PV pole rack movement and wind

jtdiesel65 · March 2008

Hi,

I built my own pole rack for 8 170W panels. It's a 6" steel pole set in concrete about 7 feet deep and extends above ground about another 6 feet. On top I fixed a 4" square tube steel using two pieces of 4x6" angle iron. This forms a "tee". Two 1/2 bolts bolt the angle irons to the pole and two 1/2 bolts bolt each angle iron to the square stock. At each end, on top of the square stock I use another 4x6" angle that extends upward to bolt on a frame made from 3" "C" channel iron. These angle iron pieces are bolted to the square stock and the "C" channel using 1/2" bolts. The panels are bolted to these frame members and the frame members are bolted together using 3/8" bolts. The frame is adjustable tilt with angle iron that extends from the "c" channel to another set of 4x6" angle bolted to the square stock.

What I am seeing is a little bit of movement, like a wobble, during high blustery wind. The whole rack and panel surface seems to move as a single unit. I think there is a little bit of rotation in the 4" square stock plus some movement due to the long length of the frame "C" channels. Visually this is amplified by the size of the array. A little movement in the center looks like alot of movement at the edges. There is no side to side movement.

I'm wondering if you see this type of movement in commercial pole racks?

If you grab the edge of the array, is there any visible wobble or movement?

autoxsteve · March 2008

Re: PV pole rack movement and wind

Can you post a picture? I am having trouble understanding the attachment of thw 4" square tube and angles to the pole...

From how I see it, your 4" x 6" angle iron pieces attach to the pole using two bolts each (vertically I presume). I suspect this can create some side to side wobble in the array.

Also from the description, the entire array is cantilevered off the Tee? This is providing a greater moment arm than if the array was centered (vertically) on the Tee.

Ralph Day · March 2008

Re: PV pole rack movement and wind

Autosteve,
you did better than me on imagining the setup described. True, it sounds like something i'd build with my welder and chop saw but easier to do than to pick off of a description.

JT, no insult intended, i have a heck of a time describing anything i've built without a picture to show. My rack has 4 180Sharp panels, the rack is an old General Instruments satellite tracker. During windy times i check the inclination adjustment (a threaded rod with nuts on either side of the terminus), sometimes one will work loose. Where the actuator arm attaches (on the 2x3foot square of 2x3inch box tubining) a nylock nut worked loose as well. THis accounted for wobble i noticed this winter in the winds. Both tied down now, but in heavy winds i still see some movement...but not at night. The lesson being, if it bothers me...i don't watch:p

My wind turbine tower also has flexibility in it during high winds, but it also is not nearly so noticable at night...same lesson as above.

Note: the movable rack is manually adjusted, not actively tracked, and there are 8 ground mount fixed panels (seasonally adjusted) as well. If ever they wiggle i'm headed for the basement!

Ralph

jtdiesel65 · March 2008

Re: PV pole rack movement and wind

Thanks for the replies.

Well, here goes..hope this works.

I went out there and watched it while the wind was raging. I think what's happened is that the holes in the pole have "wallowed" out some, so I get some movement from that. Add to this also is a little movement in the angle irons. I think I need to weld the angles to the pole and angles to the square stock. The square stock should probably have caps welded on too.

autoxsteve · March 2008

Re: PV pole rack movement and wind

Ding, ding, ding we have a winner.

The entire assembly can pivot around those two 1/2" bolts mounted vertically. Welding those brackets and the 4" square steel tube to the pole wil help immensely.

Also, if you simply put one bolt behind and forward of the two presently mounted (so the bolts create a diamond) that would help a lot too. you'd have to create some angled spacers to do that but it could be done relatively easily. (in fact, you might even be able to locate some spacers off the shelf at a good hardware store).

Also, some guy wires from the ends of the array back to the pivot point (square tube) also might help..... Of course the guy wires woudl have to be adjusted when you change the angle of the panel array...

mike95490 · March 2008

Re: PV pole rack movement and wind

jtdiesel65 wrote: »

The square stock should probably have caps welded on too.

1) nice photos. I think the 6" pipe is the right size, that's what other mounts use.

2) square stock is not nearly as strong as round. And if you put caps on the ends, how do you hold the nuts for the bolts ?

3) the arms off the "T" seem so short, they may have a lot of leveraged load on them, maybe if they were longer, they could resist more ? Welds still have a heck of a load to hold.

jtdiesel65 · March 2008

Re: PV pole rack movement and wind

I figured the nuts won't matter because I'll crank them tight a zap a weld to lock them for good and then weld the angles to the square stock anyway.

I think there is just a bit of play in the bolt holes that adds up. I'm going to weld it and see how it does. I may need guy wires too.

Seems like commercial mounts would have some movement too. The array is so big, it seems like there would be a little bending in the racking as wind gusts hit the array.

mike95490 · March 2008

Re: PV pole rack movement and wind

jtdiesel65 wrote: »

nuts won't matter because I'll crank them tight a zap a weld

Just don't use the welder to "zap" something expensive / electronic

PV pole rack movement and wind

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