Prototype Frame Assembly: Part 3

Well I ended up taking down the upper frame. It proved too expensive to source a chevron mounted bearing, after Fastenal failed to deliver on the one in their catalog, which would have been ideal. So I drilled some new holes in the frame rail and started to use a hole saw to expand the hole around the shaft, to suit a different bearing.

New bearing mount holes to the sides of the shaft, and my damage to the rail using a hole saw.

I found out 2 things from this: using a hole saw without a solid pilot hole (I used a chunk of wood for the pilot) to guide it doesn’t work well. And it is DEFINITELY loud enough to wake a sleeping toddler. I could clamp some aluminum behind it to provide a better pilot but to save time, and be lazy, I am going to leave the hole as it is and just space out the bearing. Also I think I would need to scrounge up a better hole saw. The blade I was using had some serious wobble and has little left for teeth.

I then reassembled the upper frame and did a little bounce testing.

From this testing I found that the lower frame isn’t quite as stiff as I would like. The back right hand corner will lift a little (about 1/8″, maybe less) when you really bounce your weight away from the wall. It doesn’t feel flimsy or like it’s going to tip but it is a little off-putting when you can see and hear it lift. With the planks on I doubt you will be able to see it, their weight should hold it down more, but I would still like to do something to stiffen up the frame. The wall frame feels super stiff.

Since only the one side lifts I am inclined to think that the diagonal strap on in the lower frame (the one that runs along the floor) is keeping the other corner from lifting. If this is the case it will be easy enough to stiffen things by adding an additional diagonal strap, making the lower frame symmetric. It could also be that the one brace running diagonally from the top to the bottom shown below has a stiffening, bent flange, while the other does not. This can be checked by simply swapping the braces and seeing if it has any effect.

(Note: I swapped the braces after writing this post and it did change things a bit but the difference seems to come primarily from the lower strap.)

The lower frame may benefit from adding a second cross brace to the bottom, making the stuff on the floor symmetric.

I will run some additional FEA and verify that this change will improve things.


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