Applications Manual

Transmission Circuit Recommendations

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Applications Manual
Transmission Circuit Recommendations

Revision history

2 | © Danfoss | August 2023

Table of revisions
Date

ChangedRev

August 2023

Added note about S90 and H1 ports

0202

Changed document number from 'BLN-9886' to 'BC152886483346'

XX

May 2015

Rebranding to Danfoss

BA

July 1997

Major revisionA

BC152886483346en-0202

Applications Manual
Transmission Circuit Recommendations
Contents
Introduction
Applications Manuals......................................................................................................................................................................4Transmission Circuit Components
Introduction........................................................................................................................................................................................5
Charge Pump..................................................................................................................................................................................... 5
Displacement Controls................................................................................................................................................................... 6
Reservoir.............................................................................................................................................................................................. 7
Conduit Sizing................................................................................................................................................................................... 8
Case Drain Requirements.............................................................................................................................................................. 8
Heat Exchangers............................................................................................................................................................................... 9Filters.....................................................................................................................................................................................................9
Suction Filtration.......................................................................................................................................................................10
Charge Pressure Filtration..................................................................................................................................................... 10
Loop Flushing Shuttle.................................................................................................................................................................. 12
Pressure Controls........................................................................................................................................................................... 12
Components for Special Functions
Flow Dividers/Combiners............................................................................................................................................................16
Flow Limiters....................................................................................................................................................................................16
Counter Balance Valves............................................................................................................................................................... 16
Accumulators...................................................................................................................................................................................17
Cylinders............................................................................................................................................................................................17
Anti-Cavitation Valves.................................................................................................................................................................. 17
Bypass Valves...................................................................................................................................................................................18
Control Cut Off Valve.................................................................................................................................................................... 18
Proportional PCOR Control and Brake Pressure Defeat................................................................................................... 18
Integrated Speed Limitation......................................................................................................................................................18
Remote Variable Pressure Limiter
Introduction..................................................................................................................................................................................... 20
Circuit................................................................................................................................................................................................. 20
Component and Circuit Performance.................................................................................................................................... 22
Applications..................................................................................................................................................................................... 24
Wheel Assist
Description....................................................................................................................................................................................... 26
Ganged Pumps
Introduction..................................................................................................................................................................................... 28
Circuit Recommendations.......................................................................................................................................................... 28
Case Drain Routing Variations
Routing Variations......................................................................................................................................................................... 30
One Charge Supply/Two Pumps
One Charge Supply....................................................................................................................................................................... 32
Charge From Implement Return Flow
Charge From Implement Return Flow....................................................................................................................................33
Multiple Motors
Parallel Multiple Motors...............................................................................................................................................................34
Flow Dividers and Divider/Combiners
Introduction..................................................................................................................................................................................... 35
Heat Generation............................................................................................................................................................................. 35
Design Flow Range........................................................................................................................................................................35
Loop Flushing..................................................................................................................................................................................35
Motor Life Reduction.................................................................................................................................................................... 36
Differential Flow for Steering.....................................................................................................................................................36
Motor Speed Synchronization (Non-Propel)........................................................................................................................36
Recommendations for Trouble-Free Circuit.........................................................................................................................37© Danfoss | August 2023

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Applications Manual
Transmission Circuit Recommendations
Introduction

Applications Manuals
Content included in these manuals
These applications manuals provide design theory and detailed calculations for building hydraulicallypowered machines.
The original document was written as one manual with four sections.
The current set of manuals includes the four documents listed below. The section numbers from theoriginal document are listed in parenthesis after the current document title.
• Selection of Driveline Components BC157786484430 (originally Section 1)• Pressure and Speed Limits for Hydrostatic Units BC152886484313 (originally Section 2)• Transmission Circuit Recommendations BC152886483346 (originally Section 4)• Fluids and Lubricants BC152886484524 (originally Section 3)
Other Reference Manuals• Hydraulic Fan Drive Systems Technical Information BC152886484410• Hydraulic Fan Drive Systems Design Guidelines AB152886482265

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Applications Manual
Transmission Circuit Recommendations
Transmission Circuit Components

Introduction
Transmission performance and component life are functions of the hydraulic circuit design. Circuit design
affects the ability of the various elements to control system pressure, fluid temperature and other factors
relating to the quality of the fluid. The circuit elements will interact with each other to produce either
desirable or undesirable results, depending on their arrangement in the circuit.
Circuit elements must be present to remove undesirable heat and contamination and to control the
passage of cool, clean fluid into the main power loop. Other circuit elements are present to control
pressure and, sometimes, to control flow.
A typical closed circuit schematic is shown in the figure below. The closed circuit piston pump is normallyfactory equipped with a charge pump, charge pressure relief valve, and a control. It may also beequipped with high pressure protection devices such as relief valves (shown) or pressure limiters. A loop
flushing shuttle valve can be integral to the pump or motor, or may be installed externally. Some piston
pumps are provided with additional control elements as: control over-ride, neutral start switches, and/or
neutral return mechanisms. Other components such as the reservoir, heat exchanger, and filter are addedby the system integrator.
The purpose of this manual is to advise Danfoss users on generally accepted circuit arrangements, but it
does not, and cannot, describe all possible circuits which may be appropriate for various applications.
Transmission circuits are unique designs and must be developed and tested to ensure required
performance and life.
Typical Closed Circuit Transmission Schematic

SERVO

HYDROSTATICPUMPCHARGERELIEFCHARGEPUMP

CHECK
RELIEFHIGH
PRESSURE
RELIEFCHARGERELIEFHYDROSTATIC
MOTOR
LOOP
FLUSHING
SHUTTLECONTROLVALVEHEATEXCHANGERFILTERP108872

Charge Pump
A charge pump is absolutely necessary in a closed circuit hydrostatic circuit. This topic lists the various
functions of the charge pump, while Selection of Driveline Components BC157786484430 describes charge
pump sizing considerations.
The charge pump functions associated with its volume flow rate are:• replenish loop fluid lost through volumetric inefficiency of the pump and motor(s)• replenish loop fluid lost through the loop flushing valve.
• provide make-up fluid in the loop for load-induced bulk modulus effects.

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Applications Manual
Transmission Circuit Recommendations
Transmission Circuit Components•••

provide flow to stroke the servo control piston (on units with servo controls) on pumps and motors.
provide constant flow to the Electric Displacement Control (EDC) for proper operation (on units with
"two stage" EDC’s).
provide a flow source for auxiliary circuit functions such as releasing parking brakes or shifting motor
displacement.

The charge pump functions associated with its charge pressure are:maintain correct low loop pressure to ensure sufficient “hold down” forces on the rotating groups.
provide sufficient pressure to activate the servo control system (on units with servo controls).
control loop flushing flow rate by the differential pressure between the charge relief valve and theloop flushing relief valve.
• provide a pressure source for auxiliary circuit functions such as parking brake release.

