油圧ユニットの主要構成部品とその役割

The main components of a hydraulic power unit are usually a motor-driven pump, a reservoir, pressure-control equipment, connections, and the fluid-conditioning and control items required by the application. Directional valves, coolers, heaters, accumulators, and electronic controls may be included, supplied separately, or unnecessary for a particular design. There is no single parts list that describes every power unit.

The useful approach is to identify functions first: where mechanical energy enters, where oil is stored and drawn into the pump, how pressure and flow are managed, and how fluid condition is maintained. Then match those functions to the schematic, bill of materials, and assembly drawing. This article explains the components of a conventional oil-hydraulic power unit; it does not provide a complete sizing or maintenance procedure.

目次

1. Read the unit as a set of functional groups
2. Motor, coupling, and pump
3. Reservoir and its service connections
4. Valves and manifold
5. Filters and temperature-control equipment
6. Instruments, electrical controls, and mounting parts
7. Check what the supplied assembly actually includes
8. よくある質問

Read the unit as a set of functional groups

A power unit is the supply assembly within a larger hydraulic system. The driven cylinder or hydraulic motor is often located elsewhere on the machine. The boundary can vary: one supplier may include the directional manifold and electrical cabinet, while another supplies only the motor-pump-reservoir assembly.

Functional group	Typical components	Main question
Mechanical drive and pumping	Motor, coupling or integrated drive, pump	How is hydraulic flow produced?
Fluid storage and exchange	Reservoir, filler, breather, level indication	How is the working oil inventory managed?
Hydraulic control	Relief and other valves, manifold	How are pressure, direction, and flow controlled?
流体の調整	Filters, cooler, heater where needed	How are cleanliness and temperature maintained?
Monitoring and integration	Sensors, controls, connections, frame	How is the unit connected, observed, and supported?

Large units may divide these functions into several modules; compact units may combine them in one block. Bosch Rexroth's power-unit range illustrates this variety, including options for oil conditioning and monitoring. Bosch Rexroth power units.

Count functions rather than visible housings. A manifold may contain several cartridges, and a compact pump group may conceal the coupling. For the basic boundary, see what a hydraulic power unit is.

Motor, coupling, and pump

The motor converts electrical energy into shaft power. AC induction motors, DC motors, and controlled electric drives suit different supply and duty requirements. The nameplate and drive documentation establish the relevant electrical and operating limits; motor power alone does not state delivered hydraulic performance.

A coupling transfers torque between a separate motor and pump. Its alignment, speed, torque capacity, and mounting arrangement belong to the assembly design. Some integrated units use a different internal connection, so a separately visible coupling is not universal.

The pump converts mechanical input into hydraulic flow against the pressure demanded by the circuit. Pump displacement and speed influence theoretical flow, while leakage and operating conditions affect delivered flow. Pressure develops through the interaction of load, restrictions, and control settings; it is not a fixed output determined only by the pump label.

The inlet connection supplies oil to the pump. Outlet, case-drain, and control connections depend on pump type. Do not identify every small connection as a drain or join connections based on appearance.

Motor and pump must be selected together for the operating points and duty. A larger motor does not independently increase a fixed-displacement pump's flow at unchanged speed. Detailed relationships belong in motor and pump sizing.

Reservoir and its service connections

The reservoir holds the operating oil inventory and accommodates volume changes during machine movements. Its design can also support air release, return-flow management, and heat exchange with the surroundings. Those benefits depend on geometry, flow paths, fluid condition, and operation; tank volume alone does not guarantee them.

Typical service features include a filler, breather, level indicator, drain, and inspection access. Some units have a combined filler-breather, while others use separate devices. A breather controls the path for air entering and leaving a vented reservoir; it is not an unrestricted opening to be ignored during contamination control.

The suction outlet, return connection, and drain returns have different purposes. Their position and internal piping affect circulation and local conditions. A return diffuser or baffle may be present, but neither should be assumed from an external photograph.

Level markings relate to specified machine states and filling instructions. A tank that appears full with cylinders retracted may have a much lower operating level during extension. Check the approved procedure and reservoir selection before interpreting nominal capacity as usable oil volume.

Valves and manifold

Pressure-control, directional-control, and flow-control functions are distinct. A relief valve provides a path intended to limit pressure under its design conditions. A pressure-reducing valve controls a downstream branch differently. Neither is a general substitute for the other.

