CHAPTER 11
BASIC ACTUATING SYSTEMS
INTRODUCTION
either single or double acting. Unlike the balanced
actuator, it has a single piston shaft extending from the
The actuating systems consist of the hydraulic
piston head, resulting in unequal working areas. Each
components used to direct and control the flow of
actuator used may differ considerably in size and
pressurized fluid as well as the components used to
construction.
perform the actual work. This chapter covers actuating
units and most of the various actuating system
Single-Acting Actuating Cylinder
components that are used in modern-day hydraulic
The single-acting, piston-type cylinder uses fluid
systems.
pressure to apply force in only one direction. In some
designs of this type, the force of gravity moves the
ACTUATING UNITS
piston in the opposite direction. However, most
cylinders of this type apply force in both directions.
LEARNING OBJECTIVE: Identify various
Fluid pressure provides the force in one direction, and
hydraulic actuating units.
spring tension provides the force in the opposite
direction. In some single-acting cylinders,
An actuating unit may be defined as a unit that
compressed air or nitrogen is used instead of a spring
transforms hydraulic fluid pressure into mechanical
for movement in the direction opposite that achieved
force, which performs work (moving some
with fluid pressure.
mechanism). Two types of actuating units are used in
naval aircraft--actuating cylinders and hydraulic
Figure 11-1 shows a single-acting, spring-loaded,
motors.
piston-type actuating cylinder. In this cylinder the
spring is located on the rod side of the piston. In some
TYPES OF ACTUATING CYLINDERS
spring-loaded cylinders, the spring is located on the
blank side, and the fluid port is located on the rod side
Actuating cylinders are the most commonly used
of the cylinder.
actuating units in aircraft hydraulic systems. The
A three-way directional control valve is normally
purpose of an actuating cylinder is to convert fluid
used to control the operation of this type of cylinder.
under pressure into linear or mechanical motion.
To extend the piston rod, fluid under pressure is
Actuating cylinders are generally installed in such a
directed through the port and into the cylinder. See
manner that the piston shaft (rod) end of the cylinder is
figure 11-1. This pressure acts on the surface area of
attached to the mechanism to be actuated, with the
the blank side of the piston, and forces the piston to the
other end attached to the aircraft structure.
right. This action, of course, extends the rod to the
There are two types of actuating cylinders--
right, through the end of the cylinder. The actuated unit
balanced or unbalanced. Balanced actuators have
is moved in one direction. During this action, the
equal working areas, with a piston shaft extending
from both sides of the piston head. This type of
cylinder may be a single-acting actuator, which
receives hydraulic pressure on only one side of the
piston head for movement in one direction, and some
other means of force for movement in the opposite
direction. However, it may also be a double-acting
type, which uses hydraulic pressure alternately on both
sides of the piston head to move it in the selected
direction.
The most common type of actuating cylinder used
Figure 11-1.--Single-acting, spring-loaded, piston-type,
on naval aircraft is the unbalanced type, which may be
actuating cylinder.
11-1
