The 5 Main Components of a Hydraulic System: How They Work

What do wheelchair lifts, aircraft wing flaps, and heavy-duty truck brakes have in common? They all rely on hydraulic systems to convert fluid pressure into powerful, precise mechanical motion.

A hydraulic system transmits force through pressurized fluid to perform work — from lifting multi-ton loads on a construction site to controlling the flight surfaces of a commercial aircraft. A pump-and-motor combination creates pressure, distributes it through a network of valves and actuators, then returns the fluid to a tank where it cools before recirculating. To see this cycle in more detail, read our full breakdown of how a hydraulic system works.

Compared to electrical, mechanical, or pneumatic alternatives, hydraulic systems deliver superior force from compact components, handle heavier loads without complex gear or pulley setups, and offer exceptional accuracy and control. These advantages explain why understanding hydraulic system components is essential for anyone involved in specifying, maintaining, or troubleshooting fluid power equipment. For a closer look at where these systems show up in the real world, explore our guide to what hydraulic systems are used for.

Below, we break down each of the five main components of a hydraulic system — reservoir, pump, valves, actuators, and pressure regulator — covering what they do, how they work, the types available, and what to consider when selecting them.

1. Reservoir (Hydraulic Tank)

The hydraulic reservoir is both the starting and ending point for fluid in the circuit. While it may look like a simple storage container, the reservoir plays a far more dynamic role in system health and performance.

What Does a Hydraulic Reservoir Do?

Stores a ready fluid supply — giving the pump consistent access to the volume it needs to maintain pressure and flow. Without sufficient fluid, the system can't operate efficiently and may experience pump cavitation.

Transfers heat away from the system — hydraulic fluid absorbs significant heat during operation. The reservoir provides surface area for the fluid to cool before being drawn back into the circuit, preventing the overheating that degrades seals and shortens component life.

Allows solid contaminants to settle — metal particles, sediment, and debris introduced during operation damage pumps, valves, and actuators over time. The reservoir gives contaminants space to settle to the bottom, away from the pump inlet.

Promotes air and moisture separation — trapped air causes cavitation and spongy actuator response, while moisture accelerates corrosion and fluid degradation. The reservoir's surface area and internal baffle design help both escape from the working fluid.

How to Size a Hydraulic Reservoir

A widely accepted industry guideline is to size the reservoir at two to three times the pump's flow rate (in gallons per minute). For example, a system with a 10 GPM pump would typically use a 20–30 gallon tank. This ratio provides adequate dwell time for the fluid to cool and decontaminate before recirculating.

Internal baffles separate the return line from the suction line, forcing fluid to travel a longer path — improving both heat dissipation and contaminant settling. Breather caps with built-in filters prevent external contaminants from entering as fluid levels fluctuate during operation.

2. Hydraulic Pump

The hydraulic pump is the heart of the system. It converts mechanical energy — from an electric motor or engine — into hydraulic energy by creating fluid flow. Without the pump, fluid doesn't move, and nothing else in the circuit functions. For a deep technical dive, see our complete guide on what a hydraulic pump is and how each design compares.

An important distinction: pumps create flow, not pressure. Pressure is actually the result of resistance to that flow downstream in the hydraulic circuit — from valves, actuators, and other restrictions.

Types of Hydraulic Pumps

Gear Pumps are the workhorses of the fluid power industry — economical, reliable, durable, and straightforward to install. They use meshing gears to displace fluid and are highly tolerant of contamination, making them the go-to choice for mobile equipment, agricultural systems, and less demanding industrial circuits. External gear pumps handle pressures up to 3,000–3,500 PSI; internal gear pumps offer a wider viscosity range (up to 2,200 cSt) and tend to run quieter. The trade-off is that gear pumps become less efficient over time as internal clearances wear.

Vane Pumps use a slotted rotor with sliding vanes to move fluid. They deliver smooth, consistent flow at moderate pressures, making them ideal for machine tools, injection molding equipment, and other applications that demand low noise and steady output. Vane pumps occupy a strong middle ground between the simplicity of gear pumps and the performance capabilities of piston pumps.

Piston Pumps are the performance leaders. They operate at pressures beyond 6,000 PSI with excellent volumetric efficiency, making them the standard in demanding construction, mining, and heavy industrial applications. Variable displacement piston pumps can also adjust output to match system demand in real time, significantly improving energy efficiency.

