[peteeng]

Hi All - There is some discussion about welding vs bolting vs bonding so I thought I'd put some numbers on paper. I've kept it simple. If we take a 50mm (2") wide mild steel bar 6mm (1/4") thick and weld it what do the other connections have to look like to support the same tensile load? Interesting thought. So attached is the calcs, but the summary is the welded joint yields at 75kN so this is the target for the others. If we look up the shear strength of class 10.9 bolts an M12 has a shear strength in shank of 74kN, a 5/8" grade 8 has 84kN. So a single snug fit bolt is same strength as the weld.

If we try to do this with a friction fit we need 7xM12 Class10.9 bolts to hold the 75kN. This is why aircraft use snug bolt or rivet fits to minimise the number required. Plus its why old riveted structures used so many as they were very low strength rivets and they used conservative approaches to the design.

Bonded connection. If we use a toughened epoxy or a urethane adhesive we can get 30MPa shear strength glue. There are 40MPa adhesives around as well. 30Mpa gives us a 50mm wide connection for the same strength as the weld. This is why aircraft have steadily moved into bonded connections to minimise the number of holes to make, ream and fit fasteners to. Now machine structures are stiffness driven not strength driven so it would seem that relatively small bonding areas or joint geometry is required to achieve adequate strength connections. Plus a bonded joint is visco elastic so is damper then a welded joint.

Obviously we can't butt joint an adhesive yet (we probably can with a big fillet) but if we revisit how riveted structures are made out of angles and flats we can bond these easily and use a few bolts for alignment and fixturing while the bondo sets. Then there is no heat and it will be the same geometry as fixtured when cured. Cheers Peter S

https://www.prosetepoxy.com/standard...bly-adhesives/ 30MPa epoxy to steel

[mactec54]

Hi All - There is some discussion about welding vs bolting vs bonding so I thought I'd put some numbers on paper. I've kept it simple. If we take a 50mm (2") wide mild steel bar 6mm (1/4") thick and weld it what do the other connections have to look like to support the same tensile load? Interesting thought. So attached is the calcs, but the summary is the welded joint yields at 75kN so this is the target for the others. If we look up the shear strength of class 10.9 bolts an M12 has a shear strength in shank of 74kN, a 5/8" grade 8 has 84kN. So a single snug fit bolt is same strength as the weld.

If we try to do this with a friction fit we need 7xM12 Class10.9 bolts to hold the 75kN. This is why aircraft use snug bolt or rivet fits to minimise the number required. Plus its why old riveted structures used so many as they were very low strength rivets and they used conservative approaches to the design.

Bonded connection. If we use a toughened epoxy or a urethane adhesive we can get 30MPa shear strength glue. There are 40MPa adhesives around as well. 30Mpa gives us a 50mm wide connection for the same strength as the weld. This is why aircraft have steadily moved into bonded connections to minimise the number of holes to make, ream and fit fasteners to. Now machine structures are stiffness driven not strength driven so it would seem that relatively small bonding areas or joint geometry is required to achieve adequate strength connections. Plus a bonded joint is visco elastic so is damper then a welded joint.

Obviously we can't butt joint an adhesive yet (we probably can with a big fillet) but if we revisit how riveted structures are made out of angles and flats we can bond these easily and use a few bolts for alignment and fixturing while the bondo sets. Then there is no heat and it will be the same geometry as fixtured when cured. Cheers Peter S

https://www.prosetepoxy.com/standard...bly-adhesives/ 30MPa epoxy to steel

No stress from not welding this is great, but how do you stress relieve the steel parts once it is all glued and bolted together, all the steel parts have stress from it's manufacture, just gluing and bolting a frame together does not get rid of this built in stress, so when you machine the linear rail and ballscrew mounting points the steel glued and bolted parts will move all over the place