Photos courtesy of Bumax

Potentially disastrous effects could result from galled fasteners seizing or experiencing fatigue breakage, so aircraft manufacturers must counter this threat. Not everyone in manufacturing is familiar with galling, except for those who absolutely must know: designers and engineers. With galling a serious threat to the quality and safety of expensive and crucial equipment, staff in executive management, operations and procurement would be wise to study this phenomenon also.

In galling, excessive friction between two moving metal surfaces causes mechanical wear, tearing up material and transferring it between the surfaces when load compresses the surfaces together.

Fasteners prone to galling

Standard stainless-steel bolts and fasteners tend to gall under certain conditions. Thread galling can occur with standard fasteners when pressure and friction cause the bolt threads to seize to the threads of a nut or a tapped hole. Severe galling, known as cold welding, can cause the two surfaces to fuse together, making the joint impossible to remove without cutting the bolt or splitting the nut.

Galling’s consequences should not be underestimated. A galled fastener may fail to achieve the necessary pre-load – especially in dynamic loading. This jeopardizes the fastener’s task to securely fasten two surfaces together, which is complicated by the fact that an application may involve hundreds or even thousands of stainless-steel fasteners.

If critical fasteners in the rotating parts of a chemical pump, rotor blade, or propeller gall, the joint is likely to experience fatigue breakage, requiring costly maintenance and downtime for the customer. At worst, fatigue breakage of critical fasteners might result in accident or injury. Galled fasteners are also much more susceptible to corrosion, which can ultimately result in breakages.

Five galling prevention steps

Select the correct material

Certain types of stainless steel are more prone to galling due to their atomic structure. Cold-formed, strain-hardened stainless steels provide excellent galling resistance due to improved strength and reduced ductility.

Choose fasteners with a perfect fit

A tight fastener fit reduces galling risk by minimizing movement and friction. High-quality threads have fewer surface deviations that can rub together and lead to galling.

Add lubrication

Lubrication allows materials to slide past each other without causing friction. Some leading premium fastener manufacturers use custom-made wax to ensure an optimal friction coefficient. Anti-seizing and anti-galling lubricants can also help reduce galling.

Avoid damaged fasteners

A bolt with dented or damaged threads has a significantly increased chance of galling. Check all fasteners for damage that may have occurred during shipping. Dirty bolts with debris in the threads can also greatly increase the risk of galling – so only use clean bolts.

Install carefully

Most stainless-steel is sensitive to high temperatures, so tightening bolts slowly can reduce friction and heat that can cause galling. Avoid power tools that can cause excessive friction and heat. Calculate each application to determine tools that can be used.

Catastrophic failure risk

In the aircraft industry, catastrophic failures can occur if the thread galls before enough torque has been obtained. However, galling can be overcome even in the most challenging of applications – particularly by selecting premium quality stainless-steel fasteners.

A fastener such as the Bumax 88 has a high molybdenum content to make joints extra strong, reliable, and corrosion resistant.

Premium fasteners may look similar to standard stainless-steel fasteners, but superior material properties can make all the difference with galling. Explain to your fastener supplier exactly how the fasteners will be used to ensure optimized galling resistance. Premium stainless-steel fasteners also typically offer other properties that are essential in many critical fastener applications – including corrosion resistance and high tensile strength.

Bumax

About the authors: Patrik Lundström Törnquist is managing director of Bumax, and Anders So¨derman is the company’s technical director. They can be reached at anders.soderman@ bufab.com and patrik.tornquist@bufab.com.