Applications Manual

Pressure and Speed Limits for
Hydrostatic Units

powersolutions.danfoss.com

Applications Manual
Pressure and Speed Limits for Hydrostatic Units

Revision history

Table of revisions
Date

ChangedRev

May 2015

Danfoss layout - major update

0401

July 1997

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Contents
Introduction
Applications Manuals......................................................................................................................................................................4
Pressure and Speed Limits
Introduction........................................................................................................................................................................................5
Load to Life Relationships............................................................................................................................................................. 5
Pressure Limits for Hydraulic Units............................................................................................................................................ 6
Speed Ratings for Hydraulic Units........................................................................................................................................... 10
Motor Performance at Low Speed
Motor Performance at Low Speed...........................................................................................................................................15
Bypass Valve Speed Limitations
Bypass Valve Speed Limitations............................................................................................................................................... 16
Inlet Vacuum Limits
Inlet Vacuum Limits.......................................................................................................................................................................17
Dynamic Braking
Dynamic Braking............................................................................................................................................................................ 18
Displacement Control:............................................................................................................................................................ 19
Case Drain Pressure Limits
Case Drain Pressure Limits..........................................................................................................................................................20© Danfoss | May 2015

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
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 BLN-9885 (originally Section 1)• Pressure and Speed Limits for Hydrostatic Units BLN-9884 (originally Section 2)• Transmission Circuit Recommendations BLN-9886 (originally Section 4)• Fluids and Lubricants 520L0463 (originally Section 3)
Other Reference Manuals• Hydraulic Fan Drive Systems Technical Information 520L0824• Hydraulic Fan Drive Systems Design Guidelines 520L0926

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits

Introduction
In order to properly size hydrostatic units for power transmission applications, it is necessary to select anappropriate design pressure and design speed. The selection of these design parameters are affectedby required unit life. The purpose of this guideline is to assist system integrators in the selection ofdesign pressure and design speed to optimize hydraulic unit utility and to provide satisfactorytransmission life.
Danfoss Power Solutions publishes continuous (1) ratings and maximum ratings for both pressure andspeed capabilities. Application decisions are made by considering both typical and extreme operating
conditions. Predicted unit life, based on the design pressure and speed, is compared to the required unit
life for the application. Satisfactory design criteria must be obtained for both normal rated and maximum
values.
(1) The term “continuous” refers to the speed ratings of Danfoss Power System products, but not to theirpressure ratings.
Load to Life Relationships
System pressure is the dominant operating variable affecting hydraulic unit life. High pressure, which
results from high load, reduces life in a manner similar to many mechanical assemblies such as engines
and gear boxes. There are load-life relationships for the rotating group and for the shaft bearings whichmust be considered separately.
The graph Hydraulic Unit Life vs System Pressure shows the general relationship between life expectancy
and system pressure. The upper limit of hydraulic unit life is established by fatigue and wear of therotating group, bearing fatigue, or wear and deterioration of other parts, each with their own load-life
relationship. At high system pressures, fatigue and wear of the rotating group is the limiting factor,
while bearing failure becomes the limiting factor at intermediate pressures. It is important to note theshallow slope of the life curve at high system pressures which indicates a significant decrease in life witha slight increase in pressure.
Actual life is also affected by a combination of factors relating to fluid quality as shown in Hydraulic Unit
Life vs System Pressure by the curves corresponding to less-than-ideal fluid quality. Factors relating to fluid
quality include temperature, contamination (heat, hard particle, water, chemical, and entrained air),viscosity, and lubrication properties. Most of these factors are also influenced by operating pressure.
System design should take these factors into account. Technical Information Manuals (Hydraulic Fluids
and Lubricants 520L0463, Experience with Biodegradable Hydraulic Fluids 520L0465, and Design Guideline
for Hydraulic Fluid Cleanliness 520L0467) cover this subject in more detail.

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
Rotating
Fatigue group
& Wea
r

Be

Pressure

ing

Fa

tig

ue

Normal Limitof Unit Life

nentsHydraulic Unit Life vs System Pressure

mpo

Poor Fluid
Qualityr Co

Marginal Fluid
Quality

Othe

System OperatingarLife Expectancy (Time)

