The axial travel of a screw or nut per revolution. For an ordinary single-start fastener, lead equals pitch; a double-start thread has a lead of twice its pitch, so it moves twice as far per turn with the same thread depth. Multi-start threads appear on bottle caps, quick-acting closures, valve stems and lead screws, and they are less self-locking than single-start threads of the same pitch.
It's a two-start thread, so the lead's double the pitch, that's why the cap spins on so fast.
Also heard as
- thread lead
Do not mix it up with
- Pitch: Pitch is crest-to-crest spacing; lead is advance per turn, which is larger on multi-start threads.
Related terms
Pitch
The distance from one thread crest to the next, measured parallel to the screw axis.
TPI
Threads per inch: the count of thread crests in one inch of length, used to state the pitch of inch threads.
Left-hand thread
A thread that tightens counterclockwise, used where normal rotation would loosen a right-hand fastener.