'pre-skewing' their stator to help reduce the chance of contact

[dlenox]

All,

The other week when I dis-assembled my 17' turbine I noticed a slight rub on the front of the stator from the outside rotor which was caused by a loosening of the bearings due to wear. Luckily it did not do any damage, but mearly polished the outside layer of cloth.

This rub was not at the bottom of the stator where you would think it would be due to the loosening of the bearings, but it was at the 3 o'clock position. This makes sense when you think about it, my turbine rotates CCW (looking at the front of it), and the wind causes lift and pushes at the 3 o'clock position trying to essentially skew it.

This time when I aligned the stator I took this skewing into consideration and rather than having the stator perfectly square, I mounted it so that it was pre-skewed in this same direction. This way when the bearings loosen up and the wind rotates the turbine, it should help reduce the chance of contact.

If the turbine rotated in a CW direction I would have therefore expected to see this rub on the stator at the 9 0'clock position.

Has anyone else 'pre-skewed' their stator to help reduce the chance of contact?

Dan Lenox

[Flux]

If you got rubbing at that point then doing as you suggest may give you a little more time before you run into trouble but I really wouldn't want to make too many guesses as to what is going to happen with slack bearings.

There are so many forces acting on the thing and their magnitude changes so much with wind direction, yaw direction and rate of yaw that anything could happen. If your site is one where you get good clean wind then the thrust that causes furling may be the main force. If turbulent and the thing yaws about then I would expect the gyro force to dominate and I would not expect the rub to be at 3 oclock.

If you have taper roller bearings check them after a few weeks and adjust and you will probably never need to adjust again, things do take some time to seat and settle.

Flux

[Ungrounded Lightning Rod]

Sounds like he needs a slightly larger gap, too.

Raising cutin a bit and losing a little power in low winds (and maybe gaining some in moderate winds to higher RPM and better air attachment) might be a small price to pay for avoiding damage and shutdown.

[dlenox]

All,

Currently now I have an air gap of 1.125", previous gap was about 1.050".  So this iteration I added about .060" more air gap. I am a bit hesitant too add too much air gap with the amount of wind that we seem to get occasionally at my site, and taking a cautious approach.

Also previously I used 'typical' wheel bearing grease and this time pure white lithium grease. Apparently even with keeping the turbing in stall and low rpm's still took a toal on sloppy bearings after 1yr operation.

Still playing...

Dan

[Flux]

Not sure how much clearance you have between magnets and stator. I have only used commercial hubs for small machines ( up to 8ft). When I have built my own hubs I keep the bearings a lot farther apart than a typical car type hub so the play in the bearings has much less effect. For easy starting the bearings seem to have to run without pre-load so with a fairly large diameter magnet rotor and a 17 ft prop on the front a tiny bit of slack in the bearings probably does give quite a bit of movement at magnet radius.

I have never played with clever grease, I have never had any bearing trouble on any machine except when water has got into a bearing. Any slack bearing I have encountered has shown signs of rust. If moisture is getting in then higher spec grease may help, otherwise I suspect it will make no difference.

I haven't built anything as big as 17ft and my biggest is a radial construction but that has very little clearance between magnets and stator and has given no trouble. Bearing clearance would certainly be more of an issue with axial construction. I suppose only time and experience will tell you how much clearance you need long term.

Flux

[fabricator]

When I get mine flying I intend to use the grease seal and use Mobillith AW-O, the viscosity is kinda like molasses, it's made for low speed applications, it is between grease and gear oil.

[dlenox]

Flux,

My stator measures .800" thickness, so that gives me about .150" gap on each side.

Dan

[dlenox]

Fabricator,

Yep I too am using the rear grease seal, this time trying pure white lithium grease.

Last weekend I assembled the turbine, and after spinning it with my lathe for a while noticed that I munged up the seal when putting it on. My rear rotor appears to have fused itself onto the hub. This weekend I tried a different approach and put the seal on the shaft first, then put on the hub/rotor.

Once on I managed to use some big screwdrivers to push the shaft back into the hub. The hub/rotor assembly are pretty heavy and difficult to control during assembly. Would have been easier if I could have put the hub back on separately from the rotor but this technique worked out ok.

Dan

[fabricator]

I read so much about not using the seal to aid in low wind startup, but I cant imagine this very small amount of friction making much difference on a large machine like this.

[Dave B]

Hello,

 I Just wanted to chime in here also. After tearing my 16' down for a rebuild for battery charging I noticed considerable play in my hub bearing. Plenty of air gap so no stator or rotor damage. To back up a bit here when installing the (6000 lb rated axle) spindle and roller bearings previous I had removed the tension spring over the rubber of the grease seal (it's hard to see if you didn't know it was there) and also purposely wore down the inner edge of the seal just a bit to reduce the drag but still installed it hoping to help keep water out. Also, just premium wheel bearing grease was used to properly pack the bearings.

