Figure 4-139. Irreversible Valve
Hydraulic Fuses. Fuses are incorporated in hydraulic systems to prevent continued loss of fluid in cases
of serious leakage Fuses are strictly safety devices. They are not needed for operation of the system. Hydraulic fuses
are usually installed in lines that are run in hazardous locations or that conduct fluid to components located in such
places. They can only be used in lines through which fluid flows in one direction but not continuously or in lines through
which flow is reversed at intervals. A hydraulic fuse is designed to permit only a limited quantity of fluid to pass through
it. When this limit is reached, the fuse automatically shuts off further flow. The typical fuse shown in figure 4-140
consists essentially of a housing, a sleeve, a piston, a piston return spring, and a metering plate The housing has a port
at each end, one pressure, the other, return. The sleeve is attached to the housing and is centered on the axis of the
housing. It has a polished bore in which the piston operates Holes in the end of the sleeve nearest the outlet port open
the bore of the sleeve to a space between the sleeve and the housing. The metering plate divides whatever fluid enters
the fuse Into streams of unequal size. The larger streams flow around the outside of the sleeve, the smaller streams,
through the inside.
Figure 4-140. Hydraulic Fuse Assembly
Normal flow. Whenever the fuse is not transmitting fluid, its piston is pulled to the inlet end of
the fuse by the piston return spring as shown in figure 4-141. As soon as fluid begins flowing through the fuse, the
stream of fluid that enters the sleeve reacts against the piston. This causes a slow movement of the piston toward the
outlet end of the sleeve. Meanwhile, the stream of fluid that flows on the outside of the sleeve passes through the
communication holes in the sleeve and then out of the fuse outlet port. As long as the quantity of fluid passing through
the fuse is equal to or less than the rated flow capacity of the fuse, the piston travels less distance within the sleeve than
its travel limit.
Blocked flow. If the flow of fluid through the fuse becomes greater than its rated flow capacity,
the increased pressure causes the piston to travel until the needle on its end plugs the opening at the outlet end of the
housing, thus blocking further fluid flow through the fuse (see figure 4-142). The piston is held in the blocking position
until the pressure is relieved. When the pressure is relieved, the piston return spring retracts the piston and fluid again
flows through the fuse.
Drycleaning solvent is flammable and solvent vapors are toxic. Use P-D-680, Type II Solvent in a well-
ventilated area. Keep away from open flames. Avoid prolonged solvent contact with skin.