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



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



www.ifps.org | www.fluidpowerjournal.com

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

evaluatinghydrostaticpumps fpj 01-02.11

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