fifth wheel

Definition

A type of trailer hitch, which consists of a horseshoe-shaped plate on a multidirectional pivot, with a locking pin to couple with the kingpin of a truck trailer.

Etymology

From the fact that since many vehicles such as carriages, coaches, wagons, prime movers, and trucks generally have four wheels on which they move, this component which resembles a wheel is the fifth. * (anything superfluous or unnecessary): Sense 3 refers to the fact that a fifth wheel is not needed for a four-wheeled vehicle to operate.

Example Sentences

• "In most forms of semi-trailers a great deal of time is wasted in connecting the trailer to the tractor or pulling vehicle. Most of the connecting gears are some form of fifth wheel, consisting of two horizontal circular bands of iron, one fitting on top of the other, with grease between and with a bolt or king-pin dropped through holes in the hubs or center bosses of both parts of the fifth wheel."
• "The semi-trailers are equipped with hydraulic brakes which operate through the king pin of the fifth wheel. The booster-controlled fifth wheel gives full and complete cab control of all braking and coupling operations."
• "From the blow-back tank, the fluid is piped into a larger tank commonly known as a fracturing (frac) tank or wheely tank. This is a large, 21,000-gallon tank mounted on wheels and provided with a fifth wheel for towing from jobsite to jobsite by a semi-tractor."