Pressure Limits for Hydraulic Units
The following definitions of hydraulic unit pressure capability and associated life requirements areprovided to assist in sizing transmission components. Contact a Danfoss Power Solutions representativewith your machine parameters if there is a question regarding typical load cycles.
In closed circuit hydrostatic units, System Pressure is the differential (delta) pressure between the highand low pressure work ports. It is the measure of the pressure available to accept or provide torquethrough the conversion of mechanical to/from hydraulic power. Zero System Pressure is the condition
where both sides of the hydrostatic loop are at equal pressure, typically at Charge Pressure. System
Pressure is the dominant operating variable affecting hydraulic unit life and performance. High systempressure, which results from high load, reduces expected life. Hydraulic unit life depends on the speed
and normal operating, or weighted average, pressure which can only be determined from a duty cycle
analysis.
In open circuit units, System Pressure is the gauge pressure of the working port since the low pressurereference is zero gage pressure at the reservoir.
In closed circuit hydrostatic units, it is common to provide a flow source at a moderate pressure toprovide makeup flow (charge flow) for the hydrostatic loop. The pressure of this flow source is called
Charge Pressure and is usually measured in terms of delta pressure above case pressure. The lower
pressure side of the hydrostatic power loop will typically be at, or near, charge pressure. In open circuit
systems, charge pressure may be considered to be the pressure available to the inlet of the pump or theoutlet of the motor. It may be either below atmospheric (0 Gauge Pressure) or above. If the chargepressure is above atmospheric, it is often known as Boost Pressure for a pump or Back Pressure for a
motor.
In closed circuit units, Minimum Pressure is the gauge pressure that must be maintained in the working
ports under all operating conditions to avoid excessive cavitation and to provide lubrication for sliding
components. Minimum pressure is typically less than Charge Pressure. Minimum Pressure limits are
identified in the Technical Information Bulletin for each product. Unless otherwise specified, Minimum
Pressure is defined as 10 bar [150 psig].
In open circuit units, Minimum Inlet Pressure is the minimum allowable absolute pressure in the pumpsuction line. The minimum inlet pressure must be evaluated in both steady state, and transientconditions; note that when pump displacement increases, inlet pressure is reduced due to acceleration of
fluid in the inlet line.
Application Pressure is the highest intermittent pressure defined for a given application; and is the highpressure relief, pressure limiter, or pressure compensator setting typically defined within the order codeof the pump or motor. It is determined by the maximum machine load demand. Application Pressure isthe "applied" System Pressure (delta pressure) at which the driveline generates the maximum calculated
pull or torque, or at which maximum loads move in the application. Application Pressure is typicallydetermined from the maximum design loads of the vehicle driveline or work function components.
Application Pressure is assumed to occur for a small percentage of operating time, usually less than 2% of
the total.
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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
For systems with pressure limiter valves acting to avoid frequent overloads, it may be necessary to selecta reduced maximum design pressure below that normally allowed because time at maximum Application
Pressure may be excessive.
Maximum Working Pressure is the highest recommended Application Pressure. Maximum Working
Pressure is the highest standard pressure setting allowed within the product code. Systems with
Application Pressures at, or below, the Maximum Working Pressure should yield predicted (calculated)
unit life given proper component sizing. Maximum Working Pressure limits are identified in the Technical
Information manual for each product. Application Pressures above Maximum Working Pressure will only
be considered with duty cycle analysis and factory approval. (Factory approval means Engineering
approval is needed.)Maximum Pressure is the highest allowable System Pressure under any circumstance. System Pressures
above Maximum Pressure may result in damage to the hydrostatic unit which could result in reduction in
useful life below what would be predicted by existing design/life analysis tools; and/or a permanent
reduction in the performance or efficiency of the hydrostatic unit.
Root Mean Effective Pressure is the time weighted average for a pressure duty cycle using a specified
weighting exponent relative to the internal components under consideration. RMEP is a calculated
System Pressure equivalent for a specific duty cycle. It can be used to compare the relative severity ofloading cycles and used for component life predictions.
The formula for calculating the RMEP value is shown below:

Where:RMEP = Root Mean Effective Pressure
Pressure = System Pressure average for the time step
% time = Total time for the time step expressed as a decimal percent
a = cumulative fatigue damage exponent for the unit or the component under consideration• a = 8 for rotating groups and shafts in bending fatigue••

a = 3 for ball bearings
a = 10/3 for tapered or cylindrical rolling element bearings

The exponent of 8 is derived from mechanical fatigue and is appropriate for the all rotating components
except bearings. The exponents of 3 and 10/3 are derived from the bearing industry standard practice.

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
Pressure versus Time chart showing key pressure terms

Application A - The System Pressure duty cycle stays under the Application Pressure. The unit in thisapplication should have satisfactory predicted life provided the fluid viscosity, quality and cleanliness aremaintained.
Application B - The delta system pressure starts out within the maximum working pressure but has a
spike above Maximum Working Pressure. The trace also shows a spike below charge pressure, but still
above the minimum pressure. The unit in this application would perform as expected, but has thepotential of having a somewhat shorter predicted life, dependent upon the total amount of time spentnear Maximum Pressure.
Application C - The system delta pressure trace shows a spike significantly above Maximum Pressure. The
unit in this application may suffer damage and have its expected life significantly shortened; and/or nolonger provide expected performance.
Normal working pressure is the pressure expected when performing “normal” work functions. It is theaverage pressure across the normal work load range.
The normal working pressure occurs at a regularly occurring load condition. Its design value can beestablished by calculating system pressure at the design input power and maximum pump displacement(see the following graph). For machines with variable load cycles, the design input power may be
estimated by deducting the average power to the other functions from the maximum available engine
power.
System Pressure Line
Application
Max.