•••

All of these charge flow and charge pressure requirements must be accommodated in each mode ofoperation or the system will experience poor performance or component damage. Danfoss highly
recommends that each application be scrutinized for sufficient charge flow and charge pressure toensure proper function in all modes of operation.
Most Danfoss hydrostatic pumps are offered with integral charge pumps sized for typical installation
requirements. Additional or alternate charge flow sources can be utilized. A gear pump mounted intandem with the hydrostatic pump is often used to provide additional charge flow to supplement the
integral charge pump.
Displacement Controls
The output flow of axial-piston hydrostatic pumps is controlled by varying the swashplate angle inresponse to an operator command. The two basic types of displacement controls are direct displacement
control and servo control.
Direct displacement controls simply use a mechanical lever attached to the swashplate upon which theoperator exerts a force, possibly through a linkage assembly. The force required to engage a direct
displacement control is a function of the system pressure that the swashplate is working against. There isno neutral deadband or swashplate feedback in a direct displacement control. Because the required
control force is a function of system pressure, direct displacement controls are generally only suitable tolight, to medium duty products.
Danfoss products that are included in this general category include:• DDC20, M35 and M44 Pumps• LDU20 and LDU32 U-Style Transmissions
The LPV (25cc, 30cc, and 35cc) product family includes a neutral assist mechanism that helps the pump
return to neutral for initial installation on the vehicle. Refer to LPV Technical Information Manual
BC152886483590 for additional technical specifications.
Servo controls are an interface between the operator and the swashplate. The operator input is a low
force device which ports charge pressure to the servo piston which moves the swashplate. Thus, theoperator input is not dependent on system pressure. Servo controls have a neutral deadband. Standard
servo controls also have a mechanical feedback mechanism which ensures that the swashplate position
stays where commanded. The three types of operator inputs for servo controls are manual, electric, andhydraulic.
Several Danfoss products are available with “non-feedback” servo controls which are appropriate forcertain applications. Some “Non-feedback” servo controls for pumps are generically referred to as“Automotive” controls. Command inputs to these controls may be either electric, or hydraulic. Refer to TI
Manuals Series 90 Non Feedback Proportional Electric Control (NFPE), Product Electrical Installation Tech Note
11023289, and/or H1 Automotive Control for Single Axial Piston Pumps Size 045-165 cm³ L12223856 foradditional technical specifications.
Another type of “Non-feedback” servo control for the H1 family of pumps is generically referred to as a“Fan Drive Control”; the command input to the Fan Drive Control [FDC] is via a 0 to 1800 ma (12VDC), or 0
to 920 ma (24VDC) electrical signal. With zero input, the fan is commanded to full speed in the forward
direction. With a 50% input signal, the fan is commanded to stop, and with a full input command, the
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Transmission Circuit Recommendations
Transmission Circuit Components
pump is commanded to full speed in the reverse direction. Since the FDC is basically a displacement
control, it would be possible to over-speed the fan at high engine speeds; consequently, it will benecessary to limit the maximum fan speed with the Pressure Limiter (PL) setting. This setting will bedependent on the fan’s power coefficient and the motor’s displacement. Refer to any of the displacement
specific H1-Single Pump's Technical Information Manuals and/or Hydraulic Fan Drive Systems - Design
Guidelines AB152886482265 for additional technical specifications.
Two-position medium power variable motors are available with an integral servo piston. The standard
control is a direct acting single line hydraulic control; an optional two-line control is also available. The
motor is spring biased to maximum displacement and hydraulically shifted to minimum displacement.
Minimum and maximum displacement can be set with fixed internal stops; displacements from 0% to100% of maximum displacement are available. Refer to L and K Frame Variable Motors Technical
Information Manual BC161386484850 for additional technical specifications.
Reservoir
In addition to providing the charge pump inlet with a constant source of fluid, the reservoir also helps
remove air from the system and cool the fluid. The reservoir also provides make-up fluid for volume
changes associated with fluid expansion or contraction, possible cylinder flow, and minor leakage.
The reservoir should be designed to accommodate fluid volume changes for all modes of operation andto promote de-aeration of the fluid as it passes through the tank. The total reservoir volume (in gallons)should be between 1/2 and 1-1/2 times maximum charge pump flow (in GPM) for closed circuit
applications. When designed solely for a closed circuit HST, the fluid volume should be approximately
80% of the total reservoir capacity. A large reservoir promotes de-aeration better than a small one.
However, excessively large reservoirs may contribute to difficulties with fluid temperature andcontamination control.
The reservoir may be pressurized or air breathing. Pressurizing with a low pressure relief valve helps tokeep out dust and dirt as well as to prevent air leaks in the charge pump suction line. However, theamount of entrained air in the fluid increases with pressurized reservoirs, and the pressure should be
limited to 0.7 bar [10 psig]. A "suction breaker" relief valve is required for pressurized reservoirs. In either
case, an air breather filter with adequate protection from environmental contamination (both hardparticle and moisture) must be used.
Water contamination is harmful to transmission components, and reservoirs must be designed to preventthe entrance of water into the reservoir. Since many machines must withstand high pressure washing,covers and other openings must be designed to always seal tightly, especially after maintenance
operations are performed.
The charge pump suction port must be near the bottom of the reservoir, but not so near that settled
contaminants are sucked into the charge pump. In applications where the surface of the fluid can getvery close to the suction port (due to vehicle orientation, or “sloshing”), a “vortex destroyer” should beinstalled to prevent the formation of a vortex that will introduce air directly into the flow entering thepump. This is merely a flat plate that is installed between the inlet port leading to the pump and thesurface of the fluid.
The return port(s) should be located as far from the pump inlet port as possible to provide maximum
dwell time for the fluid in the reservoir. The return fluid must always discharge below the fluid level,
preferable through a low velocity diffuser to promote de-aeration. Reservoirs should have one, or more,
baffles between the inlet and outlet ports to ensure adequate dwell time and to prevent surging andaeration of the fluid.
A filler port should be provided with a filter, which minimizes the potential for contamination entering
the system during servicing or operation.
Danfoss recommends filtering all fluids entering the reservoir, whether at initial fill, or during service andmaintenance operations.
A drain port in the reservoir is recommended to permit a complete change of hydraulic fluid without
disconnecting other hydraulic fittings. When designing a reservoir to serve multiple hydraulic systems,
stagger the elevation of the fluid outlet ports. If the fluid level becomes low during operation, the systemwith the highest outlet port will malfunction first and provide a warning to the operator.
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Applications Manual
Transmission Circuit Recommendations
Transmission Circuit Components
Reservoir Design

RETURNFLOW

SEALEDFILLER CAP

PRESUREVALVE SUCTION BREAKER
BREATHER/FI LTER

DIFFUSERWATER TIGHTCONSTRUCTION
T OP
VIEW

FLUID CAPACITY
5/8 CHARGE FLOW MIN.

SIDE
VIEW

FLUID CONTENT
1/2 CHARGE FLOW MIN.

BAFFLEDRAIN

T O CHARGE
PUMP

P108871

Conduit Sizing
Conduit sizing should be based on acceptable fluid velocity and pressure drop through the conduit.
Typical guidelines are:
Fluid Velocity (ft/sec)

Fluid Velocity (m/sec)

Main system lines:

20-30

6-9

Case drain lines:

10-15

3-5

Suction lines:

4-6

1-2

Consult hydraulic hose and tube suppliers for recommendations on calculating pressure drop and for
applicable pressure ratings.
Undersized main system lines will result in heat generation caused by high fluid velocity. Oversized main
system lines can contribute to intermittent loss of charge pressure by increasing the amount of fluid
being compressed when system pressure rises suddenly.
Lines should be sized taking into account extremes in temperature, fluid viscosity and required flow.
Charge pressure may have to be increased in order to accommodate cold fluid, long lines, or restrictions.
Undissolved air in hydraulic fluid can drastically reduce unit life. The main cause of aeration of the fluid is
air leaks, particularly on the inlet side of the charge pump. Flow restrictions such as inadequate line size,
mitered elbow fittings, sudden changes in cross-sectional area, or low ambient air pressure (i.e., at highaltitudes) can also contribute to aeration.
Case Drain Requirements
All Danfoss axial piston pumps and motors require case drain lines to relieve internal leakage. Also, flow
passing over charge relief valves and loop flushing valves must be removed through the case drain line.
The main criteria for case drain line routing are:Back pressure on the unit cases
Fluid velocity/line sizing considerations
Cooling requirements
Fill the unit cases/purge air from the case cavity

•
•
•
•

Case drain lines can be routed in series through multiple hydraulic unit cases. For circuits with loop
flushing, the loop flushing manifold would be the initiation point of case drain flow and the pump would
be the unit immediately before the heat exchanger See Typical Closed Circuit Transmission Schematic. This
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Transmission Circuit Recommendations
Transmission Circuit Components
allows flow through all unit cases for lubrication, cooling, and contamination removal. The pressure drop
through the heat exchanger is additive to the pressure drop through all of the case drain passages andthe unit case pressures. If this approach results in excessive unit case pressures, alternate routing must be
considered. The topic Case Drain Routing Variations on page 30 provides variations for case drain
circuits.
For hydraulic units with lip seals, case pressure limitations are due to seal structural limitations. Exceedingthese values, even for short time intervals, could result in premature wear out or structural failure of theseal.
For units with metal face seals, continuous case pressure capability is based on minimizing leaks from theshaft seal, and on preventing gasket blow-out during abnormal operations. This type of shaft seal is not a“leak-proof” device. There is an oil film across the seal at all times which results in normal seepage ofabout one pint per 1000 hours of operation at published case pressure limits. An increase in case pressure
will increase this seepage.
For systems with integral charge pumps, the charge pressure relief valve is referenced to case pressure,and the relevant charge pressure is actually the differential above case pressure. When case pressures areelevated, particularly during transient peaks, the differential of charge pressure above case must be
maintained. Failure to maintain charge pressure differential with respect to case pressure may
result in damage to the hydraulic unit.
Systems with remote charge pumps should be tested to ensure that charge pressure differential and low
loop pressure are always maintained. If a remote charge relief valve is not referenced to the hydraulic unit
case pressure, it is possible that the charge relief valve will not follow case pressure transients that are
due to flow surges. The charge relief setting should be verified to be sufficiently high to account for
expected peaks in case pressure.
Heat Exchangers
Heat exchangers should be sized to keep the fluid within recommended temperature limits. This isnormally done by selecting the worst continuous operating condition, and sizing the cooler for all thetransmission losses being cooled in the hottest ambient environment, at the continuous temperature
rating. For many machines, this occurs at the highest transmission output speed obtainable. An often
used "rule of thumb" is to select a heat exchanger with a capacity equal to 1/3 the horsepower of themachine. Testing to verify that temperature limits are maintained is highly recommended.
Heat exchanger sizing may be affected by other circuit elements. A loop flushing shuttle generally
decreases required capacity while flow restricting valves increase it. Frequent relief valve operation will
increase the required heat exchanger capacity.
A bypass around the heat exchanger is usually required to limit back pressure during cold starts.
Filters
To prevent premature wear of the hydraulic components, it is imperative that only clean fluid enter thehydrostatic circuit. A filter capable of maintaining the fluid cleanliness to ISO 22/18/13 or better (per ISO
4406-1999) is recommended and may be located either on the inlet (suction filtration) or discharge(charge pressure filtration) side of the charge pump. All Danfoss pumps can be used with suction
filtration, and several can be equipped with charge pressure filtration adapters.
The selection of a filter depends on a number of factors including the contamination ingression rate, thegeneration of contaminants in the system, the required fluid cleanliness, and the desired maintenanceinterval. Filters are selected to meet the above requirements using rating parameters of efficiency andcapacity.
Filter efficiency may be measured with a “Beta” (β) ratio as defined by ISO 4572 as the ratio of thenumber of particles greater than a given size upstream of the filter to the number of particles greaterthan the same size downstream of the filter. The Beta ratio applies to a specific particle size measured in
microns. For example, β10 = 20 means that for every 100 particles of 10 micron diameter entering thefilter, only 5 particles of 10 micron diameter are found downstream of the filter.
For closed circuit systems with suction filtration and open circuit systems with return line filtration, a filter
with β35-45 = 75 (β10 = 2) or better has been found to be satisfactory. For closed circuit systems which© Danfoss | August 2023

