Threaded Fasteners

Threaded fasteners used in FS are most typically made of steel but are sometimes also made of stainless, titanium, or aluminum.

Standards

There are two main threaded fastener standards: metric and inch. Within these two categories lay additional standards set by various organisations. These standards may are often intercompatible but can differ in head size, strength, alloy, coating, pitch, and local avaliability.

Metric

Metric screws are listed in the following way:M6x25 x 1mm Class 8.8. The M6 referes to the major diameter being 6mm, 25: the shaft length is 25mm, and the pitch is 1 thread per 1mm. A class 8.8 bolt is a medium strength steel bolt with a tensile strength of about 800MPa and about 80% of that being the yield strength.

ISO

Wikipedia ISO Metric Screw Threads has tables listing preferred sizes, and pitch.

Inch

Inch screws are listed in the following way:1/4-20 x 1 Grade 5. The 1/4 is the major diameter of 1/4in, a thread pitch of 20 threads per inch, and a length of 1in. A grade 5 bolt is a medium strength steel bolt with a tensile strength of about _ksi and a yield strength of _ksi.

SAE

[grades, charts, etc...]

NAS

NAS Shear bolts are often used in suspension systems because of their tight diameter tolerances, which means less compliance.

Screw Types

Photo	Names	Description
	Hex head
	Twelve point
	Socket head Allen® head
	Shoulder screw
	Set screw
	Quarter turn DZUS® fastener	A specialty fastener that allows quick installation and removal of body panels.

Nut Types

Locknut

Locknuts resist loosening. Other methods can be used to resist loosening: lock washers, safety wire, locking adhesive, or design. Most locknuts are only designed to be installed a few times otherwise their loosening resistance is diminished.

Photo	Common Name	Description
	Jam nut
	Nylon nut
	Nylok
	Distorted nut Stover nut K-nut / Jet-nut	Different components of the nut can be distorted to prevent loosening. For example: thread pitch and eccentricity. The temperature range is limited only by the parent metal or its plating and can be reused approximately 10 times before its locking capability is diminished. A K-nut or Jet-nut is ideal for high temperature applications.
	Castle nut

Fastener Characteristics (how to choose)

• Initial Tensioning
• Initial Tightening Force
  • Brings bolt close to "proof load"
• Deformation of threads (pic)
• Maintenance schedule

Torque

Why torque a bolt? Create tension in the bolt which does the following:

• Prevent joint separation
• Reduces the cyclic stress that the bolt experiences during cyclic loading (at the cost of increases amplitude of the mean stress)
• Creates friction forces between the materials being clamped. The friction force can transmit shear forces, so the bolt itself isn't loaded in shear.

Torque Accuracy

The ultimate goal of torque is to tension the bolt. Unfortunately, torque is a very inaccurate method of achieving a specific tension. The tension can be off by +/- 25% It widely used because it is cheap to apply.

Calculate Torque

The Fastenal Technical reference guide and the NASA Fastener Design Manual (Pg 17 Alternative Torque Formula) show the commonly used formula:

$T=K*F*d$

Where $T$ is Torque, $K$ is a Torque Coefficient related to friction, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle F } is the preload force, and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle d} is the diameter of the bolt. The Torque Coefficient depends on the materials, coatings, and lubricants (if present) of the connection. You can find a value from table VI of the Nasa manual, or page 24 of the Fastenal Guide.

The Lazy Way

Fastenal Torque Calculator

Further Reading

• tarkka Fasteners Introduction Youtube Video
• Machinerys Handbook
• Fastenal Technical Reference Guide
• NASA Fastener Design Manual
• Carroll Smith's Screw to Win
• McMaster-Carr
• Schaeffler Technical Pocket Guide (available in print or as an App)