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Old 05-09-2002, 02:26 PM
JimSmith JimSmith is offline
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Join Date: Jul 2001
Location: Woolwich, Maine
Posts: 3,598
Greg,

Torque to yield is a commercial practice that does leave you with a fastener that is not truly reuseable, or has very limited number of re-use cycles that are determined based on some precise length and/or diameter measurements. Most bolt materials have pretty good elasticity, as measured in the reduction in cross sectional area when you yield them until they break under tension. This characteristic means when the bolt yields slightly you get a large measure of the yield strength holding the joint closed, but the material properties of the bolt can become a little uncertain, as well as its geometry. In my experience we like to stay at 80% of yield. This leads to designs with longer fasteners as you noted to achieve a practical assembly. Longer is better as it reduces the effects of non-parallelism between bearing surfaces and perpendicularity of the tapped hole to the bearing surface. But it makes for more expensive fasteners.

Jim
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