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. Teams that use both (e.g. a team in the U.S. with metric fasteners for the engine and inch fasteners for the rest of the car) should be extremely careful not to mix up the fasteners as certain metric/inch fasteners can thread with each other and compromise a bolted joint.

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.

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

Also note that the Formula SAE / Formula Student rules may prevent countersunk, button head, or low profile bolts in certain locations.

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. Locknuts that rely on a prevailing torque, such as nylocs or distorted thread nuts, can effect the torque required to reach a desired bolt preload.

Photo	Common Name	Description
	Jam nut
	Nylon nut
	Nyloc/Nylon Insert Nut	These nuts use a nylon collar to increase friction with the mating thread. As they rely on nylon to provide the prevailing torque, their maximum use temperature is limited to <100-120°C. FSAE/FS rules may prohibit use of nyloc nuts in high temperature locations. Re-use of nyloc nuts depends on the appropriate regulations, but generally they can be re-used, provided the prevailing torque is still sufficient.
	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. These types of locknuts may strip coatings/platings off of and wear down the threads of the mating bolt. When the prevailing torque has diminished, the bolt should also be replaced.
	Castle nut

Threaded Insert

Threaded inserts are used to either repair a damaged set of threads, to create a stronger bolted joint in relatively weak materials like magnesium & aluminum, or for bolted joints that will be frequently assembled and dis-assembled. Cast aluminum parts are especially prone to being stripped when torqued, as a FSAE team will likely assemble and disassembled components such as engine casings far more frequently (and roughly!) than the manufacturers predict. Therefore, a set of helical thread inserts and associated tools are very useful for common fasteners like M6x1.0 screws for engines. Common types of threaded inserts include Helicoils, Timeserts and Keenserts. Typically, installation of a threaded insert requires a thread with a larger size than the bolt thread in parent object. Different size inserts are available based on the part's wall thickness and the required insert strength (when used to strengthen the bolted joint, larger inserts will result in a stronger joint). The male thread of a thread insert is often an uncommon thread form, and care should be taken to ensure the correct tap is used. The threaded insert is then screwed into the tapped hole and locked in place. The female thread of the threaded insert is the desired thread size.

Heat-set or heat-stake inserts can be used in parts made from thermoplastics, like FDM 3D prints. These inserts can be installed using a soldering iron, and are often used in plastic parts as a substitute for a tapped hole.

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. When following a torque spec, be careful to follow the guidelines for assembling, as the use of lubricants such as engine oil can significantly change the measured torque for a given bolt stress (which is what you ultimately care about). NEVER try to guess torque by feel, no matter how experienced you are. A good torque wrench is essential, and torque wrenches that mechanically disengage when a dialed in torque is reached are nice to have.

Calculate Torque

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

$T=K*F*d$

Where

• $T$ is Torque applied to the head of the fastener
• $K$ 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, or page 24 of the Fastenal Guide.
• $F$ is the preload tension force. Shigley's recommends $0.75*F_{proof}$ for non-permanent connections, where $F_{proof}$ 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.
• $d$ 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

• tarkka Fasteners Introduction Youtube Video
• FSAE Online Series Resources: Threaded Fasteners for FSAE If link breaks, it is found under fsaeonline.com > Series Resources > Design Event > Reference Documents.
• 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)