A directional valve routes flow among ports to command an actuator or change a circuit state. A flow-control arrangement regulates a flow path and can influence actuator speed. Actual behavior depends on the circuit, load, and valve characteristics, including whether compensation is provided.

Check valves allow flow in an intended direction and restrict reverse flow, subject to their design and leakage characteristics. Pilot-operated checks and load-control valves perform more specific functions. A valve described as “load holding” still needs correct application and placement within the machine design.

The manifold connects these functions through internal passages. Its drawing and cartridge list show what each cavity contains; a blanked port or plug may be essential to the selected circuit.

Parker's compact hydraulic assemblies demonstrate how multiple valve functions can be integrated with the motor, pump, and reservoir. Always identify the configured version rather than inferring its circuit from the family name. Parker compact hydraulics guide. For flow paths during operation, see how a hydraulic power unit works.

Filters and temperature-control equipment

A filter's location helps explain its job. Pressure-line filtration can protect downstream components; return filtration treats returning oil; an offline circuit conditions oil independently of the main working flow. Suction devices have different constraints because excessive inlet restriction can compromise pump supply.

Parker's filtration handbook distinguishes filter locations and selection considerations. A suction strainer should not be treated as proof that the whole system meets its cleanliness requirement. Select filtration around component needs and operating conditions, not a universal micron number. Parker hydraulic filtration handbook.

The filter assembly may include a housing, replaceable element, bypass valve, and condition indicator, depending on design. Confirm each feature from the documentation. An indicator does not necessarily measure oil cleanliness; it may indicate a pressure difference associated with element loading.

A cooler transfers heat from oil to air or another medium. A heater can support controlled startup conditions where required. Thermostats, bypass arrangements, fans, water connections, and control logic are part of those systems.

Neither cooler nor heater is universal. The relevant question is whether the complete unit keeps the fluid and components within their approved operating conditions over the specified duty.

Instruments, electrical controls, and mounting parts

Pressure gauges, pressure transducers, temperature sensors, level switches, and filter indicators provide different information. A gauge at the pump outlet does not automatically show pressure at a distant cylinder port. Label the measurement location as well as the displayed quantity.

A sensor can supply a signal without implementing any protective action. The controller, wiring, logic, final control element, and validation determine how that signal affects the machine. Do not equate the presence of a pressure switch with a complete safety function.

Electrical equipment may include motor starters, contactors, variable-speed drives, power supplies, terminals, and an operator interface. Verify what belongs to the unit and what the machine integrator must provide.

Mechanical items also matter: the base, brackets, tank support, vibration-control arrangements, guards, and lifting provisions make the assembly installable and serviceable. They are not interchangeable decorative accessories.

An accumulator, if supplied, stores hydraulic energy and changes the isolation and service requirements. It is an application-dependent component, not a mandatory item on every power unit. Its selection and servicing need the applicable specialist procedures.

Check what the supplied assembly actually includes

Use three documents together. The hydraulic schematic explains function; the bill of materials identifies parts; the assembly drawing shows location and interfaces. Each answers a different question, and revisions should agree.

Start at the electrical and hydraulic boundaries and trace each required function. If a return filter appears on the schematic but is absent from the delivery list, resolve whether it is supplied elsewhere or missing. If a cooler is optional, check whether the quotation's performance assumptions require it.

Record items as included, optional, supplied by the integrator, or not required. Include fluid filling, hoses, mating connectors, mounting fasteners, and commissioning information where they affect the scope. A photograph cannot settle those contractual and technical boundaries.

Ask for readable component identification and service access. Being able to replace an element or identify a valve later depends on the delivered documentation and installation space, not only the original purchase price.

Complete a power unit specification checklist with the actual configuration. This creates a practical handover from recognizing components to selecting and verifying the complete assembly.

よくある質問

Is a hydraulic cylinder part of the power unit?
Usually it is the external actuator served by the unit. Some integrated products combine both, so check the stated assembly boundary.

Does every unit need a separate oil cooler?
No. Cooling requirements depend on heat generation, duty, environment, and the unit's ability to reject heat. The absence of a cooler must be supported by the application assessment.

Are a suction strainer and a return filter equivalent?
No. They occupy different locations and have different constraints and purposes. Their presence does not remove the need for a defined contamination-control strategy.

Does a bigger reservoir fix every power-unit problem?
No. Incorrect valve behavior, electrical limits, inlet restrictions, or component selection can remain. Reservoir size is one part of the design, not a universal remedy.