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Basic Hydraulics - Modular Control Valves


CHAPTER 7 - Modular Control Valves



Figure 7.1 - modular valve

Introduction
You may already be familiar with modular valves including cartridge and stack valves. In this section, you will learn the how’s and why’s of modular valves to enhance system design and troubleshooting. For example, you will see how these valves help designers eliminate many of the problems associated with external plumbing between valves and you will increase your expertise in the operating principles and use of these valves.

Stack Valves
Stack valves are bolted together in a compact stack, eliminating the need for external plumbing between components. By eliminating the pipes, tubes and hoses typically used to interconnect the various valve components, the assembly is compact and free of potential leak points that occur with external plumbing. The standard sub-plate mounted directional control valves and cartridge style (flow control, pressure control and check valves mounted in sandwich valve blocks) are used to create stack valve circuits. Each valve block has a mounting surface on the top and the bottom.
Figure 7.2 - stack valves
Valve blocks that can only be mounted in one position have a chamfered O-ring seat around each port on the bottom-mounting surface.
Figure 7.3 - chamfered o-ring seat and o-ring
Flow control valves can be rotated to change the metering direction of the valve. A thin sealing plate holds the O-rings in place, creating the sealing surface for the flow control valve.
Figure 7.4 –flow control valve with thin sealing plate
The valve stack is attached to a sub-plate or manifold using long bolts extending through the entire valve stack. Sub-plates and manifolds have a common supply (P), a common return (T), and work ports (A, B). The surface where the valves are mounted have a P, T, A, and B port for each actuator.
Figure 7.5 - manifolds with labeled supply, return, and work ports


Assembling
The directional control valve is always located at the top of the stack. The relief valve should be placed at the bottom of the stack next to the sub-plate. Flow control valves and pressure control valves are placed between the relief valve and the directional control valve.
Figure 7.6 - stack valve assembly

Using proper procedures when assembling stack valves will produce a leak free circuit. Clean and check all O-ring seats for damage, center new O-rings in each seat, and alternately tighten the mounting bolts until all bolts are properly torqued.
Figure 7.7 - stack valve assembly

A single valve stack can be mounted on a manifold block or sub-plate, while multiple valve stacks can only be mounted on a manifold block. Each valve stack would be used to control an actuator.

Stack valves can be mounted on a manifold block in conjunction with circuits containing cartridge valves.
Figure 7.8 - manifolds with labeled supply, return, and work ports

Sub-plates and manifolds have a supply (P) port, a return (T) port, and A and B work ports which line up with corresponding P, T, A and B ports on the mounting surface. Manifolds and sub-plates with X and Y ports can be supplied when using directional control valves that require an external pilot or external drain.

Watch as the following components are converted to a schematic of a single stack that controls an actuator. The relief valve is placed next to the manifold to provide protection to the circuit. Pressure and flow controls are sandwiched in between the relief valve and the directional control valve. The directional control valve, which only has one mounting surface, is always at the top of the valve stack.

Schematic
This schematic represents a system of three valve stacks mounted to a fourstation manifold (Figure 7.10). The alternating dot and dash lines are enclosure lines. Each component outlined by an enclosure line represents a module with its internal passages. A blanking plate is used to block the ports of the unused station.
Figure 7.9 - schematic of a single stack

There are four flow paths through each valve stack. The pressure and return passages through the stack assembly connect the pressure and return ports of the directional control valve to the respective ports on the manifold. The A and B work passages through the stack assembly connect the work ports of the directional control valve to the ports of the actuator. 

The system relief valve is located in the stack closest to the common supply and return ports.
Figure 7.10 - schematic of three valve stacks mounted to a four station manifold

Sizing
Stack valves are available in four ISO sizes as shown in the table (Table 7.1). The ISO size 02 valve has a maximum pressure rating of 215 bar and a flow rating of 30 liters per minute.
Table 7.1 - standard ISO stack valve sizes

The ISO size 07 valve has a maximum pressure rating of 315 bar and a flow rating of 200 liters per minute.

