T.8.3 Positive Locking Mechanisms

Positive locking mechanisms are meant to prevent fasteners from loosening by accident (primarily through vibration).

Positive locking mechanisms have to be visible when they are installed on the car (for example, Locktite (glorified adhesive) does not count at a positive locking mechanism because even though it does a good job of keeping bolts from loosening, there is no visible way for inspectors to prove that you have used Locktite on your bolts without breaking the Locktite. 

Positive locking mechanisms are not allowed to rely on clamping force to protect them from loosening from vibration. For example, lock washers rely on a spring force (clamping force when the spring force of the washer compresses against the nut) to help the nut resist loosening due to vibrations. This only works while the washer is spring loaded against the nut. If the nut loosens enough for the washer to  no longer experience elastic deformation from the compression, then the washer becomes useless for resisting vibration. In other words, positive locking only works when the nut is flush against the washer. Nyloc locking nuts on the other hand, work no matter where the Nyloc nut is on the threads of the bolt. This is because the vibration is resisted by nylon digging into the threads of the bolt. 

Safety wiring:

Safety wiring is where you connect the heads of bolts together using wire and a specific technique that makes it impossible for the bolts to become loose as long as they are wired together. It is difficult to do and undo so safety wiring is a good thing to do once you know you will not have to access the bolts for a while.

Cotter Pins:

Cotter pins are the little pins you use to keep car hitch connectors from sliding apart. They can be used to keep dowels from sliding around, or bolts from unthreading.

Nylon lock nuts (Nyloc): 

These were mentioned earlier and are nuts with a nylon ring that deforms around bolt threads, making them resistant to loosening due to vibration.  Nylon lock nuts can be used in applications where they will stay below 80 degrees C (because nylon behaves differently at high temperature and may be less effective as an anti-vibration fastener if used in too high of a temperature).

Prevailing torque lock nuts: 

Similarly to nylon lock nuts, these lock nuts rely on material deformation between the nut and the bolt to resist vibration. The difference is that prevailing torque nuts are made entirely of metal. They do not fit the bolt perfectly or move freely like normal nuts, meaning prevailing torque lock nuts require a higher torque to tighten than normal nuts do to overcome this slight fit mismatch. This means when they are tightened on the bolt, they resist unscrewing because the bolt and nut have been deformed so that they can fit together. 

Lock washers, bolts with nylon patches and thread locking compounds (Loctite®), DO NOT meet the positive locking requirement.

This is because these fasteners do not meet rules that were laid out earlier. 

Lock washers only work when the spring force resulting from the washer being compressed is present. 

Bolts with nylon patches and thread locking compounds are not visible to the inspectors so it is hard to validate that they are present during technical inspection.