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Applications Manual
Transmission Circuit Recommendations
Transmission Circuit Components
share a common fluid reservoir with open circuit systems, a considerably higher filter efficiency isrecommended. This also applies to systems with gears or clutches using a common reservoir. For thesesystems a filter within the range of β15-20 = 75 (β10 =10) or better is typically required.
The filter capacity required depends on the amount of contaminants ingressed into the system andretained in the filter and the desired maintenance interval. As a rough guide, a capacity in grams equal totwice the charge flow in GPM or 1/2 charge flow in l/min has been found to be satisfactory for manyclosed circuit systems.
On direct displacement, non-servo, controlled pumps; a pressure drop indicator on the filter isrecommended to warn against damage caused by low charge pressure.
Since every hydraulic system is unique, the filtration requirements and performance must be determinedby test. It is essential that monitoring of prototypes and evaluation of components and performancethroughout the test program be the final criteria for judging the adequacy of the filtration system.
Suction Filtration
The suction filter is placed in the circuit between the reservoir and the inlet to the charge pump as shownin the figure Pressure Filter Circuit - Charge Relief Upstream of Filter. A non-bypass filter is recommended
with a filter contamination monitor. A vacuum gauge, located nearest the charge pump inlet, issuggested. Inlet vacuums up to 0.8 bar absolute (6 in vacuum) are acceptable during normal system
operation. An indicator set at 0.7 bar absolute (10 in vacuum) is recommended for a filter change.
Vacuums on the order of 0.3 bar absolute (20 in vacuum) are acceptable for short time intervals duringcold start or intermittent conditions. Exceeding these values may result in charge pump cavitation,
aerated fluid, and life reduction of the hydraulic components.
Charge Pressure Filtration
There are two basic types of charge pressure filtration; full flow and partial flow. In either case, thepressure filter is located downstream of the charge pump. Charge pressure filtration is often used insystems where the inlet vacuum recommendations would be exceeded if using a suction filter. Cold
weather applications and systems with a long inlet lines are prime candidates for charge pressure
filtration. Charge pressure filtration generally allows for greater filtering efficiency than suction filtration.
Charge pressure filters must be rated for at least the maximum expected charge pressure per theapplication. Generally, filter elements rated at 35 bar (500 psi) minimum collapse pressure are acceptable.
A 100μm to 125μm strainer may be located in the reservoir or in the charge pump inlet line to protect the
charge pump from coarse contaminants. Recommended practice for charge pressure filtration would be
to use a non-bypass filter element in conjunction with a differential pressure sensor that is incorporated
into the vehicles’ control system to prevent operation of the system at flows that could cause damage tothe filter element
Partial filter flow is achieved by incorporating the charge pressure relief valve ahead of the filter
element as shown in the figure Pressure Filter Circuit - Charge Relief Upstream of Filter. Only the volume of
fluid needed by the hydrostatic loop and servo control are filtered. A non-bypass filter is recommended.
Insufficient flow through the filter will result in inadequate charge pressure and will be reflected in poor
machine performance. The filter must be capable of a pressure drop equal to charge pressure without
allowing the filter Beta (β) Ratio to become less than one. A loop flushing valve is recommended toincrease the volume of “dirty” fluid being removed from the pressure loop and to increase the volume of
make-up fluid passing through the filter.
Full filter flow is achieved by incorporating the charge pressure relief valve behind the filter element asshown in the figure Pressure Filter Circuit - Charge Relief Downstream of Filter. All of the charge pump flow
is passed through the filter increasing the rate of contamination removal from the system.
With full flow filtration, a bypass valve is necessary (See warning below) to prevent filter damage and to
avoid contaminants from being forced through the filter media by high pressure differentials across thefilter element. In the event of a high pressure drop associated with a blocked filter or cold start-up
condition, fluid will bypass the filter.

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Transmission Circuit Recommendations
Transmission Circuit Components

W Warning
Operating the system with an open bypass should be avoided, and a visual or electrical contamination
indicator is recommended for both suction and charge pressure filtration. Proper filter maintenance ismandatory to prevent particle contaminants from entering the system.
Refer to Design Guidelines for Hydraulic Fluid Cleanliness BC152886482150 for additional information.
Pressure Filter Circuit - Charge Relief Upstream of Filter

CHARGERELIEFCHARGERELIEFFILTERCHARGE
PUMP
LOOP
FLUSHING
SHUTTLESCREENTO CONTROL

P108873

Pressure Filter Circuit - Charge Relief Downstream of Filter

CHARGERELIEFFILTER
BY-PASSFILTERSCREENTO CONTROL© Danfoss | August 2023

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BC152886483346en-0202 | 11

Applications Manual
Transmission Circuit Recommendations
Transmission Circuit Components
Loop Flushing Shuttle
There are two primary fluid loops in a closed circuit transmission, each having a distinct design function
and associated circuit elements. These are the main system power loop and the charge cooling loop.
Make-up fluid is transferred between these loops by leakage, and if so equipped, by the loop flushing
shuttle. Fluid quality is controlled primarily by the charge/cooling loop, but the demand for high quality
fluid is in the power loop. The loop flushing shuttle is the circuit element which is available to control thistransfer of quality fluid.
The loop flushing shuttle removes a selected volume of fluid from the power loop which must be madeup by the charge pump. The fluid removed is presumably “dirty” and/or “hot” while the make-up fluid iscool and clean. This greatly improves the heat and contamination transfer out of the power loop,
improving fluid quality and extending component life. While some hydrostatic systems can operatesatisfactorily without a loop flushing shuttle, its addition can reduce circuit problems where the powerloop generates heat or contaminants.
Danfoss offers loop flushing shuttles in several configurations. Most closed circuit motors, some LSHT
(orbital) motors, as well as the Series 42 pump family, are available with integral loop flushing. A remotemounted loop flushing shuttle is also available.
Integral loop flushing valves reduce the number of external components to be purchased and plumbed
into the system. Motor-mounted shuttle valves also increase the volume of fluid flushing thru, andcooling, the motor case.
The loop flushing shuttle is recommended for any of the following conditions:Sustained operation at low pressure and high speed. This condition can create high mechanicallosses, low power loop leakage, and build-up of heat in the power loop. In general, low pressure is
below 1000 PSI and high speed is above 2/3 full angle rated (continuous) speed.
• Cylinders in the hydrostatic circuit. Cylinders allow ingression of contaminants at a significantly
higher rate than other rotating circuit elements.
• Flow restricting valves in the power loop. These valves create heat in the loop. Note, that a loop
flushing shuttle must be placed upstream of any divider-combiner valve. (See Loop Flushing on
page 35)• Frequent operation of high pressure relief valves. This creates heat and adds it directly to the powerloop. This restriction also applies to the use of modulating relief valves in the main power loop.
• Long or reduced diameter power loop lines. Pressure drop will be higher than normal, increasing heatgeneration in the loop. In general, "long lines" exceed 150 diameters in length. Hydraulic lines withmultiple fittings, or adaptors, can also contribute to increased heat generation in the loop.
• Partial flow charge pressure filtration. A loop flushing shuttle is required to remove more fluid fromthe power loop and, therefore, to replenish higher volumes of cool, clean make-up fluid.

