In standard truck-mounted tipper applications, the hydraulic system has a relatively simple architecture in which one power source supplies one consumer—the cylinder. When semi-trailers, tandem bodies or truck-trailer combinations are introduced to increase payload and volume efficiency, however, the complexity of the hydraulic circuit and the parameters to be managed change fundamentally. In these advanced systems, it is essential to control multiple cylinders independently or in coordination and to manage fluid transfer safely between the tractor and trailer.

Hydraulic Distribution between Tractor and Trailer

A semi-trailer hydraulic architecture consists of two principal zones that are physically connected but functionally separate:

• 1. Tractor Unit Side: This is where energy is generated. The PTO connection, hydraulic pump, oil tank—generally selected as a rear on-chassis mounting—and the main directional control valve are located here.
• 2. Trailer Side: This is where the work is performed. Type K telescopic cylinders with cradle mounting geometry suitable for horizontal transport positions, together with end-of-stroke safety valves, are generally installed in this section. Other types of telescopic cylinder may also be used according to customer requirements.

The fluid connection between the two units is established through flexible high-pressure hoses and quick couplings that accommodate the coupling and uncoupling dynamics of the vehicles.

Function of Trailer Diverter Valves

In combined systems, special valve mechanisms such as the VR3600-series Diverter Valve determine which body or function receives the fluid discharged by the pump.

• 3-Way, 3-Position Configuration (VR36001040): This provides high operating flexibility. According to the pneumatic control command, hydraulic flow can be directed only to the truck’s own body, only to the cylinder on the rear trailer, or to both units for simultaneous operation.
• 3-Way, 2-Position Configuration (VR36001041): This is preferred in simpler circuits where flow is directed either only to the truck or only to the trailer/crane.

Long Line Routes and Quick-Connection Dynamics

In semi-trailer and tandem designs, hydraulic lines are physically much longer than in standard vehicles. This introduces critical engineering parameters for line management:

• Pressure-Drop Control: Fluid loses pressure through internal friction as it passes along long hose routes and through fittings. To minimise this loss, correctly sized hoses conforming to SAE J514 (R2) should be used in pressure lines, while vacuum-resistant SAE 100R4 hoses should be used in suction lines.
• Safe Disconnection (ISO 7241-A): When the tractor and trailer are separated, the hydraulic lines must be disconnected without allowing air into the system or spilling oil into the environment. Self-closing quick couplings compliant with ISO 7241-A and rated for operating pressures of 250-300 bar provide this operational safety.
• Manoeuvring Flexibility: Correct elbow geometry and hose-support systems must be planned during the design stage to prevent hoses from stretching or being pinched while the tractor turns.

Conclusion

Multiple hydraulic-line management in semi-trailer and tandem-truck applications can be stabilised through correct tandem-pump selection, flexible diverter-valve integration and hose/fitting components compliant with international standards. Sizing each part of the system through an integrated engineering approach provides high operational flexibility and long-term field reliability.