Engineering question : Max torque of bolt into 10mm 5083 aluminium plate?

@bluerm166 - I have ordered a small piece of 10mm plate of the same grade to do some testing with and see how much torque it will take.. Should be interesting to see how far off the calculator is.. Of course I will be adding Loctite to the thread so this will probably act as a lubricant on the initial tightening which may reduce the breaking torque seen on the wrench but still be squeezing just as tight..
 
A small update..

I bought a small piece of 5083 Ali plate and drilled and tapped 3 M8 holes into it.. I took thee stainless bolts and using the plate that I will be using on the tank I bolted it through and used a torque wrench to tighten it to see when it would break.. Started at 15 ft-lb, then 20 ft-lb, then 25 ft-lb, then 30 ft-lb.. It still hadn't self destructed.. At 30 ft-lb it was a little stiff to pull the bolt out so it was probably at the limit but still this is way higher than I was expecting it to take.. So based on this I figure I will tighten to about 15-20ft-lb which should be plenty to make a good seal..
 
prof pat pending said:
That sounds very high for the given material...was the head of the bolt bottoming on the same bit of plate that the thread was in?
Click to expand...

I thought it was high too but tested three holes and three bolts and all were the same.. Only possibility is if the test plate is not 5083 but it was specifically purchased and marked as such..

The threaded plate was behind a 10mm plate with clearance holes.. So the threaded plate was being pulled up against another 10mm plate with a 9mm clearance hole through it.. On one of the bolts I put a piece of rubber between the plates and got the same results..
 
Interesting topic as tank and pipework leaks are a pain. Just a couple of thoughts which I'll throw in for comment.
As you've said top and bottom plates I'm guessing the hatch is in the top of the tank. Because there will be a tendency to leak along the threads as they are parallel not taper threads, and with 38 bolts, the likelihood is that even with a thread sealant some will leak. At least if it's a top hatch, it'll just show some dampness.
As it's a 5mm? thick plastic tank, it's worth being generous with the the radius of the corners cut in the plastic to try to reduce the tendency for the plastic cracking due to fatigue from the weight of the double lid.
Regarding the bolt torque. As it's a "free" rubber gasket, i.e. not constrained in a groove, it will tend to squidge depending on the softness of the rubber. Bolt torque values are largely based on metal to metal assembles with the object of achieving an allowably high percentage of tensile strength. With this rubber interposed, I think I'd try decently hand tight (say 2 inch spanner lever), then 1/4 turn evenly opposite all round, then depending on feel, perhaps repeat. Then test for leaks. Too much torque could be self-defeating.
On industrial pipework etc flanges, fibre gaskets tend to be used...walkers etc...unless they are designed for constrained rubber seals.

Comments and criticisms appreciated!
 
Fitted the hatch to the tank today.. Here are some photos..

http://1drv.ms/SGeOEC

Was far more time consuming than I thought and had to go around multiple times tightening up the bolts.. Ended up torquing all to 20ft-lb because this actually felt quite tight with a spanner and didn't see a need for any more..

I attempted to block the filler, vent and take off and pump air into the tank to see if it held the pressure but couldn't get a good seal on the vent hose so gave up.. Going to just fill the tank and see what happens.. Worst case I have to drain it down again, best case it works and we can get back on the water.. :)
 
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