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SPRING 2021

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Distributor's Link Magazine Spring 2021 / Vol 44 No 2

100 THE DISTRIBUTOR’S

100 THE DISTRIBUTOR’S LINK LAURENCE CLAUS A “GALLING DEVELOPMENT”- WHAT EVERY DISTRIBUTOR SHOULD KNOW ABOUT THREAD GALLING from page 10 In other words, materials like hardened steel or those with hard, strong protective layers are rarely subject to galling. Stainless Steel, Aluminum, and Titanium fasteners are particularly vulnerable. These materials possess a protective natural oxide layer. If this protective layer is punctured or abraded away the resulting base metal interface is generally soft and vulnerable to galling. Galling Mechanism To better understand this phenomenon one should have a general idea of the mechanism that allows materials to gall. Galling is actually a severe form of Adhesive Wear. “Adhesive wear occurs when two metallic components slide against each other under an applied load where no adhesives are present.” [2] All metallic surfaces possess irregularities, projections, and whiskers known as “Asperities”. Considering the surface of a part or material sample, the rougher the surface is, the greater the influence of the asperities. However, even very highly polished metallic surfaces possess asperities at the microscopic level. When the contacting surfaces slide over one another the asperities bind together through metallic bonding. More heat and pressure intensifies the metallic bonding and once it reaches a threshold level the weaker of the two sliding materials experiences a shear failure which deposits itself on the opposing surface. As these deposits or lumps grow, they may penetrate the protective oxide layer on materials like Stainless Steel and Aluminum, which is working to counteract the metal bonding attraction, exposing the bare metal, and accelerating the adhesive wear. Scientists have also determined that the crystalline structure can have a dramatic impact on the tendency of the material to gall. The ease with which the crystal lattice can slip in multiple planes during plastic deformation is another indicator of galling tendency. Although there are fourteen primary crystal structures (how the atoms align themselves to form identical repeating units) possible, metals tend to only take the form of three or four of them. Scientists use terms like Face Centered Cubic (FCC), Body Centered Cubic (BCC), and Close Packed Hexagonal (CPH) to describe these crystal structures. Investigation has shown that the crystal lattice and ease of dislocation slip play a key role in galling so that many, although not all, fasteners which possess FCC structures, like Austenitic Stainless Steels, possess a greater tendency towards galling than those possessing a CPH structure, like fasteners plated with Cadmium or quench and tempered to form hardened Martensitic steel. Prevention Since certain materials, like Austenitic Stainless Steel, provide significant advantages for fasteners, it is unreasonable to suggest that designers simply avoid designing and using fasteners made of materials with strong galling tendencies. Instead, designers and users must develop strategies which minimize or eliminate the risks of deploying these materials. Factors related to design, lubrication, environment, and application can all play a role in whether fasteners will gall or not. In essence, most prevention strategies will take aim at reducing contact stress. Some of the more commonly adopted preventative measures are: ¤ Assembly Speed ¤ Lubrication ¤ Surface Roughness ¤ Surface Finishes ¤ Material Choices ¤ Thread Pitch ¤ Thread Damage ¤ Debris in Thread ¤ Thread Engagement ¤ Part Alignment ¤ Tightening Practice ¤ Insert Lock Nuts Let’s explore each of these preventative measures in greater detail. Assembly Speed One of the triggers for galling is heat. The faster that a screw thread is assembled, the greater the development of localized heat through sliding friction. When a fastener is assembled with a power driver there may not be sufficient time for the heat to dissipate through the fastener leaving high localized temperatures in the thread flank. CONTINUED ON PAGE 142

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