Hydraulic Control Lecture 6: Directional Control Valves

Types of Check Valves

• Right-Angle Check Valve: used for high flow rate applications
• Inline Check Valve: used to prevent back flow in any line where it cannot be tolerated

Check Valve Applications

• In a combination pump
• In accumulator circuit
• To protect a blocked filter by diverting flow around the filter when accumulator pressure rises
• In the tank return, to prevent fluid being sucked out of the tank when the pump is turned off

Restriction Check Valve

• Used to control the speed of the actuator by adjusting the flow rate at which a fluid is admitted to, or allowed out from, a device
• Can be pressure and/or temperature compensating to maintain the set flow rate regardless of changes in the load pressure and fluid temperature

Shuttle or Double Check Valve

• Permits flow from either of two inlet ports to a common outlet
• A free-floating metal ball shuttles according to the relative pressure at the two inlets

Pilot-Operated Check Valve

• Always permits free flow in one direction but permits flow in the normally blocked opposite direction only if pilot pressure is applied at the pilot pressure port of the valve
• A pilot-to-open check works as a regular check valve when no pressure is applied to the pilot port
• The pilot-to-close valve presents the opposite behavior

Directional Control Valves

• Specified by the number of connected lines (ports) and the number of control positions
• Example: 4/3-way valve, 3-way valve, 2/2-way valve

Three-Way and Four-Way Valves

• Three-way valves contain three ports and are typically of the spool design
• Four-way valves contain four ports and have different positions (e.g. Position 1, Position 2, No Flow, Position 3)

Hydraulic Control Valves

• Basic components of a hydraulic system: valves, hydraulic control valves
• Types of hydraulic control valves: directional control valves (DCVs), pressure control valves (PCVs), flow control valves (FCVs)

Directional Control Valves Configurations

• Seat valve: simple in design, robust, low cost, less sensitive to fluid contamination, low maintenance
• Disadvantages of seat valve: force required to operate is more, suitable mostly for low pressure applications
• Sliding spool valve: less force required to actuate, actuation force unaffected by changes in operating pressure
• Disadvantages of sliding spool valve: lower flow rate, more seal wear, short product life, more susceptible to contamination, higher cost and maintenance
• Rotary spool valve: less force required to actuate, simple and compact in size
• Disadvantages of rotary spool valve: more seal wear, short product life, more susceptible to contamination, higher cost and maintenance, higher leakage

Directional Control Valves Actuation

• Means to switch the valve from one position to another
• Classified into: check valve, pilot-operated check valve, three-way valves, four-way valves