EvaluatingHydrostaticPumps FPJ 01-02.11
Transcript of EvaluatingHydrostaticPumps FPJ 01-02.11
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AS SEEN IN JANUARY/FEBRUARY 2011
PRE-IFPEISSUEEVALUATING
HYDROSTATIC
TRANSMISSION SYSTEM
PUMPS PG.2-3
SER VO PUMP
REPLENISH
PUMP
for
High Horsepower, Heavy-Duty
Closed Circuit Applications
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REPRINTED FROM FLUID POWE R JOURNAL' S JANUARY/FEBRUARY 2011 ISSUE www.fluidpowerjournal.com | www.ifps.org
H
ydrostatic transmission drives have long been recognized
for superior power transmission when variable output
speed is required because they provide fast response,
maintain precise speed under varying loads, and allowinfinitely variable speed control from zero to maximum.
Most important, they feature a continuous power curve
without peaks and valleys and can increase available
torque without having to shift gears as is necessary with
mechanical gear transmissions.
A hydrostatic transmission is an entire hydraulic system in a single package—
pump, motor, and all required controls. Tese systems provide all of the advantages
of a conventional hydraulic system:
• Step-less adjustment of speed, torque, and power
• Smooth and controllable acceleration
• Ability to be stalled without damage
• Easy controllability
Tere are two general hydrostatic transmission configurations: split or closed
coupled. Integrated (or closed coupled) transmissions are designed to have th
hydraulic pump and motor share a common valving surface and are typically used
in light-duty applications.Split hydrostatic transmission systems (Fig. 1) consist of a power unit with the
hydraulic pump, heat exchangers, filters, valves, and controls mounted on a reser-
voir. Te hydraulic motor is remotely mounted and connected to the power uni
through hose or tubing. Split transmissions are widely used in heavy- and severe
duty closed circuit applications such as shredding, logging, and oil drilling as well a
land and sea military equipment because they provide wide flexibility in configur
ing the system for the most efficient use of space or best weight distribution.
Closed hydraulic circuits typically utilize a variable displacement pump and fixed
motor. Tis combination provides a constant torque output at a fixed maximum
pressure over the pump’s full speed range. Speed and direction are controlled with
variable displacement over-center pump. Power from overhauling loads is regener
ated back into the pump prime mover. Motor speed is limited to the maximum
speed permitted by full pump displacement.
Te decision as to which and what type of power unit design to use in a spli
hydrostatic transmission configuration depends in large part on how severe thedemands will be for the intended application. Example: shredding anything from
cars to tires, scrap steel, wood, and hot water heaters is an extremely harsh operat-
ing environment for a hydraulic pump. Here, a very fast compensator is importan
that can handle the rapid pressure spikes and rapid
pressure decay of these operations. Some pumps can
handle massive pressure spikes with ease, while other
have difficulty.
For high-horsepower, heavy-duty applications, i
is important to carefully evaluate five pump design
approach areas, specifically, the pump’s shaft and bear
ing design, valving and valve packages, controls and
control options, oil replenishment pump, and the ho
oil shuttle option for closed circuit applications.
FIG. 1: A typical split closed-
circuit hydrostatic transmission
configuration for high horse-
power, heavy-duty applications.
EVALUATING HYDROSTATIC
TRANSMISSION SYSTEM PUMPSfor High Horsepower, Heavy-Duty Closed Circuit Applications
BY DAVE EBERT,
Product Manager,
Parker Hannifin
Corporation,
Hydraulic Pump
Division
DieselEngine
Reservoir
Cooler
Pump
Motor (variableoutput speed & direction)
Filter
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SHAFT AND BEARING
DESIGN
Te majority of pump designs utilize
conventional bearings at each end of a
heavy, large-diameter shaft to support
the loading. An alternative approach
utilizes a bearing centered around the
barrel. Te position of the bearing was
determined relative to the summationof the internal rotating group forces at
one point in the barrel. With the bear-
ing centered at that point, the barrel
bearing takes the radial loading, elimi-
nating the need for conventional large
pump shafts and support bearings.
Because the radial loads generated
from the rotating group are supported
by the large barrel bearing, a smaller
diameter main shaft can be employed.
Tis design permits use of a smaller
main shaft with the pistons grouped
tighter to the center. With a smaller
diameter piston bore circle, the piston
and fluid velocity is reduced, allowingthe pump to run at higher speeds with-
out risk of losing the oil film.
SELECTION KEY: Overall, the
pump’s design can play a key role in
how well it can handle higher operating
speeds and shaft/bearing loading.
VALVING
Most closed-circuit pumps will
operate in conjunction with mul-
tiple valves, providing relief functions
along with pressure compensation.
