In hydraulic control systems, directional control valves are central components that determine the direction, speed and stopping position of fluid power. These valve mechanisms manage when pressurised oil from the pump is delivered to the cylinder and how it returns to the tank. The two principal configurations in the HMLIFT directional control valve range—open-circuit and closed-circuit system designs—define the overall operating principle and character of the hydraulic circuit.

Open-Circuit Directional Control Valve (Tipping Valve)

Valves with an open-circuit control architecture are also known in the industry as “Tipping Valves” and are defined by the behaviour of the fluid when the system is in standby (neutral) mode. In this configuration, when the valve is in neutral, hydraulic oil from the pump returns directly to the tank with low resistance and without encountering an obstruction.

The operating cycle is characterised as follows:

• Neutral (Standby) Position: The pump continues to rotate, but because the oil circulates freely back to the tank through the valve, unnecessary pressure build-up and energy consumption do not occur in the system.
• Lifting Position: When a command is issued from the driver’s cab, the spool inside the valve changes position, closes the tank return path and directs the oil straight to the cylinder.
• Lowering Position: The valve connects the cylinder line to the tank; the fluid is discharged to the tank under the external load and the weight of the body.

Closed-Circuit Directional Control Valve (Directional Valve)

Closed-circuit directional control valves are designed to block all hydraulic lines and flow paths when they are in the neutral position. While the valve is in standby mode, flow from the pump is retained in the line and is not allowed to pass freely and directly to the tank.

The operational characteristics of this architecture are as follows:

• Locking in Neutral: Because the lines to and from the cylinder are closed inside the valve, hydraulic fluid is trapped within the system. This supports mechanical locking of the cylinder in its current position and reduces the risk of unintended drift.
• Controlled Lowering Capability: Internal channels in advanced models allow the return rate of the fluid to the tank to be controlled more precisely, enabling fast and slow lowering options.

Technical Capacities and Functional Features

The HMLIFT valve family offers a range of models that can be matched to the application’s volumetric flow requirement and the system operating pressure. The open-circuit tipping-valve family covers a flow range of 140-200 L/min, while the closed-circuit directional-valve family typically operates within 110-140 L/min. Depending on the model, operating pressure is specified between 190 and 250 bar.

Some of the special functions that stand out during integration are described below:

Fast-Slow Lowering Function

Pneumatic staging available in certain valve models allows the downward movement of the body to be managed in two different speed modes. Once unloading has been completed, lowering the body quickly through the safe range shortens cycle times, while slow and controlled descent can be selected for precision operations.

Trailer Diverter Valves

In combined vehicles where a truck and trailer/semitrailer are used together, VR3600-series diverter valves direct the hydraulic flow from a single pump source to the correct unit. Depending on the requirement, these valves are available with two- and three-position options that can direct flow only to the truck cylinder, only to the trailer cylinder, or supply both units together.

System Integration and Selection Criteria

A directional control valve must not be considered in isolation within the hydraulic circuit. The pump’s maximum flow, the system’s peak operating pressure, the type of end-of-stroke safety valve to be used and the number of channels in the pneumatic joystick control in the driver’s cab must all be fully compatible with the valve selection.

Conclusion

The directional control valve configuration is a fundamental design decision that determines the operating character of a vehicle-mounted hydraulic system. The appropriate model should be selected by analysing the application geometry, operational safety expectations and flow requirements. HMLIFT technical catalogues and engineering guides are recommended as references when determining the most suitable valve integration for a project.