Galling is a common complication that arises when fastening or disassembling threaded components. It can result in damage to the threaded features or seizing of said components. Such damage or seizing can often be costly to repair or remedy in terms of manpower, scheduling delays, and lost production. Galling is a form of adhesive wear and material transfer between metallic surfaces during operations in which relative motion of said surfaces is involved. Its driving forces are adhesion and friction which result in tearing of crystal structure in the bulk material. The fastening of threaded components in which interlocking threaded features are slid past each other under high loads is an industrial operation notably prone or vulnerable to galling. Galling is a major concern in said application because the same features which promote galling, such as material ductility, metal on metal contact, friction, and high compressive loads, are not only present, but are indeed necessary features for operation.
When complimentary screw threads contact each other, the initial mating is limited to the high points on said surfaces. These asperities can cause galling at relatively low loads since localized stresses and energy densities are greater than their respective macroscopic values. These elevated local values result in increased frictional forces and infiltration of said asperities into the opposing surface upon sliding, thereby instigating plastic deformation. The combination of frictional forces and material deformation yields increased heat and energy densities which, in turn, produce elevated adhesive forces, initiation of material transfer, creation of additional protrusions, and growth of said protrusions. If the height of the protrusion grows past a critical threshold value, it may penetrate the brittle oxide layer of the complimentary mating surface. As a result, said protrusion could deform the ductile bulk material on which the oxide layer originally formed, thus creating a region of plastic flow around said protrusion. The local energy accumulation rate is greatly related to the size, shape, and material properties of the plastic zones that surround the penetrating objects. While heat generation in brittle fractures is low due to the small, transitory plastic zones, the high ductility of commonly used machine screws can be considered a requisite characteristic for galling.
In the sliding contact of nut torqueing, axial bolt load is directly proportional to potential and thermal energy in the aforementioned localized system. Thus, the high loads and relative rotation associated with the torqueing of threaded nuts onto and off of threaded counterparts are particularly susceptible to galling. Additionally, as the nut is turned further and sliding progresses, additional energy is supplied to the system. Initially there is limited energy loss in the system since heat conduction is significantly inhibited by the relatively small thermal transport cross sectional area and correspondingly low conductance on the system boundary. The result is an increase in system energy density and temperature and said energy accumulation can damage the contact surfaces and alter their plastic behavior. Furthermore, the combination of direct contact and plastically deforming flow fields can result in the constitution of a common plastic zone in which the high energy density, pressure, and temperature promote inter-surface bonding. Generally, this greatly increases apparent adhesion as well as the force needed for further nut advancement or removal. In some cases this can cause seizing of the nut onto the threaded component, and removal of said nut requires time-consuming or destructive techniques such as cutting of the nut or screw. Reducing or eliminating the compressive load between threads greatly reduces the likelihood of galling due to a decrease in localized potential energy and frictional heating in the system. Therefore, systems in which bolt load is eliminated prior to turning the nut can greatly reduce the likelihood of galling. Such systems include hydraulic tensioners, hydraulic nuts, and specialty anti-galling devices and fasteners.
Michael Psimas, Ph. D.
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