Basic Hydraulics Hydr 1305 Control Valves HYDR 1305

Basic Hydraulics Hydr 1305 Control Valves HYDR 1305 -11 Chapter 06_Hydraulic Actuators Source: Parker_Industrial Hydraulic Technology_Cleveland 1

• Introduction o The amount of pressure created by a hydraulic pump must be at least as much, and in practice more than required, to move a load with a hydraulic cylinder. § The two types of pressure control devices are the "pressure regulator valve" and the "pressure relief valve" • • Pressure Regulator Valves reduce high pump pressure to the level required by the hydraulic system. Pressure Relief Valves are mainly used to prevent a hydraulic pump from being deadheaded, but may also be used for pressure control. o Flow Control Valves control the rate of fluid flow. o Control Valves direct fluid to and from a cylinder to position a piston in a cylinder.

• Pressure Regulator Valves o Pressure regulators control downstream pressure by controlling the amount of fluid that flows into a hydraulic circuit. o A pressure regulator is a normally open device because it allows fluid to flow freely until a specific pressure at its outlet port is reached. o Basic operation of the pressure regulator valve. § The adjustment knob at the top of the regulator is used to set the desired pressure by adjusting the spring tension above the piston. § The spring holds the poppet valve 100% open (100% flow) as long as the hydraulic circuit offers no restriction to flow. § When fluid is flowing to the hydraulic circuit, the spring holds the poppet valve open enough to regulate flow and maintain downstream pressure. § The spring will close the poppet valve if the hydraulic circuit deadheads flow and pressure exceeds the desired pressure.

PRESSURE REGULATOR VALVE ADJUSTABLE PRESSURE REGULATOR SYMBOL

PRESSURE REGULATOR VALVE OPERATION (MINIMUM DOWNSTREAM PRESSURE, MAXIMUM FLOW)

PRESSURE REGULATOR VALVE OPERATION (REGULATING DOWNSTREAM PRESSURE, CONTROLLED FLOW)

PRESSURE REGULATOR VALVE OPERATION (MAXIMUM DOWNSTREAM PRESSURE, NO FLOW)

• Pressure Relief Valves o Pressure relief valves are used to control the pressure in a hydraulic circuit by removing excess fluid from the circuit. § Pressure relief valves are most often used to prevent deadheading a hydraulic pump. § Normally excess fluid is directed toward the reservoir. o A pressure relief valve is a normally closed device because it blocks fluid flow until a specified high pressure at its inlet is reached. o Basic operation of the pressure relief valve. § Spring tension is adjusted to hold the piston down against the seat when pressure at the inlet port is below the desired relief pressure. § As pressure builds up in the inlet port, it overcomes the force of the spring. § The piston pushes up, compressing the spring and opens when pressure reaches the desired high relief pressure.

PRESSURE RELIEF VALVE ADJUSTABLE PRESSURE RELIEF VALVE SYMBOL

PRESSURE RELIEF VALVE

PRESSURE RELIEF VALVE

PRESSURE RELIEF VALVE

• Check Valve. o Allows fluid flow in only one direction. o Typical applications are to bypass a component, isolate one circuit from another, and/or to prevent an accumulator from dumping its fluid back through a pump. POPPET CHECK VALVE BALL CHECK VALVE

• Flow Control Valves o A flow control valve controls the amount of fluid that flows to a valve actuator in one direction and allows free flow in the opposite direction. o Flow control valves consist of two separate valves. § One valve controls flow in either direction. § The other valve is a check valve that allows free flow in only one direction. o The symbol below shows restricted flow from port 1 to port 2 and free flow from port 2 to port 1. CONTROLLED FLOW 1 2 FREE FLOW CHECK VALVE

• Flow Control Valves o An application of a flow control valve is to allow a cylinder to lift a load at a fast rate, but lower the load at a controlled slower rate. o Operation of the flow control valve. § A movable valve stem is adjusted by the adjustment knob and controls fluid flow in one direction. § The ball check valve opens or closes the other path, depending on which side has greater fluid pressure.