How those valves are arranged and
positioned in or on the pump will
vary depending on the pump design.
One approach incorporates all valvinginto a single block (Fig. 2) that can
be quickly changed out and replaced
with a new one should there be a prob-
lem—such as system contamination
causing blocked orifices and/or stick-
ing poppets. Other designs, depending
on the location of the problem, could
require an extended system shut down
to troubleshoot and fix the problem.
SELECTION KEY: Carefully check
the pump’s valving arrangement in
light of the intended application and
what the time and cost considerations
would be to deal with a downtime
problem area.
Pump manufacturers generally have
control options to fit virtually any
application requirement. Tese include
• Adjustable displacement stops
• Manual screw adjustment
• Automatic brake and neutral
bypass control
• orque limit override
• Hydraulic stroker
• Electrohydraulic stroker
• Cylinder control
• Electrohydraulic cylinder control
• Manual rotary servo
Some controls utilize mechanical
linkages (which can wear out). Others
operate on a thin film of oil that pro-
vides a hydrodynamic balance between
the servo control and the cam. And
some control systems are designed to
permit a quick change of a pump’s
configuration by simply removing
the existing control and adding a new
one (Fig. 3). Tis chan-
geout feature avoids the
expense of having to pur-
chase a different pump to
fit the new application
requirement.
Another consideration
is control location. Some
pump designs permit
control mounting on
either side of the pump.
Others designs generally
limit control mounting
to a specific location.
In such instances, the
Recommended for closed-loop appli-
cations, hot oil shuttles can be mounted
on the pump or motor to remove hot oil
and allow the transfer of cool filtered oil
into the loop. Tis prevents the same oil
from continuously circling the loop. For
example, if the oil-replenishing pump
is capable of providing 12 gpm, and 4
gpm is lost to leakage, there is an 8-gpm
excess of replenish oil. Without the hot
oil shuttle, the excess 8 gpm of replenish-ing oil will spill across the replenishing
relief valve and generate heat.
CONCLUSION
Tere are many hydrostatic piston pump differences that must be care-
fully considered along with their features, performance capabilities, control
options and certifications. For high horsepower, heavy-duty applications,
pump and system failures are not an option. Long-term success starts with
an in-depth cost/benefit analysis.
For more information, visit www.parker.com.
only option is to rotate the pump 18
degrees, which also changes the po
configuration.
SELECTION KEY: Will the selecte
control option meet the long-term
needs of the intended application
Or is there a potential future need t
change the pump’s configuration t
fit a different application? Te abilit
to easily and quickly change contro
could save the cost of a new pump.
With the shuttle, anyt ime a pre-se
pressure differential is created from
one main line side of the system t
the other, the valve shifts so that th
excess replenishing oil mixes with th
low pressure side of the circuit an
goes back into the reservoir throug
the filters and heat exchangers that ar
part of the system.
SELECTION KEY: Built-on shutt
options are ideal to eliminate unnecessary hoses, leak point connections, an
simplify the overall hydraulic circuit.
FIG. 2: Parker’s GOLD CUP® modular valve
block provides complete closed circuit valve function
including servo and replenish (charge) pressure control. It is
removable and replaceable for ease of service.
"A" Side
Electro-hydrau
Control ("B" Sid
ReplenishPump
ServoPump
When pressure in the pump’s main
line drops below the required replen-
ishing pressure, an auxiliary pump is
used to provide for the makeup oil.
In some pump designs, this auxiliary
pump is externally mounted. Other
designs incorporate the auxiliary pump
inside the main pump body (Fig. 4)
along with the necessary check valves
for bi-directional operating. Tis design
approach reduces the overall pump
envelope and simplifies component and
circuit complexity.
Te built-in check valves are ide-
ally suited for proper replenishment
in applications such as shredding and
drilling. Te replenishment of fluid
into the system is especially critical
during high dynamic loading condi-
tions where main line pressures can
switch from one extreme to the other
in a matter of milliseconds.
SELECTION KEY: Some pump or
system designs utilize conventional
check valve configurations for oil
replenishment, which are slow to oper-
ate and have higher-pressure drops.
Others use a ring check approach that
permit high flows and react very fast
due to their low mass.
FIGURE 4: Exploded view of
a Parker GOLD CUP® pump
showing the replenishing oil
pump inside the pump body.
OIL REPLENISHMENT
CONTROLS
FIG. 3: Electrohydraulic
control mounted on the
“B” side of a Parker GOLD
CUP® pump. The same
control can be mounted t
the “A” side to quickly an
easily change the pump’s
configuration.
HOT OIL SHUTTLE OPTION
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