Background
With the rapid development of our national economy and the state's substantial investment in highway construction, road transport has become the primary mode of transportation in our country. As a key piece of equipment in road transport, the semi-trailer tractor-or "tractor unit"-has gained widespread application due to its advantages of high specialization, large payload capacity, and rapid transport speed. The fifth wheel coupling serves as the connecting device between the tractor unit and the semi-trailer. Although it may not be a particularly complex or visually prominent component, it constitutes the single most critical safety device on a semi-trailer tractor; operational errors or improper maintenance can lead to severe safety accidents.
Regarding technical specifications for connection safety, on April 27, 2026, the international standard *Commercial vehicles-Locations of electrical and pneumatic connections between towing vehicles and trailers*-revised under my country's leadership-was approved and officially published by the International Organization for Standardization (ISO). Addressing safety issues arising during the coupling of tractor units and trailers, this standard focuses on revising several core aspects, including the positioning of electrical and pneumatic connectors, installation sequences, operational clearances, and line protection measures. Its objective is to further enhance the operational safety levels of road trains through standardization.
Types
Based on whether their supporting mounts are movable, fifth wheel couplings are classified into three types: fixed, sliding (movable), and lifting. In the fixed-type fifth wheel, the left and right support brackets are bolted securely to the fifth wheel base plate, which is then rigidly fastened to the chassis frame (longitudinal beams) of the tractor unit. The fixed type is the most widely used variant. Sliding-type fifth wheels come in various designs; while their basic structure is largely similar to that of the fixed type, the key difference lies in the connection mechanism between the base plate and the tractor's chassis frame-typically employing a multi-hole or toothed rail design. This allows the fifth wheel base plate to be adjusted forward or backward as needed to accommodate the specific semi-trailer being coupled, thereby facilitating smooth and secure connections with different trailers. Some sliding-type fifth wheels are also equipped with an automatic pneumatic control system, which utilizes air pressure to drive the fifth wheel base plate back and forth along its sliding rails. Lifting-type fifth wheels are primarily utilized on tractor units operating within port terminals; these units feature a fifth wheel coupling that can be raised or lowered within a specific vertical range to accommodate semi-trailers of varying heights.
Based on their degrees of freedom, fifth wheel couplings are further categorized into single-degree-of-freedom and double-degree-of-freedom types. The single-degree-of-freedom fifth wheel is also referred to as the single-axis fifth wheel. It is capable of longitudinal tilting of approximately 8 degrees only. This type of fifth wheel offers superior driving stability and is suitable for high-speed, lightly loaded semi-trailers-particularly those with a high center of gravity-operating on well-maintained road surfaces. Its primary drawback is that, due to its inability to tilt laterally, it subjects the vehicle frame to significant torsional stress. The dual-degree-of-freedom fifth wheel is also referred to as the dual-axis fifth wheel. Capable of both lateral and longitudinal tilting, it serves to protect the frames of heavy-duty tractors operating across a variety of road conditions. However, because the dual-axis design sits significantly higher than its single-axis counterpart, it exhibits compromised lateral stability during high-speed cornering; consequently, this type of fifth wheel is rarely utilized in general road transport applications.
