The kingpin is a thick steel pin welded to the underside of the trailer's front plate. It is the primary connection point between the truck and the trailer. When coupling, the driver backs the tractor under the trailer, sliding the kingpin into the throat of the fifth wheel until the spring-loaded jaws lock around it. It must withstand the pulling force of the entire trailer weight.
Because of the extreme friction and pull forces, the kingpin must be checked regularly for wear. If it gets bent or worn thin, it can damage the fifth wheel jaws or cause a catastrophic trailer detachment. Mechanics use a kingpin wear gauge to measure its diameter. Any kingpin showing significant wear or cracks must be cut out and replaced by a certified welder.