Cartridge Valves
Cartridge valves can be used to perform flow control, pressure control, and directional control, load holding and logic functions. They are a very compact design that must be installed into special cavities in valve blocks or manifolds. Cartridge valves, used in combination with a manifold, can be used to create a compact circuit, free of potential leaks associated with external plumbing. Cartridge valve can be divided into two design categories: screw-in and slip-in.
Figure 7.11 -cartridge valve and manifold

Screw-In
The screw-in style cartridge valve uses a threaded base to secure the valve in the cavity. Screw-in cartridges use either a spool, poppet or ball valve element. The spool element can be used for 2-way, 3-way or 4-way flow functions while the poppet and ball elements provide for only a 2-way flow function. Screw-in cartridge valves are typically used in low flow systems where the flow is 35 gpm or less. Some of the available screw-in valve configurations are: check valve, relief valve, needle valve, solenoid operated directional control valve, and manually operated directional control valves.
Figure 7.12 - screw-in style cartridge valves in a manifold block

Slip-In
Slip-in cartridge valves use an insert consisting of a poppet, spring, seals and a sleeve, which slips into a cavity machined into a manifold block. A cover assembly bolted to the manifold block secures the insert in the manifold. The cover assembly usually contains one or more internal flow paths, which allows the cartridge valve to interact with another valve in the manifold to create a specific function within the manifold. A directional control valve used to control the function of a slip-in cartridge can be mounted on the surface of an interface cover.
Figure 7.13 –exploded view of a slip-in cartridge valve assembly

Some cover assemblies also contain other options, such as orifices, stroke adjustors or a direct acting relief valve, to create additional valve functions. 

These control covers function like the pilot section of a two-stage valve, while the slip-in cartridge functions like the main stage of a two-stage valve. Slip-in cartridges are typically used in circuits with flows of 30 gpm or more.

The poppet element of the slip-in cartridge is available in several area ratios. The first poppet has a 1:1 area ratio, the second has a 1:1.1 ratio, the third has a 1:2 area ratio and the last poppet has a 1:2 area ratio with metering notches.
Figure 7.14 - diagram of the different area ratios for slip-in cartridge valves

Notice how the symbols are drawn to show the differences in area ratios. The last ratio also shows the metering notches. The ports are typically labeled A, B, and Ap. A and B are the work ports and Ap is the pilot port. The pressure at each port, area ratios, and spring force will determine whether the valve will be open or closed. This schematic represents a cylinder being controlled by four slip-in cartridge valves, each of which is controlled by a directional control valve. By actuating or de-actuating the directional control valves in different combinations, twelve potential flow path combinations are possible.

Several examples are:
• Energizing valves 2 and 4 will allow flow from the P port to the A port and return flow from the B port to the T port so the cylinder will extend.
• Energizing valves 1 and 3 will allow flow from the P port to the B port and return flow from the A port to the T port so the cylinder will retract.
• Energizing valves 2 and 3 will create what is called a regenerative circuit, which will cause an increase in the cylinder extend rate. Flow is from port P to port A with the return flow from port B combining with the flow going to port A.
• With all four valves de-energized, the cylinder will be held in the position where it is stopped. Pressure from the A port of each directional control valve holds the respective cartridge valve closed. This blocks both the A and B ports of each cartridge valve. Since they are indeed check valves, they have zero leakage so there will be no cylinder drift.
• Energizing all four valves will cause the P, T, A and B work ports to be in common or open to each other. This will allow pump flow to return to the tank and the cylinder to float.

In accordance with ISO 7368 or DIN 24324, slip-in cartridge valve configurations are rated by size in millimeters. This table shows values for slip-in cartridges ranging in size from 16 millimeters to 100 millimeters. Each valve has a nominal flow rating and is measured as flow through the valve at a pressure drop of 72 psi. For example, at 72 psi a 25 millimeter valve would have a nominal flow rating of 119 gallons per minute.










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