Knowing the mechanics of thread galling, the factors that will increase its likelihood, and methods to reduce or eliminate galling problems can help optimize fastener performance in service.

- Sep 07, 2018-

Fine Threads. Fine threads tend to gall more than coarse threads. This is because fine threads are more prone to nicks or other damage from handling, and

thread nicks can contribute to galling. Fine threads further require more rotation and higher torque for the same stress on the larger tensile stress area during tightening, resulting in more area under the torque angle curve and more heat energy input.

Thread Damage. Threads with significant nicks or other damage can gall. If there is interference between the mating threads, there will be some level of prevailing torque during the entire tightening process. In some cases, the geometry of the damaged area may be such that it initiates galling in the immediate vicinity of the damage. Excessive Tightening Torque. Proper tightening torque is essential to develop the needed preload in a fastener, and preload is critical to prevent vibration loosening. Tightening torque should never be reduced below the proper value for the sake of galling prevention. However, tightening torque can contribute to galling. Mating Part Alignment. Mating parts should be lined up as well as possible before final assembly to reduce the amount that a fastener has to be turned during tightening. Otherwise, thread galling can occur. 

Tightening Sequence Issues. If a fastener is tightened with enough clamp load to keep the pieces from moving before all the other fasteners are started, some clearance holes may not be in the proper position. This can cause screws to enter on an angle.

In some cases, tightening on one side causes components to a gap on the opposite side. The fasteners on the gapped side will need to draw the gapped components together, forming a soft joint. A better approach is to start in the middle of the workpiece and progressively tighten fasteners on alternating sides of the center, finishing at the far ends. Low-quality and Inconsistent-quality Fasteners. Threads not perpendicular to the bearing face of nuts can create the same galling issues as screws entering on an angle, because of hole size or position problems. And fasteners with inconsistent yield strength can yield from excessive tightening torque.

Methods to Reduce Galling Sometimes, fastener design features prone to galling simply cannot be eliminated. The next line of defense against galling is the assembly technique, but even with the best assembly practices, some galling can occur. When this happens, one of the following additive methods might help:

Add lubricant at the assembly. Many different lubricants and anti-seize compounds are available. Some, such as USP-grade castor oil, are very safe and still provide excellent lubrication. An anti-seize compound is recommended for stainless threads.

• Specify fasteners with a lubricant. It is common to specify dry film lubricants on all-metal, prevailing torque locknuts, but these same dry film lubricants can be applied to nonlocking fasteners, too. Typically, only one of the mating threads needs to have a lubricant to prevent galling. When possible, it is preferable to apply the dry film lubricant to the external threads for complete coverage.

• Specify fasteners with anti-galling plating. Some metals deposited by electroplating have inherent lubrication properties. And as with lubricants, typically only one of the mating threads needs to have a plating to prevent galling. Note that most stainless steel and aluminum fastener threads are not produced with an allowance to accommodate the plating thickness. This usually means that a fastener manufacturer can’t simply take existing parts from stock and have them plated. Instead, parts need to be specially made with the needed thread allowance.

• Specify fasteners made from anti-galling materials. While this option typically may require that fasteners be made to order, it is especially viable for high-volume, long-product-life applications. Address Galling Early While these guidelines can help prevent thread galling, particular applications may present particular challenges. Regardless, resolving galling issues at the outset can make all the difference on the road to optimized fastener performance.