Danfoss Pressure and Speed Limits User guide
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 Changed Rev
May 2015 Danfoss layout - major update 0401
July 1997 D
2 | © Danfoss | May 2015 BLN-9884 | BC00000160en-US0401
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 BLN-9884 | BC00000160en-US0401 | 3
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 hydraulically
powered 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 the
original 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
4 | © Danfoss | May 2015 BLN-9884 | BC00000160en-US0401
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 an
appropriate design pressure and design speed. The selection of these design parameters are affected
by required unit life. The purpose of this guideline is to assist system integrators in the selection of
design pressure and design speed to optimize hydraulic unit utility and to provide satisfactory
transmission life.
Danfoss Power Solutions publishes continuous
speed 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 their
pressure ratings.
Load to Life Relationships
(1)
ratings and maximum ratings for both pressure and
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 which
must 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 the
rotating 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 the
shallow slope of the life curve at high system pressures which indicates a significant decrease in life with
a 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.
©
Danfoss | May 2015 BLN-9884 | BC00000160en-US0401 | 5
Rotating group
Fatigue & Wear
Bearing Fatigue
Other Components
Normal Limit
of Unit Life
Marginal Fluid
Quality
Poor Fluid
Quality
Life Expectancy (Time)
System Operating
Pressure
Applications Manual
Pressure and Speed Limits for Hydrostatic Units
Pressure and Speed Limits
Hydraulic Unit Life vs System Pressure
Pressure Limits for Hydraulic Units
The following definitions of hydraulic unit pressure capability and associated life requirements are
provided to assist in sizing transmission components. Contact a Danfoss Power Solutions representative
with 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 high
and low pressure work ports. It is the measure of the pressure available to accept or provide torque
through 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 system
pressure, 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 pressure
reference is zero gage pressure at the reservoir.
In closed circuit hydrostatic units, it is common to provide a flow source at a moderate pressure to
provide 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 the
outlet of the motor. It may be either below atmospheric (0 Gauge Pressure) or above. If the charge
pressure 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 pump
suction line. The minimum inlet pressure must be evaluated in both steady state, and transient
conditions; note that when pump displacement increases, inlet pressure is reduced due to acceleration of
fluid in the inlet line.
6 | © Danfoss | May 2015 BLN-9884 | BC00000160en-US0401
Application Pressure is the highest intermittent pressure defined for a given application; and is the high
pressure relief, pressure limiter, or pressure compensator setting typically defined within the order code
of the pump or motor. It is determined by the maximum machine load demand. Application Pressure is
the "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 typically
determined 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.
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 select
a 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 of
loading 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.
©
Danfoss | May 2015 BLN-9884 | BC00000160en-US0401 | 7
System Pressure Line
Design Input
Power
Output Flow
Rated
Design Tractive Effort
at Maximum Design
Speed
NLHI
Full Load Speed
Max.
Application
System
Pressure
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 this
application should have satisfactory predicted life provided the fluid viscosity, quality and cleanliness are
maintained.
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 the
potential of having a somewhat shorter predicted life, dependent upon the total amount of time spent
near 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 no
longer provide expected performance.
Normal working pressure is the pressure expected when performing “normal” work functions. It is the
average pressure across the normal work load range.
The normal working pressure occurs at a regularly occurring load condition. Its design value can be
established 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.
8 | © Danfoss | May 2015 BLN-9884 | BC00000160en-US0401
Loading...