# Bearing strength of square tubing

I'm designing a single axis PV solar tracker. Square tubing will be the structural member of the axis. U-bolted cross members will support the solar panels. Each section of square tubing will be 10'-0" long between supports. Four cross members and four solar panels will attach to each 10' section. The total weight on each 10' section will be about 175# plus the weight of the square tubing. The apparatus will rotate about the central axis of the square tubing, so the weight will be borne upon the tubing in a number of orientations. The four bearing points of the cross members will be 12" from each end and 12" on either side of center, and each point will bear the same weight ~44#. Can anyone help me calculate the size and gauge of the square tubing in steel or aluminum. The cross pieces will be aluminum and the u-bolts will be stainless, so aluminum tubing seems preferable. I hope the square tubing can be either 1.5" OD or at most, 2.0" OD. Can someone help me figure the size and thickness of tubing needed?

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nvn
Homework Helper
mmoreland: Each of the perpendicular aluminum cross members, and solar panels, is symmetric about the main square tube longitudinal axis, correct? And it sounds like the main square tube will be at an arbitrary angle to the ground. Or is the main square tube at a known fixed angle to the ground?

The main square tube will be level, or parallel to the ground. It will rotate through about 60 degrees around its long axis as if it were a long square axle with bearings on either end. mm

Hello NVN, I suspect I didn't make it any clearer with that last reply. The main square tube will be oriented east-west, be level, and be supported at each end. There will be a number of such square tubes each connected end to end through bearing supports, and they will all rotate together. One long rotating tube, perhaps 63' long altogether, oriented e-w and level. The cross members will all be attached so as to create a plane-like surface on one side of the long square tube, and the solar panels will be affixed to the cross pieces so that they will form a plane surface. Does that help form a picture? mm

nvn
Homework Helper
Is each segment of the main square tube cut, at the roller bearing supports? Or is the main square tube actually an uncut, continuous square tube, passing through the bearing supports? Regardless, you could design it as a simply-supported beam, if you wish.

Do you mean the main square tube can rotate +/-30 deg about its longitudinal axis, from the horizontal position? Or do you mean +/-60 deg? Does your 196 N weight per cross member connection point include the effect of wind? What is the maximum wind gust velocity? Would that be a hurricane (such as v = 50 m/s)? You could go ahead and compute the cross member weight, and add it to the 196 N solar panel weight. The density of aluminum 6061-T6 is rho = 2715 kg/m^3.

Could you give us the dimensions and spacing of your solar panels? And the distance of the main square tube from the ground?

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Hello nvn, Thank you for your careful questioning. The square tube will rotate as a unit, but it is not continuous. It will be bolted to each bearing location. First the dimensions: The entire array will be 61.4" wide and approximately 63' long, and it will consist of 24 pv solar panels oriented facing south, the axis of the tube being east-west. They will rotate about the long axis from 16 deg from horizontal to 73 deg, so the 60 deg rotation was somewhat overstated. It will be able to be placed in a horizontal position for ground maintenance under the array. The axis will be approximately 42" above ground level. In it's steepest position (73 deg), the top of the panels will be slightly less than 72" above the ground, and the bottom edge somewhat more than 12" above. The center of gravity of the panels and attachments will not coincide with the center of the long axis, but will be offset above the axis approximately 4".

I can't be very accurate regarding wind speed, but certainly wind is a concern. Wind is from the NW or SW and is usually less than 10 m/s, but I have witnessed winds as high as 27 m/s. There will be a solid fence 5'-0" tall next to and on the north side of the array. At a distance of 30' there are low buildings and low foliage to the south of the array. We are in an area surrounded by redwood and fir trees, and the array will be in an open space within it, but surrounded at a distance of several hundred feet by tall trees.

Michael Moreland
Mendocino, CA

Hello nvn, I failed to include spacing of the solar panels. Each panel is 61.4" x 31.5". They will be placed in pairs in landscape orientation with about 0.5" gap between each panel and between each pair. I'm working on a configuration that will allow them to be mounted in portrait orientation, but it requires an additional layer of support members. Either orientation will result in a total array of ~62" wide and ~63' long. mm