Design Input
Power

Full Load Speed

SystemPressure

Design Tractive Effort
at Maximum Design
Speed

Output Flow

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
The preferred method of selecting an Application Pressure requires duty cycle information for theapplication. A duty cycle is a means of quantifying the pressure and speed demands of a particularsystem on a percent time basis. The duty cycle data is weighted to reflect the preponderant influence ofpressure on unit life. Using the rotating group and shaft bearing load-life tools developed by Danfoss
Power Solutions for the various product families, the optimum Application Pressure can be selectedbased on the required life expectancy for the application.
If an external side load is expected on the shaft, its magnitude and direction with respect to theswashplate must be known when calculating expected bearing life.
It is very important that the duty cycle information be as accurate as possible due to the exponential
effect that pressure has on the life of the rotating group.
In the absence of duty cycle data, allowable pressure limits for design can usually be established basedon the maximum and normal working load in the drive. This assumes that the duty cycle falls within atypical band width. By using this approach, two “pressure limit” values can be used to predict a successful
application when machine life requirements are known.
The following table is provided to aid the engineer in understanding and selecting appropriate designpressure for a given component life requirement. These values are derived from the load-life relationshipfor the rotating groups of the various Danfoss Power Solutions product families at 1800 rpm shaft speed
and no external shaft loads. Unit life is given in terms of L20 hours (80% survival) for the rotating groupbased on published recommendations for fluid cleanliness and viscosity. Using these guidelines, thesystem must meet both the maximum and continuous pressure limits to be considered acceptable.
Prototype test programs to verify operating pressures and predicted unit life are strongly recommended
prior to finalizing any hydrostatic drive design.

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
Design Pressure Recommendations as a Function of Required Life
Required Life
(L20 Hours)

Series 15

Series 40

KV/LVSeries 42

Series 90

H1P

Displacement (cc/rev) & Root Mean Effective Pressure, at Full Displacement, Psid [dbar]
3675[253]
L25
4150[286]
284150[286]424900[338]454500[310]
4150[286]
L30 3650[252]323650[252]55
4900[338]
53
3900[269]
4025[277]
L35 2875[198]414150[286]75
4900[338]60
4500[310]
3675[253]
K38 3650[252]513650[252]100
4900[338]
68
3900[269]
3675[253]
K45 2875[198]1304900[338]69
4900[338]
3450[238]180
4900[338]78
4900[338]250
4900[338]89
4900[338]100
4900[338]115
4900[338]130
4900[338]
147
4900[338]
165
4900[338]
210
4900[338]250
4900[338]

1500

15

3200[220]

25
35
44
PV046
MV046
MF046

5000

15

2750[190]

25
35
44
PV046
MV046
MF046

3175[219]
3600[248]
3450[238]
3175[219]
3175[219]
2975[205]

L25
L30
L35
K38
K45

3600[248]
3125[215]
2475[171]3125[215]
2475[171]

28
32
41
51

3600[248]
3125[215]
3600[248]
3125[215]

42
55
75
100
130
180
250

4225[291]
4225[291]
4225[291]
4225[291]
4225[291]
4225[291]
4225[291]

45
53
60
68
69
78
89
100
115
130
147
165
210
250

15000

15

2400[165]

25
35
44
PV046
MV046
MF046

2775[191]
3125[215]
3000[207]
2750[190]
2750[190]
2600[179]

L25
L30
L35
K38
K45

3125[215]
2725[188]
2150[148]
2725[188]
2150[148]

28
32
41
51

3125[215]
2725[188]
3125[215]
2725[188]

42
55
75
100
130
180
250

3650[252]
3650[252]
3650[252]
3650[252]
3650[252]
3650[252]
3650[252]

45
53
60
68
69
78
89
100
115
130
147
165
210
250

Series 51

H1B

60
80
110
160
250

4900[338]
4900[338]
4900[338]
4900[338]
4900[338]

60
80
110
160
250

>4150[286]>4150[286]>4150[286]>4150[286]>4150[286]3850[265]
3375[233]
3850[265]
3375[233]
4225[291]
4225[291]
4225[291]
4225[291]
4225[291]
4225[291]
4225[291]
4225[291]
4225[291]
4225[291]

60
80
110
160
250

4225[291]
4225[291]
4225[291]
4225[291]
4225[291]

60
80
110
160
250

>3600[248]>3600[248]>3600[248]>3600[248]>3600[248]3350[231]
2925[202]
3350[231]
2925[202]
3650[252]
3650[252]
3650[252]
3650[252]
3650[252]
3650[252]
3650[252]
3650[252]
3650[252]
3650[252]

60
80
110
160
250

3650[252]
3650[252]
3650[252]
3650[252]
3650[252]

60
80
110
160
250

>3125[215]>3125[215]>3125[215]>3125[215]>3125[215]

Speed Ratings for Hydraulic Units
The applied speed of hydraulic units is limited by several design and application factors which havevarying effects on life. In general, hydraulic unit life varies inversely proportional to speed at, or below,
the (continuous) speed limit. Operation at speeds in excess of the continuous speed limit causes less thannormal life but is permissible at less than full power. The maximum speed limit cannot be exceeded
without significant reduction in life and without risking immediate failure resulting in a loss of hydrostatic
power.

