When a shuttle valve is in the normal operating position, fluid has a free flow from the normal system inlet port to the unit outlet port. The shuttle is seated against the emergency inlet port, and held there by the shuttle spring or by normal system pressure. The shuttle remains in this position until the emergency fluid, gas, or air is released under pressure by the emergency control valve. The application of emergency pressure at the emergency inlet port forces the shuttle from the emergency inlet port seat to the normal system inlet port seat. The emergency pressure then has a free flow to the unit outlet port, but is prevented from entering the normal system by the shuttle.

Shuttle valve maintenance is generally limited to repairing leakage. External leakage may generally be repaired by tightening the end caps. If this does not stop excessive leakage, the end cap O-rings should be replaced.

Internal leakage can usually be repaired by removing and flushing the unit with clean hydraulic fluid. Excessive heating is a good indication of internal leakage through a shuttle valve. Excessive cycling of the emergency system pump is also an indication of a leaky shuttle valve.

After an emergency system has been operated, all emergency system pressure should be bled off as soon as possible, and the normal system restored to operation.

Restrictors are used in hydraulic systems to limit the flow of hydraulic fluid to or from actuators where speed control of the cylinders is necessary to provide specific actions. If control in one direction only is desired, a one-way restrictor is used. If restricted fluid flow both to and from an actuating cylinder is necessary, a two-way restrictor is installed.

Figure 8-15.Shuttle valve.


Figure 8-16.Restrictors.