You have been warned.
Armed with some information (the welded joints need to withstand 500 foot-lb of torque) I can now design a test. It may be destructive, but hopefully not
Destructive or not, I need to know.
I have measured torque with a home-built rig in the past, so I have a starting point. Here's what I think I need:
1 - a sturdy wall, that can accommodate connection to the aluminum tube and joint to be tested
2 - a hydraulic jack, to apply the steady pressure to be tested
3 - a scale, to measure the applied pressure
4 - a short section of rebar or something else handy to extend the hydraulic jack up to the joint being tested
The idea is to take a single joint between 2 tubes, fix one tube to the wall at an appropriate height, and orient it to allow testing of the joint at the end of a measured lever. Since the scale only works in one direction, up or positive in the Z direction, the tubes and joint must be moved and re-attached to test a different joint direction. The first test is to bend forward, followed by backward - these use the same mounting holes on the wall. Bend to the passenger side, then to the driver's side requires horizontal mounting of the tube and new holes in the wall. The last test re-uses the first set of mounting holds. Plus an extra piece of aluminum bolted in place to twist the joint at the specified 500 foot-lbs.
Instead of cutting another piece of tube, I connected two of my existing 60 inch lengths of tube with a bolted connection as described in previous updates and videos.
For your reference - Bolt together demo - YouTube Explanation of bolt together frame idea - YouTube
The bolted joint is 3/8 to 1/2 inch from the end of the tube. The contact point for the hydraulic jack is about 36 inches from the center of the bolt, at 36.5 inches from the joint end of the piece. This gives me approx 36 inches or 3 times the weight shown by the bathroom scale.
The analog bathroom scale that I had used for my previous work was apparently recycled through the local Salvation Army. I guess I'll have to invest $20 and get another one. Perhaps I can find a more rugged version at a thrift store? The bed on that scale flexed so I had used a 2 x 8 on top of it to prevent the bed from bending.
The local Value Village comes through with an analog bathroom scale for $6. This looks like the one that I was searching for in my garage .. and could not find. So I am familiar with the ways around the flexing of the bed. Onward!
The 60 inch aluminum tube that has the joint bolted to it has 2 extra holes drilled close to the ends (3/4 and 7/8, since I was not paying attention!). I had planned to use the walls of my garage as the mount points for the test. My wife requested that I 'dig holes' in one of our shed walls instead. I had not really thought of that, but I promised to try. The shed is much smaller in weight and built of smaller dimensional lumber - 2 x 4 instead of 2 x 6 - so I hope the wall does not flex and mess up my test.
I started out using 2 lag bolts 3.5 inch x 5/16 to hold the aluminum tube to my shed wall. The placement of the holes did not allow use of a socket/ratchet combination or my cordless drill. Using a wrench to screw the lag bolts in was very time consuming. And if things go OK than I will be faced with doing this many times ... time for something different. I used a 2 x 4 bolted to the aluminum, and lag bolts through the 2 x 4 to the shed wall. I'm not sure that it will hold, but it's a WHOLE lot easier to put on. If it breaks, I'll come up with something better.
The first position is on the corner of the shed.
Now for some overkill. I am targeting a test of 150 lbs on my scale, at a distance of approximately 4 feet, giving me approx 600 foot-lbs of torque. With this value I am still confident that the joint has withstood 500 foot-lbs. Even if all of my errors add in the same direction. And even if Murphy's Law is followed so that the errors add in the worst possible way. The test value of 150 lbs is also close (2/3) to the calibration weight(me) with my other bathroom scale, so I am cautiously confident that the scale reading is reasonably accurate.
The analog bathroom scale was rough-calibrated with my digital bathroom scale using my weight (223.8 on my digital scale, with my safety gear on, versus 221 read on the analog scale). The analog bathroom scale is within 2 lbs of my digital scale - close enough for me. The hydraulic jack is weighed (25 lbs without the handle) and the scale is reset to 0 with it still on the platform.
Refer to the pictures - 612a tube bolted to a 2 x 4, 2 x 4 lag bolted to the wall, joint sitting low, tube extending 5 feet from the wall. 613a - jack beneath, sitting on bathroom scale. Bathroom scale leveled and zeroed. The top of the jack is a 1.75 inch circle. Distance from the center of the bolt on the bolted connection to the closest contact point of the hydraulic jack is 36 inches.
Set up the video to capture anything exciting.
The way that the shed is set, and the small deck that is attached, makes 3 feet much easier to implement than 4 feet. So I'll need to use 200 lbs at 3 feet instead of the 150 lbs at 4 feet that I listed above.
The pressure is slowly increased - slowly does not really do justice to the pumping that is done to extend the hydraulic jack. Somewhere around 7 pumps per 5 lbs pressure. At 50 lbs, I notice that the joint is no longer square but do not dig into it. After raising the pressure to 66.6 lbs (200 lbs torque), the joint deforms enough to have the weight on the scale drop off. There is some close-up video of the joint. More close-ups to show where it deformed - the middle wall of the tube mounted on the wall, where the bolts go through. The shed did not move. The lag bolts held, at least for the limited torque exerted so far. With this failure, I see no reason to proceed with the other directions of the test.
After unloading the joint, the square shows that most of the movement was permanent. 5 mm was measured as permanent. there may have been an additional 1mm that was elastic.
Video - (long and boring, good resolution so it takes a while to download!) Update Saskatoon Sep 23, 2013 - YouTube
What to do from here?
Since the deformation was in the middle of the tube wall, I guess I should avoid that area for bolting in the future. With the 2 inch tube that I am using for Weston`s truck (his parents cautioned me that a car was not `cool`. All of the toys for little boys are trucks, or tractors, or quads. So the model SGT01 frame will be modified slightly to look more like a truck) .. where was I? Oh - I don`t think that the 2 inch tube (1.75 inch inside dimension) has the room for 4 of the 3/8 bolts side by side. 4 * 3/8 or 0.375 = 1.5 inches ... which leaves a total of 0.25 inches clearance between the bolts. That divides down to 0.083 inches (1/12 of an inch) per hole drilled for alignment issues, not drilled plumb and square, marking accuracy ... significantly tighter than my present construction methods will tolerate!
Perhaps I can use two bolts (instead of the one I used in this test) to connect each of the 1.75 inch tubes to the main tube, and a single bolt (instead of the 2 I used in this test) to connect the second tube at the joint? That should minimize any distortion on the side wall of the main tube, as happened in this test.
What about the 1.25 inch tube being used for the motor module? 2 bolts from 1 direction and a third from the other direction adds up (3 * 3/8) to more than the 1 inch size. Perhaps a single bolt could be used with a washer to reinforce the wall of the aluminum tube?
Looking ahead to the 4 inch tube, moving the bolts as far as possible to the edge of the tube wall, and using 2 bolts per 3.75 inch tube (for a total of 6) should again minimize the problem with distortion on the unsupported edge of the tube. Will it be enough? Who knows? We`ll see!
For the near future, after I get the changes implemented on the 2 inch tube and they successfully pass the 500 foot-lb test, the 4 inch tube will be next! it may take a couple of tries to get the 2 inch joint to pass the test. That's one of the reasons why I'm testing on smaller and easier to handle joints.
Whew! If you're still reading, I'm impressed!