A common question asked by people outside of the trucking industry is, “How do you connect a road tractor to a semi-trailer?” The union between these two transport equipment is made by means of two connection devices. One is the “fifth wheel” which is mounted on the rear frame of the road tractor. The other is called a “center bolt,” which is located at the front of the truck trailer. Here is a brief explanation.
What is a fifth wheel?
Originally, a fifth wheel was a wheel-like steering mechanism that allowed the front axle of a horse-drawn carriage to turn when turning. Around 1910, John C. Enderock invented a device for attaching trailers to motor vehicles. In their initial use, the trailers were towed by Ford Model “T” passenger cars. This coupling scheme required three men to hitch and unhitch the car’s trailer. In 1918 Enderock designed a new coupling device that allowed a single driver to easily connect the tractor and trailer. The new fifth wheel design used a clamp with a spring-loaded locking device mounted on the fifth wheel plate that was attached to the tractor frame.
Today, the fifth wheel serves as a coupling device connecting a road tractor to a semi-trailer. A fifth wheel is a heavy metal disc with a “V” shaped groove. It is fixed horizontally to the chassis above the rear axles of the road tractor. The “V” groove is located at the rear and contains a locking device. The fifth wheel mechanism used to connect tractors and trailers today is very similar to the Enderock design of the early 20th century.
What is a Kingpin?
A kingpin is a heavy metal cylindrical pin that sits under the front of the trailer. The kingpin is the trailer mechanism that attaches it to a road tractor. It is typically positioned between 18″ and 48″ from the nose of the trailer. An oiled metal grill plate surrounds the kingpin. This rack plate allows the fifth wheel to slide under the trailer. When the tractor is backed up towards the front of a trailer, the fifth wheel turns and slides under the trailer’s grill plate. The fifth wheel then locks onto the trailer kingpin.
When attached to the kingpin, the fifth wheel allows the trailer to turn at the point where the fifth wheel and kingpin meet. The swivel attachment allows the tractor/trailer combination to make turns and provides stability and maneuverability on the road. When the load is delivered, the driver disconnects the tractor from the trailer by lowering the legs of the trailer dolly. He then pulls a lever to disengage the fifth wheel locking mechanism and moves the tractor away from the trailer.
For more information on semi trailers, check out the American Trailer Exchange
