On Sun, 03 Feb 2008 22:42:47 -0500, Ned Simmons wrote:

On Sun, 3 Feb 2008 17:22:03 -0800 (PST), "
wrote:

whats better for strength? i have four fasteners to choose from:

8-32

10-24

10-32

1/4"-20

If diameter is fixed, whats stronger, more or less threads?

The load will be shear, i.e., it will try to shear the bolts off,
putting stress on them in that direction.

If the thread is in the shear plane, the screw with the largest root
diameter under the thread will be strongest. For a given OD, a finer
thread will have a larger root diameter. If the shear is carried by the
unthreaded portion of the screw, the thread pitch won't matter.

If the fasteners are torqued tightly enough that load is carried by
friction in the joint rather than shear, the fine thread again has the
advantage because its greater root diameter makes the bolt stronger in
tension, allowing it to apply more force to the joint.

Among your choices, the 1/4-20 is strongest; a 1/4-28 would be stronger
still.

Assuming that you have enough depth in the aluminum to fully engage the
threads, and that you're using a decent tap.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
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