W Warning
Danfoss Power Solutions strongly recommends that a secondary means of braking be provided to stop a
runaway load caused by block lift or any other condition..
Speed capability is affected by flow rate and mechanical loading of the rotating group, and therefore, is afunction of hydraulic unit displacement (swashplate angle). The graph Speed Ratings for Piston Pumps andMotors shows the typical speed rating versus displacement relationship for axial piston and bent axis
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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
hydraulic units. In general, speed capability increases as displacement decreases, up to a peak value atintermediate displacements.
The Rated Continuous Speed Limit is the highest speed recommended at full power conditions. Thisrating defines the highest speed at which normal life can be expected. Continuous full power operation
applies at the conditions defined by continuous pressure.
Operation above the rated speed limit is expected at reduced power, as engine speed increase (forpumps) and reduced volumetric loss (for motors) will produce increased motor/vehicle speed at low, or
negative, power. Some reduction of life would be expected at full power when operating above the rated
speed limit. For normal systems, working power level, and resulting system pressures will be low whenapplied properly, and time above the rated speed limit will be low.
The rated speed limit varies with swashplate angle up to the reduced angle continuous maximum value.
The upward sloping speed characteristic is derived from the kinematic design parameters of axial piston
pumps and motors. They define the conditions at which normal operation is expected, based on
sufficient charge pressure. Normally, pumps are not applied at reduced angle speed ratings because theyfrequently must operate at the highest speed and maximum angle simultaneously.
The maximum speed rating is the highest operating speed recommended and cannot be exceeded
without severe reduction of unit life, or without risking premature failure and loss of drive. Operating
conditions between continuous and maximum speed limits should be restricted to less than full power
and to limited periods of time. For most drive systems, maximum motor speed occurs during downhillbraking or negative power conditions.
The maximum speed rating varies with swashplate angle up to the reduced angle maximum value. At
this speed, conditions of marginal lubrication may exist when system pressure is low. Also, rotational
forces are high and may cause abnormal mechanical loading and stress on the rotating group.
For all axial and bent axis units, exceeding the maximum speed limit is a safety concern when thetransmission is providing retarding power. At high swashplate angles, the maximum speed is limited byblock lift which causes a loss of drive power. This could allow dangerously high, and uncontrollable,
output speed. Operation above the maximum speed limit must be avoided under all modes of operation
including engine speed-up and reduced volumetric loss during downhill braking.

W Warning
Danfoss Power Solutions strongly recommends that a secondary means of braking be provided to stop a
runaway load caused by block lift or any other condition.

© Danfoss | May 2015

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
Speed Ratings for Piston Pumps and Motors

ReducedAngle
Maximum
Speed
ReducedAngle Rated(Continuous)
Speed

Operation Not Permitted
Intermittent
Operation

Block
Lift Limit

Continuous
Full PowerOperation

Swash Angle

Full Angle
Maximum
Speed
Full Angle
Rated(Continuous)
Speed
Max Disp
P108902

The highest permissible rated (continuous) and maximum speed limits for Danfoss Power Solutions
closed circuit piston pumps and motors are provided in the table Figure : Speed Ratings for Closed Circuit
Pumps and Motors on page 13. Variable motors have speed ratings for both “maximum” and “minimum”
displacements. The “minimum” displacement speed limit applies at approximately half of full
displacement, but this varies with product family and frame size. Minimum displacement speed ratings
are not provided for pumps, because pumps are typically expected to operate at fixed input speeds at full
displacement.

12 | © Danfoss | May 2015

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
Speed Ratings for Closed Circuit Pumps and Motors

P u m psF i x e d M o to r s

V a r i a b l e M o to r s
F u l l D i s p.

P r o duc t

R a te dMax

R e d u c e d D i s p.

