The Galling Phenomenon: What exactly is galling?

Galling is a common complication encountered during the fastening or disassembly of threaded components. It can result in damage to the threads or even cause the components to seize. These failures are often costly in terms of manpower, schedule delays, and lost production. Galling is a form of adhesive wear caused by material transfer between metallic surfaces in relative motion. It is driven by adhesion and friction, which can tear the crystal structure of the underlying material.

Threaded fastening, which involves sliding interlocking threads under high load, is particularly prone to galling. This is due to the inherent conditions of the process - such as ductile metals in contact, metal-on-metal sliding, friction, and high compressive loads - all of which not only promote galling but are essential to the operation itself.

When complementary screw threads make contact, the initial mating occurs at the asperities (microscopic high points) of the surfaces. These asperities concentrate stress and energy at a local scale, far exceeding average values. As sliding begins, these concentrated forces lead to plastic deformation, raising local temperatures and energy density. This accelerates adhesion, material transfer, and the formation of protrusions. If these protrusions exceed a critical size, they can breach the oxide layer of the mating thread and deform the underlying ductile material. The result is a plastic flow zone around the protrusion, where galling begins to propagate.

The rate at which energy accumulates in this localized system depends heavily on the size, shape, and material properties of the plastic zone. In contrast to brittle fractures, which generate little heat due to their small plastic zones, the ductility of most common machine screws makes them especially susceptible to galling.

During nut torqueing, axial bolt load contributes directly to energy buildup in the contact zone. As the nut turns and relative motion continues, energy accumulates due to limited heat dissipation - restricted by the small cross-sectional area available for thermal conduction. This results in rising energy density and surface temperature, which in turn alters plastic behavior and further promotes adhesion.

At a critical point, the combination of plastic deformation and sustained contact can create a shared plastic zone between the threads. This zone is characterized by high energy, pressure, and temperature, which can cause the two surfaces to bond. Once this occurs, continued rotation requires significantly more force and may even result in seizure of the nut. Removing a seized nut often requires destructive techniques such as cutting or drilling.

One of the most effective ways to prevent galling is by eliminating compressive thread load before turning the nut. Doing so reduces potential energy and frictional heating at the interface. Tools and systems designed to achieve this - such as hydraulic tensioners, hydraulic nuts, and anti-galling technology like the Velocity Washer - can significantly lower the risk of galling in critical applications.