I am building an Auto Rotisserie to cook the under coating off a the bottom of the body. Don’t want it dripping on me. All joking aside. Plan on 2”x2” by 1/8 tube steel. I need to know if that will be strong enough to hold a 70 body. Thoughts? Anyone build one?

 

I tend to over-engineer everything but 1/8" steel sounds awful thin. I would be using at least 3/16"

 

Sounds a little light to me also. If you dig around or call a local fabrication shop you can find "Load Tables" they'll give you acceptable load limits on what type of steel your using size and acceptable length it will hold the load at. Or find an engineer at your local bar.

 

After doing some more looking 3/16 is the min.


Thanks.

 

A 2x2 square tube with 1/8" walls made from even the crappiest steel you can get will support 35,000 lbs in shear and over 50,000 lbs in compression. The two issues you will have are deflection, bearing loads from any bolts, and weld tear-out.

The deflection may or may not be an issue depending on the design of the rotisserie and where there are gussets or other features.

The bolt bearing is a different issue. Depending on the bolt size you pick, you may have a problem. If you are copying the popular design were the adapters that bolt to the body have adjustability relative to the pivot head that spins, then you'll have bolts that go through the upright and pivot head. A 3/4" bolt will only have about 3000 lbs of bearing load capability in a 1/8" thick tube. That's only one side, so the capability would be double IF the parts were perfectly machined and the loads balanced. Typically they are not, and add to that the fact that there is no safety margin on these numbers. Typically you'd want a safety factor of 3:1, and preferably 5:1, so from that standpoint it's woefully underdesigned. This suggests that even 3/16" might be inadequate for areas where there are through bolts.

Weld tearout is another issue and is much more difficult to analyze. For this reason alone, I'd also recommend 3/16" wall on the main tubes.

 

Well stated, Joe.

I would add that the size of the tubing matters greatly, it's not a 1:1 ratio.

2x as large is NOT 2x as strong.... more like 4x or 8x depending on what strength we are talking about. So, if you happen by the local scrap yard, and 2x2 isn't available but 3x3 is, then go for it.

At high load points like the anti rotation pin you may wish to enhance the thickness with welded on patches. Ditto for corners and joints.

I have an engine stand with a pinch joint for anti-rotation, and I like it A LOT- it's adjustable from a slight drag to impossible to turn, and every shade of gray in between. I used 3x3 tube for that I think, because it was handy.