Background
With my country's rapid economic development and substantial national investment in highway construction, road transport has become a major mode of transportation in the country. Semi-trailer tractors, as the main equipment in road transport, are widely used due to their advantages of high specialization, large loading capacity, and fast transportation. The towing seat is the connecting device between the tractor unit and the semi-trailer. Although it is not a particularly complex or eye-catching component, it is the most important safety device for a semi-trailer tractor unit. Improper operation or maintenance can lead to serious safety accidents.
Regarding the technical specifications for connection safety, on April 27, 2026, the international standard "Commercial Vehicles - Electrical and Pneumatic Connection Positions Between Tractor and Trailer," revised under my country's leadership, was approved and officially released by the International Organization for Standardization (ISO). This standard addresses safety issues during the connection of tractor and trailer, focusing on revising several core points such as the arrangement, installation sequence, operating clearances, and pipeline protection of electrical and pneumatic connectors, aiming to further improve the operational safety level of truck-trailer combinations through standardization.
Types
Based on whether their supports can move, they are divided into fixed, movable, and lifting types. Fixed tow hitches have two supports bolted to the tow hitch base, which is then fixed to the tractor's main beam. The fixed type is the most widely used. Mobile tow hitches come in various styles, with structures similar to the fixed type. The main difference is the multi-hole or toothed design used in the connection between the tow hitch base and the tractor's main beam. The tow hitch base can be moved forward and backward as needed to connect with different trailers. Some mobile tow hitches are also equipped with automatic pneumatic control devices, allowing the tow hitch to move forward and backward within a chute using pneumatic pressure. Lift-type tow hitches are mainly used on tractors in ports and docks. The tow hitch can move up and down within a certain range to accommodate semi-trailers of different heights.
Based on degrees of freedom, tow hitches are divided into single-degree-of-freedom and double-degree-of-freedom types. A single-degree-of-freedom tow hitch, also known as a single-axle tow hitch, can only tilt longitudinally by about 80 degrees. This type of tow hitch offers high driving stability and is suitable for high-speed, lightly loaded semi-trailers with a high center of gravity that travel on good road surfaces. The disadvantage is that because it cannot swing laterally, the frame bears a larger torque. A dual-degree-of-freedom tow hitch, also known as a dual-axle tow hitch, can swing laterally and longitudinally, protecting the frame of heavy-duty transport tractors in various road conditions. However, because dual-axle tow hitches are significantly taller than single-axle tow hitches, their lateral stability is inferior during high-speed cornering, so this type of tow hitch is rarely used in ordinary road transport.