•

A higher degree of system reliability is obtained with a loop flushing shuttle. A shuttle valve can improve
an unsatisfactory condition of built-in contamination or marginal cooling. A motor-mounted shuttle canalso reduce the length of the case drain line from the transmission to the reservoir. A shuttle valve canpotentially reduce cooler size. The final determination is made by evaluating trade-offs between the
shuttle, cooler, filter, configuration of lines and expected life.
Pressure Controls
Closed circuit hydraulic systems require a means to limit or control the maximum loop pressure to somedesign value. Since hydraulic unit life decreases substantially at high pressures, unit life may beunacceptably short if system pressure is not controlled. The type of pressure protection selected willaffect the response time and power dissipation of the system.
Relief valves are fast acting and typically do not have large pressure overshoots, provided the flow
capacity of the valve is not exceeded. Full flow relief valves are uncommon in most hydrostatic pumps.
However, high pressure relief valves dissipate power, sometimes in considerable amounts, and this is asource of heat generation in the system. Hydrostatic units should be sized and pressure relief valves set
so that the maximum pressure is reached less than 2% of total operating time.
The pressure limiter, shown in the following illustrations, a pressure limiter circuit is a nonpower
dissipative high pressure control, and therefore limits the heat generated in the system. The pressure
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Transmission Circuit Recommendations
Transmission Circuit Components
limiting function can be used as an integral part of the machine control system. However, if the pressure
limiter setting is high, the amount of time at the pressure limit must be balanced against required life.
Unlike a relief valve, the pressure limiter changes the displacement of the pump in response to highsystem pressure. When system pressure reaches the pressure limiter setting, the pressure limiter ports
control fluid to the servo system to destroke the swashplate to the position required to maintain thedesired maximum system pressure. Pressure limiters can be used to control winch payout loads because
the swashplate will travel into the opposite direction if required to maintain the desired system pressure.
In a typical propel application, the pressure limiters act to reduce pump displacement during accelerationor forward driving. During deceleration or dynamic braking, the pressure limiter acts to increase pumpdisplacement up to, and including, the maximum displacement.
H1 Pressure Limiter Circuit
M14

M6 1

2

R2

R1
C2

C1

M3 L2

MA
A

F00B F00A

M4
CW
B

M5
S

MB

L4

P003418

Series 90 Pressure Limiter Circuit

M3

A

A

B

B

L2

M3

M1

M1
M4
M5

M

M2
S

L2

L1

M2

P104286

The response time and overshoot of the pressure limiter are a function of the pump displacement andthe rate of pressure overload. In normal pumping mode, if the pump is at maximum displacement when
the pressure limiter activates, it will take longer for the pump to destroke than if it was at half stroke.
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Transmission Circuit Recommendations
Transmission Circuit Components
Similarly, if the system experiences a high pressure spike, there will be greater overshoot than if the load
developed slowly. To dissipate these overshoots in the short time interval before the pressure limiter
becomes fully activated, the pressure limiter is equipped with an internal relief valve to trim pressure
overshoot, so an external relief valve is usually not necessary. However, systems with control valves in the
power loop are an exception to this since they have fast flow change characteristics. High flow, highpressure relief valves are required with directional or switching valves in the power loop. Testing is
recommended to determine the amount and duration of pressure overshoot when using control valvesin the main power loop.
The pressure override, shown in the following illustration Pressure Override Circuit, is another means oflimiting system pressure in a hydraulic circuit. Like the pressure limiter, the pressure override acts todestroke the swashplate in response to high system pressures. When the set pressure is reached, theservo supply (charge) pressure to the displacement control is blocked, and both servo pistons areconnected to case pressure via the displacement control valve. The pressure override does not destroke
the swashplate as quickly as the pressure limiter, but again, the response time and pressure overshootare a function of pump displacement and the rate of pressure overload.
High pressure relief valves are required to trim pressure spikes in the short time before the pressure
override becomes fully activated. The recommended relief valve setting is 70 bar [1000 psi] higher thanthe pressure override setting to provide for adequate response and minimize interference between thetwo pressure controls.
Braking is also a special situation regarding pressure limiting devices. During downhill braking, thepressure limiter or pressure override will act to increase vehicle speed if the pressure setting is reached.
This may decrease the amount of braking power available from the transmission at high vehicle speed ascompared with a relief valve pressure control system. Because the pressure limiter and pressure override
valves are nondissipative by design, the engine absorptive power limits the amount of high speed
braking effort. With relief valves, some of the power required for high speed, steep slope braking can be
dissipated over the reliefs by destroking the pump.
Continued braking over relief valves will heat the hydraulic fluid and may damage the transmission, so
only emergency situations should be considered for the use of relief valves. Vehicles must also beequipped with mechanical brakes which are suitable for emergency stops, emergency relief valve braking
is usually redundant.

W Warning
A braking system which is independent of the hydrostatic transmission must be provided which isadequate to stop and hold the vehicle should the condition develop.
Machines with dual path transmissions which depend on differential output speed to steer mustverify that the pressure control device does not prohibit adequate steering control. This applies
for both driving and braking.

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Transmission Circuit Recommendations
Transmission Circuit Components
Pressure Override Circuit

DISPLACEMENTCONTROL

PRESSURE
OVERRIDE

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Applications Manual
Transmission Circuit Recommendations
Components for Special Functions
Some applications may require additional circuit components in order to perform specific functions. A
few of the frequently used components are discussed in this article which includes suggestions andprecautions on their use.
Flow Dividers/Combiners
Flow dividers are used in circuits where two motors in parallel are supplied from a single pump. Typically,
the flow divider is expected to provide equal flow rates to the motors to ensure positive traction or toprovide synchronized motor speeds. On propel circuits, the flow divider also provides overspeed
protection if one motor loses traction.
Vehicle steering usually requires that the leftside and rightside motors operate at different speeds. The
flow divider must have the capability to allow this differential flow split. Motor speed ratings must not be
exceeded during steering maneuvers.
The pressure drop through the flow divider generates heat in the system and should be considered whensizing heat exchangers. Motor-mounted loop flushing valves should not be used. Instead, an externalloop flushing valve should be located between the pump and the flow divider to ensure proper system
performance and correct flushing flow. The combining mode should be reserved for the least-used travel
direction.
In the divide mode, the volumetric tolerance stack-up of the motors and the flow divider may cause one
motor inlet to cavitate. Volumetric tolerance stack-up includes the slight difference in motor
displacement, the difference in motor volumetric efficiency, and the flow division capability of the flow
divider. Anti-cavitation check valves may be required to protect the motor inlets.
Thorough system testing is essential to verify stable charge pressure and motor inlet pressures
under all operating modes.
This includes all combinations of:forward and reverse travelvehicle steeringvarious pump displacements
various pump speeds
various motor displacements

•
•
•
•
•

See also Flow Dividers and Divider/Combiners on page 35 for additional information on flow dividers andcombiners
Flow Limiters
As their name implies, flow limiters are used to limit the flow rate to a set value. In hydrostatic circuits,
they are used to prevent motor overspeed, especially in multiple motor applications in the event of a lossof traction. Because the flow limiter does not regulate flow below the maximum setting, they do notmake good positive traction valves.
To prevent the possibility of inlet cavitation with an overrunning load, the flow limiter must be mounted
on the outlet side of the motor. Flow limiters should be sized to be active only during an overspeed eventto minimize the heat generation in the system. When the flow limiter is active, the flow restriction willcause higher-than-normal back pressure on the motor. Although this is not usually detrimental to themotor, it can affect loop flushing flow and motor life.
When flow limiters are applied, system testing must be conducted to verify correct performance.
Counter Balance Valves
Counterbalance valves are normally used in open circuit applications to hold a load while the pump is
idling and to control over-running loads. However, some hydrostatic winch , propel, and linear actuatorapplications use counterbalance valves to maintain a payout tension control of the load.

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Transmission Circuit Recommendations
Components for Special Functions
While active, the counterbalance valve generates considerable amounts of heat due to the flow rate andpressure drop across the valve. Because the valve generates high back pressure on the motor, a motormounted loop flushing valve cannot be used. An external loop flushing valve should be considered.
Accumulators
An accumulator is rarely necessary in a closed loop circuit. However, the two functions that anaccumulator could serve in a hydrostatic system are to absorb shock in the high pressure loop or toprovide instantaneous low loop make-up flow. This instantaneous make-up flow requirement isassociated with fluid compressibility problems, sometimes called the bulk modulus effect. Systems withlong lines or applications which experience load induced pressure spikes are vulnerable to fluid
compressibility and normally require supplemental low loop make-up fluid for short periods of time.
However, the accumulator volume adds to the total volume of the system which is subject to the fluid
compressibility during a load induced pressure spike.
Cylinders
Generally, open loop circuits are chosen to control hydraulic cylinders. However, a closed loop circuit canbe utilized if the correct precautions are observed.
One type of cylinder found on many mobile vehicles is the hydraulic release parking brake. A 2-position
3-way valve is typically used to either release the brake with charge pressure or to apply the brake byventing the cylinder to tank. This brake control sub-circuit is actually an open circuit which simply utilizes
charge pressure as a convenient pressure source for brake release. Although there is no sustained charge
flow requirement, there is an instantaneous flow requirement. In the vast majority of installations, thischarge flow requirement is insignificant. However, it could become significant if the brake release line isvery long, if there are multiple brakes being released, or if there are other charge flow demands occurringat the same time.
A closed circuit installation of a double ended cylinder is similar to a hydrostatic motor installation in thatthe return flow from the cylinder is the same as the input flow. Generally, no special considerations are
required for charge pump sizing, but the influence of both bulk modulus and larger control volumes
should be taken into consideration. Obviously, pump control and high pressure protection is critical to
avoid system damage or heat generation when the cylinder has reached the end of its stroke.
A closed circuit installation of a single ended cylinder requires special considerations due to the unequalvolumes of the rod and head ends of the cylinder. When the cylinder is activated from the head end, theinput flow is greater than the exhaust flow. If the charge pump is not able to make-up this volume
difference, the low loop side must be supplemented through a check valve from an external source.
Conversely, when the rod end is pressurized, the return flow is greater than the input flow.
Generally, the internal charge relief valve is unable to pass the increased low loop flow, and an external
shuttle and relief valve are necessary to dump the flow to tank. The conventional charge circuit that
includes shuttle check valves to direct charge flow to the low pressure side of the loop will block any
excess flow from reaching the charge relief valve. The return flow from the head end of the single ended
cylinder must be directed to an external relief valve through a pilot-to-open check valve that isconnected to the “head end” flow passage. The pilot pressure to open the P.O. Check Valve is sourced
from the opposite side of the loop.
As mentioned in a previous section, closed circuit applications involving linear actuators often require
counterbalance valves to maintain control of the load. With single ended cylinders, give special attention
to the selection of the external low pressure relief valve and the loop flush relief valve to prevent valve
interactions.
Anti-Cavitation Valves
When axial piston motors are applied into open circuit applications, pay particular attention to the backpressure requirements for the return side of the motor; refer to the Technical Information Bulletin for the
specific product family being applied. In addition, anti-cavitation valves should be used to ensure thatthe minimum pressure requirement is always satisfied on the supply side of the motor© Danfoss | August 2023

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Applications Manual
Transmission Circuit Recommendations
Components for Special Functions
Bypass Valves
Low and Medium Power axial piston pumps are equipped with a loop bypass valve that connects the A
and B sides of the working loop, or connect both sides of the working loop directly to the charge gallery.
The bypass function allows a machine to be moved without rotating the pump shaft. For example: you
may move a disabled vehicle to a service location or winch it onto a trailer without operating the prime
mover. Consult the product specific Technical Information Bulletins for bypass valve operating
instructions.

