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Stainless Steel Fastener Torque and Galling

Types of Fasteners Prone to Torque and Galling

Fasteners made of stainless steel, aluminum and titanium are those most prone to galling when tightened. So what exactly is galling and what harm can it do to your products?

Galling describes the seizing or abrading of screw threads. This can occur over an extended period of time when joint elements jam during assembly, if threads are damaged. Galling is often the result of a breakdown of the protective oxide layer, which exposes the base metal, resulting in interlocking surfaces. Different materials behave differently in terms of galling, and it is important to take them into consideration. Galling can occur anywhere from seven seconds to 58 seconds at a specific load.

Where Torque Comes into Play

When designing a joint, we target a specific clamp load or tension to ensure that the joint members stay tight and perform their intended function. Since we cannot typically measure tension during tightening, we use torque, which is easily measured, to achieve the desired load. This torque/tension relationship is greatly affected by friction. If friction is higher than expected, as would be the case with galling, then our resulting tension is reduced. A joint that does not have enough tension can fail as a result of fatigue if external forces are high enough and dynamic in nature.

Galling can also be an issue if parts require regular service and need to be dis-assembled. If joints are locked together as a result of galling during assembly, they often break when removed making the joint difficult or impossible to repair.

Preventing Galling When Working with Stainless Steel

A simple way to prevent galling when working with stainless steel is to make sure that fasteners are properly lubricated prior to assembly. This can be achieved manually with paste or grease, or by application of a coating to the fasteners such as Bossard ecosyn-lubric®.

To learn more about minimizing the risk of galling, contact us at ProvenProductivity@bossard.com.

Stainless Steel Fastener Torque and Galling by
March 21, 2014

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