R a tedMax

R a te dMax

R a te d

Max042

4 2 00

4 6 004 2 00

4 6 00

---

---

---

---

055

3 9 004 2 503 9 004 2 503 9 004 2 50

4 6 005 1 00075

3 6 003 9 50

3 6 003 9 50

3 6 003 9 504 2 504 7 00100

3 3 003 6 503 3 003 6 50

---

---

---

---

130

3 1 003 4 003 1 003 4 00

---

---

---

---

180

2 6 002 8 50

---

---

---

---

---

---

250

2 3 002 5 00

---

---

---

---

---

---

21

3 5 003 8 003 5 003 8 003 5 003 8 004 4 005 2 5022

3 2 003 9 003 2 003 9 003 2 003 9 004 0 004 8 5023

2 9 003 3 502 9 003 3 502 9 003 3 503 7 004 5 5024

2 7 003 1 002 7 003 1 002 7 003 1 003 4 504 1 0025

2 4 002 9 502 4 002 9 502 4 002 9 503 1 003 7 5026

2 1 002 5 002 1 002 5 002 1 002 5 002 8 003 4 0027

1 9 002 0 50

1 9 002 0 50

1 9 002 0 502 4 502 9 50

060

---

- --

- --

---

3 6 004 4 005 6 007 0 00

080

---

- --

- --

---

3 1 004 0 005 0 006 2 50

110

---

- --

- --

---

2 8 00

3 6 004 5 005 6 00

160

---

- --

- --

---

2 5 003 2 004 0 005 0 00

250

---

- --

- --

---

2 2 002 7 003 4 004 2 5028

3 4 003 9 00

---

---

---

---

---

---

41

3 4 003 9 0 0*

---

---

---

---

---

---

M2 5

4 0 005 0 004 0 005 0 004 0 005 0 005700

M3 5

3 6 004 5 00

3 6 004 5 00

3 6 004 5 005300

M4 4

3 3 004 1 003 3 004 1 003 3 004 1 004850

M4 6

4 0 004 1 00

3 6 00

3 6 004 0 004 1 005000

4 0 004 2 004 0 004 2 00

---

---

---

---

B D P -1 0 L

3 6 00

3 6 00

---

---

---

---

---

---

B D P -2 1 L

3 6 00

3 6 00

---

---

---

---

---

---

S er ies 90

S er ies 20

S er ies 51

S er ies 42

S er ies 40

S er ies 15
15
S er ies 70* Any operation above rated speed requires application approval© Danfoss | May 2015

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
The following equations are provided to calculate motor speed limits for intermediated displacements.
Although the calculated values will exceed the tabulated values at lower swashplate angles, speed limits
from the table should never be exceeded. The equations are valid for “maximum” as well as “rated” speed
limits and are provided for convenience of calculation.
Axial Piston Motors
NV = NM (TM / TV)1/2
or
NV = NM (DM / DV)1/2
where:
NV = Motor speed limit at reduced angle (RPM)
NM= Motor speed limit at maximum swashplate angle from Table 2 (RPM)
TV = Tangent of reduced swashplate angle
TM = Tangent of maximum swashplate angle
Tan 15° = 0.268 (Series 15 and Series 40-M25)

•
•
•
•

Tan 16° = 0.287 (Series 40-M35 and Series 40-M44)
Tan 17° = 0.306 (Series 90 and Series 40-M46)
Tan 18° = 0.325 (Series-20, Series 42, and H1P)

DM= Motor displacement at maximum angle (cc/rev or in3/rev)
DV = Motor displacement at reduced angle (cc/rev or in3/rev)
Bent Axis Motors
NV = NM (SM / SV)or
NV = NM (DM / DV)
where:
NV = Motor speed limit at reduced angle (RPM)
NM= Motor speed limit at maximum angle from Table 2 (RPM)
SV = Sine of reduced angle
SM = Sine of maximum angle, Sin 32° = 0.530 (Series 51 and H1B)
DM= Motor displacement at maximum angle (cc/rev or in3/rev)
DV = Motor displacement at reduced angle (cc/rev or in3/rev)

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Motor Performance at Low Speed

Motor Performance at Low Speed
Some applications require controlled motor operation at low speeds. Generally, speeds less than 5% ofrated are considered “slow” for axial piston motors. For bent axis motors, speeds less than 2-3% of rated
are considered “slow”. Like high speed applications, a common requirement for low speed operation iscontinuous smooth output shaft rotation. However, axial piston type motors have inherent limitations
which may provide unacceptable performance at low speeds.
The main limitation is related to the interaction of the pistons as they transition from high to low pressure
across the valve plate. As each piston makes the transition, there is a variation in the motor’s total
displacement. Even if system pressure remains constant, the variation in displacement results in a “torqueripple” at the motor output shaft, which is generally described as shaft cogging. As system pressure
increases, the speed at which cogging occurs also increases. Other system parameters affecting thisinherent phenomenon include inertia of the load, load fluctuations, and bulk modulus of the system (linediameter and length, line type, and fluid compressibility).
Another inherent limitation to low speed operation, common to all types of driveline components, issimply mechanical friction. For piston motors, this is largely a function of the design of the unit and the
manufacturing tolerances of the rotating group parts.
As a rule of thumb, most Danfoss Power Solutions axial piston motors will operate smoothly down to~150 rpm. Some axial piston motors may run smoothly at less than 150 rpm, but performance must be
verified for the specific application. Danfoss Power Solutions does not recommend that any axial piston
motor be size to operate continuously at speeds less than 100 rpm.
The H1B and Series 51 variable displacement bent axis motors have demonstrated smooth performanceat speeds down to 50 rpm in most applications, but acceptability must be judged based on specificapplication requirements.
Because the limitation to low speed operation is inherent to all piston type motors, Danfoss Power
Solutions cannot offer “special hardware” to improve low motor speed performance. Low speed
applications should be critiqued carefully at the design phase and thoroughly tested for acceptableperformance at the prototype phase.