 I forgot to take photos but the inner bearing was a red rusty mess as well as the grease. It had enough drag at times that it actually had spun on the spindle. The outer bearing faired much better but both went into the metal scrap bucket along with both races (cones) and grease seal.

 I had suspected a problem with reduced performance maybe a couple weeks prior to bringing it down, obviously none too soon and right on the annual planned date.

 It is all back together again with new races, bearings and grease seal that I have left as is. It seems like considerable drag when hand turning the rotor but my blades and hub are already balanced and indexed from before so I won't have to try to balance these with the additional drag. I already like the idea of that grease seal in there as it normally would be and for kicks I used Mobil 1 synthetic grease to see what that looks like next time down for inspection (1 year planned again)

  Lots of improvements and modifications and soon to go back up with the new purpose of charging a 24 V battery bank. I'll load the 3 phase AC with water heating elements until I can purchase the batteries, it will be a different animal again and should be interesting.  Dave B.

[dlenox]

Dave,

Thanks for your input. Do you have any specs on stator thickness and air gap to share with us?

My hub is rated about the same as yours, but I wish that they made one with a larger front bearing as these big machines have a lot of weight to them. As long as water stays out it seems like once broken in the front bearing takes most of the abuse.

I used Timken #204508 rear seal (2.000" ID, 3.062" OD, .250" wide) which is the standard part number, it does not have the spring on it and adds negligible friction. When I took my turbine apart it looked as if only a slight amount of water droplets got in but did not do any real damage.

This weekend I finished up making a fiberglass tail for my machine. Paul Schreiner from PS Composites helped as he has all the equipment to do vacume bagging. We used 2 sheets of masonite waxed and PVA'ed to sandwich the layup.

The new tail consists of glass cloth, glass matt, 1/8" foam core, glass matt and glass cloth. It's very strong but I think that if I were to make it over again I would add one layer of polyester cloth to each side to make it even stiffer.

The new tail vane weighs in at about 11lbs and is 1/4" thick, while the 3/8" plywood was about 30lbs, so it should help a lot with my furling issues. Worst case is that I always can easily add weight.

Hoping to get it back up and flying soon I miss not seeing it up on the tower.

Dan Lenox

[ghurd]

I have an idea not related to direction of rotation.

The wind is blowing the off set back, trying to skew the axle to an angle to the wind.

The prop seeking force is trying to aim the blades into the wind.

The rub point would be at 3:00.

It makes sense in my head.

G-

[dlenox]

ghurd,

ok except my urbine is offset to the left and blades turning CCW both looking from the front - so it is pulling turbine forward, not pushing back

Dan

[Dave B]

Hi Dan,

  My stator is approx. 5/8" thick. I had run 3/16 - 1/4" gap from the magnet surface to the stator on both rotors previously when I finally got things running well as a water heater. My magnets are 1 x 2 x 3/4 (20x2) and now with my new stator it's going back up with the same gap, I had back figured from the previous voltage measurements to design my 24 v stator and it tests near right on for what I wanted.

 I like the larger air gap and after tearing it down I won't go any closer. Sure I'm missing out on some power by not squeezing in the largest wire and the fewest turns with a close gap but after reading about crashes, rubs and burn-ups it's nice to know that with a bit of expected wear I won't be grinding things up.

 Like you say, our blades and hub are quite heavy (I think I'm at about 65-70 lbs for my Ash blades and hub plates alone). Like you, I can't wait to get this back up and flying, I miss it too.  Dave B.

[ghurd]

You did know this would result in a sketch?  LOL

For this, the direction of rotation does not matter.

If the offset was set to the right, then the rub would be at 9:00.

The wind is pushing back on the stator / axle / yaw / etc.  The tail keeps it facing the wind.

The prop's wind seeking force is pulling the prop to be parallel with the wind.

The 9:00 area is being pulled apart.

The 3:00 area is being squeezed together.

Not sure if it is correct, but it still makes sense to me.

G-

Did not show the rear magnet disk.

[dlenox]

Glen,

Great explaination - thanks for the clarification.

Dan

[dlenox]

Dave,

Thanks for the info! What about offset from center of yaw bearing? Mine is about 8-3/4".

So it would seem that our magnet-to-magnet (airgap) is almost exactly the same, minor difference in stator thickness.  My previous gap was 1.070" and this iteration I increased it to 1.125".

Am hoping that this will help to minimize my previous stall situation, as well as lightening the tail to help with furling.

Dan Lenox

[dlenox]

Dave,

I just double checked and the magnets I am using are 1-1/2" x 3" x 3/4" N42 so that is the biggest difference in our turbines.

I expect that I could use bigger air gap, and new gap should increase blade speed by about 10%, but I think that I will try it before going larger - as this will give me some good information on which to base this decision.

