Section 6.5

Locking Devices

Split pins, lockwire, self-locking nuts, tab washers, circlips and keys — positive and friction locking.

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Main ideas

Locking devices stop fasteners loosening under vibration — a fastener is not secure until it is positively locked. They fall into friction locking (spring washers, self-locking nuts), positive locking (split pins, lockwire, tab washers) and specialised methods.
Split (cotter) pins give a positive lock through a castellated nut on a drilled bolt; the legs are bent to retain them and they are single-use. Lockwire (safety wire) ties fasteners so any loosening tendency tightens the wire.
Self-locking nuts use a fibre/nylon insert (temperature-limited) or an all-metal deformed thread to create friction; tab and locking-plate washers are bent up against a flat to positively lock.
Circlips and retaining rings hold parts on shafts or in bores; keys and keyways transmit torque and prevent rotation of a component on a shaft.
Golden rules: use the specified device, install lockwire so it pulls in the tightening direction, and never re-use single-use items (split pins, lockwire, spent self-locking nuts).
⚠ Exam trap: safety/lock wire must always be routed so that any tendency of the fastener to loosen TIGHTENS the wire; and split pins and lockwire are single-use — a removed one is discarded, never refitted.

Solved examples

Two bolts are to be lockwired together. How is the wire routed, and why does direction matter?
The wire is fitted so that if either bolt tries to unscrew, the pull it exerts on the wire tends to tighten it (and the other bolt). Routed the wrong way, a loosening bolt would slacken the wire and defeat the lock. New wire is used each time and twisted evenly with the correct tension.

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