The flow configuration of the valve in the center position often requires careful consideration. Many configurations are possible, from all ports being blocked to all ports being interconnected. Figure 4 illustrates some common arrangements:

1. All Ports Blocked (a): This configuration blocks pump flow at the ‘P’ port, allowing the pump flow to be used elsewhere in the system. It also prevents flow into or out of an actuator, effectively locking it in place. However, depending on the valve type, leakage between ports might still occur, potentially allowing the actuator to creep. For hydraulic motors, internal leakage across ports and into the external drain connection means even a leak-proof valve may not fully lock the motor. In applications like clamping or using an accumulator to maintain pressure on a cylinder, a leak-proof valve can be particularly beneficial.

2. Automotive Lift Repair Tampa Florida  Open Center Valve (b): In this configuration, the pump flow is unloaded back to the tank when the valve is in the central position, allowing the actuator to float freely under external forces. While this reduces waste heat generation, it also means that the pump flow is not available for other functions while the valve is centered.

3. P Port Blocked, A and B Ports Open (c): This setup allows the pump flow to be blocked while keeping the ‘A’ and ‘B’ ports open to the tank. This enables the actuator to float while still using the pump flow for other functions.

4. Automotive Lift Repair Tampa Florida  Tandem Center Condition (d): This configuration unloads the pump but prevents actuator movement. Similar to other configurations, leakage into or out of the ‘A’ and ‘B’ ports may still occur when the valve is centered.

The most suitable configuration depends on whether the pump flow needs to be unloaded and the specific requirements of the actuator and its load. 

Some manual directional valves allow for progressive opening and closing of flow paths, offering additional control over actuator speed and its acceleration and deceleration rates. 

Finally, determining the valve’s pressure and flow ratings is straightforward once system design parameters are known. For differential area cylinders, remember that flow rates through the valve will vary due to differences in piston areas on either side. For example, with a 2:1 area ratio, exhaust flow from the annulus side will be half the inlet flow to the full bore side during extension, and during retraction, it will be twice the inlet flow to the annulus side. Thus, the valve’s flow rating may need to exceed the pump flow. Additionally, when controlling differential area cylinders, potential pressure intensification, which occurs when restricting exhaust flow from the annulus side, must be considered when determining the valve’s pressure rating.

When a cylinder is subjected to a negative (“runaway”) load or is decelerating a high-inertia load, it may require a valve with a pressure rating higher than the normal system operating pressure at the pump outlet.

VALVE OPTIONS

Once the basic specifications for the Automotive Lift Repair Tampa Florida  directional valve are established, the next step is to select the type of valve that best meets these specifications. Valve construction options include:

– Sliding spool valves

– Rotary spool valves

– Ball valves

– Poppet valves

– Rotary shear valves

SLIDING SPOOL VALVES

Automotive Lift Repair Tampa Florida  Sliding spool valves, as their name implies, operate by moving a spool linearly within a closely fitting body to connect or block ports machined into the body, as illustrated in Figure 5.

These valves are available in various configurations, including two or three positions, spring-offset, spring-centered, and detented. They also offer multiple options for the spool center condition. Sliding spool valves can be individually mounted, typically on a standard ISO 4401 / CETOP / NFPA manifold interface, or mounted together as valve segments when multiple valves are needed. Manifold-mounted valves often use components common to solenoid valves, which are mass-produced by many manufacturers, resulting in lower manufacturing costs. As a result, sliding spool valves are among the most cost-effective manual directional valves available.

Typically, these Automotive Lift Repair Tampa Florida  valves range in size from 30 L/min (8 gpm) to 120 L/min (32 gpm) and beyond. For valves designed to fit a standard interface, the maximum pressure rating is determined by the interface itself and typically ranges from 250 to 350 bar (3600 to 5000 psi).

As with all sliding spool valves, sealing between ports is achieved by the close fit of the spool within the body. The clearance between the spool and the body represents a compromise: it must be tight enough to minimize leakage while allowing the spool to slide freely without sticking, which is particularly important for spring-return spools. For a valve rated at 75 l/min (20 gpm), typical leakage rates from the P port to the T port can be around 150–200 mL/min (10–12 in³/min) when operating at maximum pressure.

The clearance between the spool and the body also makes these valves vulnerable to contamination. Particles forced into this gap by pressure differences can cause wear and potential malfunctions, such as failure of the spring return mechanism. Even particles smaller than the clearance can accumulate over time, leading to spool jamming—a phenomenon known as ‘silting.’

ROTARY SPOOL VALVES

Automotive Lift Repair Tampa Florida  Rotary spool valves also use a spool fitted within a sleeve, but in this design, the spool is rotated to select the flow paths rather than sliding (see Figure 7).

Inlet flow typically enters at the end of the spool and travels through a central passageway within the spool. As the spool is rotated by the hand lever, holes and slots in the spool either open or block off the ports in the body, creating the desired flow paths.

Automotive Lift Repair Tampa Florida  Rotary spool valves are often produced as ‘screw-in cartridge’ valves, allowing them to be integrated into manifold blocks with other control valves or mounted individually. In some cases, a rotary lever is less prone to accidental activation, and when panel-mounted, the pipe connections and valve body can be positioned behind the panel, with only the operating lever visible on the panel’s front.

The leakage and contamination sensitivity of rotary spool valves are similar to those of sliding spool valves, so they are not ideal for applications requiring a leak-proof function.