screw torque for thin material 2

[vbk]

Hello,

I'm specifying torque values for screws on a steel assembly (using standard torque tables). For some, i reduced the torque values where the screws are large relative to the materials being clamped & the holes threaded, e.g., when a 3/4" bolt is clamping a 1/4" thick steel plate to a 1/4" steel plate with a threaded hole, i reduce the torque spec. Also, when the stretch length of the fastener is very short relative to the material thickness, is there a general need to use a higher torque value (assuming a hex nut is used)? I think using thick washers can help but excepting for that, is torquing closer to yield of the bolt generally more important for clamping of thin materials?

 

[MintJulep]

Sometimes using a larger number of smaller diameter fasteners is a really good idea. This might be one of those times.

 

[vbk]

thanks. Yes, that's a theory i often use when fastening fairly thin flanges with large moment loads. The situation here is that some of this was designed by another engineer & now it's left to me to recommend torques on the screws.

 

[3DDave]

Use a low torque and thread locking compound.

 

[MintJulep]

If we guess that your 3/4" bolt is UNC then in 1/4" material you would be lucky to have 2 good threads tapped. Not very much. The biggest risk would be stripping of the threads in the plate.

Longer grip length --> greater stretch for the same tension. Greater stretch helps reduce loss of preload due to embeddment.

The load path of the joint and the loads that the joint needs to resist are much more important than the thickness of the materials in the joint.

 

[vbk]

Thanks for all the help. I misspoke about the threaded hole being in 1/4" thick steel, it's in 1/2" so not great but better. Today I torqued these to about 200 ft*lbs. I have them in small patterns of 4 screws loaded in shear with about 7500 lbs of shear force on each group of 4.