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Forum Index : Windmills : Jacobs var. pitch-can we do it?
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| domwild Guru  Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Dreaming: Now that we have pipe mounting is the following a feasibility? The Jacobs variable pitch system relies on the centrifugal force to pull a spring-loaded prop radially away from the centre BUT a pin slides along a curved spline in the prop holder and turns the prop. Fine pitch at low RPM, coarse at high RPM. A more elegant system than the one with weights to turn the prop. Cutting that groove (spline) would require a CNC three-axis milling machine and may not be worthwhile for a single F&P. A kind of MPPT - not that topic again! 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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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
I dont think you really, really need a CNC three-axis milling machine to cut a slot like that, but it might be quite a challenge to make two or three that are the same! 
I would start by making a milling spindle for my lathe, I dont have one so that would be a good incentive to make one. The spindle would mount on the lathe cross slide. Then I would rig up a suitable gear reduction between the main spindle of the lathe and the feed screw. Lets say the feed screw is 8 turns per inch pitch and we want 90 degrees of rotation for each inch of travel on the pitch changing mechanism. That means we want 8 turns of the feed screw for 90 degrees on the spindle, i.e. 32:1. That should not be too hard to do. You could not use the lathe motor for what would be a very slow speed instead I would put a handle on the end of the leadscrew. I would mount the piece I want to cut the spline in in the lathe chuck and start the milling spindle. I would use the cross slide to bring the spinning cutter (slot drill?) in contact with the work and slowly turn the feed screw to make the cut. The cross slide would be advanced for the next cut, and so on. If you wanted to do it, I am sure it could be done.  |
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MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
domwild I was coached by several people not to waste my time on a variable-pitch blade system, but I had to find out the hard way (did it anyway) and I'm here to tell you, "Don't waste your time!" It's a fun project, but so far as I can tell, Marcello Jacobs' design is the only one that worked and stood the test of time. His mills were run in the Antarctic under 24/7 severe conditions and I think one of his contraptions was still spinning something like 50 years after it was left there. If you do wind up making a feathering device, the trick is to make it work THE SAME on each blade at the same time. If you don't, it creates an imbalance and the whole thing will shake apart. Since that's what you're trying to avoid in the first place, it doesn't make much sense introducing not one but three new variables into the system, with the liklihood of all three working synergistically almost a sure failure. The bottom line here is this: There is not much gained by feathering your blades over what can be done using a furling tail or tip spoilers, so spending a lot of time and energy on making something fancy is just a fun project. I'm not one to take advise well, so I tried several designs, but in the end, wound up building a furling tail! . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Yeah I tend to agree with Mac, steer away from a viable pitch if you can find a better way to control the mill. Its something I havn't built, but if I did, I would use a system where the blade angle is controled by a shaft through the middle of the alternator. Pushing the shaft back and forward would adjust the pitch, and I would do this with some sort of mechanish that senses wind speed, like a paddle. I would imagine the ideal setup would sit the blades at 45 degrees or so for easy startup, then bring the blades closer to the ideal angle in normal wind conditions. But then in stronger winds you want the blades to start tilting back again to stop it overspeeding, and in excessive winds turn the blades to 90 degreees to protect the turbine. Could be done with a paddle and some cleverly though out levers. But no easy task. Hey John to make the spline could you use a length of square solid and heat/twist it into shape? Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Hi Glenn I guess you could twist a length of square though it might be a bit tricky to incorporate into the machine. I have often wondered if there is a passive aerodynamic way of doing all this. Some sort of aerofoil that would be effective at only one speed and would go into a high speed stall when the wind got up too high. BTW, there was an early, European origin I think, type of variable pitch propellor for aircraft which had a smaller diameter, (servo?) airscrew on the end of the prop shaft. It was entirely automatic and did not require pilot control. It was just a little thing which looked a bit like a 'pin wheel' or maybe a distorted lemon squeezer (yea, sounds weird). I have searched but not been able to find any reference to it online. |
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Raptor50 Newbie  Joined: 17/08/2011 Location: SwedenPosts: 8 |
So this thing is'nt worth spening some time on? I wish the day had more hours.. |
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Rastus Guru Joined: 29/10/2010 Location: AustraliaPosts: 301 |
Hi Raptor50, It looks like you have the time and Mac suggested to go for it if you have the inclination to.Cheers Rastus see Rastus graduate advise generously |
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| Raptor50 Newbie Joined: 17/08/2011 Location: SwedenPosts: 8 |
Oops.. Should have said in the first message perhaps, i don't have it. Just the picture. But i saved the picture becouse i was looking for something like this. But if it's not worth to try to get it, i'll start looking for some other soulution. I wish the day had more hours.. |
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