Catalog 1630-8/USA Hydraulic Accumulators

Applications Piston and Bladder Type

*Safety Note:
In any accumulator circuit, a means should be available
of automatically unloading the accumulator when the
machine is shut down. Such a valve could be located at
this point in the circuit.

Control of Usable Volume

The usable volume of an accumulator should be
discharged at a controlled rate. If an accumulator is
required to maintain system pressure, this controlled *
rate is automatically achieved by the leakage fluid it has
to replace. However, an accumulator which is used to
develop a pressurized flow can discharge its usable
volume too rapidly as a downstream directional valve is
shifted. For this reason, accumulators in this application
are often equipped with a flow control and bypass
check at their inlet-outlet port.

Pump Unloading in
Accumulator Circuits
To keep a pump/electric motor fully unloaded until it is
required to re-charge an accumulator, an electric M
pressure switch can be used.
In the circuit illustrated, a pressure switch senses Pressure
accumulator pressure sending and cutting-out electrical Switch
signals at various pressure levels. The electrical signals
are transmitted to a normally-open, solenoid operated
2-way valve which vents and de-vents a pilot operated *
relief valve. When the accumulator is being charged,
the pressure switch sends an electrical signal to the 2-
way valve solenoid. With the accumulator charged, the
pressure switch cuts out the signal, venting the relief Pilot Operated
valve and unloading pump/electric motor. The setting of Relief Valve
the pressure switch determines the pressure range (Simplified
within which a pump/electric motor works. Symbol)

Using a pressure switch to vent a relief valve results in

a pump/electric motor being fully unloaded when
system conditions dictate.
In the circuit illustrated, an unloading valve is used to M
dump a flow back to tank once an accumulator is
charged to the unloading valve setting.
Once the valve closes, pump/electric motor must there-
fore generate power to recharge the accumulator to the
unloading valve setting.

Unloading Valve

119 Parker Hannifin Corporation

Hydraulic Accumulator Division
Hydraulics Rockford, IL 61115
Catalog 1630-8/USA Hydraulic Accumulators
Applications Piston and Bladder Type

Differential Unloading Relief Valve

Instead of using a pressure switch and solenoid valve
to vent a relief valve while an accumulator is charged,
one hydraulic component can be used — a differential
unloading relief valve.
A differential unloading relief valve is specifically
designed for use with accumulators. As its name
implies, the valve unloads a pump/electric motor over a
differential pressure range. *
A differential unloading relief valve consists of a pilot Differential
Unloading
operated relief valve, check valve, and differential piston Relief
in one valve body. The valve body includes pump, tank, Valve
Differential
Pilot Valve
and accumulator passages. Piston
Dart

Maintaining Pressure Passage to

Accumulator
Accumulators are used to maintain pressure. This can
Check
be required in one leg of a circuit while pump/electric Valve Orifice Main Valve
motor is delivering flow to another portion of the Passage
Spool
system. From
Pump
In the circuit illustrated, two clamp cylinders are re-
quired to hold a part in place. As the directional valves Passage to
Tank
are shifted, both cylinders extend the clamp at the
pump’s compensator setting. During this time, the Valve A
accumulator is charged to the setting also.
System demands require that cylinder B maintain
pressure while cylinder A retracts. As directional valve A Cylinder A
is shifted, pressure at the pump, as well as in line A,
drops quite low. Pressure at cylinder B is maintained M
because the accumulator has stored sufficient fluid
under pressure to make up for any leakage in line B.
Valve B
Accumulators not only maintain pressure by compen-
sating for pressure loss due to leakage, but they also *
compensate for pressure increase due to thermal fluid Cylinder B
expansion or external mechanical forces acting on a
cylinder.
In the illustrated circuit, assume that the cylinder is
operating near a furnace where ambient temperatures
are quite high. This causes the fluid to expand. With an
Extra Volume
accumulator in the circuit, the excess volume is taken Absorbed
up, keeping the pressure relatively constant. Without an
accumulator, pressure in the line would rise uncontrolla-
bly and may cause a component housing, fitting, or Heat Expands
conductor to crack. * Fluid
˚F ˚F Curing
Press
The same situation can also occur if an external
mechanical force acts to retract the cylinder. Assume
now that the cylinder is clamping a curing press. As
curing occurs, heat within the press causes it to expand
resulting in a force acting to retract the piston rod. The
accumulator once again absorbs the additional volume,
˚F ˚F Force To
maintaining the pressure at a relatively constant level. Retract

*Safety Note:
In any accumulator circuit, a means should be available
for automatically unloading the accumulator when the
machine is shut down. Such a valve could be located at
this point in the circuit.

120 Parker Hannifin Corporation

Hydraulic Accumulator Division
Hydraulics Rockford, IL 61115
Catalog 1630-8/USA Hydraulic Accumulators
Applications Piston and Bladder Type

Developing Flow
Since charged accumulators are a source of hydraulic
potential energy, stored energy of an accumulator can
be used to develop system flow when system demand
is greater than pump delivery. For instance, if a M
machine is designed to cycle infrequently, a small
displacement pump can be used to fill an accumulator
*
over a period of time. When the moment arrives for the 100
GPM
machine to operate, a directional valve is shifted
downstream, and the accumulator delivers the required
pressurized flow to an actuator.
Using an accumulator in combination with a small
pump in this manner conserves peak horsepower. For
instead of using a large pump/electric motor to
generate a large horsepower all at once, the work can
be evenly spread over a time period with a small pump/
electric motor.

Absorbing Shock
Load Bounces
Hydro-pneumatic accumulators are sometimes used to
absorb system shock even though in this application
they are difficult to properly design into a system.
Shock in a hydraulic system may be developed from
the inertia of a load attached to a cylinder or motor. Or,
it may be caused by fluid inertia when system flow is
suddenly blocked or changed direction as a directional
valve is shifted quickly. An accumulator in the circuit will
absorb some of the shock and not allow it to be trans-
mitted fully throughout the system. *
Shock may also occur in a hydraulic system due to
external mechanical forces. In the circuit illustrated, the
load attached to the cylinder has a tendency to bounce
causing the rod to be pushed in and shock generated.
An accumulator positioned in the cylinder line can help
reduce the shock effects.
Restriction Controls
Accumulator Discharge

*Safety Note:
In any accumulator circuit, a means should be available M
for automatically unloading the accumulator when the
machine is shut down. Such a valve could be located at
*
this point in the circuit.

Developing System Flow

121 Parker Hannifin Corporation

Hydraulic Accumulator Division
Hydraulics Rockford, IL 61115