When vibration is a factor, which screw thread series is used?

When vibration is a factor, which screw thread series is used?

• A. UNR (Unified National Extra Fine)
• B. UNC (Unified National Coarse)
• C. UNF (Unified National Fine)
• D. Metric

Answer: (A)

Vibration tends to loosen fasteners by allowing tiny relative movements between threads. An extra-fine Unified thread profile increases the number of engaged threads in the same length, raising the friction and distributing the load over more threads. That tighter, denser engagement helps resist the dynamic motion caused by vibration, keeping the clamping force more stable. In other words, the extra-fine UNR series is chosen when vibration is a factor because it provides better resistance to loosening under dynamic loads. Coarse threads have fewer engaged surfaces and are more prone to loosening in vibration, and metric threads aren’t part of the Unified system.

Vibration tends to loosen fasteners by allowing tiny relative movements between threads. An extra-fine Unified thread profile increases the number of engaged threads in the same length, raising the friction and distributing the load over more threads. That tighter, denser engagement helps resist the dynamic motion caused by vibration, keeping the clamping force more stable. In other words, the extra-fine UNR series is chosen when vibration is a factor because it provides better resistance to loosening under dynamic loads. Coarse threads have fewer engaged surfaces and are more prone to loosening in vibration, and metric threads aren’t part of the Unified system.