Alternative Designs of the NAS1291 Series

Due to the popularity of NAS1291 lock-nuts in the aerospace industry there is a high demand for an alternative lock-nut that is less susceptible to failure.

One of the main criticisms of the NAS1291 design is its thin wall design. As a result of this thin wall design, manufacturers must heat treat the nut to high Rockwell hardness to meet the tensile requirements of the standard. However, high hardness requirements make the material more susceptible to cracking and induced hydrogen embrittlement.

Alternative designs, that increase the wall thickness of the NAS1291 nut, allow for lower hardness values to meet the tensile requirements of the standard.

NAS1291 Bell Flange Design 'Style B'

One such alternative design is to increase the wall thickness around the base of the NAS1291 nut. It is widely accepted that 80% of the load is carried by the first two threads in conventionally threaded fasteners. As a result this increase of wall thickness around the base two threads makes for a much stronger design that can be heat treated to lower values to meet the tensile requirements.

The primary advantages of this design is that it is dimensionally within the specifications of the NASM21042 & NAS1291 standard. As such, this 'Bell Flange Design' can be used without redesign of the application, or the use of 12-point tools for installation.

Thread Diameter	NAS1291 'Style A'	NAS1291 'Style B'
.4375"
	MS21042-7	MS21042-7B
.500"
	MS21042-8	MS21042-8B
.5625"
	NAS1291-9	NAS1291-9B
.6250"
	NAS1291-10	NAS1291-10B

The combined drawings of the style A and B nut can be found below for comparison:

NAS1291 Series LMC | Style A

NAS1291 Series LMC | Style B

A 12 Point Design

EASA has issued the replacement of MS21042-4 nuts with NAS1805-4 in the swash plate support of helicopters. NAS1804 and NAS1805 lock-nuts both have a higher wall thickness and height. However, as a result of their 12-point design they must be installed with special tools. Additionally, due to their higher height and wall diameter, they may not fit in all of the same application as NAS1291 nuts.

Finally the design does not provide a replacement NAS1291 nuts with a thread diameter smaller than .1900" .

Least Material Conditions for the NAS1805 & NAS1804 series can be found below:

NAS1804 Series LMC

NAS1805 Series LMC

Alternative Thread Types

Conventional thread designs such as those used in the NAS1291 series are known to carry as much as 80% of the load on the first two threads. However, alternative fastener threads have been designed that more evenly distribute the load.

Thread designs such as those designed by Spiralock® distribute the load more evenly in addition to preventing self loosening in high load and vibrational aerospace applications.

Spiralock® Advantages

The disadvantage is that the use of these thread designs are currently not common in the aerospace industry.