Гидравлические системы для мобильной техники: области применения и требования к конструкции

Hydraulic systems for mobile equipment convert engine or electric-drive power into controlled work, steering, and, on some machines, travel. Their design must account for changing shaft speed, simultaneous functions, external loads, limited installation space, and outdoor operating conditions. A pump’s maximum pressure and flow ratings cannot describe how the complete machine will behave throughout its work cycle.

The useful starting point is a function map: what must move, how fast, against which load, and which other actions may occur at the same time. This guide connects those application requirements to circuit choices, flow allocation, load control, thermal conditions, and verification. It provides a basis for discussion with a system engineer rather than a construction circuit or model-specific adjustment procedure.

Содержание

1. Map the machine functions and work cycle
2. Choose the circuit architecture by function
3. Define simultaneous operation and priority
4. Control external loads and machine motion
5. Check flow and power across the speed range
6. Design for temperature, contamination, and movement
7. Coordinate electronic control and fault behavior
8. Turn application needs into a verification plan
9. Часто задаваемые вопросы

Map the machine functions and work cycle

List the machine’s functions before selecting components. A loader may need lifting and tilting while approaching a load. Agricultural equipment may combine implement positioning with continuous motor-driven work. A material-handling machine may need precise low-speed motion and controlled load holding. These are representative application questions, not claims about a particular machine design.

Function group	Typical requirement to define
Work cylinders	Force versus position, speed, stroke, holding, and lowering
Rotary work drives	Torque, speed range, starting load, reversal, and stopping
Travel, where hydraulic	Tractive demand, speed range, grade, and controlled deceleration
Steering	Required response, available supply, and interaction with work functions
Attachments	Pressure-flow range, return and drain conditions, interface compatibility
Auxiliary services	Cooling fan, pilot supply, or other machine-specific demands

Record normal cycles and difficult combinations, including low-speed operation, starting a loaded function, and transitioning between actions. State which functions must operate together and which may be deliberately sequenced.

Compared with hydraulic systems for industrial machinery, mobile applications require explicit attention to machine motion, onboard power availability, and changing environmental conditions. Neither category guarantees a simple or constant duty cycle.

Choose the circuit architecture by function

An open circuit normally draws fluid from a reservoir and returns working flow to the reservoir. “Open center” describes a valve’s neutral flow arrangement and is not another name for every open circuit. A closed-center valve arrangement can form part of an open-circuit hydraulic system.

Architecture concept	Application question	Important limitation
Fixed-displacement pump with an open-center valve arrangement	Can a relatively simple supply and valve path meet the work cycle?	Flow follows speed; neutral losses and simultaneous-function behavior need review
Variable-displacement, load-sensing supply	Should pump delivery respond to work-function demand?	Pump and valve controls must match; supply remains limited
Closed-circuit hydrostatic drive	Is a dedicated pump–motor power loop suitable for a travel or rotary function?	Charge supply, heat removal, pressure control, and other supporting functions remain necessary
Separate or combined circuits	Which functions should share supply and which require separation?	Packaging, energy use, failure interactions, and interfaces must be evaluated

In a conventional hydraulic load-sensing arrangement, the pump controller responds to a load signal and the pressure difference between that signal and pump outlet. The Danfoss Series 45 explanation illustrates this relationship. The appropriate control margin comes from the selected system, not a universal value.

In a closed-circuit hydrostatic drive, the main working flow circulates between pump and motor. Supporting circuits replace lost fluid and manage fluid condition; “closed” does not mean that the system needs no reservoir or cooling. These supporting functions are described in Danfoss transmission circuit guidance. For terminology, start with hydraulic system fundamentals.

Define simultaneous operation and priority

Suppose two work functions request flow at the same time. Their combined request may exceed available pump delivery, particularly at low shaft speed or when a power limit reduces pump displacement. The design must state what should happen then: slower combined motion, priority for a particular function, or an approved sequence.

Load sensing and flow sharing describe different capabilities. A load signal helps the supply respond to demand. The way limited supply is distributed depends on the valve arrangement and controls. Do not assume that every load-sensing valve bank will preserve the same speed relationship between functions when demand exceeds supply.

Этот Danfoss PVG 100 documentation describes flow-sharing behavior for its particular valve design under sufficient and insufficient pump flow. That capability must be verified for the selected configuration rather than inferred from the words “proportional valve.”

Steering priority is a separate requirement. Where steering and work functions share one supply, a suitable priority arrangement can reserve supply for steering before allocating the remaining flow to work. Danfoss’s priority-valve explanation describes this application. The complete steering function still requires machine-specific engineering and validation.

Use an action-combination table to define required behavior, then perform the detailed work in matching a pump, valve, and cylinder.

Control external loads and machine motion

Mobile actuators often encounter loads that change with position and machine attitude. During lowering or deceleration, an external load can drive an actuator rather than merely resist it. This overrunning condition requires appropriate motion control; supplying less flow alone does not establish controlled movement.

Define the load states: lifting, holding, lowering, impact, reversal, and loss of a command or energy source. Include linkage geometry and changing mechanical advantage. Cylinder pressure by itself does not establish the machine’s permitted load, structural capacity, or stability.

Load-holding arrangements must suit the actuator, load, pilot conditions, return pressure, and required motion. HAWE’s LHT documentation illustrates the role of a load-holding valve in preventing uncontrolled load-driven movement while permitting controlled lowering. Its settings and application limits are specific to the selected design.

A load-holding valve, mechanical support, brake, and machine stability assessment serve different purposes. Define their roles and interactions explicitly. Do not treat a generic valve symbol as proof that every failure mode is controlled. If a machine already exhibits unwanted movement, use the evidence-based approach in hydraulic cylinder drift and the approved machine service procedure.