How to Choose the Right Hydraulic Pump

When comparing a gear pump vs. piston pump, the decision typically comes down to pressure requirements and budget. Gear pumps win on cost and simplicity; piston pumps win on pressure capability and efficiency. For applications between 2,000–3,000 PSI where noise is a concern, vane pumps often represent the best value.

3. Hydraulic Valves

Hydraulic valves are the traffic controllers of your fluid power circuit. Commonly regarded as the most indispensable component in the hydraulic system, valves manage every aspect of fluid flow — starting it, directing it, regulating its volume, controlling its pressure, and stopping it entirely. For a full technical primer, read our guide to hydraulic valve principles and applications.

Directional Control Valves (DCVs)

Directional control valves serve three fundamental purposes: stopping fluid flow, allowing it, and changing its direction between hydraulic cylinders and motors. They are the most common valve type and are available in spool configurations including 2-way, 3-way, and 4-way designs with manual, solenoid, hydraulic, or pneumatic actuation.

The spool's center position determines how the valve behaves at rest — a critical design decision for system performance and safety. Open center allows free flow when idle, reducing heat buildup. Closed center blocks all ports, holding actuators in position. Tandem center routes flow back to the tank while keeping actuator ports blocked. Need help choosing? Our article on hydraulic control solutions covers system architectures in depth.

Pressure Control Valves

Relief valves — the primary safety device in any hydraulic system. They open when pressure exceeds a set limit, routing excess fluid back to the reservoir. Every hydraulic circuit should have at least one.

Pressure reducing valves — maintain a lower, consistent pressure in a specific branch of the circuit, allowing a single pump to serve actuators with different pressure requirements.

Sequence valves — ensure operations happen in the correct order by requiring a set pressure threshold before allowing flow to the next actuator downstream.

Flow Control Valves

Flow control valves govern the volume of fluid passing through a circuit section per unit of time, directly controlling actuator speed. They are essential in applications where precise speed regulation is required — such as controlling the descent rate of a crane boom, the feed rate of a hydraulic press, or the travel speed of a conveyor drive.

Valve Actuation Methods

How a hydraulic valve is operated depends on the application's automation level: manual (hand levers, foot pedals), solenoid (the standard for automated systems), hydraulic pilot (for shifting larger high-flow valves), pneumatic (air-actuated), or mechanical (cams, rollers, springs).

4. Actuators: Hydraulic Cylinders and Motors

The actuator is where hydraulic energy converts back into mechanical energy to perform the machine's actual work — lifting, pushing, pulling, turning, and clamping.

Hydraulic Cylinders (Linear Motion)

Hydraulic cylinders translate fluid pressure into straight-line push and pull motion. They are the most visible component on many hydraulic machines — the extending arms on an excavator, the lift mechanism on a forklift, or the press ram on a stamping press.

Single-acting cylinders — fluid drives the piston in one direction; the return stroke relies on gravity or a spring. Used in simpler applications like hydraulic jacks. Double-acting cylinders — fluid powers both extension and retraction, providing full control in both directions. This is the most common type across industrial and mobile equipment. Telescopic cylinders — multiple nested stages deliver a long stroke from a compact retracted size. Common in dump trucks and crane booms.

Cylinder longevity depends heavily on seal condition, rod surface quality, and fluid cleanliness. For diagnostic techniques and failure prevention, see our technical deep-dive on hydraulic cylinder analysis.

Hydraulic Motors (Rotary Motion)

Hydraulic motors convert fluid pressure into rotational torque, functioning essentially as the inverse of a hydraulic pump. They are used wherever continuous rotation is needed — conveyor drives, wheel motors, winches, augers, and drum drives. Like pumps, hydraulic motors come in gear, vane, and piston types, with piston motors delivering the highest torque at the highest pressures.

Cylinder vs. Motor: When to Use Each

5. Pressure Regulator

The hydraulic pressure regulator does exactly what its name implies — it regulates the fluid pressure throughout the system. If pressure rises above a specified safe level, the regulator diverts excess fluid back to the reservoir until system pressure drops to the correct range.

Uncontrolled pressure can lead to blown hoses, ruptured seals, damaged pumps, and serious workplace injuries — making this one of the most critical safety components in any hydraulic circuit.

How Hydraulic Pressure Regulators Work

A pressure regulator continuously monitors system pressure against a preset value. When pressure exceeds the setpoint, an internal passage opens to divert excess fluid to the tank. When pressure falls below the setpoint, the passage closes, allowing pressure to build again. This constant balancing act protects every downstream component.