© Danfoss | May 2015

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Bypass Valve Speed Limitations

Bypass Valve Speed Limitations
Most Danfoss Power Solutions axial piston pumps have a bypass valve which allows system fluid to be
cross-ported between the main “A” and “B” ports when pump input shaft rotation is not possible. Thisvalve requires manual engagement and disengagement via an external device on the pump, such as an
adjustment screw or plunger. The intent of this valve is to allow a vehicle using a closed loop propel
circuit to be moved a short distance at slow speeds. Typically, this translates to loading a vehicle onto a
trailer, getting a malfunctioning machine away from the work area, or out of a traffic lane.
Cross-port bypassing is required to prevent hydraulic lockup as the motor becomes a flow source whilethe vehicle is being moved (assuming the motor remains coupled to the ground via tires or tracks). The
bypass valve should not be used or referred to as a “towing valve” which implies high speed and long
duration usage.
Vehicle speed limitations associated with cross-port bypassing are a function of flow rate and duration oftime. Exceeding one, or both, of these criteria can lead to failure of the transmission. Flow rate is criticalbecause the bypass valve may become saturated due to its pressure rise rate creating excessive heat. The
temperature rise of the fluid as it passes thru the bypass valve is proportional to the square of the flow
rate. (i.e., if you double the flow rate, you will increase the temperature rise rate by 4-times).
Duration is critical because there will be no charge fluid to makeup leakage in the circuit, and the systemlines will become evacuated. Because the flow rate being bypassed is a function of the motor
displacement, final drive ratio, and rolling radius; it is impossible to publish strict speed ratings for the
bypass mode. Also, the circuit design will affect how quickly system fluid is lost to leakage. However, thefollowing rule of thumb for flow rate and duration has been used successfully to quantify the limitations
during a bypass mode.
Do not operate the transmission in bypass mode at more than 10~15% of rated motor speed anddo not exceed a duration of 3~5 minutes, depending on the product. Refer to the product specificTechnical Information Manuals for details and limits.
During normal transmission operation, the bypass valve must remain fully closed. Partial opening of thevalve will cause sluggish performance and will create excessive heat in the system.
The bypass function is standard on all Series 90 pumps, Series 40 pumps and H1P pumps. It is an optionalfeature on Series 42 pumps and Series 15 pumps. Please refer to the technical information bulletins forthe specific pump families for details on the use of the bypass function.
The bypass function is not available for Tandem H1 Pumps.

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Inlet Vacuum Limits

Inlet Vacuum Limits
Flow enters the charge pump through the charge inlet. Unless this inlet is pressurized, a vacuum will exist
due to the resistance to flow through the inlet line leading to the inlet port of the charge pump.
The degree of inlet vacuum affects the ability of the charge pump to fill properly. Starving the charge
pump can cause the inlet fluid to reach its “air release” pressure level which results in cavitation.
Cavitation is the formation and collapse of vapor and/or air bubbles in the fluid. When the bubbles
collapse, they pit or erode the metal surfaces on which they are in contact resulting in reduced
Volumetric Efficiency. Charge pumps are usually less susceptible to erosion damage because of the
reduced pressure in the charge cavity. Typically, damage occurs at the valve plate, cylinder block andpiston slippers due to the high pressures in the system circuit. Volumetric losses can become so extremethat the charge pump will be unable to meet the demands of the system. Loss of charge pressure cancause serious degradation of system performance as well as component failure.
As a general recommendation, continuous inlet vacuum should not exceed 6 inches of Hg. for any
gerotor style charge pump during normal operation. Intermittent limits for cold start situations are morea function of the capability of the charge pump. Danfoss Power Solutions medium and high power pumpfamilies are generally capable of achieving upwards of 24 inches Hg. for short duration events. Series 15
products are capable of vacuums on the order of 20 inches Hg. for short duration events. Refer to the
product specific Technical Information Manuals for details and limits.
Normal operating inlet vacuum in the range of 6 to 10 inches Hg is generally associated with a suction
filter in need of replacement.
Inlet suction specifications are based on the use of mineral base fluids at sea level. Fluids with a higher
specific gravity than mineral base fluids, such as fire resistant or synthetic fluids; are more resistant toflow and are more likely to cause cavitation. Some of these fluids may require inlet vacuum limits on theorder of 3 inches Hg for continuous operation. Please refer to Hydraulic Fluids and Lubricants 520L0463,
Experience with Biodegradable Hydraulic Fluids 520L0465, and Design Guideline for Hydraulic Fluid
Cleanliness 520L0467 for more details on the subject.
Operation at altitudes above sea level also changes charge pump inlet vacuum limits. As a rule of thumb,for every one thousand feet above sea level, the inlet vacuum limit should be reduced by 1 inch Hg.
Pressurized reservoirs can be used to compensate for lost atmospheric pressure in closed circuit hydraulicsystems. Refer to the product specific Technical Information Manuals for details and specific inlet
vacuum limits.

