Fasteners in Motorsport

In this article, I will be looking into fasteners that are commonly used in motorsport, and, for that matter, most automotive sector such as the requirements of fasteners, metric vs imperial, threads and thread fasteners, types of nuts available as well as strength of nuts and bolts. Please feel free to skip to the parts most relevant to you,


Requirements

For motorsport purposes, you will usually find that the fastener requirements are laid out in the rule book. For example, for Formula Student, the fasteners 'must meet, or exceed, SAE Grade 5, Metric Grade 8.8 and/or AN/MS' with 'button heap cap, pan head, flat heat or round head screws or bolts in critical locations prohibited'. More on fastener requirements can be found by looking at the Formula SAE Student 2015 Rules.


Metric vs Imperial

The confusing thing about fasteners is that they can come in metric or imperial units. This is mainly because Europe likes to use metric whereas America likes to use imperial.
When it comes to choosing metric or imperial, make sure you check the availability, cost and quality of the fastener. It is also important that once you have chosen metric or imperial, you stick to it since changing from metric to imperial or vice versa can cause a whole load of problems!


Threads and Thread Fasteners

  • The pitch of a thread is the distance between the identical points on the thread profile.
  • The screw minor diameter and  nut major diameter are used to calculate the thread undercuts.
  • The tap drill diameter is the nearest standard drill size to the mean minor diameter of the internal thread.
  •  The tapping allowance is roughly the equivalent that is untapped. a C1 tapping allowance is roughly 4 pitch lengths of incomplete thread. The minimum allowance using a bottom tap will be 2 pitch lengths.
  • The screw end clearance is an arbitrary selection and can be varied depending on the number and tolerance on the clamped components on the bolt length. The screw end clearance should be longer when soft materials are being used.
  • There must be clearance between the clamped components and bolt shank so that threads can pass without difficulty.
  • The minimum thread depth specified equals the length of engagement plus the screw end clearance.
  • For a critical metal to metal face joint, counter drill or counter bore one pitch depth at the clearance diameter so that the first thread cannot deform preventing the faces from sealing.
  • On any 2D technical drawings, it is vital to specify the tap drill diameter and thread designation as well as the minimum full thread depth.
To tap a whole, a 3-piece tap set is commonly used:
  1. The taper tap
  2. Second tap
  3. Finish tap
This helps to remove the metal bit by bit to make the thread as accurate and of high quality as possible.

Representation of Typical Hexagon-Headed Screw Assembled In a Blind Hold

Strength of Fasteners

For bolts and nuts, the strength is grade a decimal number. For example, the strength of a mild steel bold is 4.6 and nut at 4. For high tensile bolt, it would raise to 8.8 What does this mean though?

  • The first part of the number makes clear the bolt strength grade.
  • The second part of the number (decimal) is the fraction that the yield stress.
For ISO metric course threaded fasteners:
  • Bolts should be tightened to 85% of the bolt proof load (load which would cause minimal permanent strain of the bolt).
  • Only around 10% of the torque applied to tightening a bolt actually is effective in producing the pre-load. The 90% is absorbed in the friction of the threads and under the bolt head (this is why it is always best to lubricate when tightening to reduce friction and reduce the torque required to produce the pre-load).
Lubrication not only reduce the amount of friction when tightening, it also decreases bolt failure during installation and increases the bolt's life span. The variation in the frictional coefficients affect the amount of pre-load that is achieved at a specific torque. The higher the friction the less conversion of torque to pre-load.


Types of Nuts

There are a range of non-locking lots such as:
  • Hex nuts
  • Jam hex nuts
  • Wing nuts
  • Weld nuts
  • Riv nuts
  • Anchor nuts
There are also a range of locking nuts such as:
  • K nuts
  • Philidas nuts
  • Stover nuts
  • Binx nuts
  • Nyloc nuts
It is important to remember that locking nuts can only be used once. Once they are used, they will need replacing since locking nuts plastically deform upon being used.

There are also other types of ways you can lock bolts in such as:
There are also some other useful fasteners you can use such as:
  • Dzus fastener - Convenient way to secure body panels.
  • Single action ball lock pins - A 2 ball mechanism with a ring handle.
  • Aluminium rivets.