Thread-binding issues sometimes may arise when assembling with self-clinching fasteners made from stainless steel. One such issue is galling, which is the seizing or abrading of threads caused by adhesion between the sliding surfaces of a fastener’s mating threads. Although thread galling is not typically a wide-spread problem with threaded fasteners, it can cause significant fastener damage and even fastener failures. Even more vexing, sometimes galling of mating threads will go undetected at assembly until it’s too late, when screws break during removal for service or other reasons. With most loose hardware, failed fasteners can be replaced easily with new fasteners, and seldom does the galling damage extend to the attached parts. However, a failed self-clinching fastener may require that the panel into which it is installed be scrapped since self-clinching fasteners become permanent parts of an assembly. Interrupted production and added costs then follow. Knowing the mechanics of thread galling, the factors that will increase its likelihood, and methods to reduce or eliminate galling problems can help optimize fastener performance in service.
Mechanics of Thread Galling The surface of a thread flank is not flat but, instead, composed of high and low spots. The high spots are called asperities. When a thread flank is loaded in straight tension with a tensile testing machine, the asperities yield, compress in height and increase in size until the total contact area can support the load without further yielding. When a threaded joint is tightened, the asperities are crushed as load increases, but there also is sliding motion between the thread flanks. This sliding motion can cause the tips of asperities to shear off. On metals such as stainless steel and aluminum.
This installed self-clinching, flush-head stud with galled threads create attachment difficulties with mating hardware (the nut at the top).
that readily form oxides on their surface, the shearing away of the oxide layer exposes bare metal. With adequate heat from friction and pressure from the induced load, these bare spots can weld together. Welds of this type have been found to be stronger than either base metal. Therefore, with increased torque as the tightening continues, the welds themselves will not break, but instead, chunks of base metal will be pulled out of the weaker base metal.
This initiates galling. As more tightening torque is applied, the contact area increases to support the higher induced load. This creates larger, stronger welds, which pull out even larger chunks of the weaker base metal. In extreme cases, the torque necessary to continue relative rotation of the mating threads can exceed the torsional strength of the external thread and cause it to fail in torsion. Even in less extreme cases, galling significantly increases the coefficient of friction on the galled surface; with common torque-controlled tightening (that is, all fasteners are tightened to the same torque), preload may be significantly less than required for a reliable joint.
Although the role of heat input in galling has not been fully explored, high heat input is known to increase the chance of galling. The source of heat during tightening of a threaded assembly is friction. Screw threads are very inefficient, with only 10 to 15 percent of the tightening torque used to induce load, and the rest used to overcome friction. Factors Contributing to Galling A variety of factors can increase the likelihood that some percentage of mating threads will gall when assembled. Soft Connection Elements. Soft elements require more rotation during tightening. This increases the area under the torque-angle curve, which causes more heat energy input from friction.
In engineering terms, an element is softer when it has lower stiffness, and a threaded connection may contain one or more soft elements. While fastener stiffness is generally fixed for a given thread size and material, the stiffness will vary with grip length or the space between the nut face and the underside of the screw head. The longer the grip, the lower the stiffness. High-speed Tightening. This action can generate very high temperatures on the thread flanks before the heat can be dissipated by the rest of the fastener mass. The slower speed will allow more time for heat dissipation. Material and Finish Combinations. Decades of experience have shown that, in most thread galling cases, both threaded components have been made from 300 series stainless steel. Materials that form natural protective oxides (such as stainless steel, aluminum, and titanium) have been found to be more prone to galling. This is because the protective film provides corrosion
resistance without any plating or other additive finish. These oxides tend to give the surface a higher friction coefficient than plating materials typically used on steel fasteners. This means higher torque is required for a given clamp load, which means more area under the torque-angle curve and more heat energy input. If the oxide is very thin, it will follow the surface roughness on the thread flank and be completely removed when the tip of an asperity is sheared. This action exposes pure base metal with excellent welding characteristics. This is especially true for stainless steel, where the chromium oxide layer is only several angstroms thick. The abrasive nature of the sheared-off oxides also may contribute to thread galling. Materials of the same type generally are more prone to galling, possibly because they are more easily welded than dissimilar materials. Materials of the same hardness also will generally be more prone to galling, since the breaking down of asperity tips is happening to nearly the same degree on both materials. In turn, plated materials will generally be less prone to galling.