C Caution
Damage to the hydraulic system may result from operating without charge flow. Bypass valves are formoving a machine or vehicle for very short distances at very slow speeds. They are NOT tow valves.

C Caution
Do not move the machine faster than 20% of maximum speed or for more than 3-minutes. Towing fasteror longer than described may result in damage to the drive motor(s). To return the vehicle to normal
operation - carefully close the bypass valve.
Control Cut Off ValveThe H1 tandem pump offers an optional control cut off valve integrated into the pump center section.
This valve will block charge pressure from the displacement control valve in both pumps, allowing theservo springs to de-stroke both pumps regardless of the pump’s primary control input. There is also a
hydraulic logic port which can be used to control other machine functions, such as spring applied,pressure release, brakes.
• When the control cut off valve is energized; charge flow and pressure is allowed to reach the pumpcontrols and the hydraulic logic port.
• When the control cut off valve is de-energized, the control passages and the logic port are connected
and drained to the pump case.
• The charge supply side of the control cut off valve is internally screened to protect the valve fromcontamination.
Refer to H1 045/053 Tandem Axial Piston Pumps Technical Information BC152886483958 for additional
technical specifications.
Proportional PCOR Control and Brake Pressure Defeat
Danfoss bent axis variable displacement motors are available with a pressure compensator control. The
motor is held at minimum displacement by the servo piston until a set value of supply pressure isreached; then the motor displacement increases proportionally with pressure until it reaches maximum
displacement. A Variable Pressure Compensator Over-Ride Valve includes an electric proportionalsolenoid which overrides the pressure compensator and strokes the motor to maximum displacement. In
addition, the proportional solenoid changes the pressure compensator setting to allow different, on thego, settings.
For propel applications, use the electric Brake Pressure Defeat (BPD) option in conjunction with the PCOR
option. The BPD consists of an electric off/on solenoid driving a two-position, three-way porting spool
valve. The applied logic allows the pressure compensator control to operate normally during accelerationand cuts off the supply pressure to the PCOR during deceleration or overrun.
Refer to H1 Bent Axis Variable Motor Electric Proportional PCOR Control P1, P2 Product Electrical Installation
BC152886483381 for technical specifications.
Integrated Speed Limitation
In many applications, vehicle weights are getting bigger, while engines are being down-sized. More andmore situations arise by over-speeding of the diesel engine in self-propelled machines which areequipped with hydrostatic propel drives systems. The engine over-speeds when the machine works in
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Transmission Circuit Recommendations
Components for Special Functions
downhill, or in braking, mode; due to the limited braking torque capacity of the diesel engine. To avoid
this, Danfoss has developed a system which prevents over-speeding by integrating valves into the pump.
This feature is called Integrated Speed Limitation (ISL) and is offered for H1 pumps.
Advantages for the ISL system are:• Sufficient deceleration of the vehicle when braking

•
•
•
•
•

© Danfoss | August 2023

Protect the diesel engine and the hydraulic pump against over-speed
Ensure optimal use of the diesel engine’s brake capability
Acts independently without operator input
Saves mechanical brakes
No additional hydrostatic components are required for this function

BC152886483346en-0202 | 19

Applications Manual
Transmission Circuit Recommendations
Remote Variable Pressure Limiter

Introduction
A single sided Remotely Variable Pressure Limiter (RVPL) circuit can be created by properly connecting ahigh pressure pilot-operated relief valve to a standard Series 90, or H1 pump. Adding a RVPL circuit allows
for a pressure limiter setting that is adjustable and lower than the internal pressure limiter. This added
control feature is useful for applications requiring adjustable constant pressure control, such as drill rig
pull-downs and constant tension winches. More detailed information about applications is presented
after the explanation of the circuit.
Servo Gage Ports
Servo gauge port M4(port M5 on other side)
Servo gauge port M4

Servo gauge port M5

Series 90 top view

H1 PumpP018877

Circuit
The following schematics illustrate the external circuitry required for a single sided RVPL. A high pressureline must be routed from the appropriate system pressure gage port on the pump (or other systempressure source) to the inlet of a remotely mounted pilot pressure relief valve (the RVPL valve). The
output of this valve is then returned to the appropriate servo pressure gage port on the pump. The figure
above Servo Gauge Ports shows the series 90 and H1 pump gage port locations. Note that the gage port
logic is different for Series 90 and H1, and is dependent on the control type. Please see schematic ortechnical information to verify gage port logic and function.
Consult product specific Technical Information Manuals for the servo pressure gage port locations onother Danfoss servo controlled pumps.
The external RVPL valve becomes a second pressure limiter valve which is connected in parallel with theinternal pressure limiter valve. Of these two valves, the one which is set at a lower pressure will limit
system pressure. The internal pressure limiter should be set at a maximum desired system pressure, and it
will limit maximum system pressure, even if the external RVPL valve is adjusted to a higher setting. Lower
system pressures are maintained by adjusting the external RVPL valve to settings lower than the internalpressure limiter.

C Caution
The servo mechanisms of Danfoss variable displacement axial piston pumps are not designed toaccommodate full system pressure. Consequently, a relief valve mechanism must be provided to ensurethat the servo mechanism is not over-pressurized. The upper pressure limit is product specific. Consult
the product specific Technical Information Manuals, or your Danfoss technical information representative
for these pressure limits.

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Transmission Circuit Recommendations
Remote Variable Pressure Limiter
Remotely Variable Pressure Limiter Circuit For Series 90 Pumps (Counterclockwise Rotation)
B N A
M3

M4

M5

OPTIONAL
ORIFICEA

M1
REMOTE LY VARIABLE
PRESSURE LIMITER
M2
B

L1

L2

S

P108878

Remotely Variable Pressure Limiter Circuit For Series 90 Pumps (Clockwise Rotation)
B NA
M3

M4

M5

OPTIONAL
ORIFICEA

M1
REMOTELY VARIABLE
PRESSURE LIMITER
M2
B

L1

L2

S

P108879

The external RVPL valve will always be connected to the system port in which it is desired to have the
RVPL function. However, the appropriate servo connection is dependent upon pump rotation. For
example the table Pump Logic Chart below shows the respective system ports and servo gage ports forboth CW and CCW rotations of the Series 90 pump.

W Warning
If the servo gage port plumbing is reversed, the remote pressure limiter will not function.
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Applications Manual
Transmission Circuit Recommendations
Remote Variable Pressure Limiter
If a two-level pressure limiter is desired without readjustment of the external RVPL valve, the externalvalve can be switched in and out of the circuit with an on/off valve (manually, hydraulically or solenoid
operated) placed in series with the RVPL. This valve will switch pressure limiter operation between theinternal and the external valve settings. This logic can even be extended to more than two pressure
limiter levels, using additional external RVPL valves and on/off valves.
Do not attempt to control the system delta pressure near zero bar [psi]. The system gage pressure must
always be above charge pressure and preferably no less than 50 bar [750 psi].
If independent control of both system pressures is required, the RVPL circuit shown can be separately
applied to both sides of the pump. It is also possible to control both system pressures to the same levelwith a single external RVPL valve. However, the external circuitry becomes more complex, and Danfoss
Application Engineering should be contacted for assistance.
Series 90 Pump Logic Chart
Rotation
RH (Clockwise)
LH (Counterclockwise)

Port

System Gauge Port

Servo Gauge Port

A

M1

M4

B

M2

M5

A

M1

M5

B

M2

M4

Component and Circuit Performance
Ideally, the external RVPL valve should have flow characteristics close to that of the internal pressure
limiter valve. The critical characteristic is the pressure rise rate versus flow, which is the slope of apressure versus flow curve for the RVPL valve. The desired pressure rise rate is 4.5 bar/lmin [250 psi/gpm].
A pressure rise rate is illustrated in Desired Flow Characteristics of External RVPL Circuit below. If thepressure rise rate of the RVPL valve is higher (more than 4.5 bar/lmin [250 psi/gpm]), response of the
RVPL will be slower than that of the internal pressure limiter. If the pressure rise rate of the RVPL valve issignificantly lower (less than 4.5 bar/lmin [250 psi/gpm]), the RVPL response will be faster.

