Background
With my country's rapid economic growth and substantial state investment in highway infrastructure, road transport has become the country's primary mode of transportation. Semi-trailer tractors, serving as the core equipment for road transport, are widely utilized due to their specialized capabilities, high load capacity, and efficient transport speeds. The fifth-wheel coupling-the connecting mechanism between the tractor and the semi-trailer-is a critical safety component; although it may not appear complex or conspicuous, improper operation or inadequate maintenance can lead to serious safety accidents.
Regarding technical specifications for connection safety, an international standard titled "Commercial vehicles - Location of electrical and pneumatic connections between towing vehicles and trailers"-revised under my country's leadership-was approved and officially released by the International Organization for Standardization (ISO) on April 27, 2026. Addressing safety concerns during the coupling process, this standard focuses on key aspects such as the positioning and installation sequence of electrical and pneumatic connectors, operational clearances, and line protection, aiming to further enhance the operational safety of tractor-trailer combinations through standardization.
Types
Based on the mobility of the mounting assembly, they are classified into fixed, sliding (movable), and lifting types. In the fixed type, the coupling assembly is bolted to a base plate, which is in turn secured to the tractor's chassis frame; this is the most widely used configuration. Sliding fifth-wheel couplings come in various designs but share a similar structure to the fixed type; the primary difference lies in the connection between the base plate and the chassis frame, which utilizes a multi-hole or toothed design. This allows the assembly to be repositioned longitudinally to accommodate different trailers. Some sliding models feature an automatic pneumatic control system that uses air pressure to slide the assembly along its rails. Lifting fifth-wheel couplings are primarily used on tractors in port terminals; they can move vertically within a specific range to accommodate semi-trailers of varying heights.
They are also classified by their degrees of freedom into single-degree-of-freedom and dual-degree-of-freedom types. A single-degree-of-freedom fifth-wheel coupling is also known as a single-axis coupling. It allows for only longitudinal tilting (pitching) of approximately 80 degrees. This type of fifth wheel offers high driving stability and is suitable for high-speed, lightly loaded semi-trailers with high centers of gravity operating on good road surfaces. Its drawback is that, due to the lack of lateral oscillation, the vehicle frame is subjected to significant torsional stress. The two-degree-of-freedom fifth wheel-also known as a dual-axis fifth wheel-allows for both lateral and longitudinal oscillation, thereby protecting the frames of heavy-duty transport tractors across various road conditions. However, because dual-axis fifth wheels are significantly taller than single-axis versions, they offer inferior lateral stability during high-speed cornering; consequently, this type is rarely used in standard highway transport.
