Ed Godin
Technical Services Manager
Mike Schubert
Marketing Manager
Parker Hannifin
Hydraulic Accumulator Div.
Rockford, Ill.
Edited by Kenneth Korane
Hydropneumatic accumulators are
widely used in industrial and mobile
hydraulic systems because they provide
auxiliary power during peak periods.
This lets designers select smaller
pumps, motors, and reservoirs in the
main system.
Accumulators are also widely used
in leakage compensation and holding
applications and, on off-road equipment,
in braking, ride control, steering,
dead-engine pilot, and tensioning
systems.
Beyond their basic use, accumulators
are finding their way into new
applications due to an ability to save
energy, reduce initial and operating
costs, handle high payloads, and prolong
equipment life.
With today’s high fuel costs, a
promising new application with
significant payoffs and benefits uses
accumulators as rechargeable hydraulic
batteries. These recover and store
previously lost energy and use it to
supplement pump flow in mobile and
stationary equipment.
ENERGY RECOVERY
Excavators, for instance, are prime
candidates for energy-recovery systems.
The machine’s massive arms, when lowered, generate a great deal
of force on the rod end of the lift cylinders.
In turn, this exerts huge forces
on the fluid and blind end of the
cylinders. Fluid forced from the cylinders
— at several hundred psi, in most
instances — throttles through valves
and returns to the reservoir. The result:
potential energy in the elevated
lift is lost as heat when the arm lowers.
Energy-recovery systems, in contrast,
use the high-pressure fluid being
expelled from the cylinder to drive a
hydraulic motor. The motor, in turn,
drives a hydraulic pump that charges
an accumulator. Energy stored in a
piston or bladder accumulator can
subsequently be harnessed to supplement
pump flow and help lift the
excavator arms and load on the next
work cycle.
For example, if lowering the excavator
arms displaces 7 gallons of fluid at 1,000 psi, the hydraulic motor can
drive a hydraulic pump and develop
2,175 psi or more. This high-pressure
fluid can then be stored in an accumulator
for use in the next lift cycle.
Such energy-recovery systems
make it possible to reduce pump size
by 25%, with resulting fuel cost savings
as high as 30 to 35%.
Excavators with energy recovery
features generally command about a
15% premium over the cost of standard
machines, depending on fuel or
electricity prices. In addition to excavators,
energy recovery generally produces
the best payback on loading and
trenching machines. Forestry excavators
with accumulators, for example,
have a fuel-cost savings of about 25 to 30% because they can use smaller
diesel engines. Typical machines used
in construction applications yield a
10% savings.
In addition, most loading cycles
have about 20 sec when full pump
flow is not used. As a result, the primary
pump operates in a partially
compensated mode and draws less
power. Pump flow during this “dwell”
time can also charge an accumulator.
Then, when the hydraulic system
requires full flow, the accumulator
can supplement flow from the pump.
Here, designers taking advantage of
this concept can significantly reduce
the size of the primary pump.
A typical accumulator system
stores 8.5 gallons of fluid at pressures
between 1,150 and 2,175 psi. Fluid
discharge from the accumulator takes
about 5 sec, or a flow rate of 102 gpm.
Combining the accumulator’s
8.5-gallon flow with the output of an
80-gpm pump generates flow rates
of 108.7 gpm during the 5-sec time
span. The instantaneous flow of fluid
stored in the accumulator combined
with that of the primary pump results
in a faster, more-responsive system.
Generally, excavators use a 50-gallon
piston accumulator supplemented
by 50-gallon gas bottles
— pressurized cylinders that increase
capacity — for a total gas capacity
of 100 gallons. Smaller machines
use about 30 gallons of gas capacity.
Some manufacturers of smaller machines
gang five 6-gallon piston accumulators.
Others OEMs prefer six 5-gallon bladder combination units.
Using gas bottles in conjunction
with an accumulator can significantly
reduce costs as well as save space. Gas
bottles can mount remotely and in
any orientation. Excavator designers
typically position accumulators on
the rear and replace a portion of the
counterbalance weight.
Sizing accumulators
Sizing an accumulator is fairly
straightforward once designers know
the volume recovered from the cylinder
output after intensification by the
secondary pump. Basically, accumulators
are sized to supplement pump
flow, so engineers need to know minimum
and maximum system pressures
and how much fluid the accumulator
must deliver.
Some accumulator manufacturers
offer software-based calculators
to streamline the process. Parker’s
inPHorm Accumulator Sizing and
Selection software, for example, performs
the necessary calculations and
eases the process of sorting through
catalog charts, tables, and drawings.
The software includes calculations
for using accumulators as an auxiliary
power source such as supplementing
pump flow. It’s available at parker.com/accumulator.
Make Contact
Parker Hannifin
parker.com/accumulator