What Is A Shuttle Valve?

A shuttle valve (also known as a double-pressure valve or selector valve) is a type of directional control valve in hydraulic systems.  Its primary function is to automatically switch between pressure sources, ensuring the system selects the required pressure or flow from multiple inputs.

Types of Shuttle Valves

1.  Pneumatic Shuttle Valves

Pneumatic shuttle valves are used in compressed air systems to control airflow.  They isolate air between two independent streams, maintaining isolated and constant outlet pressure even with supply variations.  They regulate air movement in pneumatic systems for tools and actuators.

2.  Hydraulic Shuttle Valves

Hydraulic shuttle valves control fluid flow in hydraulic systems.  Like their pneumatic counterparts, they divert hydraulic oil to ensure stable outlet pressure despite fluctuations in demand.  They utilize high-pressure sources for smooth operation and redundancy.

Key Applications

1.  Pressure Source Selection

When a system has two independent pressure sources, the shuttle valve automatically selects the higher-pressure side as the output, ensuring critical actuators (e.g., brakes, locking cylinders) always receive sufficient pressure.

Example: Travel hydraulic systems in construction machinery, redundant pump systems.

2.  Redundant Backup

If the primary system fails (e.g., pump malfunction), the shuttle valve switches to a backup pressure source (e.g., an accumulator), enhancing system reliability.

Typical Case: Aircraft landing gear hydraulic systems, industrial safety circuits.

3.  Pressure Retention for Single-Acting Cylinders

In locking circuits, shuttle valves work with hydraulic pilot-operated check valves to maintain the position of actuators (e.g., hydraulic cylinders), preventing unintended load movement.

Example: Crane outriggers, mold locking in injection molding machines.

4.  Priority Signal Transmission

In control circuits, shuttle valves transmit priority signals (e.g., pilot pressure) for logic control or multi-channel operations.

Example: Signal switching for different actions in excavator pilot control systems.

Hydraulic Shuttle Valve Structure

Working Principle

Structure: A shuttle valve has two inlets (A, B) and one outlet (C). An internal spool (or steel ball) moves laterally.

Operation:

When inlet A pressure > inlet B pressure, the spool moves toward B, sealing port B. Pressure from A flows to outlet C.

Conversely, if inlet B pressure is higher, the spool seals port A, and pressure from B flows to C.

If pressures are equal, the output depends on design (typically the sum or the higher value).

Differences from Other Valves

Directional Control Valves: Shuttle valves select pressure automatically without external control, while directional valves require manual or electrical control to change flow direction.

Safety Valves: Safety valves limit pressure, whereas shuttle valves switch between pressure sources.