Rolled threads have improved physical characteristics, greater accuracy and a high degree of surface finish. They are uniformly produced at high rates of production with no wasting of material. The following major advantages account for the increased use of thread rolling.
The cold forging that threads receive during the rolling process strengthens them in tension, shear, and fatigue.
The cold working of the surface increases the tensile strength of the metal worked, and static tensile test have frequently recorded increases on the order of 10% in the breaking strength of the parts.
When a thread is rolled the fibers of the material are not severed as they are in other methods of screw thread production, as shown in Fig. 3, but are re-formed in continuous unbroken lines following the contours of the threads, as in any good forging as shown in Fig. 4. Rolled threads resist the stripping because shear failures must take place across rather than with the grain.
Thread rolling increases the part’s resistance to fatigue failure in several different ways. Rolling between smooth dies leaves the thread with smooth burnished roots and flanks, free from tears, chatter or cutter marks that can serve as focal points of stress and, therefore, starting points for fatigue failures.
Rolling also leaves the surface layers of the thread, particularly those in the roots, stressed in the compression. These compressive stresses must be overcome before the tensile stresses can be built up, which alone, can cause fatigue failures. This increase in root hardness, up to 30%, adds considerably to the parts resistance to fatigue.
It has been repeatedly demonstrated that any fastener that is properly tightened when it is installed, and remains tight throughout its life, is less likely to fail by fatigue than one that is assembled loosely, or that becomes loosed in service.
Threads produced by any of the cutting methods have a surface condition consisting of partly torn-away particles that gradually bear down in service permitting the fastener to loosen. Rolled threads, on the contrary, which are compacted and burnished during threading, are less prone to loosen, and, thus ordinarily have longer fatigue lives.
Rolled threads show no loss of fatigue strength when heated for several hours to temperatures up to 500° Fahrenheit; whereas, fatigue strengths of threads produced by other means are lowered by as much as 25% by the same treatment.
Improved fatigue strength, resulting from all the above factors, is reported to be on the order of 50-75%. On heattreated bolts from Rockwell C36 to 40 hardness, that have the threads rolled after heat-treatment, tests show increased fatigue strength of 5 to 10 times that of cut threads.
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