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• Thread Galling Problems

Prevention of Thread Galling when using Hydraulic Bolt Tensioning Tools
Thread galling is a common, yet seldom understood problem with larger diameter threaded fasteners and hydraulic bolt tensioning equipment. Galling is a cold welding process which can occur when the surfaces of male and female threads come in contact with heavy pressure. Hydraulic nuts and detachable bolt tensioning tools can be much heavier than normal hexagon nuts, this extra weight will increase the pressure on the mating thread surface which can lead to galling. Stainless steel fasteners are particularly susceptible to thread galling, although it also occurs in other alloys which self generate an oxide film for corrosion purposes, such as aluminium and titanium. During the assembly of the bolt tensioning tool to the fastener a pressure builds between the contacting thread surfaces and breaks down the protective oxides. With the absence of the oxide coating, the metal high points can shear and lock together. Minor galling may cause only slight damage to the thread surface and the tensioner may still be removed. However, in severe cases galling can completely weld the hydraulic tensioning tool and bolt together and prevent dis-assembly. Another factor that increases the risk of thread galling is thread surface texture. A smoother surface texture will lead to less frictional resistance. As previously mentioned, friction increases the possibility of galling. Most external threaded fasteners feature rolled threads provide a relatively smooth surface. Unfortunately, an easy method of rolling internal threads has yet to be found therefore most bolt tensioning tools are manufactured using a single point thread cutting process. This invariably leads to a rougher surface finish on the tensioning tools internal threads. Unfortunately, even with the understanding of the mechanism of galling, little is known on how to successfully control it. However, probability of galling