Global Fastener News

November 28, 1994 FIN – Jim Taylor, vice president of Elmhurst, Illinois-based Semblex Corporation, told the Southeastern Fastener Association fall meeting in Charlotte, NC, that in comparison to machine screws and thread cutting screws in a pilot hole, the advantages of thread-rolling include the largest shear area, greater resistance to vibration loosening, stronger mating threads with uninterrupted grain flow; extra strength from work-hardening method of forming; stronger fastening; and 65% engagement.
A machine screw has 75% thread engagement; the tapped pilot hole with cut threads shows weakening by interrupted grain flow; chamber depth and chip space are needed in a tapped hole; the area is subject to shearing; and tolerances required on the male and female threads contribute to looseness; Taylor said.

The thread-cutting screw requires a longer point; the tapped hole with cut threads show weakening by interrupted grain flow; the unsymmetrical point contributes to difficult starting and re-use; and champfer depth and chip space are needed in the tapped hole.

Taylor’s list of the most frequently asked questions and his answers:

• What is the unique shape of the thread-rolling screw?
Trilobular.

• What are the benefits of this shape?
(A) Will produce higher percentage thread engagement;
(B) Produces prevailing torque, which helps prevent screw loosening;
(C) Produces resistance to loosening caused by vibration in assemblies.

• What are the advantages of “thread-forming” with thread-rolling screws?
(A) Cold-formed exactly fir internal threads;
(B) Internal threads are formed to tap dimensions;
(C) Formed threads are stronger due to the “work-hardening” effect and uninterrupted grain flow;
(D) No chips, no burrs.

• Why do thread-rolling screws provide stronger assemblies?
(A) High resistance to stripping
(B) Tensile strength of 135,000 PSI;
(C) Increased thread depth allows higher tensile loading of the assembly.

• What advantages do the thread-rolling screws bring to the production line?
(A) Low driving torque;
(B) Excellent drive-to-strip ratio to aid in power driving of fasteners;
(C) Overwhelmingly reduces fastener installation costs;
(D) No chips, no cleaning.
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