Valve Operation With no Pressure. In A, Figure 2-11, the ratchet valve is shown with no pressure
applied. The piston is centered in its bore and both ball check valves are closed. This locks the
actuating cylinder in position by trapping all fluid in the cylinder.
Valve Operation With Pressure Applied. In B, Figure 2-11, the ratchet valve is shown with pressure
applied to port 1. This forces the piston to the right where it unseats ball check valve b. Pressure
entering port 1 also unseats ball check valve a on the left side. Fluid then flows through the ratchet
valve and the piston moves to the right.
CHECK VALVES
A check valve is installed in a hydraulic system to control the direction flow of hydraulic fluid. The
check valve allows free flow of fluid in one direction, but no flow or a restricted one in the other
direction.
There are two general designs in check valves. One has its own housing and is connected to other
components with tubing or hose. Check valves of this design are called in-line check valves. In the
other design, the check valve is part of another component and is called an integral check valve. It will
not be covered because its operation is identical to the in-line check valve. The two types of in-line
check valves, simple and orifice, are described in the following paragraphs.
Simple In-Line Check Valve. As illustrated in Figure 2-12, the simple inline check valve consists of
a casing, inlet and outlet ports, and a ball-and-spring assembly. The ball and spring permit full fluid
flow in one direction and block flow completely in the opposite direction. Fluid pressure forces the ball
off its seat against the spring pressure, permitting fluid flow. When flow stops, the spring forces the ball
against its seat, blocking reverse flow.
Orifice In-Line Check Valve. The orifice check valve shown in Figure 2-13 is used to allow free
flow in one direction and limited flow in the opposite direction. This is accomplished by drilling a
passage in the valve seat connecting the inlet side of the valve to the outlet side.
SEQUENCE VALVE
A sequence valve, shown in Figure 2-14, is placed in a hydraulic system to delay the operation of one
portion of that system until another portion of the same system has functioned. For
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