Installation

Many of the recent failures of the MS21042 & NAS1291 series can be attributed to improper installation of the fastener. Over-torquing or improper selection of the male fastener threads can result in galling.

Galling is a form of adhesive wear that can occur in fasteners as they are tightened. As the fastener tightening continues, these localized cold welded joints shear, tearing off metal particles. Debris and particles from the sheared joints film are entwined in the sliding surfaces which exacerbate the adhesive wear. The process can ultimately lead to seizing of the fasteners and breakage if tightening continues. Seizing can even occur during the loosening process.

With today's use of high speed, high powered torque wrenches, the considerations to avoid galling are increasing necessary.

Galling is best prevented by:

• Install using calibrated hand tools or torque wrenches with low RPMs
• Install the threaded male fastener with no more that three threads past the nut. Continuing to thread the nut past this point creates unnecessary friction and wear.
• Use Lubrication, or anti-galling compounds prior to thread assembly.
• Carefully evaluate the thread fit of the male mate. Due to maximum material conditions there could be zero clearance between the mating threads.
• Carefully evaluate different alloys with different hardness values in threaded joints. Corrosion resistant materials such as stainless steel, aluminium, titanium and nickel based alloys self-generate a protective oxide surface film which more readily causes galling.

Another common cause of failure of the NAS1291 & MS21042 series due to installation is 'shanking' of the nut by the male fastener. This is caused by an improper selection of the male fastener length, which allows the body of the fastener to 'shank out' the threads of the nut. As a result of this possibility, the male fastener should not be installed with no more the two or three threads past the nut.