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The fifth wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. Some recreational vehicles use a fifth wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle. The coupling consists of a kingpin, a 2-or-3 1⁄2-inch (50.8 or 88.9 mm) diameter 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. Today's fifth wheels allow the trailers to slide into the fifth wheel and lock into it, and are a very reliable unit when maintained and serviced properly. The engagement of the king pin into the fifth wheel locking mechanism is the only means of connection between tractor and trailer, no other device or safety mechanism is used. Couplers and pintle hooks will use safety chains in the event of a trailer separation while going down the road. Trailer-to-trailer connection can also be made by using fifth wheels.
The invention of the fifth wheel is often credited to U.S. inventor Charles H. Martin of the Martin Rocking Fifth Wheel Co. who invented the device in 1911. The earliest documented patent of a fifth wheel is in patent number 2,053,812 issued to Charles E. Bradshaw of Wellville, VA filed March 18, 1936 and granted September 8, 1936. One third of the patent was assigned to Charles Martin, also of Wellville, VA.
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.