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Dynamic Braking

Dynamic Braking
By definition, a closed circuit hydrostatic transmission is capable of transmitting power through both the“A” and “B” sides of the power loop. In a typical mobile vehicle system, there are two modes of powertransmission: propel and braking. Propel implies that the power is transmitted from the engine (or prime
mover), to the pump through to the motor, through the final drive gears and wheels, and finally to theground. Power transmitted during the braking mode is opposite from the propel mode. Braking implies
that the power from vehicle inertia is transmitted from the ground, through the wheels and final drive
gears, to the motor, to the pump and back into the engine. All closed circuit hydrostatic systems withduty cycles that include accelerations and decelerations for a load are subjected to both the propel andbraking modes of operation. On mobile equipment applications, the braking mode will typically be
encountered when the vehicle is commanded to decelerate quickly or when it travels down an incline of
sufficient grade to overcome the rolling resistance of the vehicle. These modes of operation are often
referred to as “dynamic braking” and “downhill braking”, respectively.
It is important to note that, although dynamic braking is an inherent feature of closed circuit hydrostatic
systems, the pump and motor only act to transmit the power; and that the braking power of the vehicle islimited to the braking power of the engine. The ability of the vehicle to dynamically brake is only afunction of the power absorption capability of the engine while remaining within the maximum speed
limits of the engine, pump, and motor. Most engines have the capacity to absorb 10~15% of their rated
power. Braking power transmission above the absorption capability of the engine may cause the engine
to over-speed and risk premature failure. Consult the engine supplier for the braking power absorption
capability.
Some high inertia applications may require the addition of a special dynamic braking valve to prevent
engine over-speed. The Integrated Speed Limitation Valve improves vehicle deceleration while
protecting the engine from failure due to over-speed. Please consult H1 Axial Piston Pump 115/130
147/165 ISL Integrated Speed Limitation 11053026 for additional information that applies to the H1 Seriesof pumps. It is recommended that a test program to verify safe, and controllable, performance during allmodes of dynamic and downhill braking is completed.
Although the closed circuit hydrostatic system is capable of transmitting dynamic braking power back to
the engine, the loss of hydrostatic driveline power in any mode of operation may cause the loss ofhydrostatic braking capability. Therefore, a braking system which is independent of the hydrostatictransmission must be provided which is adequate to stop and hold the vehicle should the condition
develop.
As implied above, the roles of the pump and motor are reversed during dynamic braking. The motor
become a flow source, while the pump receives the flow and offers some resistance to it based on its
displacement and engine frictional load. The resistance, or course, determines the system pressure in thebraking loop. The pump transmits hydraulic power (flow at pressure) back to the engine where it isdissipated as frictional heat.
During a downhill operating mode, the vehicle will experience an increase in travel speed for two
reasons. The first reason involves volumetric leakage of the pump and motor. In the propel mode (asdefined above) normal pump and motor leakage reduce the output speed (i.e., vehicle speed) relative tothe input speed. However, during downhill braking, when the pump and motor roles are reversed, theleakage acts to increase the vehicle speed.
The second, and most significant issue, involves the internal forces acting on the swashplate as a result ofbraking pressure in the power loop. In the propel mode, internal swashplate forces act to reduce thedisplacement of the pump. The magnitude of the resultant force or moment is a function of systempressure, input speed, and servo spring force, swashplate angle and valve plate design. In the braking
mode, however, the swashplate forces act to increase the displacement of the pump. If the pumpdisplacement increases, the motor can rotate faster since more flow is accepted by the pump, with the
net result being increased vehicle speed.

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Dynamic Braking
Displacement Control:
The mechanical feedback mechanism in the displacement control ultimately limits the amount of pumpswashplate angle increase, which is called over-travel. However, the pump displacement control, likemost controls, has a dead-band within which no error signal will be received from the feedback
mechanism. Thus, the control will regulate swashplate position only after the swashplate has traveled
outside of the dead-band. In other words, the displacement control limits the amount of swashplate
over-travel, but cannot eliminate it completely. While the pump is operating within the dead-band
during a downhill condition, the resulting effect on vehicle performance is generally considered
unfavorable.
There are two ways to improve the downhill speedup characteristics of a pump: reduce the deadband of
the displacement control or increase the internal swashplate forces. Because downhill speedup tends tobe more pronounced in larger, heavier vehicles, Danfoss Power Solutions has addressed these issues with
the Series 90 and H1 pump families.
Danfoss Power Solutions has also created a special set of hardware identified as a “Downhill Kit” that
improves the internal forces during the downhill condition for the Series 90 pump family. The “Downhill
Kit” includes nested servo springs (one spring nested inside another) and a special valve plate. The“Downhill Kit” improves the ability of a vehicle to minimize downhill speedup, but does not eliminate thepotential for speedup on severe grades. This situation can only be considered during the design phase of
the vehicle powertrain.
Because the downhill kit increases the neutral return forces on the swashplate, the actual charge pressuremust be at least 24 bar (350 psi) to allow the pump to reach full displacement.
Because it is difficult to quantify all of the parameters associated with downhill speedup, it is difficult forthe designer to predict whether to be concerned for a given application. In general, however,applications involving large inertial loads on off-highway grades should be critiqued carefully for
downhill performance.