W Warning
The faster RVPL (high flow capability) can create instability or overpressurize the servo.
Applications attempting to utilize faster RVPL responses should be thoroughly tested to insure adequate
performance and life.
If it is desired to utilize an RVPL valve with an inadequate pressure rise rate (flow capacity too high), thedesired pressure rise rate can be approximated by placing an orifice in series with the external RVPL
valve. This configuration is shown as being optional on the circuit diagrams (shown above). If the RVPL
valve has very little pressure rise, an orifice of 1.8mm [0.070 inch] diameter will provide an acceptablepressure rise rate in the RVPL circuit. If the pressure rise rate of the RVPL valve is an intermediate value(low but not negligible), a larger orifice may be used. The desired final result is a combination of theexternal RVPL valve and the orifice which has a pressure rise rate of 4.5 bar/lmin [250 psi/gpm] at a flowrate of 3.8 lpm [1 gpm]. The valve will typically operate correctly while maintaining constant systempressure.
To minimize poor RVPL response with cold oil, 10mm [3/8 inch] diameter (-6) lines are recommended toand from the external RVPL valve, particularly for long runs (3m [10 ft.] or more). The pump gage ports are“-6” Metric, or SAE, straight thread O-ring boss ports.
With the desired 4.5 bar/lmin [250 psi/gpm] pressure rise rate of the external RVPL valve, the swashplate
destroking performance of the RVPL will be similar to that of the internal pressure limiter. However, it
should be noted that the RVPL will experience more pressure overshoot (pressure rise above its steadystate setting) during load transients requiring rapid destroking. This will occur because the internal
system pressure relief valve will not operate in tandem with the external RVPL, as it does with the internalpressure limiter.
The system relief valve limits the pressure overshoot of the internal pressure limiter when swashplate
response is not sufficient to limit system pressure to 14-28 bar [200-400] psi above the limiter setting.
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Transmission Circuit Recommendations
Remote Variable Pressure Limiter
When controlling with the external RVPL, the swashplate destroking response rate will be a function of
the system pressure overshoot above the steady state RVPL setting. If the pressure rise rate of the RVPL
circuit is a constant 4.5 bar/lmin [250 psi/gpm], the following table gives estimated 17° to 0° destroking
times for various levels of overshoot (assumes constant level of overshoot during destroking). Note thatthe use of a high flow RVPL valve with a series orifice will result in slower response, particularly at thehigher overshoot levels. This is because the pressure rise rate for an orifice is nonlinear, requiring morepressure overshoot to obtain a given flow.
Destroke Time (17° to 0°) for Pressure Overshoot
Series 90 Pump

250 psi

500 psi

750 psi1000 psi

42cc

0.21 sec

0.11 sec

0.07 sec

0.05 sec

55cc

0.35 sec

0.18 sec

0.12 sec

0.09 sec

75cc

0.45 sec

0.23 sec

0.15 sec

0.11 sec

100cc

0.65 sec

0.32 sec

0.22 sec

0.16 sec

The proposed RVPL circuit has the capability to push the pump swashplate over center, even at low
system pressures, to maintain system pressure near the RVPL setting. This is required in constant tension
winch applications, where the cable must pay out as well as pay in to maintain constant pressure. In thisapplication, the pump displacement control will be biased for full pay-in speed, and the RVPL will push
the pump swashplate over center to allow pay out.

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Transmission Circuit Recommendations
Remote Variable Pressure Limiter
Desired Flow Characteristics of External RVPL Circuit

1000

System Pressure Rise Above Steady State Limiter Setting (PSI)

800

Pressure Rise Rate ofHigh Flow RVPL Valve with
0.070 in. Diameter Orificein Series

600

250 PSI / GPM Desired
Pressure Rise Rateof RVPL Valve

400

200

0
1

-200

2

3

4

Steady State
Operating Point-400

RVPL Valve Flow (GPM)

P108880

Applications
As mentioned, the RVPL circuit is useful for applications requiring adjustable constant pressure control,
such as drill rig pulldowns and constant tension winches. The circuit is normally used with a displacement
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Transmission Circuit Recommendations
Remote Variable Pressure Limiter
control (manual, hydraulic or electrical). The displacement control provides speed control when the RVPL
is set at a pressure higher than is required to move the load. This allows precise load positioning when
the system is not in the constant pressure mode. When operating at constant pressure, the RVPL isadjusted to maintain a desired pressure level, and the displacement control is set to limit the maximum
load speed. The RVPL is designed to behave in a manner similar to the internal pressure limiter to reduce
pump displacement when the pressure reaches the RVPL setting.
In drilling applications, motor output torque is regulated by limiting system pressure with the RVPL. The
RVPL can be used to control drilling torque, with the internal pressure limiter set at a higher pressure.
On a constant tension winch, the displacement control is typically set to allow full pump displacement to
run full speed cable pay in, while constant cable tension is maintained by means of the RVPL. The abilityof the RVPL to drive the pump swashplate over center for a winch pay out condition makes it especiallysuitable for a mooring winch application. On a mooring winch, the pump’s displacement control is set to
pay in the cable at a certain speed. If the cable tension increases to the point of the RVPL setting, thepump’s displacement will decrease to maintain the pressure setting. If the tension is great enough, the
RVPL will drive the swashplate over center and the winch will pay out cable. Consideration must be given
if the load can drive the swashplate to its maximum displacement when paying out cable. Any flow
generated by the load that the pump cannot take in will either drive the pump faster or drive thepressure higher. A relief valve can be used to limit the maximum pressure condition, but excessive heat
will be produced. In most cases the prime mover, acting in a dynamic braking mode, will be thelimiting factor. A separate method of braking the load must be provided to prevent engine orpump overspeed.
High inertia vehicles, such as railroad maintenance machines, need control of deceleration forces. The
pump has a capability to be driven to a neutral swashplate condition before the vehicle reaches a zero
speed. Without some method to prevent this from occurring, the internal pressure limiter will drive thepump towards maximum displacement during dynamic braking. The pump, now acting as a motor, will
feed a large amount of torque into the engine due to the pump’s high pressure and displacement. Thismay be more torque than the engine can handle in a braking mode. A circuit utilizing the RVPL, whilesensing travel direction, can be used to limit system pressure while decelerating. By setting the RVPL at apressure level less than the internal pressure limiter, torque can be absorbed by the prime mover at a safe
level. The system is designed such that the internal pressure limiter controls the acceleration torquewhile the RVPL controls the deceleration torque.
Certain applications require that maximum torque levels be controlled, whereby the load is dependent
on a second hydraulic drive. For example, a drilling operation may require the maximum torque on the
drill head to be controlled. The torque, however, will depend on the rate of pull-down of the drill head.
An RVPL circuit could be used to limit torque in the following manner: both the drilling pump and thepulldown pump would have their pressure limiter settings set above the RVPL. The RVPL would sense
system pressure from the drilling circuit, but instead of feeding the RVPL signal back into the drilling
pump, the signal would feed into the pull-down pump to bring it out of stroke. Drill torque is thencontrolled by the pulldown rate of the drill head.
As another example, a harvesting machine may include two hydraulic systems: one system to propel themachine and one system to feed crop into the machine. In this application, the RVPL would sense thepressure of the system gathering crop into the machine, but its signal would be tied to the propel system.
When the gathering system pressure increases due to heavier crop, the RVPL will operate to slow down
the vehicle speed.
When two or more pumps are used in parallel, an external pressure limiter is also recommended. For
more detailed information for this kind of application, see Ganged Pumps on page 28.

© Danfoss | August 2023

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Applications Manual
Transmission Circuit Recommendations
Wheel Assist

Description
Additional traction is often needed on machines working in poor traction conditions. The need may only
be to prevent stalling of the machine, or to increase the drawbar pull or to assist in improved steering.
Axles, or wheels, not usually used for the propel function can be fitted with hydraulic motors to give
auxiliary tractive force. Each vehicle has its own particular needs and each must be analyzed according tothose needs.
Some of the considerations that will influence the size or type of wheel assist are:weight distribution due to loading or grade
coefficients of traction
auxiliary drive wheel speed
wheel diameter
final drive ratios

•
•
•
•
•

As a general rule, the wheels which are offering the additional traction must not lose traction (spin out)
before the main drive axle wheels spin out. This is especially true when fluid from the main propel circuit
is used for the wheel assist function. The traction coefficient must be known, as well as the load
distribution or the weight shift distribution. Variations in the traction coefficient and load distribution
may exist due to changes in traction and operating grades.
Wheel speeds will change due to shifting into a different final drive ratio or to changes in variable motor
displacements.
Often the assist and main drive wheels are of different diameter which then requires a different rpm forany given vehicle speed. Likewise the turning radius affects a speed change between the inside wheel
and the outside wheel. Maximum speed limits of the wheel assist motors must be considered closely due
to these variations as well as any downhill speedup. Flow dividers may be necessary to prevent full flowfrom going to any one motor in any circumstance.
Controlling a wheel assist function can be done in a variety of ways. Variable motors, using main system
drive flow as the power source, can be stroked for additional traction, manually or automatically, at somedesignated pressure setting. Pressure compensated motors stroked at a pre-selected pressure may work
well when traction coefficients are rather consistent. But, if the main drive wheels are losing traction at apressure less than the wheel assist pressure set point, the wheel assist motors would not shift offering
additional traction.
Sensing a percentage of main drive wheel slip can be the signal to the control system to add wheel assist
traction. Typically, a separate pump and motor arrangement would be used. In this case, the main drive
could be mechanically or hydraulically driven.
Application assistance is always available from Danfoss technical representative when wheel assist drives
are encountered.
The wheel assist shown in the figure Operator Controlled Wheel Assist is accomplished with two variablemotors and wheel ends on the steerable wheels. These variable motors are typically capable of both zeroand full displacement. While in a mode of low torque requirements, the variable motors are in zerodisplacement. When more tractive force is required, the motors are shifted toward full displacement.

