means of an actuating unit. A device called an actuating cylinder is used to impart powered straight-line
motion to a mechanism.
Hydraulic systems must also have devices to control or direct the fluid pressure to the various
components. Such devices include selector valves, check valves, ratchet valves, irreversible valves,
sequence valves, and priority valves. Each is described in the paragraphs that follow.
A basic actuating cylinder consists of a cylinder housing, one or more pistons and piston rods, and one
or more seals. The cylinder housing contains a polished bore in which the piston operates and one or
more ports through which fluid enters and leaves the bore. The piston and rod form an assembly which
moves forward and backward within the cylinder bore. The piston rod moves into and out of the
cylinder housing through an opening in one or both ends. The seals are used to prevent leakage between
the piston and cylinder bore, and between the piston rod and housing. The two major types of actuating
cylinders are single-action and double-action.
Single-Action Actuating Cylinder. The single-action actuating cylinder, shown in Figure 2-1,
consists of a cylinder housing with one fluid port, a piston and rod assembly, a piston return spring, and
When no pressure is applied to the piston, the return spring holds it and the rod assembly in the
retracted position. When hydraulic pressure is applied to the inlet port, the piston, sealed to the cylinder
wall by an O-ring, does not allow the fluid to pass. This causes the piston to extend.
As the piston and rod extend, the return spring compresses. A vent on the spring side of the piston
allows air to escape. When pressure is relieved, the return spring forces the piston to retract, pushing the
fluid out of the cylinder. A wiper in the housing keeps the piston rod clean.
The cylinder can be pressure-operated in one direction only. A three-way control valve is normally
used to control cylinder operation.
Double-Action Actuating Cylinder. The double-action actuating cylinder consists of a cylinder with
a port at either end and a piston and rod assembly extending through one end of the cylinder (Figure 2-