hydraulic valves Flashcards
characteristics of pressure reducing valves
pilot pressure to close the valve is sensed downstream . inlet port is connected to system pressure . outlet port is connected to the circuit to be operated at a reduced pressure . drain line is required to allow internal leakage to return to the reservoir
all relief valves have the following characteristics no matter where they are located in the system
a relief valve is always normally closed . pilot pressure to open the valve is connected in parallel directly upstream of the valve . inlet is connected in parallel with the system or circuit it protects . outlet port is connected to the reservoir
circuit relief valves
limit pressure in one branch of a system
three types of back pressure valves are
counterbalance valves . brake valves . lock valves
the amount of oil flow that passes through an orifice is determined by
size and shape . pressure drop across the orifice . the viscosity of the oil
two other advantages of pilot operated control valves
much less prone to chatter . can operate through a much wider pressure range than a direct acting valve
three common designs of normally closed direct acting pressure control valves
ball type . poppet type . guided poppet type
flow control valves control
the speed of hydraulic pumps and motors
guided poppet type normally closed direct acting pressure control valve
closely fitted into the body making it more stable than the other two designs
ball type normally closed direct acting pressure control valves
the simplest . cannot handle a large flow of oil without chattering severely
what is valve modulation
the constant movement from open to closed balancing spring force against hydraulic pressure determined by the changing oil flow
characteristics of a sequence valve
normally closed . pilot pressure to open valve is sensed up stream . inlet port is connected in parallel with function to be operated first . outlet port is connected to function to be operated second . drain line is required to allow internal leakage to return to tank . heck valve is required for rapid return of reversible actuators
the component common to all flow control valves is
the orifice
counterbalance valves
a safety device used to control the lowering of a vertical load .
the greatest disadvantage of pressure control valves
is that a high amount of pressure override is unavoidable when they are passing high rates of oil flow
what is pressure intensification
when a restriction on the rod end of a cylinder allows pressure in the system to be greater than normal operating ranges
why can brake valves have a pressure setting less than a counterbalance valve
because it is operated by actuator inlet pressure rather than outlet pressure
unloader valves differ from check valves
are always used in conjunction with check valves . pilot pressure to open the unloader is sensed from the high pressure side of the check valve
priority flow dividers
divert all of the pump oil flow to a primary circuit until flow requirements of that circuit are met . excess flow not required by the primary are delivered to other circuits
what is a (meter out circuit )
flow control valve is installed on the return side of a actuator restricting the return oil flow and regulates speed no matter the load acting on it
lock valves are normally not serviceable or adjustable
true . a malfunctioning lock valve will result in loss of actuator control or erratic actuator movement . you must replace the valve is any of these conditions exist
counter balance valves do not work well with varying loads because
power is wasted opening the valve when loads a are lighter than maximum . static oil pressure in the actuator may open the valve is maximum load is exceeded resulting in a loss of control
back pressure valves
are pressure control valves that control actuator movement by restricting return oil from the actuator
pressure control valve applications
pressure reliefs . unloader reliefs . pressure reducing valves . sequence valves . backpressure valves
two basic gas filled accumulators are
bladder and piston
what is a high low type pump configuration
a demand type hydraulic system that provides oil flow in relation to demand on the system . flow rate is high when pressure is low . when load on the system is high flow decreases and pressure rises
the difference between cracking and full flow pressure is known as
pressure override
unloader valves are normally pilot operated valves because they must
pass full pump flow without excessive override
in a direct acting pressure control valve the force created by hydraulic pressure is directly opposed by
spring force
gas filled bladder type accumulators
uses a rubber bladder to separate the nitrogen from the oil . oil entering the accumulator acts on the surface area of the bladder compressing the nitrogen and causing pressure to rise . a spring loaded poppet valve in the oil inlet port of the accumulator stops the bladder from being pushed out if gas pressure is higher than oil pressure
what is one of the most significant factors in determining pressure override
spring constant of the valve . the greater the spring constant the greater the pressure override
pressure reducing valves
are a pressure control that allow one circuit to have a max operating pressure lower than the rest of the system
the override pressure of a pilot operated vs a direct acting control valve is much less because
the strong spring on the pilot operated only needs to pass a small amount of flow . the majority of the flow passes by the main poppet which is held by a light spring
sequence valves
cause actions in hydraulic system to happen in a certain order . maintain a predetermined minimum pressure in the primary hydraulic function while secondary is operated
similarities to unloader valve and relief valves
are always normally closed . inlet port is always connected in parallel with the system or circuit . the outlet is connected to tank
three types of accumulators
weighted . spring loaded . gas charged
weighted accumulators
