plug is the fulcrum. A little balance theory now: There are 3 basic actions (movements) of the beam about the fulcrum. These are influenced by the centre of gravity in relation to the fulcrum. The first action is Accelerating. This is bought on by the centre of gravity being above the fulcrum. When balanced, if the beam is deflected from balance, it will continue to move at an accelerating rate to the limit of it's travel. The second action is Neutral. This is bought on by the centre of gravity being on the fulcrum. If deflected from balance, the beam will remain in that position. The third and the one we use is Vibrating. This is where the centre of gravity is below the fulcrum. If deflected from balance the beam will return to the balance state. As the centre of gravity is moved from just under the fulcrum to a much lower point, the beam goes from being Sensitive to Dead. This means that more weight is required to bring about the same change in balance. Here is the stub off an F&P shaft I made to plug the hole and provide a fulcrum with a centre of gravity adjustment. The main thing to note if you make your own, is that for consistency, the thread must be firm and the string must be tight through the string hole. 
string hole (I have two. one each end for different size strings) 
knot hole 
Method: Pick a dead calm day or go indoors where no draughts. Fit the plug to the prop. Lower the C of G (centre of gravity) for a dead action. Add weight for a horizontal balance. Raise the C of G and adjust with weight as required. Repeat this step until a very sensitive action is achieved. Never fear, most will go to far to a neutral or accelerating action and get odd and confusing readings. Just go back a little. Well that's how I do mine. Hope you have fun. 
was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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martinjsto Senior Member Joined: 09/10/2007 Location: AustraliaPosts: 149 |
nice job on those blades gill, they look like timber, very neat hub to are they the plasmer cut ones? i used the same method of ballencing my blades, and also after doing it your way then set them up on a vertical spigot and slowly rotated and added small weights untill they didnot stop at the same position ie no backwards rotation. i found using both methods gave me a near perfect ballance, i only needed about 10 grams of counterballance weights on 2 of the props. great forum gill thanks for your efforts martin free power for all McAlinden WA |
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Tinker Guru  Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Hi Gill, Let me see if I got that 
1. what you call the 'fulcrum' is the top end of the threaded rod, where the string exits? 2. the balancing 'action' positions are adjusted by screwing the threaded rod in and out (up or down actually) on the splined stub? 3. your propellor hangs backside up and the washers (or lack of them) are your balancing weights? Did I get it? 
Very clever, your method that is. Klaus Klaus |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Gill, Well done! Thanks. Showing how one can get a centred string for the system is also a great help. Regards, Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
Thanks all for the response. Martin, Blades are the old fibreglass Chinese type, modified to my own dimensions but similar to the mod described in this site. The prop hub has been posted here before but here is a link on to how to make one. PropHub Tinker, Your spot on working out what I didn't say. It's a YES to each question though the backside facing upward is because the CofG is closer to that face on this prop. If the blades protruded forward and the C of G was to be near or even further forward than the front of the hub, then that face would need to be uppermost. In such cases the threaded rod can protrude past the splined stub to position the fulcrum in correct relation to the C of G even if the C of G appears to be in no-mans-land. was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5116 |
Hey Gill thats very good. Can I add this story and photos to the web site as a way to ballance a turbine. I dont have a lot of info about balancing, and this story would help other readers. Is there anything else you would like to add? Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
Go ahead Glenn, my pleasure. I've done a little edit as one line didn't read quite right. Also added a little note on plug tollerances, so right to go. By all means edit as you see necessary. was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5116 |
Thanks, will put it together this weekend and let you know. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5116 |
Hey Gill and Klaus. I've put together a page about the balancing. I did start by re-writing it, but the discussion between you guys was spot on, easy to understand, so I've just cut and paste with a little formating. The address is http://www.thebackshed.com/windmill/articles/turbinebalance. asp There is no link from the main site yet. Let me know if your happy with the page and I'll add a link from the projects and info page. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
G'day Glenn, Great work as usual. Well presented. Makes me quietly proud to have helped the site. Thanks was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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vasi Guru Joined: 23/03/2007 Location: RomaniaPosts: 1697 |
Nah, no good! Too simple... You need a special alien device to make this right 
I like your method Gill. No more special workbench and no ladder if propeller diameter is too big . Hobbit name: Togo Toadfoot of Frogmorton Elvish name: Mablung Miriel Beyound Arduino Lang |
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