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

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

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

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)

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.

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

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

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.

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.

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.

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.

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.

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

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

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∙ 10𝑔𝑝𝑚 1714 =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 1714 =0.81ℎ𝑝

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

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