Causes of Metal Galling

Metal Features

• Nut and bolt fasteners are usually made from titanium, stainless steel, or aluminum. All of these metal types are alloys, or metal mixtures. Alloys have a natural outside film made of oxides. The oxides protect the fastener from corrosion, which is one of the main reasons alloys are preferred for nut and bolt manufacturing. However, this protective oxide film can contribute to metal galling.

Pressure

• Nuts and bolts are typically tightened to specific torque values. However, over tightening the metal parts together can easily break the oxide film. As a result, the oxide film produces tiny metal shavings. The shavings become trapped within the nut and bolt threads, causing a clog. Ultimately, the metal parts freeze or seize together, forming metal galling.

Temperature

• The metal shavings that become clogged cause friction between the nut and bolt threads. As a result of the friction, heat builds up along the metal bodies. The heat causes the metal to expand. The expanding metal places more pressure against the nut and bolt threads, as well as the shavings. Ultimately, more damaging galling occurs from the excessive heat and subsequent metal expansion.

Defects

• Improperly manufactured threads along the bolt can cause galling. Threads with a slight roll over, or bend, along the high spots can easily crack and break off when the nut is spun onto the bolt. These broken pieces act like the metal shavings and clog the threads, resulting in a seizure of the nut and bolt assembly.

Rapid Movement

• Workers using high speed driver hand tools can contribute to metal galling by using the rapid speed for nut and bolt installations. The rapid movement produces excess heat along the nut and bolt assembly, causing metal expansion and subsequent galling.

Prevention

• Simple actions can prevent metal galling. Nuts and bolts should be shaded, if the installation is outside. The sun's rays can easily heat nuts and bolts, making them more prone to galling. Workers can use slower speeds on hand tools for installing the nut onto the bolt. The slower speed will help alleviate the excess heat that occurs during high speed installation. Dry film lubricants should be applied to the bolt's threads prior to installation. The lubricant will prevent galling by aiding in a smooth movement between the nut and bolt threads.