190303Fa Flashcards
How do hydraulic valves assert their control over the system?
- Controlling pressure
- regulating the amount of oil that flows
- determining the path of oil flow
What are the three categories of hydraulic valves?
- pressure control valves
- flow control valves
- directional control valves
What are hydraulic valves rated for?
Maximum pressure capability, as well as the pressure drop they create
What are the two classifications of pressure controls?
- Normally open
- Normally closed
Pressure control valves perform what tasks?
Limit or regulate pressure
create a pressure condition required for control cause actuators to operate in order
What two forces must be balanced in a pressure control valve?
Spring force vs. hydraulic force (pressure)
How can spring force be adjusted in a pressure control valve?
With an adjustment screw (mechanical mean)
True of False: A normally open pressure control valve allows flow from inlet to outlet?
True
Where does a drain valve allow oil to flow to on a pressure control valve?
Usually to tank/reservoir
What does the drain port prevent on the spring side of a pressure control valve?
Hydrostatic lock
True or False: A normally closed pressure control valve allows oil flow from inlet to outlet port?
False
What is the constant movement in a valve called?
Valve Modulation
What is the stiffness of a spring called?
Spring Constant
What is the pressure called, when oil just begins to flow to the outlet port?
Cracking Pressure
The difference between cracking and full flow pressure is called?
Pressure override
Out of the three styles of direct acting pressure control valves which style can handle the least amount of flow?
The ball type (11 L/min)
Why does the guided poppet type of direct acting pressure control valve have holes drilled in the poppet?
allow the oil to flow from inlet to outlet port, limits max flow capacity of the valve
What does excessive override do?
Wastes power and created unwanted heat
What is the greatest disadvantage of direct acting pressure controls?
a high amount of pressure override is unavoidable when passing high flow rates
What is another name for a pilot operated pressure control valve?
Compound pressure control
The main poppet is held closed by a light spring or strong spring?
Light spring
What happens when cracking pressure is achieved in a pilot operated pressure control valve?
forces the pilot poppet off its seat
When the main poppet is open is oil flow allowed to go to the outlet ports?
Yes
What is used to adjust the pressure setting in a pilot operated pressure control valve?
Pressure adjusting screw
Is the override pressure of a pilot operated valve high or lower than the direct acting valve?
override pressure is lower
What are some advantages of pilot operated pressure control valves?
- Less prone to Chatter
- operate at a much wider pressure range
- less override pressure
Do the majority of flow travel pass the pilot poppet or main poppet in a pilot operated valve?
Pass the main poppet
Are pressure relief valves normally open, closed or either?
Normally CLOSED
Does the outlet of a pressure relief valve return to tank?
YES
What are two common applications of relief valves?
System relief and Circuit relief
What is a system relief valve connected to?
In parallel with the pump outlet line
With the system relief valve cracked what happens to the actuators?
- Flow is diverted
- Slow speeds
- excessive heat
What is the main difference between the two types of pressure relief valve applications?
Location in system and basic construction
What are three types of hydraulic accumulators?
Weighted, Spring-Loaded or gas-charged
What are three styles for gas-charged accumulators?
Bladder, Piston or Diaphram
In a gas-charged accumulators what is used as the “gas”?
Hint: Oxygen, compressed air or nitrogen gas (dry)
Nitrogen gas (dry) N²
In an gas-charged accumulator the hydraulic oil can mix with N²?
No, unless it is faulty
What will be the symptom if the gas-charged accumulator is faulty?
You will not see aeration or any sign of the leaked N² because it will go to tank and leave
No shock load absorption - broken pins, spools, control valves
Poor operation
Are spring accumulators holding a constant pressure?
No, the spring will be compressed depending on oil
What are some functions of an accumulator?
Energy storage
Shock absorption
Gradual pressure build up
Constant pressure maintenance
What are the 5 basic pressure control valve application? Bonus 2 extra
Pressure relief valves
Unloaded valves
Pressure reducing valves
Sequence Valves
Backpressure valves
Bonus: Counter balance valve
Brake valve line lock
In a pressure relief valve, is the pilot pressure connected in parallel and upstream of the valve?
Yes
System relief valves are usually connected in parallel where in a system?
Usually at pump outlet
What is the pro to using a pilot operated system relief valve?
Little pressure override and giving FULL FLOW
What does a circuit relief valve protect?
A circuit or branch in a system
Are all relief valve springs adjustable?
Not all, some need to be replaced to get a different pressure
Are circuit relief valves usually direct acting?
Yes, Low flow
When adjusting system relief valve should the engine be at full throttle or idle?
Full throttle and stall actuator
What are two ways to adjust the system relief valve if it is adjustable?
Screw or add/remove Shims
What are we adjusting in a circuit relief valve?
Cracking pressure
When adjusting a circuit relief valve what should the engine be doing?
Low idle, Minimum flow
What are some faults that can happen in a relief valve?
Poppet stuck open
Poppet seized closed
broken spring
damaged o-rings
contamination
When replacing a relief valve what should be done?
