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Flashcards in Pressure Controls Deck (36)
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Pressure control valves preform what 3 functions

Limit or otherwise regulate pressure
Create a pressure condition required for control
Cause actuators to operate in a particular order


What are the 2 classifications of pressure controls

Normally open
Normally closed


Spring constant

Expressed in pounds per inch of compression, indicates the stiffness of the spring


Pressure override

Is the difference in pressure between cracking pressure and full flow pressure. it is determined by the spring constant . the greater the spring constant the greater the pressure override


Direct acting valves

Force created by hydraulic pressure is directly opposed by spring force. Oil will only act against 1 spring to make a change. Can however use a pilot sense line


What are 3 names for pilot operated pressure control valves

2 stage


Pilot poppet is designed to handle

High pressure and low flow


Main poppet spring is designed to handle

High flow and low presurre


What is the main advantage of a pilot operated valve over a direct acting valve

Pressure override is greatly reduced


What are the 5 basic pressure control valve applications

Relief valves
Unloader valves
Pressure reducing valves
Sequence valves
Back pressure valves


What 4 common characteristics do a relief valves have

Always normally closed
Pilot pressure to open valve is connected in parallel directly upstream
Inlet port is connected in parallel to system is protects
Outlet port is connected to reservoir


What are 2 most common applications of relief valves



Relief valves

-Normally closed valve
-Pilot pressure to open the valve is connected in parallel directly upstream of the valve
-Inlet port is connected in parallel to the system or circuit that it protects
-Outlet is connected to the reservoir
-1 turn of the adjusting screw is generally close to 1000 psi
-A circuit relief will usually be set higher then the main system relief
-Relief are generally mounted in the directional control valve
-Main reliefs are set with engine at full throttle, dead head an actuator, and full flow going through relief


Circuit reliefs are usually set ______ then system relief valves



System relief pressure should be set how

Engine full throttle
Max flow through relief
Dead head a actuator


What is a second name for a back up ring

Anti excrussion ring


What is the primary function of an accumulator

Stores hydraulic energy in the form of a volume of liquid under pressure


What are the 3 types of hydraulic accumulators

Spring Loaded
Gas charged


Weighted accumulators

Has weights on top of a piston
Maintains a constant pressure regardless of the oil volume in the accumulator
more commonly used in industrial applications then mobile equipment


Spring loaded accumulators

Use a coil spring acting on an accumulator piston to exert force on oil
The more oil that is forced in the higher the pressure will get at the compression force in the spring increases


Gas charged accumulators use what gas

Nitrogen gas is used as it will not react with the hydrocarbons in the oil


3 styles of gas accumulators



What is the purpose of a accumulator located between the pump and DC valve

Stores energy


Piston accumulators

One chamber with a piston in the middle that separates the gas from the oil
Piston is fitted is fitted with piston sealing rings to prevent internal leakage


Pressure in a gas chamber

As more oil is forced into accumulator, the pressure rises due to compression of gas


What happens if orifice in pilot operated pressure reducing gets plugged?

Valve stays open, over pressure results in outlet


What are the 4 reasons accumulators are installed on equipment

Energy storage
Shock absorption
Gradual pressure build up
Constant pressure maintenance


Unloader valve

-Allows full pump flow to return to the reservoir while maintaining a minimum pressure at the pump outlet
-Normally-closed valve
-Inlet is connected in parallel with the system or the circuit it unloads
-Outlet is connected to tank
-Are always used in conjunction with a check valve
-Pilot pressure to open the unloader is sensed from the high pressure side of the check valve
-Are normally pilot operated valves – cannot afford excessive override


Pressure reducing valve

-A pressure control that allows one circuit of a hydraulic system to have a lower max operating pressure then the rest of system
-Normally open valve
-Pilot pressure to close the valve is sensed downstream of the valve
-The inlet port is connected to system pressure
-The outlet port is connected to the circuit to be operated at reduced pressure
-A drain line from spring cavity is required
-Will modulate


What happens when the orifice gets plugged in a pressure reducing valve

The valve will stay open and not limit the maximum pressure