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Transmission Circuit Recommendations
Wheel Assist
Operator Controlled Wheel Assist

AXLE MOTORENGINEPUMP

VARIABLE MOTORS
W/ WHEEL DRIVES

P108881

The wheel assist shown in the figure Pressure Controlled Wheel Assist can accomplish the additional
tractive force required with a separate designated pump for the assist function. This system can produce
a fixed or variable tractive force by the use of an electronic pressure control. By controlling pressure levelfrom zero to maximum by operator input, just the right amount of assist traction is possible. At a zeropressure command, vehicle coast is possible.
Pressure Controlled Wheel Assist

AXLE MOTORENGINE

PUMP PUMPFIXED MOTORS
W/ WHEEL DRIVES

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Applications Manual
Transmission Circuit Recommendations
Ganged Pumps

Introduction
The full through drive torque capability of Danfoss axial piston pumps is often used to eliminate the needfor multi pad pump drives. These pumps have found additional success with the through torque drive by“ganging” pumps together. A ganged pump is defined normally as two or more pumps bolted together
with the output flows combined to drive one load. The objective of the ganged pump is to simulate theperformance of one large pump with the flow of two or more smaller pumps. Two smaller pumps will
have higher individual speed capabilities than a large pump and therefore could offer an input speed
advantage. “Ganging” pumps has created an entirely new set of larger pump displacements for
additional application flexibility.
Circuit Recommendations
In order for two pumps to operate and perform as one, it is critical that each pump reacts similarly to thecommon load. For example, the Series 90 Pressure Limiter automatically adjusts pump displacement tolimit pressure when the limiter setting is reached, but it is not practical to assume that the valves in two
pumps will be set exactly the same. Therefore, the lowest valve in either pump would attempt to regulate
the pressure of the entire system by controlling the displacement of one pump. To prevent thisundesirable effect, a single external valve should be used to control both of the ganged pumps
simultaneously.
The concept shown in the figure below is commonly referred to as a “Remote Variable Pressure Limiter”(RVPL) and details can be obtained from Danfoss Application Engineering. Electrical Displacement
Controls (EDC’s) are shown in the figure. The check valves are necessary to isolate the pump servos fromone another when the system pressure is less than the RVPL setting. The remote pressure limiter is asingle side circuit. Two RVPL circuits are necessary to control both sides of the loop.
Both pumps must have a displacement control and it is recommended that they be of the same style(manual, hydraulic or electric). Do not use a cover plate and slave the second pump to the first. The
displacement control adds the necessary swashplate feedback so that both pump displacements arematched closely to one another within the accuracy and tolerance of the two controls. At pressures
below the RVPL setting the displacement control response time for each pump is not affected by the
RVPL circuit. However, when the RVPL does function, the system pressure limiter response time willapproximately double due to the total volume of two servos and added plumbing that needs to bepressurized before the pumps will destroke.
A ganged pump with the RVPL regulating port “A”

RVPL VALVEPORT "A"LO

LO

EXTERNAL
HPRV

C

ED

C
ED

HI

HI
PORT "B"P108883

Since the internal pressure limiters (PL) of each pump needs to be functionally removed, it is necessarythat they be ordered, or adjusted, to avoid any interference with the remote valve. The internal PL’sshould be set at a minimum of 35 bar (500 psid) above the setting of the remote pressure limiter valve.
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Transmission Circuit Recommendations
Ganged Pumps
Since the high pressure relief function of the Series 90 Multi Function Valve is also eliminated when the
integral PL’s are functionally removed, it is recommended that an external high pressure relief valve
capable of short duration high flow be added to the circuit at a pressure of 35 bar (500 psid) above theremote pressure limiter setting.
It is recommended that both pumps be controlled together. For example, an idle pump in neutral would
be forced over center when system pressure is created by a second individually controlled pump.
Additionally, the displacement change resulting from the operation of the pressure limiter is a result of a
combination of forces acting on the pump swashplate. The pumps should be controlled together so the
net forces on each pump are similar and, therefore, the performance of the pressure limiter optimized.
The table Series 90 Pump Logic Chart on page 22 identifies the appropriate system pressure and low
pressure servo gage ports that are used in the RVPL circuit based on input rotation. The port labels shownin the table are consistent with the Series 90 Technical Manual, pump outline and installation drawings.
Consult product specific Technical Information Manuals for the servo pressure gage port locations onother Danfoss servo controlled pumps.
Other control concepts and configurations are possible, but their acceptance would depend on theapplication and performance expectations. Any ganged pump application should be tested thoroughly
for control and driveline performance. Ganged pumps have been used successfully in many applications.
For application assistance, or information on other control concepts, please consult your Danfoss
representative.

© Danfoss | August 2023

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Applications Manual
Transmission Circuit Recommendations
Case Drain Routing Variations

Routing Variations
The routing of case drains in a closed circuit system should allow for adequate cooling of the hydraulicfluid, but with minimum external plumbing. The figures in this section show the recommended casedrain routings for various pump/motor configurations.
The following figure shows the simplest method for routing case flows for a pump/motor combination
without loop flushing. It shows that all of the case flow goes through the motor housing. To ensure thecases of the pump and motor are full of fluid at all times, case outlets should come out at the top of theunit and case inlets should come in at the bottom.
Case drain routing without loop flushing

MOTOR
HYDROSTATIC
PUMP
HEAT
EXCHANGER

P108884

The following figure shows a method of case drain routing for a system which incorporates loop flushing.
This method combines case flows for both the pump and the motor before they flow into the heatexchanger.
Case drain routing with loop flushing

MOTOR
HYDROSTATIC
PUMP
HEAT
EXCHANGER

P108885

An alternative routing for a loop flushing system is shown in the figure below. Here all of the case flow
goes through the pump. This system uses somewhat simpler plumbing than the previous method.
However, it has the disadvantage that the pump case temperatures will be greater.

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Applications Manual
Transmission Circuit Recommendations
Case Drain Routing Variations
Case drain routing with alternate loop flushing

MOTOR

HEAT
EXCHANGER

P108886© Danfoss | August 2023

BC152886483346en-0202 | 31

Applications Manual
Transmission Circuit Recommendations
One Charge Supply/Two Pumps

One Charge Supply
It is possible to have one charge pump supply charge flow to two separate pumps, providing charge flow
is great enough. The main advantage is that a single line from the reservoir may simplify plumbing. The
figure below shows that only one of the pumps is equipped with a charge pump, which supplies charge
flow to the second pump through its charge pressure gage port. The charge pump must be sized large
enough to provide adequate charge flow to both pumps for all operating conditions. This is especially
important at low speeds, since charge flow is less and leakage losses are a greater portion of the chargeflow.
One pump is equipped with a charge pump

PV

PV

CHARGEINLET

CHARGE PUMPGAGE PORT

MV

MV

P10888732 | © Danfoss | August 2023

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Applications Manual
Transmission Circuit Recommendations
Charge From Implement Return Flow

Charge From Implement Return Flow
Often charge flow for a closed circuit system can be provided by an implement pump, whose primaryfunction is to power an open circuit load such as a cylinder, motor, etc. The figure below shows such anarrangement, whereby one or more pumps are without charge pumps and receive all of their charge flowfrom the implement circuit return flow. The main advantage to this method is the single inlet suction linefrom the reservoir. Care must be taken that the implement pump return flow from the load always
exceeds the closed circuit charge flow requirements for any operating condition. Also, a pressure filter
should be placed between the charge inlet and the return flow to keep contaminants from theimplement circuit out of the hydrostatic system.
Charge Flow From Implement Return

