Fluid Power Systems (ME353)

Fall 2012

Lecture 8

Controlling the System

Pressure, Direction, and Flow

Primary Control Functions in a Hydraulic System

Control valves allow hydraulic systems to produce the type of

motion or level of force needed to complete the functions

expected of a hydraulic circuit

A variety of valves can control actuator direction, speed, and

force output

The three basic types of control valves are:

– Pressure control

– Directional control

– Flow control

Pressure control valves can:

– Protect the system from damage due to excessive pressure

– Sequence motion

– Limit pressure in selected sections of a circuit

Directional control valves direct

fluid flow to establish and control

actuator movement

Flow control valves control the operating speed of actuators

They provide a means to vary the rate of fluid flow

Basic Structure and Features of Control Valves

A valve body serves as the holder of the working elements of a valve

– Body can be a special casting or machined from standard stock materials

– Typical parts of the body include precision bores, ports, and fittings for

mounting

Fluid control valves incorporate several internal elements to provide a

desired operation

The elements allow, direct, meter, or stop the flow of fluid

The elements include fixed orifices, needle valves, spools, poppets, and

sliding plates

A fixed orifice is a precision hole either:

– Machined into the valve body or a component

– Pressed as a separate part into a valve passageway

Both designs are used to control fluid flow

A spool is a cylindrical metal piece fitted into the bore of a

valve body

The spool is used to block or direct fluid through a valve to

produce a desired fluid flow characteristic

Internal and external forces are used to position the various

valve elements

– Springs are typical internal forces used to operate valve

elements

– Manual, pilot pressure, and electromagnetic force are

common external forces used for operation

Precision fit, rather than separate seals, is used to prevent

excessive internal leakage in most hydraulic control valves

Internal leakage must be drained from valve chambers

– Fluid buildup causes backpressure

– Backpressure prevents the proper operation of internal valve

elements

Internal and external drains are used to remove internal leakage

– Internal drains may be used when the outlet line is not subjected to

system pressure

– External drains are connected to low-pressure return lines leading to the

reservoir

Normal valve position refers to the position the internal

elements assume when a hydraulic system is shut down

– Normally open

– Normally closed

Symbols for normally open and normally closed valves

Directional control valves may be referred to by the number of

distinct flow positions provided by the valve

– Two position

– Three position

Valve Operation (Springs, Fluid Pressure, and Orifice fluid flow): Springs, fluid pressure, and fluid flow are very important in the operation of

hydraulic system control valves

Springs are used in control valves to:

– Move spools and other internal elements

– Establish the maximum operating pressure

– Serve as a biasing force

Fluid pressure is used in control valves to:

– Directly open or close valves

– Remotely operate a valve element

– Operate a compensating device to obtain desired fluid flow

Fluid flow through an orifice is used in control valves to establish

differences in pressure

- These pressure differences combined with balancing pistons and biasing

springs are commonly used in the operation of pressure and flow control

valves

Pressure Control Valves

Pressure control valves may be grouped into one of five types

– System maximum pressure control

– Actuator sequence control

– Restrained movement control

– Pump unloading control

– Reduced pressure control

Control valves can be classified by internal modes of operation

– Direct operation

– Balanced-piston operation

Direct-operated valves depend on heavy internal springs to

establish valve operating pressure

Direct-operated relief valves use system pressure to generate force to

compress a spring

This opens a ball or poppet valve, allowing excess fluid to return to the

reservoir

Balancing-piston valves (compound relief valves) use lighter

springs and system pressure acting on internal valve

mechanisms to establish the desired operation

Compound relief valves consist of pilot- and balancing-piston sections

– Combined into a single valve

– More efficient and quieter than direct-operated relief valves

The pilot section of the compound relief valve contains a small,

direct-operated relief valve

The pilot section indirectly establishes maximum system

pressure by controlling the pressure in the balancing-piston

section of the valve

The balancing-piston section of the compound relief valve uses

a metering orifice and a balancing spring to create pressure and

force differences

These differences correctly position the piston to produce a

desired maximum system operating pressure

Maximum system pressure control devices are referred to as:

– Relief valves

– Safety valves

– Hydraulic pressure fuses

Relief valves are normally closed valves

They open when system pressure approaches the set maximum

operating pressure

The operation of relief valves can be classified as:

– Direct operated

– Balancing piston (compound)

Safety valves are used to prevent damage to the hydraulic

system if the system relief valve should fail to open

Typically, safety valves are direct-operated relief valves

Safety valves are generally set 25% higher than the normal

system operating pressure

A typical hydraulic pressure fuse

Hydraulic pressure fuses function as a pressure-limiting device by using a

disk that ruptures at a predetermined pressure

They act as a positive-pressure-limiting device for systems where system

pressure limits are critical to safe system operation

Sequence valves allow the automatic sequencing of two or more actuators in

a hydraulic circuit

– Primary actuator moves as soon as fluid flow is directed to the actuator

section of the circuit

– Sequence valve blocks flow to the secondary actuator until a

predetermined pressure is reached, then allows fluid flow to the actuator

A sequence valve is typically

fitted with an integral check

valve

This allows free flow of fluid

around the valve when the

direction of the actuator is

reversed

Sequence valve with integral

check valve

A circuit containing a sequence valve

Restrained movement control valves are used in circuits to

prevent unexpected actuator movement. Often called:

– Counterbalance valves when used with cylinders

– Brake valves when used with motors

Counterbalance valves prevent unexpected lowering of the boom

Counterbalance and brake valves are normally closed valves

A check valve is required to permit free reverse flow around to

valve

Counterbalance valve with integral check valve

Counterbalance valves allow the downward movement of loads supported

by a cylinder only when the system pump is operating

Force generated by system pressure and the force created by the weight of

the load are needed to move the load downward

Circuit containing a counterbalance valve

Pump unloading controls hold a desired operating pressure

while the pump operates at near-zero pressure

This reduces energy consumption and maintenance costs

Pump unloading controls use an unloading valve and a dual

pump or accumulator to maintain desired system pressure while

dumping unneeded pump output to the reservoir at very low

pressure

A typical unloading valve

Unloading valves are normally closed valves with external pilots

– Unloading relief valves are used with accumulator circuits

– Direct-operated valves can be used in typical high-low dual-pump

circuits

Circuit containing an unloading relief valve