Types of Hydraulic Pressure Regulation

Direct-acting relief valves — the simplest type, suitable for low-flow applications. A spring holds the valve closed until system pressure overcomes the spring force.

Pilot-operated relief valves — deliver more precise pressure control with less pressure override, preferred for high-flow systems where tight regulation is important.

Pressure-reducing valves — unlike relief valves (which limit maximum system pressure), reducing valves maintain a constant lower pressure in a subcircuit, even as main system pressure fluctuates. Learn more in our detailed article on hydraulic pressure reducing valve applications.

Essential Supporting Components

While the five core components form the backbone of every hydraulic system, several supporting parts are critical to long-term performance and reliability.

Hydraulic Fluid

Hydraulic fluid transmits energy, lubricates moving parts, dissipates heat, and carries contaminants to the filter. The three main categories are petroleum-based oils (most common, cost-effective, wide temperature range), water-based fluids (specified where fire resistance is required — steel mills, underground mining), and synthetic fluids (engineered for extreme temperatures, higher cost). Choosing the wrong fluid type or viscosity grade is a leading cause of premature wear and inefficiency.

Filters

Since hydraulic systems operate under high pressure, even microscopic contaminants can cause significant damage. In fact, fluid contamination is responsible for roughly 80% of all hydraulic system failures. A well-designed filtration strategy includes suction filters (before the pump), pressure-line filters (after the pump), and return-line filters (before the reservoir).

Hoses, Pipes, and Fittings

These are the arteries of the hydraulic system, conducting pressurized fluid between components. Proper selection depends on operating pressure, temperature, fluid compatibility, and routing. Reinforced rubber hoses provide flexibility for moving connections; rigid steel pipes handle high-pressure, fixed routing. Using undersized or poorly rated fittings is a frequent — and costly — source of system failures. For a full cost analysis, see our article on the real cost of hydraulic line fittings.

Coolers and Heaters

Temperature management is essential. Coolers (air-type or water-type) prevent overheating during heavy use, while heaters bring fluid to proper operating viscosity in cold-start conditions. Hydraulic fluid that runs too hot breaks down and loses lubricating properties; fluid that runs too cold becomes too viscous to flow efficiently.

How All 5 Components Work Together

Understanding each component individually is important, but seeing how they function as an integrated hydraulic circuit is what enables effective system design and troubleshooting. Let's trace the complete cycle on an excavator every time the operator moves the boom:

1. Reservoir — Hydraulic fluid rests in the tank, cooled and conditioned. Contaminants have settled, air has separated, and the fluid is at proper operating viscosity.
2. Pump — The engine-driven piston pump draws fluid from the reservoir through a suction filter and pressurizes it, creating flow throughout the circuit.
3. Valves — The operator's joystick controls a directional control valve, routing pressurized fluid to the correct cylinder port. A pressure relief valve diverts excess flow if system pressure spikes.
4. Actuator — Pressurized fluid enters the boom cylinder, extending or retracting the piston rod to raise or lower the boom with smooth, controlled force.
5. Pressure Regulator — Throughout the operation, the regulator ensures the system stays within safe parameters, protecting every component from damaging pressure spikes.
6. Return — Fluid returns to the reservoir carrying absorbed heat and contaminants, where it cools, gets filtered, and the cycle begins again.

This cycle repeats continuously at high speed, delivering the smooth, powerful, and precisely controlled motion that hydraulic equipment is known for.

Advantages of Hydraulic Systems

Superior force-to-size ratio — hydraulic systems generate enormous force from compact components, outperforming comparable electrical and mechanical systems.

Zero-backlash power transmission — unlike solid machine linkages, there is no slack or play in hydraulic power transfer, enabling precise positioning.

Extended component life — with proper fluid maintenance, hydraulic components can deliver years of continuous, reliable operation with minimal wear.

Smooth, vibration-free operation — properly specified hydraulic systems deliver consistent action unaffected by load variation.

Easy speed and force control — by adjusting valve settings or pump displacement, operators can fine-tune speed, force, and direction with exceptional precision.

Built-in overload protection — relief valves provide inherent safeguards against overload, unlike mechanical systems that can suffer catastrophic damage from unexpected forces.

Frequently Asked Questions

This guide is part of Pozoom's Hydraulic Knowledge Center. Check back regularly for new technical articles, troubleshooting guides, and industry resources for fluid power professionals.