Threaded Fasteners

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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
Hexhead mcmaster.png Hex head
12point mcmaster.png Twelve point
Sockethead mcmaster.png Socket head

Allen® head


Shoulder mcmaster.png Shoulder screw
Setscrew mcmaster.png Set screw

Quarter turn


DZUS® fastener[1]

A specialty fastener that allows quick installation and removal of body panels.

Reduced Loadability

Countersunk, button head, and low profile bolts have weaker heads than hex head or socket head bolts, which can cause them to fail at the base of the head instead of the shank, and at lower load then the shank could withstand. The ISO/DIN standards state that the metric bolts will have "reduced loadability" of 80%, and will be labeled with a "0" before the class. ie. Class 08.8, 010.9, or 012.9.

  • Countersunk head: ISO 10642:1997(E) Hexagon Socket Countersunk Head Screws. The title of table 3 states "Minimum ultimate tensile loads (...) 80% of the values specified in ISO 898-1"
  • Low Profile Socket Head: DIN 7984. The title of table 3 states "Minimum ultimate tensile loads (...) 80% of the values specified in ISO 898-1"
  • Button Head: ISO 7380 Hexagon Socket Button Head Screws. The title of table 3 states "Minimum ultimate tensile loads (...) 80% of the values specified in ISO 898-1"
Caution:

There are many countersunk, button head, and low profile bolts produced that are

  • not made according to one of the aforementioned standards. If they are not advertised as conforming to one of those standards, then their load capability cannot be confirmed (Or look up the standard they are made to).
  • not made to a strength standard. These will not have 08.8, 010.9, or 012.9 etc printed on the head.

Nut Types

Hex Nut

Weld Nut / Riv-Nut

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[2]. A K-nut or Jet-nut is ideal for high temperature applications.
  Castle nut  

Threaded Insert

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%.[3] It widely used because it is cheap to apply.

Calculate Torque

The Fastenal Technical reference guide[4] and the NASA Fastener Design Manual[3] (Pg 17 Alternative Torque Formula), and Shigley's[5] show the commonly used formula:

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 T=K*F*d}

Where

  • 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 T } is Torque applied to the head of the fastener
  • 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 K } is a Torque Coefficient related to friction. 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,[3] or page 24 of the Fastenal Guide.[4]
  • 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 tension force. Shigley's recommends 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 0.75 * F_{proof}} for non-permanent connections, where 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_{proof}} is the proof load of the bolt. Proof Stresses for common fasteners are listed in Shigley's, which can be used to calculate proof load. The tables in Shigley's show that proof stress is often 70%-80% of the yield strength of the bolt.[5]
  • 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.
Locknuts with Torque Calculations

Note that using nylon insert "locknuts" or prevailing torque nuts will increase the torque required for a given bolt tension. This torque is independent of the tension, so you can measure how much additional torque is required to overcome this locking feature, and simply add it on top of the calculation.

The Lazy Way

Fastenal Torque Calculator

Further Reading

References

  1. ↑ https://en.wikipedia.org/wiki/Dzus_fastener
  2. ↑ Barrett, Richard. "Fastener Design Manual", Nasa Reference Publication 1228 (1990) pp7
  3. ↑ 3.0 3.1 3.2 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009424.pdf NASA Fastener Design Manual
  4. ↑ 4.0 4.1 Fastenal Technical Reference Guide
  5. ↑ 5.0 5.1 Budynas, Nisbett. Shigley's Mechanical Engineering Design 9th edition


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