why my bands always came loose

I've been fighting a long time with my egg bands loosening and misalignment issues especially after I do a high temp cook. Today I think I have finally figured it out.

 I replaced my bolts a year ago with some from a local hardware store, because I had an over bite and remembered I over torqued on my original assembly as I got some info wrong on the correct torque. Also I saw the post of the guy who lost his lid off the balcony. That day I re-aligned my dome and put in the new bolts and torqued to 125-130 lb.in.

Over time I steadily developed an over bite which I would try to correct without a full disassemble/reassemble. Recently I finally got to the point of air leakage. It was time to completely readjust my bands again. That and my hinge was rubbing on my table as the lower band had visibly slid down. Thinking I will have to get used to the fact my bands will need constant maintenance I ordered a dozen gr.5 5/16" carriage bolts from the same store as previous. This time the bolt heads were marked differently. They had the traditional 3 lines to indicate grade 5. The ones in my egg had A307. I did a quick search and found that A307 is not an equivalent gr.5 (which I was told the first time) and has a yield (these are approximate not exact to every bolt) of 60ksi and a proof load (the point at which the bolt will start to deform but spring back) of about 40ksi. Gr.5 has a yield of about 110ksi and a proof of 80ksi.

40 ksi or 40,000 lbs/sq. in sounds like a lot so I kinda thought this wasn't my issue and they were plenty strong until I remembered how the bolts bend when torqued, so I did a few quick calculations (its winter I was bored).

The cross-sectional area of a 5/16-18 bolt is .0524sq.in. At 130 in.lb of torque there is about 2150lbs of tensile force on the bolts. Using a proof load of 40ksi and multiplying by the area (.0524) you get 2096lbs. This is the point the bolt will deform. So applying 2150lbs will result in deformation.

The same formula applied to the gr.5 bolt results in a proof load of 4192lbs double that of the A307, and no where near the elastic deformation point.

With this knowledge I performed a test. I removed only one bolt (bottom band) and replaced with the grade 5 and torqued. The bolt did deform, but no where near what the A307 did. Why did it deform when it shouldn't have? I believe its because the band tabs are not 180 degree surfaces, so the load is applied off center on the bolt, and forces are much greater on the one side of the bolt. How much more exactly? I don't know, but now I believe its to the point of bringing the A307 into the yield zone (the point at which any stretching will not "spring back".

To verify my findings I did a couple of cooks and cycled the egg. One 350 degree cook for an hour and another high temp 600 degree run for an hour. After each heat cycle I checked the bolts with my torque wrench. The gr.5 stayed tight, but the A307 needed an extra half turn each time to regain the proper torque back.

My conclusion is that the A307 or Gr.2 bolt is in the yield zone and any extra load applied by heating/cooling of the egg stretches the bolt further, but it will not bounce back. If I keep re-torqueing eventually it will neck and break. If I don't re-torque, the band will loosen and I will develop some misalignment. The gr.5 bolt does however stretch slightly but it returns or springs back as it is not into the yield zone yet, but it is proofing.

The importance of this all??? make sure your hardware store gives you the correct bolts, and doesn't just give you the "this is good enough" line. Also why not avoid this all and just go gr.8? well if there is no proofing of the bolt (elastic stretch) it can loosen up quite easily, or if you get brave and crank it down until it does stretch, how much extra tension do you now have on your ceramic?? Will it handle it? maybe but I don't want to be the one experimenting.

If you read all this, you must be as sick of winter as I am