Hydraulic Valves Part 1 Flashcards
190301fA
How do hydraulic valves assert their control over the system?
- controlling pressure - regulating flow through the branches of a system - determining the path of oil flow
When selecting a pressure gauge to read hydraulic pressures what is a good rule of thumb?
Select a gauge with a range twice that of the max system spec for more accurate results.
Do you need to run a main system relief in a system that also runs an unloader valve?
Yes, when the unloader valve closes and pump pressure is allowed to build and run actuators the main relief still maintains maximum system pressure, the unloader is only used to relieve the strain on a pump in a closed centre hydraulic system when the directional valve is in the closed position.
How is a counterbalance valve different from a brake valve?
Pilot pressure is sensed at the outlet of a counterbalance valve whereas pilot pressure is sensed on both inlet and outlet of a braking valve.
What’s is spring constant?
The stiffness of a spring in PSI, the greater the spring constant the greater the pressure overide
Are pressure reducing valves normally open or closed?
Open
Accumulators are installed for what 4 reasons?
-energy storage -shock absorption -gradual pressure buildup -constant pressure maintenance
How does a counterbalance valve allow the reversed flow of oil?
A check valve
What are the 7 common types of pressure control valves?
-pressure relief valve -unloaded valve -pressure reducing valve -sequence valve -counter balance valve -brake valve -line lock valve
3 types of accumulators are:
-weighted -spring loaded -gas charged
How is the second actuator in a sequence valve connected in the circuit?
Always in series with and downstream of the valve.
What flow control valve is this?
Unloader valve
Are pressure relief valves normally direct acting or pilot operated?
Direct acting (one spring)
Where is a counterbalance valve usually located physically?
Attached to the base end of a cylinder
What does the term pressure override refer to?
The difference between cracking pressure and full flow pressure
How is a counterbalance valve often used in a circuit
A safety device on a linear actuator
What flow control valve is this?
Lock valve
What flow control valve is this?
Counterbalance valve
How does a hi-low pump operate?
2 pumps run in parallel when high flow is required, as system pressure rises an unloaded valve set to a specific pressure will dump 1 pump to tank and allow the prime mover to deliver max horsepower to the other pump to deliver more torque.
How do unloader valves differ from relief valves?
- they are always used in conjunction with a check valve
- pilot pressure used to open the valve comes from the high pressure side of the check valve
When is a brake valve used?
On the return line of a hydraulic motor to prevent the load from over speeding the motor, and provide controlled deceleration.
What is another term for a pilot controlled pressure control valve?
A compound pressure control valve
How does a pressure reducing valve function?
Oil is allowed to flow freely through the valve to an actuator until the pilot line sees enough pressure to close the valve and Maintain pressure to that actuator while diverting the flow of oil out of the valve to send full system pressure to another actuator
What similarities does a unloader valve share with a relief valve?
- always normally closed
- inlet port is in parallel with the circuit it unloads
- outlet always connected to the reservoir
A technician replaced the brake valve on a picker truck but did not reconnect the pilot line that is in parallel with the inlet. The driver complains that the load won’t lower. Why?
Inlet pilot pressure needs to be seen to aid the outlet pressure and act upon the spool to open and allow the flow of oil.
How does a sequence valve allow the reverse of flow?
A check valve
What is a lock valve sometimes referred to?
Pilot operated check valve
In a modern hydraulic system where are circuit relief valves often located?
The directional control valve
Why is it important for an unloader valve to have an accumulator upstream of the valve?
once pump flow is directed away from the outlet port the accumulator maintains pressure on the check valve inside the valve, thus keeping the pump unloaded
How does a lock valve operate?
Flow to the inlet of the valve through a check valve and also acting on a shuttle valve that opens the check valve attached to the return circuit, and vice versa. Without flow directed through either inlet our outlet, flow on the other side of the valve is locked.
How do sequencing valves function?
Pilot pressure is sensed upstream of the valve. the inlet port, which is connected to the pilot, has to operate first in the sequence. When pilot pressure builds, the spool shifts to allow oil to flow to the outlet and operate the second sequence
What flow control valve is this?
Brake valve
To test circuit relief settings how should system relief be set?
Higher than system pressure settings, with care taken to only actuate the circuit in question, and always following the service manual.
What flow control valve is this?
Pressure reducing valve
Two basic forms of pressure control valves are:
Direct acting and pilot operated
How is the flow of oil reversed in a brake valve?
Check valve
3 types of gas charged accumulators are:
-bladder -piston -diaphragm
Why is a braking valve important on hoisting equipment?
To eliminate the chance of the motor stopping abruptly when the flow of oil stops, which could cause severe shock load
What is the biggest disadvantage of using direct acting pressure control valves?
Direct acting valves have a higher pressure overide. A higher pressure overide leads to unwanted heat and wasted power generated in the circuit
How does a counter balance valve function?
Oil is allowed to flow through the valve to fill the base end of a cylinder. The oil is held in the base end of the cylinder until enough force is excerpted on the rod end to build enough pressure on the intensifier piston inside the valve and force the spool to overcome spring pressure, open, and allow the cylinder to lower. When the rod end pressure drops the spool closes, and the cylinder stops lowering.
How does a brake valve function?
pilot pressure is sensed on both the inlet and outlet of the valve, when lowering, pilot pressure is sent to the inlet and outlet and opens the valve and allows oil to flow to turn the motor, when the directional control valve is in the closed position pressure drops in the inlet and the outlet pressure alone cannot keep the valve open and the spool is closed slowly braking the motor.
Are sequencing valves normally open or closed?
Closed
Can system pressure be tested with a hand actuated hydraulic pump?
No: only circuit pressures can be tested with a hand actuated pump.
On a pressure relief valve where is pilot pressure drawn from in the system
Always upstream of the valve.
The constant movement of a valve under spring and oil pressure is known as?
Modulation
Hydraulic pumps in tandem are sometimes referred to as
Hi-low pump
How is a brake valve often used
As a safety device on a rotary actuator