FREE FLOW PORT 1 TO PORT 2 Flow Control Valves NO FLOW PORT 2 TO PORT 1 § With the valve stem closed the valve simply acts like a check valve and allows fluid flow from Port 1 to Port 2 to flow freely, § But not from Port 2 to Port 1

FLOW RESTRICTED PORT 2 TO PORT 1 FLOW NOT RESTRICTED PORT 2 TO PORT 1 Flow Control Valves § With the valve stem is partially opened, fluid flows from Port 2 to Port 1 at a rate determined by the position of the valve. § The pressure at Port 2 is more than at Port 1, thus the check valve is closed. § With the valve stem opened more, the rate of fluid flow increases from Port 2 to Port 1. § The check valve remains closed.

• Accumulators. o Accumulators are components which can be used to perform the following functions: § § Store energy (potential energy). Absorb system shocks or spikes. Build gradual pressure. Maintain constant pressure. • Piston Accumulator o Nitrogen is a common gas used PISTON ACCUMULATOR Rated at higher pressures than because it is nonflammable and bladder accumulators very compressible. o Fluid is pumped into the bottom of the cylinder, increasing pressure, and forcing the piston assembly up and compressing the gas (potential energy). o If the fluid is allowed to flow out of the accumulator, the gas expands which forces the fluid from the cylinder (kinetic energy) and if pressure decreases enough will close the piston valve.

• Bladder Accumulator. o The same principle applies to a gas-charged bladder accumulator. o Whereas a piston was used to separate the gas and hydraulic fluid in the piston accumulator, a bladder separates the gas and hydraulic fluid in a bladder accumulator. • Spring Accumulator. o A spring accumulator uses a spring instead of gas to store hydraulic energy. o Spring accumulators are not rated as high a pressure as gas accumulators. SPRING ACCUMULATOR BLADDER ACCUMULATOR Rated at lower pressures than piston accumulators

• Shuttle Valves o Shuttle valves are automatic flow path selectors which allow the higher of two pressures to flow through the valve. o A typical use is to allow two separate hydraulic control valves to control one single acting cylinder. o In the illustration, fluid from control valve B seats the shuttle ball to the left and pushes the piston in the cylinder to the right which compresses the piston spring. o If control valve A is being used to control the piston position than the shuttle ball will be forced against the seat on the right and fluid from A valve will be directed to the cylinder. SHUTTLE VALVE SYMBOL

SHUTTLE VALVE

• Control Valves o Illustrated to the right is a very simple exploded view of a spool valve. o Spool valves are the most common type of valve used to control fluid flow. o Spool valves can be quite complicated with pilot valves, solenoid valves, and other types of valves built into them. o Rotary control valves are also frequently used.

• 2/2 Valve: Two-Port, Two-Position Spool Valve o Used for On-Off fluid flow control. o Used to isolate one circuit from another circuit.

• 2/2 Valve: Two-Port, Two-Position Spool Valve o Schematic: • • • The motor driven pump takes suction from the reservoir through the filter. The pressure relief valve drains fluid back to the reservoir when the control valve is closed The pressure relief valve drains fluid back to the reservoir when the control valve is open and back pressure is greater than the pressure relief valve's setting.

• 3/2 Valve: Three-Port, Two-Position Spool Valve o Used to position a single acting cylinder actuator. o Piston cannot be placed in mid-travel; either fully extended or fully retracted. o Ports: • • • Common port A is connected to cylinder. Port P is connected to hydraulic fluid supply. Port T is connected to the reservoir. T A P

• 3/2 Valve: Three-Port, Two-Position Spool Valve o Schematic: • • The motor driven pump takes suction from the reservoir through the filter. The pressure relief valve drains back to the reservoir when the piston pressure in the system exceeds its pressure setting. • Fluid is deadheaded when the piston is at its full travel to the right "A" and when the control valve is closed "B". "A" "B" A P A T P T