Dan Lenox

[dlenox]

oops - not to mention 24v VS 48v stators...

My sassafrass blades came in at just over 20lbs, plus about 50 for the 1/4" retaining plates on both sides of blades.

Dan

[SparWeb]

Ghurd,

His yaw mount is built like a dump truck.  It looks too stiff to bend this much.

Using the yaw moment loads from Bergey's Excel (NREL test report TP-500-38550) to guess at the loads on Dan's, I'm going to assume about 10,000 inch-pounds moment (833 foot-pounds) have been applied either in a rapid yaw or furling event.  With a 2" spindle (at the root) that is 6" long, then the tip of the spindle will bend only very slightly.  The clearance between rotor and stator will change by less than 1/64".

Both yaw and furling pivot around the tower's axis.  Since this is a big gyroscope, the force that acts on the hub is 90 degrees to the applied force.  That means the action is to nod up or down, when the force is left-or-right.  The stator would have scuffing at 12 o'clock or 6.

The windmill is free to yaw left or right, so vertical gusts do not apply much moment to the spindle before the windmill turns left or right, relieving the load.

Dan, I still don't know what could have happened to cause that scuffing.  Didn't you say your windmill has over-sped and produced a lot of current?  Is there any sign of heat damage in the stator?  When the wire heats up the epoxy can flex, too.  More like sag, actually.  In the way you mounted the stator, is there a chance that the stator mounting bolts push the stator to bend?  Though when cold the stator is stiff and the bend is negligible, when the stator is hot perhaps the pre-load can make the stator flex more.

Just ideas off the top of my head, because I don't believe it's a gyroscopic load problem.

[Dave B]

Dan,

  Previously I was running 18' and had the over speed and blade crash scenario. My offset is still at 9" for my 16' blades. I reduced the furling tilt angle and lopped off a foot of blade. My boom length is now 8' and I'm not sure of the total weight with the vane. My vane is aluminum sheet folded over on the edges for added stiffness with a bracket up the middle similar to the Dan's. No wheres near as heavy as plywood which helped dial in my furling a bit easier. This was for direct heating and now I know I will have a different machine and if anyhting I'm thinking it may furl earlier. Adding weight will be much easier than the major surgery before. Hope this helps, how long before yours goes back up ?  Dave B.

[Dave B]

I don't think he's talking about bending but rather bearing wear and then seeing the movement to one specific side. Maybe I'm wrong but what ever the case I say open the gap and maybe set the maintenance check for 6 months. It's a serious fix blowing out a stator but we see it here all the time. Be a rebel, change things. An ounce of prevention ...      Dave B.

[Ungrounded Lightning Rod]

OK, let's try this:

 - Weight of the blades is windward of the bearing's center of support.

 - That causes gravity to try to pull the rotor down to face the earth.

 - This force on the rotor causes it to precess as a gyroscope.

 - Rotor rotates CCW as viewed from the front.

 - Precession is caused by the rotor "carrying" the torque force through 90 degrees as it becomes translated to rotary motion.

 - So the right hand side tries to move back (as viewed from the front) and you get your rub at the front at 3:00.  (If the stator were a tad farther back you'd get it on the rear at 9:00.)

Does this make sense to the rest of you?

[ghurd]

Correct Dave.

I had a great explanation written, if I do say so myself...

"Vista" decided to edit it.

Then this is what I ended up with.

It has nothing to do with any rotational forces.

Assume very loose bearings.

Total forces of the prop try to push everything that yaws back around the tower.

Wind seeking force of the prop tries to keep the blades facing the wind.

The wind is pushing everything back around the tower.

The seeking force is pulling the blades into the wind.

They clash.

Combined forces make...

The front magnet disk hit the front of the stator at the side near the tower (3:00).

OR

The rear magnet disk hit the rear of the stator at the side away from the tower (9:00).

The prop is pushing everything back, but it is pushing on one side harder than the other.

Static 2-D example.

Loosen the bearings.  A lot.

Push back on the blade mount with 500 pounds of force (total wind force).

Continue that amount of force, while twisting (seeking force) the 3:00 side toward the stator (tower) with 100 pounds of pull toward the wind, and the 9:00 side away from the stator (tower) with 100 pounds away from the wind.

G-

Edit to my post with the sketch:

Change Parallel to Perpendicular.

[dlenox]

Dave,

Just finishing up the vacume bagged tail vane, after that it depends on free time and weather, and it can be tough to coordinate the two.

As you know I have to climb the tower and do all my work at the top.

The turbine has been down for 2 months now - after getting it off the tower my focus was on getting the solar tracker made and putting 720 watts of panels up. That got completed a short time ago.

Right now weather forcast is for more wind and rain so hopefully I can get it up in a couple of weeks.

Dan

[SparWeb]

Does this make sense to the rest of you?

Yes it does, and you've brought to my attention something that I think I've been missing, too...