The bolt-together joint was built.
As described in the previous post, the new pieces were aligned on new pieces of 2 inch tube. 4 holes were drilled through into the 2 inch tube and bolts were installed, threads facing out. There was room to install lock washers this time.
The second 2 inch tube was pounded into place (still a very tight fit). A single hole was drilled through and a 3/8 by 2.5 inch bolt put through to secure the tube in place. I may not be getting better at this, but I'm getting faster!
The tube was mounted on the shed (not quite vertical, as it turns out ... but close). The same equipment was used to test:
- 2 x 6 base
- analog bathroom scale
- 2 x 4 on the scale to prevent denting the deck
- 20 ton hydraulic jack
- square to check deflection of the test joint
- caliper to measure deflection
I took video. The first is a summary
http://youtu.be/gsqUvlhQ-R0
The next 4 are exhaustive video so that someone could reproduce the tests if they were so inclined. I don't recommend watching it if you are easily bored. 4 videos so that it fits into the Youtube maximums.
http://youtu.be/TSndXAVtQr0
http://youtu.be/Aj0R2x9KMus
http://youtu.be/XAzm_fGinrg
http://youtu.be/q2EblcLIDsY
The tests show that the tubing I am using - 2 inch square 6061-T6 aluminum - will not withstand the required 500 foot-lbs of torque. The joint flexes out of square almost immediately (30 lbs force) but the flex or deformation accelerates above 300 lbs force.
The failure, or deformation, occurs in the 1/8 inch wall of the tube that is bolted to the wall. That is the main tube of the car frame, the part that everything else is attached to.
The tube wall thickness could be increased, but increasing the wall thickness of this tube would substantially increase the weight of the frame.
That portion of the tube wall could be reinforced with angle or a welded element that is annealed in an oven to restore it's temper. That requires a skilled aluminum welder and an oven with a control system to restore the aluminum tempering. And the additional reinforcing would block access to the tube, where access is required for bolts, wiring, etc.
For Weston's vehicle, I expect that this frame will be sufficient for a few reasons:
- Weston weighs 30 lbs, and I am designing for his uncle to ride with him in a modified seat, behind Weston and sitting atop the 'motor module'. Total weight 300 lbs. Note that maximum speed will be GREATLY reduced wit this much weight on the vehicle!
- Each joint can withstand 300 lbs. The weight in each axis will be distributed across at least 4 joints on the front and 4 joints on the back
- The failure of the joints, when they deform permanently, is based on the imposed force. And it is not a catastrophic failure. The 90 degree joint was tested to 300 lbs of force. I was expecting the Aluminum to 'tear' but I did not reach that point. At the speed that this vehicle will be limited to, the tires will go far out of alignment but the structure will not fall to the ground at a dangerous speed.
- There will be a full roll cage for Weston (not for an adult riding behind him, though). I have not sourced a 5 point harness or a seat to accommodate that as yet.
- I still have the option of adding another 1/8 inch of aluminum reinforcing on the inside of the 2 inch tube, for a bit of extra design factor
- That's enough design factor for me until I can do more testing!