PV

CHARGEINLET

PRESSURE
FILTER
IMPLEMENTPUMPLOAD(CYLINDER, MOTOR, ECT.)P108888© Danfoss | August 2023

BC152886483346en-0202 | 33

Applications Manual
Transmission Circuit Recommendations
Multiple Motors

Parallel Multiple Motors
Several motors can be combined in the same closed circuit, but are normally only used in parallel. Series
operation of axial piston or bent axis motors loads the bearings excessively causing substantial
reductions in life expectancy. Contact Danfoss if series circuits are under consideration.
Several motors driven in parallel from one pump allow a potential motor overspeed situation. If one
motor is at a reduced load due to poor traction or other unusual situation, it may overspeed and damage
the unit if not sized to accept all the pump flow. Flow dividers, divider-combiners or flow limiters havebeen used successfully to prevent motor overspeed.
Motor overspeed is a safety concern when the transmission is providing retarding power. For swashplate
motors, maximum speed may be limited by block lift and accompanying loss of drive. Operation above
maximum speed is not permitted.
Flow dividers create heat, even during normal operation, as they continuously act during vehicle turns,
with load differentials, or due to normal tolerance effects. Care must be used in sizing heat exchangers
when using flow dividers.
Flow divider performance is sensitive to the droop built into the valve. A large droop should be used to
improve control stability and reduce the amount of time the valve regulates. A flow droop of at least 30%
prior to the valve restricting at the pressure limit is recommended. A cross-valve orifice may be required
to achieve this. This may affect motor sizing and maximum speed expected and should be accounted forwhen sizing units.
A divider-combiner valve can pressurize both motor ports when operating in the combining mode. The
frequency of this occurring must be anticipated as this condition greatly reduces expected hydraulic unit
life. The combining mode should be used in the least frequent travel direction. Danfoss recommends
testing to determine the level and duration of both motor ports being pressurized.
A loop flushing shuttle should be used to keep cool fluid flowing into the power loop. Flow in a loop
flushing shuttle must be accounted for when using flow dividers or divider/combiners in a multiple
motor circuit. Because the charge relief valve in the shuttle allows oil flow, a differential flow can exist
from one side of the loop to the other. If the shuttle is installed between one motor and the flow control
valve, charge flow causes the valve to operate, even though all motors may be at the same speed. A loop
flushing shuttle should be installed between the pump and flow control valve in order to avoid thisproblem.
Motor speed may be controlled with the use of flow limiters, which operate at a set maximum flow. This
type of valve only functions if overspeed is imminent, and it typically operates a small amount of time.
Flow limiters should be installed to restrict motor return flow to avoid cavitating the motor inlet when
they function. Since this will pressurize both motor ports and reduce expected life, frequency ofoperation must be determined.
Danfoss technical support is available for obtaining suggestions on transmission circuit design. Contact
your Danfoss representative if questions of suitability exist. In any case, Danfoss recommends testing thefinal circuit for sufficient time and under severe enough conditions to determine acceptable performanceand life.
Multi-Motor Circuit With Flow Limiters

HYDROSTATICPUMP

HYDROSTATIC
MOTOR
LOOP
FLUSHING
SHUTTLE34 | © Danfoss | August 2023

P108889

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Applications Manual
Transmission Circuit Recommendations
Flow Dividers and Divider/Combiners

Introduction
This article describes how flow dividers and divider-combiner valves are used in closed circuit
applications and how special considerations must be taken when using these components.
Flow dividers and flow divider-combiner valves are normally used in vehicle propel circuits where two
motors are plumbed in parallel and supplied by a single flow source. In such installations, they serve
potentially two purposes; to prevent one motor from overspeeding if it loses contact with the ground,and to ensure uniform output speed of both motors.
The basic difference between flow dividers and flow divider-combiners is simply that the flow divider is
not designed to combine flow. In the divide mode, their performance is effectively the same. A circuit
using flow dividers must either be a unidirectional system or it must bypass the divider during reverse
flow. Because most vehicle propel systems are bi-directional, this article focuses on the divider-combinerwith the understanding that the comments made with respect to the divide mode of operation are valid
for both types of valves.
The information presented in this article is based on field experience using flow divider-combiners inclosed loop hydraulics. Valve suppliers should be consulted regarding the performance, ratings, andsizing for each application.
Heat Generation
The pressure drop across the divider-combiner is typically on the order of 200 to 600 psi in the divide
mode. Many valves will have an even greater pressure drop in the combine mode. This pressure drop is asource of heat generation in the circuit which should be accounted for when sizing the heat exchanger.
The heat generation can be minimized if the divider-combiner is by-passed during modes of operation
when it is not needed in the circuit.
Design Flow Range
Flow divider-combiners are designed for optimum performance within a specified range of flow rates.
Operation outside of the specified range will significantly reduce the reliability of the valve and will likely
cause circuit problems as well. Consult the valve supplier for correct sizing on each application.
Loop Flushing
If loop flushing is desired in a circuit using a flow divider-combiner, it must be located between the pumpand the flow divider-combiner. If loop flushing is attempted at the motors, problems can occur during
both the divide and combine modes of operation.
During the divide mode of operation, the low loop side of the motor should be at charge pressure.
However, on propel applications where the two motors are effectively tied together by the ground, thehigh pressure side of one motor can actually be subjected to a vacuum (see Recommendations forTrouble-Free Circuit on page 37) due to volumetric discrepancies in the divider-combiner and the
motors. If this occurs, the loop flushing shuttle will shift and attempt to draw fluid from the inlet side of
the motor, compounding the vacuum problem.
During the combine mode of operation, the low loop pressure at the motors is elevated by the pressure
drop across the divider-combiner, which can be significant. Thus, the loop flushing relief valve at the
motor, being subjected to this considerably higher pressure, would dump significantly more fluid than in
the divide mode of operation. This could potentially cause increased case pressures and/or a loss ofcharge pressure in the combine mode of operation.

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Applications Manual
Transmission Circuit Recommendations
Flow Dividers and Divider/Combiners
Loop Flushing When Used With a Divider-Combiner
ON/OFF
TRACTIONVALVE

DIVIDING FLOWCROSS-BLEEDORIFICE
LOOP
FLUSHING
VALVE

MF
MF

COMBINING FLOW
P108890

Motor Life Reduction
Hydraulic unit life is highly dependent on system pressure. In the combine mode of operation, when the“low loop” pressure is significantly higher than charge pressure, the motor life can be adversely affected.
Although the delta pressure across the motor (the pressure that transmits torque) may be at someprescribed design value, the "effective" pressure seen by the motor rotating group is the sum of the highand low sides of the loop. Therefore, the frequency of operating in the combine mode must be
anticipated and included in the duty cycle to calculate a predicted unit life. Operation in the combine
mode should be minimized if long motor life is critical.
Differential Flow for Steering
On a typical propel system, the motors must rotate at moderately different speeds in order for the vehicle
to negotiate a smooth turn. To accomplish this, the flow divider-combiner must allow differential flows.
The combine mode is often much less capable of handling this differential flow during a steering event.
Motor speed ratings must not be exceeded during the steering event. Consult the valve supplier for
specifications on differential flow in both divide and combine modes of operation.
Motor Speed Synchronization (Non-Propel)
Several non-propel types of applications require two (or more) motors operating at the same speed tocorrectly perform a work function. Depending on the output speed accuracy required, a flow dividercombiner may or may not be an acceptable means of accomplishing the objective.
The system performance will be limited by the volumetric tolerance stack-up of the motors and thedivider-combiner. This includes the slight difference in motor displacement, the slight difference in motor
volumetric efficiency, and the flow division capability of the divider-combiner. Because the flow divider is
not a perfect device, it will not send exactly the same volume of fluid to the two motors. Because themotors are not perfect devices, their speed given the same volume of fluid will be slightly different. As
the system loads up, the volumetric efficiency of the two motors will decrease at slightly different rates.
The result of these imperfections is that the two motors will probably not rotate at exactly the same
speed. The acceptability of a system can only be determined by testing.

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Applications Manual
Transmission Circuit Recommendations
Flow Dividers and Divider/Combiners
If a system requires a high level of accuracy, the best way to control two (or more) motors at the same
output speed is with a one pump per motor. A microcontroller used in conjunction with speed sensors at
the motors and EDC’s at the pumps would provide the best synchronization.
Recommendations for Trouble-Free Circuit
Several circuit problems may arise when using flow dividers in closed loop hydraulics. These problems
seem to be very sensitive to system dynamics and do not surface on each and every installation. The
severity of the problems may fluctuate on a given machine with fluid temperature, vehicle speed, andgrade. To combat these problems, the following recommendations are provided with the figure Loop
Flushing When Used With a Divider-Combiner to help design trouble-free circuits.
• The loop flushing valve must be located between the pump and the flow divider-combiner per the
discussions presented above.
• Bypass the divider-combiner in the combine mode of operation with either check valves or ON/OFF
valves. When the combiner is eliminated from the circuit, it will not provide motor speed regulation
or limitation in the event of a wheel spin-out.
• Provide anti-cavitation check valves at the inlet port of both motors. This is necessary because thevolumetric tolerance stack-up of the motors and the divider-combiner can cause one motor tocavitate in a typical propel application. If one motor requires slightly more fluid to rotate at the same
speed as its partner motor (and if the two motors are tied together by the ground) then it will starve.
Because its inlet pressure drops, the motor is doing no work. On some machines this could go
unnoticed indefinitely. The same scenario occurs if the divider-combiner were to split the flow
unevenly.
• Work closely with a divider-combiner supplier to select the most appropriate valve for theapplication. Take into account flow and pressure ratings, pressure and temperature compensation,differential flow capability, and volumetric effectiveness.
• Test the vehicle extensively in all modes of operation to ensure correct functional performance aswell as correct circuit performance.

© Danfoss | August 2023

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Danfoss Power Solutions is a global manufacturer and supplier of high-quality hydraulic andelectric components. We specialize in providing state-of-the-art technology and solutions
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BC152886483346en-0202