I have some fasteners that have mating threads, internal and external. Often, I have a problem loosening and separating the parts. Why is this happening, and how can it be avoided?
The most likely cause of this condition is thread "galling" or seizing. This occurs when the surfaces of the mating parts are very abrasive. Galling occurs only on metallic fasteners, and is more likely to occur when the external threads are cut, rather than rolled. Cut threads have rougher surfaces because of the machining process. Galling can also result from surface oxidation in some materials. When the parts are engaged, microscopic particles break loose from the roughened surfaces and lodge between the mating parts. This results in mated parts "sticking" together and is responsible for "galling". Thread galling can become severe enough that the mating parts seize, making it virtually impossible to separate them.
Galling is a genuine problem which should be considered in design. Galling can best be avoided by manufacturing mating parts of different materials and hardnesses. Another way to prevent thread galling is to apply a lubricant to the threaded portion of the fastener.
Accurate Screw uses a heat cured solid lubricant per MIL-L-46010. The lubricant will help prevent oxidation and give the threads a smoother surface, with easier engagement.
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