Danfoss Pressure and Speed Limits User guide

Applications Manual

Pressure and Speed Limits for Hydrostatic Units

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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

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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 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

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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 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 (1) ratings and maximum ratings for both pressure and 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

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.

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Applications Manual

Pressure and Speed Limits for Hydrostatic Units

Pressure and Speed Limits

			Rotating		group
			Fatigue
				& Wear
						Bearing
	<![if ! IE]> <![endif]>System Operating					Fatigue
		<![if ! IE]> <![endif]>Pressure	Normal Limit
			of Unit Life			Other
			Marginal Fluid
						Components
			Quality
			Poor Fluid
			Quality

Hydraulic Unit Life vs System Pressure

Life Expectancy (Time)

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.

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.

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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 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.

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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 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.

	System Pressure Line
Application
Max.		Design Input
		Power			Full Load Speed
System					Design Tractive Effort
Pressure					at Maximum Design
					Speed
					NLHI
		Output Flow	Rated

8 | © Danfoss | May 2015

BLN-9884 | BC00000160en-US0401