By storing energy in the form of fluid under
ACTUATORS (ACTUATING CYLINDERS)
pressure, an accumulator can supplement the
An actuator is a device that transforms fluid
pump's output when the pump is under peak
p r e s s u r e i n t o m e c h a n i c a l f o r c e t o m ove s o m e
load.
mechanism. For example, a hydraulic motor is an
actuator that converts fluid under pressure into rotary
In the event of normal hydraulic system failure,
motion.
the energy stored in an accumulator can be used
The most common actuator used in support
to actuate a unit of the system. For example,
equipment is the actuating cylinder. This actuator
s u ffi c i e n t e n e rg y c a n b e s t o r e d i n t h e
c o nv e r t s f l u i d u n d e r p r e s s u r e i n t o l i n e a r o r
accumulator for several applications of the
reciprocating mechanical motion. There are two main
brakes of a self-powered vehicle.
types--the single-action actuating cylinder and the
double-action actuating cylinder.
Several types of accumulators are shown in figure
8-35. Of these, the cylindrical (cylinder) type is the one
NOTE: You will find the terms piston and ram
most often found in ground support equipment. This
both used when referring to the movable elements of
type of accumulator is used in the hydraulic systems of
actuating cylinders. The term ram is normally
weapon loaders, maintenance stands, engine stands,
associated with movable elements that have no piston
and aircraft spotting dollies to name a few.
rods and are used primarily for push functions. (The
piston and rod usually are approximately the same
A cylindrical accumulator consists of a cylinder
diameter.) Movable elements that are attached to
and a piston assembly. Attached to both ends of the
piston rods and are used for both push and pull
cylinder are end caps. An internal piston separates the
functions are called pistons.
fluid and nitrogen chambers. In one end cap, a
hydraulic fitting is used to attach the fluid chamber to
Single-Acting Actuating Cylinders
the hydraulic system. In the other end cap, an air filler
valve provides a connection to an air/nitrogen source.
Single-acting actuating cylinders are designed to
In operation, the compressed gas chamber is charged to
allow piston (ram or shaft) travel in one direction
a pressure that is something less than the system
through the use of fluid under pressure. Piston travel in
operating pressure. This initial charge is called the
the opposite direction is provided by some other force,
accumulator preload.
usually gravity (weight). For example, in the actuating
Assume that the cylinder-type accumulator
cylinders of hydraulic jacks, workstands, and some
pictured in figure 8-35 is designed for a nitrogen gas
models of forklifts, hydraulic fluid provides the force
preload of 700 psi in a 1,500-psi hydraulic system.
to raise the rams, and the weight of the ram and the load
When the initial charge of 700 psi is introduced into the
lowers the ram.
unit, hydraulic system pressure is zero. As nitrogen
T h r e e ex a m p l e s o f s i n g l e - a c t i n g a c t u a t i n g
pressure increases, the piston is pushed toward the
cylinders are shown in figure 8-36. Figure 8-36, view A
opposite end until it bottoms. Then, with the nitrogen
shows a single ram cylinder. This type of cylinder is
at 700 psi, the pressure in the hydraulic system must be
used in hydraulically operated workstands and as the
increased to something greater than 700 psi before the
lift cylinder on some models of forklifts. Fluid is
hydraulic fluid can actuate the piston. Thus, at 701 psi
forced into the bottom of the cylinder to extend the
in the system, the pressure begins to move the piston
ram. Then, to retract (lower) the ram, fluid is allowed to
back toward the gas end of the cylinder, compressing
flow out of the cylinder to the reservoir. The weight of
the nitrogen as it moves. At 900 psi, the piston will
the ram and any load that it might carry forces the ram
have moved several inches. At 1,500 psi, the piston will
to retract into the cylinder.
have moved back to its normal operating position,
A telescoping ram cylinder assembly is shown in
compressing the nitrogen until it occupies a space less
view B of figure 8-36. This type of actuating cylinder
than one-half the length of the cylinder. Then, when
is used in some hydraulic jacks, and operates much
actuation of a hydraulic unit lowers the pressure in the
like a single ram cylinder. Fluid is forced into the
system, the compressed nitrogen expands against the
bottom of the cylinder to extend the rams. Since the
piston, forcing fluid from the accumulator, providing
fluid pressure acts on the entire cross-sectional area
an instantaneous supply of fluid to the hydraulic
system.
of ram 1, this ram extends first. When the lip of ram 1
8-28
