1. Hydraulic Pressure Control
MECH 1300
Hydraulic Pressure Control
Topics
Pressure Relief Valves
Unloading Valves
Pressure-Reducing Valves
Sequence Valves
Counterbalance Valves
Brake Valves
Pressure –Compensated Pumps
Pressure Control Valve Mounting
Pressure Control Valve Specifications

2. Pressure Control Valves
MECH 1300
Pressure Control Valves
Allows for controlling the pressure:
Enables control of the output force of a cylinder.
Enables control of the output torque of a motor.
Control the maximum pressure level in a hydraulic circuit
Safety

3. Pressure Relief Valves
MECH 1300
Pressure Relief Valves
Limit the maximum pressure by providing an alternate path for fluid to flow.
Direct-Acting type – basic, uses a ball or poppet

4. Pressure Relief Valves
MECH 1300
Pressure Relief Valves
Pilot operated relief valve – uses a small relief valve and a main relief valve and operates in two stage process (or compound).
The pilot relief valve opens first, once it receives the maximum pressure the main relief valve can open.
Generally smaller than direct acting valves and have a wider range of maximum pressure ratings. They can also be operated remotely using vent port (direct acting)

5. Pressure Relief Valves
MECH 1300
Pressure Relief Valves
Cracking Pressure – The pressure at which the relief valve begins to open. (some flow through the relief valve). Higher cracking pressure is advantages because once the relief valve cracks, we lose flow over the relief valve. Lost flow results in reduced actuator speed and lost power.
Full Flow Pressure – The pressure at which the relief valve is completely open. (all flow is through the relief valve and is the max pressure of the system)
Pressure Override – The difference between Cracking Pressure and Flow Pressure.

6. Pressure Relief Valves
MECH 1300
EX: Compare two types of relief valves operating at 2000 psi and 2250 psi
At 2000 psi
Direct Acting: 9gpm is lost across the relied valve, 41 gpm goes to the system so 9/50 X 100 = 18% of the system flow is lost.
Pilot Operated: 0gpm is lost across relief valve, 50 gpm goes to the system so 0/50 X 100 = 0% of the system flow is lost

7. Pressure Relief Valves
MECH 1300
EX: Compare two types of relief valves operating at 2000 psi and 2250 psi
At 2250 psi
Direct Acting: 25gpm is lost across the relied valve, 25 gpm goes to the system so 25/50 X 100 = 50% of the system flow is lost.
Pilot Operated: 3gpm is lost across relief valve, 47 gpm goes to the system so 3/50 X 100 = 6% of the system flow is lost

8. Pressure Relief Valves
MECH 1300
Shifting shock – Occurs when directional control valves are shifted into the closed position. Shifting the valve closed can cause a pressure spike on the outlet line.
Cross-Over relief valve – Used to prevent shifting shock in motor circuits. Two pressure relief valves combined into one package and are mounted in parallel 180 degrees to each other.
When the DCV is shifted into neutral, some oil is allowed to flow from outlet to inlet through the cross-over relief, providing a shock absorbing or cushioning effect.
Two valves are required because the motor is bi-directional and the motor inlet and outlet will switch depending on the direction of rotation.

9. Unloading Valves Similar in construction to pressure relief valves
MECH 1300
Similar in construction to pressure relief valves
Read pressure in an external line, not their own line
Can isolate flow from two pumps based on psi
EX:
Supplies a cylinder from both pumps until a certain psi (say its set for 500)
At 500 psi, the unloading value opens for pump 1 and unloads the pump back to the tank while pump 2 continues to supply the cylinder. Pump 1 is isolated from the system.

10. Pressure Reducing Valves
MECH 1300
Pressure reducing valves maintain a reduced pressure level in a branch circuit.
The reducing valve is normally open while the relief valve is normally closed.
The reducing valve has an external drain, while the relief valve does not.
Reducing valve can close partially to create a pressure drop across a valve.

11. Sequence Valves
MECH 1300
A valve that is a pressure control valve that is used to force two actuators to be operated in sequence.
Sends flow to a branch circuit, not back to the tank.
Can operate remotely
EX: Clamp cylinder extends to hold a work-piece,
Then a second cylinder extends to bend the work-piece.

12. Counterbalance Valves
MECH 1300
A valve that is a pressure control valve that is used to prevent a load from accelerating uncontrollably. Prevents the load from driving the cylinder.
Apply only to cylinders
Can happen in vertically mounted cylinders where the load is weight.
Can operate remotely.
Can drain internally, does not have an external drain.

13. Brake Valves
MECH 1300
A valve that is a pressure control valve that is used to prevent a load from accelerating uncontrollably, just like a counterbalance valve, except that brake valves only apply to hydraulic motors.
Prevents the motor from being driven by the weight, by closing the inlet just enough to allow enough back pressure on the outlet to keep the motor in control

14. Pressure Compensated Pumps
MECH 1300
Limit the maximum pressure by reducing their displacement.
When pressure reaches a preset level (firing pressure), the displacement is reduced to prevent further rise in pressure
Flow will decrease rapidly to zero
EX: Hydraulic Press:
Presses something together until it reaches its firing pressure

15. Pressure Compensated Pumps
MECH 1300
EX: A hydraulic press circuit operates at 3500 psi and 10 gpm. When the cylinder is fully extended and under pressure, the leakage in the system totals 0.4 gpm. Compare the power consumption when the cylinder is fully extended with and without a pressure compensated pump.
Without: 9.6 gpm is forced over the relief valve and 0.4 gpm is lost due to leakage, both at 3500 psi. Total energy waste is
𝐻𝑃 𝐻 = 𝑝∙𝑄 1714 = 3500 𝑝𝑠𝑖∙ 9.6𝑔𝑝𝑚 =20.42ℎ𝑝
With: When the cylinder bottoms out, the pumps displacement will be reduced to 0.4 gpm. The total wasted energy is
𝐻𝑃 𝐻 = 𝑝∙𝑄 1714 = 3500 𝑝𝑠𝑖∙ 0.4 𝑔𝑝𝑚 =0.81ℎ𝑝

16. Pressure Control Valve Mounting
MECH 1300
Inline mounting style – fittings that are screwed into the valve
Sub-plate mounting style - have unthreaded connections on the bottom of the valve

17. Pressure Control Valve Specifications
MECH 1300
What you will see:
Maximum Pressure
Maximum Flow
Filtration level
Fluid Type and viscosity range
Physical size, mounting and porting