Check flow and power across the speed range

For a displacement pump, theoretical delivery is:

Q_theoretical [L/min] = displacement [cm³/rev] × speed [rpm] ÷ 1000.

For an illustrative calculation, assume a fixed displacement of 40 cm³/rev and a volumetric efficiency of 0.90 at both operating points. These are invented design assumptions, not manufacturer performance data.

Shaft speed	Theoretical flow	Illustrative delivered flow
1000 rpm	40 L/min	36 L/min
1800 rpm	72 L/min	64.8 L/min

Actual volumetric efficiency varies with the pump and conditions; a variable-displacement pump may also operate below its maximum displacement. Use performance data for the intended pressure, fluid state, and speed.

If two work functions request 30 and 25 L/min, their total request is 55 L/min. The illustrative low-speed supply of 36 L/min cannot satisfy both requests fully. The actual allocation depends on the control design and any other demands. A higher-speed supply of 64.8 L/min clears this simple flow comparison, but does not by itself prove acceptable machine performance.

Hydraulic power across a defined pressure difference is:

P_hydraulic [kW] = Δp [bar] × Q [L/min] ÷ 600.

At an illustrative pump pressure difference of 200 bar, the two delivered flows represent 12 and 21.6 kW of hydraulic power. Required shaft input is higher after pump losses. Engine or motor capability must be checked at the actual speed, including torque limits and power reserved for other services.

For an electric mobile machine, also consider the motor drive, battery operating limits, and work-cycle energy. Peak power and operating duration are different requirements. See hydraulic power unit selection for related power-source inputs.

Design for temperature, contamination, and movement

Specify the operating environment as a range of conditions. Cold starting, sustained hot operation, dust, water exposure, washing, vibration, and changes in machine attitude can affect different parts of the installation. Translate each condition into a component requirement and a verification case.

Check fluid suitability across the intended temperature range, pump inlet conditions, filter behavior, and the cooling arrangement. Do not choose an oil grade solely because it is common on another machine. The selected components and expected conditions must support the choice.

Reservoir design should account for actuator volume changes and fluid movement during the permitted machine attitudes. Inlet supply, air separation, sealing, and service access all matter. Danfoss’s transmission circuit guidance discusses these reservoir and contamination-control concerns for hydrostatic circuits; its numerical recommendations should not be generalized to every mobile system.

Hoses and cables need routing that accommodates full movement without unacceptable bending, rubbing, or tension. Verify attachment return and drain interfaces as carefully as the pressure connection. A mechanically compatible coupling does not establish hydraulic compatibility.

Estimate and then validate heat rejection over representative cycles. Peak hydraulic power is not automatically the cooler’s required heat-removal rating. For an existing heat problem, see hydraulic system overheating.

Coordinate electronic control and fault behavior

Electronic control can coordinate operator commands, pump delivery, valve actuation, and sensor feedback. Define the physical quantity being controlled: valve position, actuator speed, load pressure, or machine position. Feedback from one of these does not automatically verify all the others.

Document command limits, response requirements, mode transitions, sensor plausibility checks, and communication dependencies. A “closed-loop controller” refers to feedback control and should not be confused with a closed hydraulic power circuit.

Define the required machine behavior after loss of electrical power, a missing sensor signal, communication failure, or a hydraulic supply fault. There is no single generic safe state for every moving or suspended load. The machine’s engineering assessment must determine which motion stops, which load is held, and which recovery functions remain available.

Configuration and software versions belong in the system documentation. A replacement controller with different settings can change the behavior even if the hydraulic hardware is unchanged. Connect the control interfaces to the circuit and operating sequence described in basic hydraulic system design.

Turn application needs into a verification plan

An application brief should contain more than a component shopping list. Provide the action table, loads and geometry, speed range, simultaneous-operation combinations, duty cycle, supply limits, fluid conditions, environment, and space constraints.

Requirement package	Verification evidence to define
Individual and combined actions	Measured speed, force or torque, and response at agreed conditions
Low-speed and power-limited operation	Actual supply and the intended priority or slowing behavior
Load holding and controlled movement	Approved machine-specific tests and acceptance conditions
Thermal and environmental operation	Representative cycles and relevant condition records
Attachment and control interfaces	Correct connections, configuration, and defined fault responses
Service documentation	Circuit revision, settings, fluid requirements, test points, and records

Specify the measured quantities, locations, acceptance rules, and responsible reviewers before testing. Component bench results support the design but do not demonstrate every aspect of full-machine behavior. The capabilities needed for component testing can be organized using test bench pressure, flow, and power requirements.

Keep verified operating limits distinct from assumptions that still need testing. This makes a supplier discussion, prototype review, or design change easier to evaluate.

Часто задаваемые вопросы

Are open-center and open-circuit systems the same?

No. Open center describes a valve’s neutral flow path. Open circuit describes the main fluid path through supply, actuators, and reservoir. Use both terms precisely when specifying a system.

Does load sensing guarantee smooth simultaneous motion?

No. Available flow, valve compensation, flow-sharing behavior, load interactions, and control settings determine the result. Define and test the required action combinations.

Can a larger pump solve poor low-speed operation?

It may change available flow, but it also changes drive requirements and interactions across the speed range. Check torque, inlet conditions, losses, priorities, and controls before changing displacement.

Do electrically powered mobile machines still use hydraulics?

They can. An electric motor can drive the hydraulic supply, while cylinders or hydraulic motors perform machine functions. The complete choice depends on the application. Prepare the function map, operating envelope, and verification requirements before committing to an architecture.