Control Components In Hydraulic Systems

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Fluid Power Systems

CONTROL COMPONENTS IN HYDRAULIC SYSTEMS

UNIT - 2

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Fluid Power Systems

Hydraulic Valves

Fluid power is controlled prim arily through the use of valves Selection of these valves involves not only the type but also the size, actuating technique and rem ote control capability There are three basic types of valves: 1. Directional control valves 2. Pressure control valves 3. Flow control valves

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Directional control Valves

Fluid Power Systems

Check Valve

Directional Control Valves control the direction of fluid flow in a circuit. The simplest DCV is a Check Valve as shown in figure above. A check valve allows flow in one direction, but blocks flow in the opposite direction.

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Directional control Valves

Fluid Power Systems

Pilot-operated Check Valve

It 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

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Directional control Valves

Fluid Power Systems

Three-way Valve

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Directional control Valves

Fluid Power Systems

Three-way Valve The radial clearance between the land of the spool and bore is around 5-10 µm The spool can be positioned manually, mechanically, by using pilot pressure, or electrical solenoids Three-way valves are typically used to control the flow directions to and from single-acting cylinders

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Directional control Valves

Fluid Power Systems

Valve Actuation Methods

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Directional control Valves

Fluid Power Systems

Four-way Valve

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Directional control Valves

Fluid Power Systems

Four-way Valve Four-way valves are typically used to control the flow directions to and from double-acting cylinders The graphic symbol shows only one tank port T even the actual valve m ay have two, as shown in the schem atic drawing

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Directional control Valves

Fluid Power Systems

Manually Actuated Valves

Manually actuated two-position, spring offset, four-way valve Notes.NieRoyalMech.com 11

Directional control Valves

Fluid Power Systems

Manually Actuated Valves

Manually actuated, spring centered, three position, four-way valve Notes.NieRoyalMech.com 12

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Directional control Valves

Fluid Power Systems

Mechanically Actuated Valves

Mechanically actuated, spring offset, two-position, four-way valve Notes.NieRoyalMech.com 13

Directional control Valves

Fluid Power Systems

Air-pilot - Actuated Valves

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Directional control Valves

Fluid Power Systems

Solenoid - Actuated Valves

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Directional control Valves

Fluid Power Systems

Solenoid - Actuated Valves The most common method of actuating a spool valve is by using solenoids When an electric coil (solenoid) is energised, it creates a magnetic force that pulls the armature into the coil. This causes the armature to push on the push pin to move the spool of the valve

Solenoid valve has a flow capacity of 50 litres/min and a maximum operating pressure of 250 bars

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Directional control Valves

Fluid Power Systems

Centre Flow Path Configurations

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Directional control Valves

Fluid Power Systems

Centre Flow Path Configurations

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Directional control Valves

Fluid Power Systems

Centre Flow Path Configurations Most three position valves have a variety of possible flow configurations, each suited for a particular application

path

An open-centre type connects all ports together. Pump flow can return directly back to the tank at essentially atmospheric pressure. At the same time, the actuator can be moved freely by applying an external force In closed centre design, all ports are blocked and pump flow can be used for other parts of the circuit. At the same time, the actuator connected to ports A and B is hydraulically locked. Very often, closed centre design forces the pump to produce flow at the high pressure setting of the pressure relief valve. This not only wastes pump power, but heats up oil and promotes wear and shortens pump life. The tandem design also results in a locked actuator. However, it also unloads the pump at essentially atmospheric pressure.

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Directional control Valves

Fluid Power Systems

Shuttle Valves Shuttle valve consists of a floating piston that can be shuttled to one side or the other of the valve depending on which side of the piston has the greater pressure Shuttle valves may be spring loaded (biased) in one direction to favour one of the supply sources or unbiased as shown in figure so that direction of flow through the valve is determined by circuit conditions

One application of shuttle valve is for safety in the event that the main pump can no longer provide hydraulic power to operate emergency devices. The shuttle valve will shift to allow fluid to flow from a secondary backup pump.

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Pressure control Valves

Fluid Power Systems

Pressure control valves control the m axim um pressure level in a hydraulic circuit, which provides control of the m axim um output force or torque. They also protect the circuit from excessive pressure, which could dam age components and possibly cause serious injury.

Pressure control valves m ay control the m aximum pressure of the entire circuit, or sim ply one branch of the circuit. Types of pressure control valves 1.Pressure Relief Valves, 2.Unloading Valves 3.Pressure Reducing Valves, 4.Sequence valves

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Pressure control Valves

Fluid Power Systems

Simple Pressure Relief Valve

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Fluid Power Systems

Pressure control Valves Simple Pressure Relief Valve

It is a norm ally closed valve whose function is to lim it the pressure to a specified m axim um value by diverting pump flow back to the tank When the system pressure reaches the pre-set value, the resulting hydraulic force exceeds the spring force and the poppet is forced off its seat. This perm its the flow through the outlet to the tank The pressure relief valve provides protection against any overloads experienced by the actuators in the hydraulic system . Obviously, one im portant function of a pressure relief valve is to lim it the force or torque produced by hydraulic cylinders and m otors

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Fluid Power Systems

Pressure control Valves Compound Pressure Relief Valve

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Pressure control Valves

Fluid Power Systems

Pressure Reducing Valve

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Pressure control Valves

Fluid Power Systems

Pressure Reducing Valve This valve (normally open) is used to maintain reduced pressure in specified location of hydraulic system It is actuated by downstream pressure and tends to close as this pressure reaches the valve setting. If downstream pressure is below the valve setting, fluid will flow freely from the inlet to the outlet There is an internal passageway from the outlet, which transmits outlet pressure to the spool end opposite the spring. When the outlet (downstream) pressure increases to the valve setting, the spool moves to the right to partially block the outlet port Then, just enough flow is passed to the outlet to maintain its present pressure level.

