Pressure-reducing valves are used in hydraulic systems where it is necessary to lower the normal system operating pressure a specified amount. Figure 8-17 shows the operation of a pressure-reducing valve. View A of figure 8-17 shows system pressure being ported to a subsystem

Figure 8-17.Pressure-reducing valve operational schematic.

through the shuttle and sleeve assembly. Subsystem pressurized fluid works on the large flange area of the shuttle, which causes the shuttle to move to the left after reaching a specified pressure, thus closing off the normal system. The valve will stay in this position until the subsystem pressure is lowered, at which time the shuttle will move to its prior position and allow the required amount of pressurized fluid to enter the subsystem. During normal operation of the subsystem, the pressure-reducing valve continuously meters fluid to the subsystem.

HYDRAULIC FUSES 

A hydraulic fuse is a safety device. Fuses may be installed at strategic locations throughout a hydraulic system. They are designed to detect line or gauge rupture, fitting failure, or other leak-producing failure or damage.

One type of fuse, referred to as the automatic resetting type, is designed to allow a certain volume of fluid per minute to pass through it. If the volume passing through the fuse becomes excessive, the fuse will close and shut off the flow. When the pressure is removed from the pressure supply side of the fuse, it will automatically reset itself to the open position. Fuses are usually cylindrical in shape, with an inlet and outlet port at opposite ends, as shown in figure 8-18. A stationary sleeve assembly is con-tained within the body. Other parts contained within the body, starting at the inlet port, are a control head, piston and piston subassembly stop rod, a lock spring, and a lock piston and return spring.

Fluid entering the fuse is divided into two flow paths by the control head. The main flow is between the sleeve and body, and a secondary flow is to the piston. Fluid flowing through the main path exerts a force on the lock piston, causing it to move away from the direction of flow, This movement uncovers ports, allowing fluid to flow through the fuse.

The movement of the locking piston also causes a lock spring to release the piston subassembly stop

Figure 8-18.Fuse, operational view.

rod, thus allowing the piston to be displaced by fluid from the secondary flow. If the flow through the fuse exceeds a specified amount, the piston, moving in the direction of flow, will block the ports originally covered by the locking piston, thus blocking the flow of fluid.

Any interruption of the flow of fluid through the fuse removes the operating force from the lock piston. This allows the lock piston spring to return the piston to the original position, which resets the fuse.

NOTE: Although the following reference was current when this TRAMAN was published, continued currency cannot be assured. You therefore need to ensure that you are studying the latest revision.

Fluid Power, NAVEDTRA 12964, Naval Education and Training Program Management Support Activity, Pensacola, Florida, July 1990, Chapter 10.