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January 1999

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Subject:
From:
KELLY M SCHRIVER <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 7 Jan 1999 15:14:00 -0500
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Hi Todd -

Most of the engineering data for threaded fastener torque values deals with
joining two or more plies of metal, which is for the most part considered
incompressible under these loads.  The principle of torquing is to stress the
fastener material to about 60% of  the yield point of the fastener metal
involved.

When a composite material, such as a circuit card, comes into play, the torque
values are no longer applicable or appropriate.  First, the composite will
generally change form and size (thickness) under sustained load, resulting in
a loose fastener.  The composite, may also undergo compressive fracture, if
enough force is applied over a small enough area.

Split washers and the like, are generally dependent on being able to penetrate
the surfaces being joined, slightly.  They are also not always shakeproof and
may loosen in vibration, such as shipment.

A more reliable solution may be to use either: a locking nut; a screw with
preapplied locking compound or nylon insert; or use a thread sealant at time
of assembly.  Dependent on design, you may also want to consider a screw with
a dual lead, or deformed thread.

Good luck - Kelly

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