Fifth wheel
The fifth-wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. Some camper trailers use a fifth-wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle, and "fifth wheel" is therefore sometimes used as a synonym for such campers in North America. The coupling consists of a kingpin, a 2-or-3 1⁄2-inch-diameter (50.8 or 88.9 mm) steel pin on the front of the semi-trailer, and a horseshoe-shaped coupling device called a fifth wheel on the rear of the towing vehicle. The surface of the semi-trailer (with the kingpin at the center) rotates against the surface of the fixed fifth wheel, which does not rotate. To reduce friction, grease is applied to the surface of the fifth wheel. The configuration is sometimes called a turn-table in Australia and New Zealand, especially if it is a rotating ball-race-bearing type. The advantage of this coupling is improved towing stability.
The term fifth wheel comes from a similar coupling used on four-wheel horse-drawn carriages and wagons. The device allowed the front axle assembly to pivot in the horizontal plane, to facilitate turning. Basically a wheel was placed on the rear frame section of the truck, which back then only had four wheels; this wheel that was placed on the frame was the "fifth wheel", hence the name. The trailer needed to be raised so that the trailer's pin would be able to drop into the central hole of the fifth wheel.
Fifth wheels were originally not a complete circle and were hand forged. When mass production of buggy parts began in the early 19th century, fifth wheels were among the first products to be made. There were a number of patents awarded for fifth-wheel design. Edward and Charles Everett, Quincy, Illinois patented a type of fifth wheel in 1850, followed by Gutches' metallic head block and fifth wheel in 1870 and Wilcox fifth wheel in 1905.
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