have a piston in a barrel with weights on top . pressure works on the bottom of the piston to overcome the weights and fill the chamber as pressure the weights overcome the oil and force it back out of the chamber
excessive pressure override in a system relief would cause
the valve to crack within the systems operating pressure causing some of the pump flow to be diverted from the actuators slowing their speed and generating unwanted heat
the vast majority of system relief valves are pilot operated because they must be able to
pass the full flow rate of the pump with as little pressure override as possible
functions and applications of accumulators
energy storage . shock absorption . gradual pressure buildup . constant pressure maintenance
if the poppet of a relief valve what sticky or seized
you would get higher system pressure than normal
what is the spring constant
indicates the stiffness of a spring
hydraulic valves assert their control over the system by
controlling pressure . regulating the amount of oil that flows through branches of the system . determining the path that oil flows
unloader valves
allow full pump flow to return to tank while maintain minimum pressure at the pump outlet . taking the load off the pump and engine driving it
what is cracking pressure
when pressure has risen just enough to lift the valve off its seat and oil can flow to the outlet port
pressure control valves preform the following tasks
limit or otherwise regulate pressure . create pressure condition required for control . cause actuators to operate in particular order
a pilot valve is designed to deal with
high pressure and low flow rates
pressure compensated flow control valves
maintains a constant flow through its orifice by maintaining a constant pressure drop across it
non pressure compensated flow control valves
the flow through them varies as the pressure downstream of the flow control changes . they do not accurately control flow because flow varies with the load on the cylinder
circuit relief valves protect
components from shock loads that produce pressure spikes . and pressure caused by expanding hydraulic oil due to heat
gas filled piston type accumulator
uses a piston honed to the inside of the accumulator and uses seals to separate it form the oil . as the chamber fills with oil the nitrogen on the other side of the piston is compressed causing pressure to rise
back pressure valve characteristics
normally closed . pilot pressure to open the valve varies depending on the valve type . back pressure valves are connected in series with the actuator return line . check valve is required to allow reversible actuators to return
a pilot operated pressure control valve offers
much more accurate pressure control with significantly reduced pressure override
lock valve
hold a suspended load in the event of a hydraulic line rupture . prevent the load from drifting due to leakage in the directional control valve . prevent the load from over speeding the actuator
direct acting pressure control valves are designed to deal with
high flow rates and low pressures
two basic pressure control valve configurations are
direct acting and pilot operated
internal leakage in a relief valve could be caused by
broken o-ring contamination keeping poppet open damaged poppet seat
spring loaded accumulator
consists of a large spring in a chamber . as oil pressure rises it overcomes spring pressure and fills the chamber . force acting on the oil from the spring is not constant the more the spring compresses the more force is exerted on the oil
two applications for unloader valves are
used in a closed center system with a fixed displacement pump . and a type of hydraulic system using tandem fixed displacement pumps
gas charged accumulators
uses nitrogen gas to exert force on the oil because it will not react with hydrocarbons in the oil
two most common applications of relief valves
system reliefs and circuit reliefs
pressure controls are most often used in applications where
they encounter low flow rates and infrequent duty cycles
the amount of pressure intensification depends on
the amount of restriction in the return line . the ratio of and rod end and base end piston areas . force increase created by the load on the rod end
two classifications of pressure control valves are
normally closed and normally open
system reliefs
protect the complete hydraulic system from being damaged by excessive pressure
if the poppet of a relief valve was stuck open or the valve spring was broken you would get
lower system specification pressure than normal
pressure control with excessive override will
waste power and generate unwanted heat
what is a ( meter in circuit )
controls the amount of oil flowing into the base end of a cylinder controlling extension
flow dividers
split a single flow of oil between two or more actuators
hydraulic valves are rated for
maximum pressure capability as well as the pressure drop they create when a given flow rate passes through them
poppet type normally closed direct acting pressure control valve
cone shaped design tends to center it in the oil stream
two types of flow dividers
priority flow dividers . proportional flow dividers
brake valves
are used in return line in hydraulic motors to prevent the load from over speeding the motor and provide a controlled deceleration of the load
why must you never use oxygen or compressed air to charge a gas filled accumulator
because oxygen and air can mix with the hydrocarbons of the oil and cause an explosion
proportional flow dividers
or more circuits splits a single flow among two circuits .the flow divider is pressure compensated to ensure oil flow is constant regardless of the load on either circuit
three categories of hydraulic valves
pressure control valves . flow control valves . directional control valves
the speed of an actuator is determined by
internal capacity . input flow rate
a pilot operated pressure control is often called
a compound pressure control because it consists of two direct operating pressure controls acting together as one unit
circuit relief valves pressure setting is set
normally higher than the system relief so they wont crack under normal working pressure