• 4/2 Valve: Four-Port, Two-Position Valve o Used to position a double acting actuator. o Piston cannot be placed in mid-travel; either fully extended or fully retracted. o When the spool valve is shifted to the right. § Fluid from ports P & B pushes the piston to the left in the cylinder. § And the left side of the piston drains to the reservoir via port A & T. P T A B P T

• 4/2 Valve: Four-Port, Two-Position Valve o When the spool valve is shifted to the left. § Fluid from ports P & A pushes the piston to the right in the cylinder. § And the right side of the piston drains to the reservoir via ports B & T. P T

4/2 Valve: Four-Port, Two-Position Valve B A T P

• 4/3 Valve: Four Port, Three Position Spool Valve o OPEN CENTER SPOOL VALVE § All ports are open when the valve is in center position. § Operating principle: • Piston can move freely when the control valve is in the center position.

• 4/3 Valve: Four Port, Three Position Spool Valve o OPEN CENTER SPOOL VALVE Move spool to left PISTON EXTENDS AB P T § Ports P & A are common. § Ports B & T are common.

• 4/3 Valve: Four Port, Three Position Spool Valve o OPEN CENTER SPOOL VALVE Move spool to right PISTON RETRACTS AB P T § Ports P & B are common. § Ports A & T are common.

• 4/3 Valve: Four Port, Three Position Spool Valve o CLOSED CENTER SPOOL VALVE PISTON LOCKED IN PLACE § All ports closed when valve in center position. § Operating principle: • Piston is locked in place since fluid is trapped on each side of the piston.

• 4/3 Valve: Four Port, Three Position Spool Valve o CLOSED CENTER SPOOL VALVE PISTON EXTENDS Move spool to left § Ports P & A are common. § Ports B & T are common.

• 4/3 Valve: Four Port, Three Position Spool Valve o CLOSED CENTER SPOOL VALVE PISTON EXTENDS Move spool to RIGHT § Ports A & T are common. § Ports P & B are common.

• 4/3 Valve: Four Port, Three Position Spool Valve o TANDEM CENTER SPOOL VALVE PISTON LOCKED IN PLACE § Fluid flow from P, through the spool bore hole, and to A when valve in center position. § Operating principle: • Piston is locked in place. • Fluid is allowed to flow when system is idling.

• 4/3 Valve: Four Port, Three Position Spool Valve o TANDEM CENTER SPOOL VALVE PISTON EXTENDS Move spool to left § Ports P & A are common. § Ports B & T are common.

• 4/3 Valve: Four Port, Three Position Spool Valve o TANDEM CENTER SPOOL VALVE PISTON RETRACTS Move spool to right § Ports P & B are common. § Ports A & T are common.

• Rotary Control Valves o Rotary control valves can perform many of the same functions as spool valves. o They consist of a core with passages inside a sleeve with ports o A handle or some other type of rotary positioning device rotates the core. o Detents act to line up the core with ports in the sleeve as it is rotated from one position to another.

Rotary Control Valves 2/2: 2 PORT, 2 POSITION 3/2: 3 PORT, 2 POSITION 4/2: 4 PORT, 2 POSITION

Rotary Control Valves

• Solenoid Operated Valves o When a hydraulic circuit and electrical circuit must work together a solenoid valve is used. o Some solenoids open and close a valve directly, but are limited on their pressure rating. 1. 2. 3. 4. 5. 6. 7. Valve body Inlet Outlet Solenoid assembly Winding Coil leads Armature valve plunger 8. Return spring 9. Valve seat SOLENOID OPERATED ON/OFF VALVE

• Solenoid Operated Valves o Some solenoids open and close a valve by operating a pilot valve. o The control valve operates under normal system pressure at its P port, but the solenoid pilot valve operates at a lower pressure. o This enables the solenoid valve to change the direction of fluid flow of a much higher pressure.

SOLENOID CONTROLLED PILOT OPERATED VALVE