Relieve all hydraulic pressure
Cleanliness
Replace O-ring seals
Torque to spec
Back off relief valve before initial start up then adjust
Unloader valve are more energy efficient than a relief valve?
Yes
Are unloader valves normally open?
No
Where is the outlet port on an unloader valve connected to?
Tank/Reservoir
What are the two application versions of a unloaded valve?
Closed center or demand type
Closed centre hydraulic systems with an unloader valve can use a fixed displacement pump?
Yes
What does a check valve in an unloader valve provide?
It holds high pressure downstream (accumulator, control valve)
Pressure reducing valves are normally open?
Yes
Why does the spring cavity need to have an external drain?
High pressure on both sides would not let the spring internally drain
In a pressure reducing valve is the pilot connected to the inlet or outlet of the valve? (upstream or downstream)
The pilot is sensing downstream from the valve
In a sequence valve the pilot is sensing from upstream or downstream from valve?
Pilot is sensing upstream (input) from valve
Is a pressure reducing valve is it normally open or closed?
Normally open
Where is the outlet connected to on a pressure reducing valve?
Outlet is connected to the circuit
Can a pressure reducing valve be “remotely” controlled?
Yes, connect into port near drain
Is a sequence valve normally closed?
Yes
Why is a drain line needed in a sequence valve?
To allow internal leakage to return to tank/reservoir
A check valve is required for rapid return of reversible actuators in a sequence valve, correct?
Yes
Is pilot pressure in a sequence valve sensed from upstream?
Yes
What is a sequence valve used for?
Cause actions in a hydraulic system to occur in a certain order,
Maintain a predetermined minimum pressure in the primary hydraulic function while the secondary function is in operation
What allows rapid return of reversible actuators in a sequence valve?
Check valve
Does a sequence valve need to have a reversible check valve in the valve body?
No doesn’t have to be in the valve, has to be a separate check valve (cartridge)
What is used with a one way sequence valve?
- One way motor
- Flow must only go in one direction
- From pump to directional control valve
If pressure is controlled in one direction in a sequence valve what happens in the opposite direction if a check valve is installed?
Freeflow in “reverse” direction
What are three backpressure valves?
Counterbalance, brake valve and lock valve
What does a counterbalance valve control when lowering a vertical load?
Freefall, restricts or prevents
How are sequence valves adjusted?
By installing a pressure gauge in the primary circuit and adjusting the valve spring force until proper primary circuit pressure is achieved just prior to secondary function movement
What characteristics are shared by all backpressure valves?
- Normally closed
- Pilot pressure to open valve varies.
- connected in series in the actuator return line
- check valve is required to allow reversible actuators to return
What changes the amount of pressure intensification?
Amount of restriction in the return line
ratio of base end and piston end areas
force increase created by the load on the rod
Why should you be careful when installing or adjusting valves installed on the rod end of a double acting cylinder?
Pressure intensification, can exceed max system pressure
What is the purpose of a brake valve?
Prevent load from overspeeding the motor
Provide a controlled deceleration of load
What makes a lock valve easier to identify?
Double ball check valve
What does a lock valve do if the system is suspending a load and a hydraulic line ruptures (fails)?
The lock valve will Hold
Whats another name for the lock valve?
Pilot operated check valves
What are the functions of a pilot operated check valves (lock valve)?
Hold a suspended load in the even of a hydraulic line ruptures
Prevent the load from drifting due to leakage in the directional control valve
Prevent the load from overspeeding the actuator
What MUST a counter balance valve have?
Check valve - reverse
Is a counter balance valve normally closed?
Yes
In a system using a counterbalance valve; if the operator lets go of the control valve and it returns to its normal position, what will happen to the “load”?
If the control valve returns to normal position with no flow going to the “cylinder”, the load will hold if the counterbalance valve is operating correctly
What is the purpose of the check valve in a counterbalance valve?
Allow free flow to reverse through the valve, for example when lifting the load
What is the visible difference between a counterbalance valve and a sequence valve?
On a sequence valve the spring is externally drained and a counterbalance valve spring is internally drained
How is the speed of an actuator determined?
Internal capacity of the actuator
input flow rate supplied to the actuator
What is the easiest way to control actuator speed?
Varying flow rate
What is the common component in a hydraulic flow control valve?
Orifice
What is a difference in pressure called?
Pressure drop
Where is pressure always highest in relation to an orifice?
Upstream of the orifice
What happens with pressure downstream from an orifice?
It lowers
Where is the lowest pressure found? In regards to an orifice
Right in the orifice
How is the amount of flow through an orifice determined?
Orifice size and shape
pressure drop across the orifice
Viscosity of the oil
What type of flow control is used to have accurate control of actuator speed?
Pressure compensated flow control
What can be added to a non-compensated flow control to make it pressure compensated?
Compensator
Where is excess flow directed to in a bypass type pressure compensated flow control valve?
To reservoir
Meter in circuit controls the speed of cylinder extensions by controlling _______?
Controlling the amount of oil flowing into the base end of the cylinder
