Fifth Wheel Coupling
From RitchieWiki
A fifth wheel coupling is used to connect a trailer to a semi-truck so it can be supported and towed. It allows the truck and semi-trailer to pivot horizontally at turns.
It consists of two plates. The upper plate is attached to the trailer and has a kingpin (or coupling pin). The lower plate is connected to the truck and has locking jaws, which grip the kingpin. The plates are lubricated with grease to help with turning.
