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desired) and regardless of the system load, if the load does not exceed the designed capacity of the reducer. [Figure 12-46] 

## Shuttle Valves 

In certain fluid power systems, the supply of fluid to a subsystem must be from more than one source to meet system requirements. In some systems, an emergency system is provided as a source of pressure in the event of normal system failure. The emergency system usually actuates only essential components. The main purpose of the shuttle valve is to isolate the normal system from an alternate or emergency system. It is small and simple; yet, it is a very important component. [Figure 12-47]  The housing contains three ports—normal system inlet, alternate or emergency system inlet, and outlet. A shuttle valve used to operate more than one actuating unit may contain additional unit outlet ports. 

Enclosed in the housing is a sliding part called the shuttle. Its purpose is to seal off one of the inlet ports. There is a shuttle seat at each inlet port. When a shuttle valve is in the normal operation position, fluid has a free flow from the normal system inlet port, through the valve, and out through the outlet port to the actuating unit. The shuttle is seated against the alternate system inlet port and held there by normal system pressure and by the shuttle valve spring. The shuttle remains in this position until the alternate system is activated. This action directs fluid under pressure from the alternate system to the shuttle valve and forces the shuttle from the alternate system inlet port to the normal system inlet port. Fluid from the alternate system then has a free flow to the outlet port but is prevented from entering the normal system by the shuttle, which seals off the normal system port. 

<img src='content_image/551945.jpg'>

<img src='content_image/551943.jpg'>

The shuttle may be one of four types: 

1. Sliding plunge 

2. Spring-loaded piston 

3. Spring-loaded ball 

4. Spring-loaded poppet 

In shuttle valves that are designed with a spring, the shuttle is normally held against the alternate system inlet port by the spring. 

## Shutoff Valves 

Shutoff valves are used to shut off the flow of fluid to a particular system or component. In general, these types of valves are electrically powered. Shutoff valves are also used to create a priority in a hydraulic system and are controlled by pressure switches. [Figure 12-48] 

<img src='content_image/551944.jpg'>