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Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Case Drain Pressure Limits

Case Drain Pressure Limits
Both steady state and transient case pressure limits can have an impact on the success of the applicationof the hydrostatic pump and/or motor in the system. It is rare that case pressure will affect theperformance of the system in a measurable way; but, it can have an impact on the success of the shaft
seals and various seals/gaskets to prevent external leakage.
Case pressure may be above, or below, ambient pressures; depending on where the internal casepressure is referenced. For example, case pressure above, or below, ambient atmospheric pressure may
impact the seals/gaskets at the various joints between housings and end caps.
Likewise, case pressure above, or below, the pressure in the driveline coupling cavity can have an impact
on the shaft seals' performance. In general, individual shaft seals are designed to seal against an internalpressure that is greater than the external pressure. If the external pressure is greater than the internalpressure, then the fluid in the external cavity, air, or other forms of contaminant can enter the case cavity.
Some Danfoss Power Solutions products have minimum case pressure limits to ensure that all of theinternal components are lubricated over the entire operating spectrum of the system. In other products,
only the maximum case pressure limits are considered.
Please consult the specific product Technical Information Bulletins for the maximum, and minimum, casepressure limits for all Danfoss Power Solutions products in the system.
In some applications, particularly those with small diameter and/or long case drain hoses; case pressure
can be influenced significantly by both viscosity of the fluid and by rapid changes in flow velocity. Under
cold start conditions, when the viscosity is highest; the pressure in the case cavity can be reduced by
reducing the velocity of the fluid in the hose and/or the length of the hose.
Short duration, transient, pressures should also be considered by the system designer. If there is a sudden
acceleration of the fluid in the case drain line, for any reason; then there will be a transient pressurechange inside the case cavity of the pump, or motor. If there is a positive acceleration of the fluid in thecase drain line, there will be a short duration, transient pressure rise above the normal operatingpressure. These pressure pulses can have the effect of reducing the life of the shaft seal. If there is anegative acceleration of the fluid in the case drain line, there will be a short duration, transient pressure
drop below the normal operating pressure. These pressure pulses can have the effect of drawing
contaminant (fluid, air, water, or abrasive particles) into the case cavity across the sealing interface of the
shaft seal.
For an incompressible fluid, the peak value of the pressure required to accelerate the fluid in a circularpassage if related to the mass density of the fluid, the length of the line, the diameter of the line and thefluid’s acceleration. Reducing the line length will reduce the transient pressure linearly; increasing thediameter of the line will reduce the transient pressure by the square of the ratio of diameters considered.

Frequently, the line length is constrained by the location of the various components in the vehicle; but, it
may be relatively easy to change the line diameter to keep case pressure within the limits defined in the
product technical information manuals.
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Applications Manual
Pressure and Speed Limits for Hydrostatic Units

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Products we offer:

•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

Bent Axis Motors
Closed Circuit Axial Piston
Pumps and Motors
Displays
Electrohydraulic Power
Steering
Electrohydraulics

Danfoss Power Solutions is a global manufacturer and supplier of high-quality hydraulic andelectronic components. We specialize in providing state-of-the-art technology and solutions
that excel in the harsh operating conditions of the mobile off-highway market. Building on
our extensive applications expertise, we work closely with our customers to ensure
exceptional performance for a broad range of off-highway vehicles.
We help OEMs around the world speed up system development, reduce costs and bringvehicles to market faster.
Danfoss – Your Strongest Partner in Mobile Hydraulics.

Hydraulic Power Steering

Go to www.powersolutions.danfoss.com for further product information.

Integrated Systems

Wherever off-highway vehicles are at work, so is Danfoss. We offer expert worldwide support
for our customers, ensuring the best possible solutions for outstanding performance. And
with an extensive network of Global Service Partners, we also provide comprehensive global
service for all of our components.

Joysticks and Control
Handles
Microcontrollers andSoftwareOpen Circuit Axial Piston
Pumps
Orbital Motors

Please contact the Danfoss Power Solution representative nearest you.

PLUS+1® GUIDE
Proportional Valves
Sensors
Steering
Transit Mixer Drives

Comatrolwww.comatrol.com

Local address:

Turollawww.turollaocg.com
Hydro-Gear
www.hydro-gear.com
Daikin-Sauer-Danfoss
www.daikin-sauer-danfoss.com

Danfoss
Power Solutions (US) Company
2800 East 13th Street
Ames, IA 50010, USA
Phone: +1 515 239 6000

Danfoss
Power Solutions GmbH & Co. OHG
Krokamp 35
D-24539 Neumünster, Germany
Phone: +49 4321 871 0

Danfoss
Power Solutions ApS
Nordborgvej 81
DK-6430 Nordborg, Denmark
Phone: +45 7488 2222

Danfoss
Power Solutions Trading(Shanghai) Co., Ltd.
Building #22, No. 1000 Jin Hai Rd
Jin Qiao, Pudong New District
Shanghai, China 201206
Phone: +86 21 3418 5200

Danfoss can accept no responsibility for possible errors in catalogues, brochures and other printed material. Danfoss reserves the right to alter its products without notice. This also applies to products
already on order provided that such alterations can be made without changes being necessary in specifications already agreed.
All trademarks in this material are property of the respective companies. Danfoss and the Danfoss logotype are trademarks of Danfoss A/S. All rights reserved.

© Danfoss | May 2015

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