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Pressure control Valves

Fluid Power Systems

Unloading Valve

NVR’s sketch for dummies (?)

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Pressure control Valves

Fluid Power Systems

Unloading Valve An unloading valve is used to unload the pump connected to port A when the pressure at port X is maintained at the value that satisfies the valve setting. The high flow poppet is controlled by the spring loaded piston and the pressure applied to port X Flow entering at port A is blocked by the poppet at low pressures. When the pressure rises to the maximum value permitted by the adjustable spring, the poppet lifts and flow goes from port A to port B The pressure signal to port X acts against the control piston and moves it further up. This opens the tank port and oil is discharged to the tank at almost zero pressure, as long as pilot pressure is maintained Hence, the pump has essentially no load. This is not the case with a pressure relief valve because the pump is delivering full pump flow at the pressure relief valve setting and thus is operating at maximum power conditions

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Pressure control Valves

Fluid Power Systems

Sequence Valve

NVR’s sketch for dummies (?)

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Pressure control Valves

Fluid Power Systems

Sequence Valve A sequence valve’s primary function is to direct flow in a predetermined sequence. It operates on the principle that when main system pressure overcomes the spring setting, the valve spool moves up, allowing flow from the secondary port It may be direct or remote pilot operated

The pressure in the primary line is effective on the end of the spool. This pressure will force the spool against the main bias spring and at the preset value of the spring, allows passage from the primary to the secondary port Ex: Clamp and feed sequence in sheet metal working When the clamp cylinder moves out and clamps the workpiece, resistance is met and pressure in the system increases. When the pressure reaches the spring setting in the sequence valve, it opens, and flow is directed out of the secondary port to operate the work cylinder

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Pressure control Valves

Fluid Power Systems

Counterbalance Valve

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Pressure control Valves

Fluid Power Systems

Counterbalance Valve The purpose of a counterbalance valve (CBV) is to maintain control of a vertical hydraulic cylinder to prevent it from descending due to the weight of its external load

The primary port of this valve is connected to the bottom of the cylinder, and the secondary port is connected to the DCV The pressure setting of CBV is higher than is necessary to prevent the cylinder load from falling due to its own weight When pump flow is directed through DCV to the top of the cylinder, the pressure in the CBV increases. Once this pressure reaches the pre-set value, spool of CBV moves up and opens the path to DCV and then on to the tank When raising the cylinder, an integral check valve opens and allows free flow for the upward motion of the cylinder

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Fluid Power Systems

Flow control Valves Flow control valves control the flow rate of fluid in a circuit.

They accomplish this by incorporating a variable orifice into the circuit that acts like a faucet, closing the flow control valve orifice reduces the flow rate and opening the orifice increases the flow rate. The speed of an actuator depend directly upon the flow rate in the system. Controlling the flow rate therefore allows us to control the speed of the actuator.

Needle valve Notes.NieRoyalMech.com 33

Needle Valve Q = C0A

Fluid Power Systems

çP/Sg

Where Q = volum e flow rate (m 3/sec) C0 = a constant for the orifice A = area of orifice opening (m 2) çP = pressure drop across the orifice (KPa) Sg = specific gravity of oil Since area A can be varied, we can rewrite the equation as, Q = Cv

çP/Sg

Where Cv = AC0 = capacity co-efficient, where A is the area of orifice when fully open The value Cv is determ ined experimentally for each type of valve in the fully open position and is listed as the ‘rated Cv ’ in m anufacturer’s catalogues Notes.NieRoyalMech.com 34

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Needle Valve

Fluid Power Systems

Problem : A needle valve is placed in the outlet line of a hydraulic system to control the extension speed of the hydraulic cylinder. The load acting on the cylinder during extension is 5000N. The following data are available:

Desired cylinder speed = 0.25 m /sec Cylinder piston diam eter = 5 cm Cylinder rod diam eter = 2.5 cm Specific gravity of oil = 0.9 Pressure relief valve setting = 35 bars Determ ine the required capacity co-efficient of the needle valve

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Flow control Valves

Fluid Power Systems

Flow control valve (Needle valve with integral check valve)

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Fluid Power Systems

Flow control Valves Non-pressure-compensated Valves In this type, system pressure is relatively constant and motoring speeds are not too critical They work on the principle that the flow through an orifice will be constant if the pressure drop remains constant The design also involves a check valve, which permits free flow in the direction opposite to the flow control direction

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Flow control Valves

Fluid Power Systems

Pressure-compensated Valves

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Flow control Valves

Fluid Power Systems

Pressure-compensated Valves This design uses a hydrostat that maintains a constant 8-10 bars differential across the throttle, whose orifice opening can be adjusted by an external knob setting. The orifice area setting determines the flow rate to be controlled Hydrostat is held normally open by a light spring. It starts to close as inlet pressure increases and overcomes the light spring force. This closes the opening through the hydrostat and blocks of all flow in excess of throttle setting. As a result, the only oil that passes through the valve is the amount that 810 bars can force through the throttle. Flow exceeding this amount can be used by other parts of the circuit or diverted to the tank through PRV

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