Products & Characteristics

The Aircraft Locknut Manufacturer's Association (ALMA) is comprised of the leading North American manufacturers of self-locking nuts for the aerospace industry. The Association works closely with other national and international military and industry technical organizations in developing specifications, standards, and other technical practices. Several member companies participated as task group members and technical resource personnel in the preparation of the Industrial Fastener Institute's handbook "Recommended Practices for Statistical Process Control".

Statistical Process Control

Although Statistical Process Control (SPC) has recently received much attention in the technical community, ALMA Member Companies have been implementing SPC practices for the last several years as a manufacturing tool to reduce or eliminate scrap, rework, and customer returns.

The fact that SPC is a must for any manufacturer in today's environment is a given. When the process is controlled, the product will meet the features necessary for the form, fit, and function of the final product.

ALMA has now completed a project to define those product characteristics which have the greatest effect on form, fit, and function of self-locking nuts. Attached is a listing of those characteristics, with illustrations, of a variety of styles of self-locking nuts.

This project was undertaken by ALMA in order to help those less familiar with self-locking nuts to recognize the characteristics that should be considered for inspection and/or design to assure proper or required product performance. We hope that this document will be used as an educational tool and guide for other interested parties by those already familiar with the products.

Manufacturers know at which point in their processes SPC has to be exercised, and what needs to be controlled for continuous improvement based on product knowledge, method of manufacture, process capabilities, and other factors. The concerns of both the manufacturer and the customer must be considered to ensure that meaningful data are generated, and that the parts will function as intended. The goal of the ALMA Member Companies is to control their processes to ensure the manufacture and timely delivery of quality products.

Product Characteristics With the Greatest Effect On Form, Fit and Function of Self-Locking Nuts

All Self-Locking Nuts
• Thread fit
• Locking torque
• Tensile strength
• Overall height
...with additional characteristics for each of the product types below.
Wrenchable Nuts
• Wrench fit after incorporation of locking feature
• Base diameter
• Wrench element height
Castellated Nuts
•  Width and location of castellation slots
Fixed Anchor/Plate Nuts
•  Rivet hole diameter and spacing
Floating Anchor/Plate Nuts
•  Rivet hole diameter and spacing
• Nut element float
Gang Channel Assemblies
•  Bolt hole spacing
• Nut element float
Shank/Clinch Nuts
•  Shank diameter and length
• Dimension from centerline to anti-rotation feature
Fixed Rivetless Anchor/Plate/Clinch Nuts
•  Shank diameter and length
Floating Rivetless Anchor/Plate/Clinch Nuts
•  Shank diameter and length
•  Nut element float
Fixed Barrel Nuts
•  Barrel diameter
Floating Barrel Nuts
•  Barrel diameter
•  Nut element float
For additional industry information, check out our
ALMA Resources page!