A hydraulic pilot check valve allows free flow in one direction and blocks flow in the opposite direction until pilot pressure is applied. Here's a breakdown of its operation:
1. Normal (Blocked) State:
- The valve acts as a regular check valve, allowing fluid to flow freely in one direction.
- In the opposite direction, a poppet (or ball) is held against a seat by a spring and hydraulic pressure, preventing flow. This provides a positive fluid lock with minimal internal leakage. The video excerpt mentions this positive fluid lock as a key characteristic.
2. Pilot Pressure Applied (Unblocked State):
- When sufficient pilot pressure is applied to the pilot port, it overcomes the spring force and the hydraulic pressure holding the poppet closed.
- This pilot pressure pushes the poppet away from its seat, creating a flow path for fluid to pass through the valve in the previously blocked direction.
In essence, a hydraulic pilot check valve is a check valve that can be "unlocked" by applying pressure to a pilot port. This allows for controlled movement and secure holding of hydraulic cylinders. For example, in a vertical cylinder application, the pilot check valve can prevent the cylinder from drifting down due to gravity when the directional control valve is in the neutral position.
Analogy:
Think of it like a door with a spring-loaded latch. Normally, the door only opens one way. However, if you push a button (the pilot pressure), the latch releases, and you can now push the door open in the opposite direction.
Key Components:
- Body: Contains the valve's internal components.
- Poppet (or Ball): The moving element that blocks or allows flow.
- Seat: The surface against which the poppet seals.
- Spring: Provides force to keep the poppet seated.
- Pilot Port: The connection point for the pilot pressure signal.
Applications:
- Cylinder Locking: Preventing cylinder drift in vertical applications.
- Load Holding: Maintaining a load position in hydraulic circuits.
- Sequential Circuits: Controlling the order of operations in hydraulic systems.
