Why a compressor isn't part of a hydraulic system and how actuators, reservoirs, and valves power hydraulic circuits.

Which of the following is NOT a component of hydraulic systems?

• A. Compressor
• B. Actuators
• C. Reservoirs
• D. Valves

Answer: (A)

In hydraulic systems, the primary components include actuators, reservoirs, and valves, which work together to control the flow and pressure of hydraulic fluid. Actuators convert hydraulic energy into mechanical energy to perform work, reservoirs store the hydraulic fluid, and valves regulate the flow and direction of the fluid within the system. A compressor, on the other hand, is typically associated with pneumatic systems rather than hydraulic systems. It is used to increase the pressure of gases and is not involved in the hydraulic process where liquid fluids are utilized for power transmission. Therefore, identifying the compressor as not being a component within hydraulic systems is accurate, as it does not serve any function in the operation of hydraulic circuits.

Hydraulic power systems are the quiet workhorses behind many machines you’ve bumped into, from dump trucks to industrial presses. They feel simple on the surface—just fluid, pressure, and motion—but dial into the details and you’ll see a careful balance of parts that keep everything moving smoothly. If you’re studying topics around the ASA hydraulic and pneumatic systems, you’ve probably bumped into questions that feel like tricky little riddles. Here’s one that often pops up: which item isn’t a component of hydraulic systems?

The quick answer is A: Compressor. But let me unpack why that is, because the distinction matters a lot in real-world applications and in how you read hydraulic diagrams.

First, a quick mental model

Think of a hydraulic system as a circulatory system for fluid. The heart is the pump, pushing oil through pipes and valves. The blood is the hydraulic fluid, and the various organ-like components do different jobs to keep everything efficient and safe.

• Pumps (the heart): A hydraulic pump creates the flow and pressure needed to move the fluid around. This is how energy stored in the electric motor or engine gets transferred into usable hydraulic power. Pumps come in several flavors—gear, vane, and piston types—each with its own quirks and best-use cases.

• Actuators (the muscles): These convert hydraulic energy back into mechanical work. You’ll find cylinders (linear motion) and hydraulic motors (rotational motion) doing the heavy lifting or precise positioning on a machine.

• Reservoirs (the fuel tank): Hydraulic fluid isn’t just a bunch of liquid; it’s a carefully managed resource. The reservoir stores oil, helps with cooling and sediment settling, and provides a buffer so the system isn’t starved or flooded with oil.

• Valves (the traffic cops): Valves control where the fluid goes, how fast it moves, and in what direction. Directional control valves, pressure relief valves, check valves—these are the brains (and the safety rails) of a hydraulic setup.

Now, what about a compressor?

On the pneumatic side—systems that move and control compressed air—compressors are essential. They take in atmospheric air and compress it to a higher pressure so that valves, actuators, and tools can use that pressurized gas to do work. In other words, compressors are stars of the pneumatic world.

In hydraulic systems, those same compressors aren’t the right tool for the job. Hydraulic systems rely on incompressible or nearly incompressible liquids like mineral oil or synthetic fluids. The goal isn’t to compress the fluid; it’s to move it and regulate its pressure. If you tried to run a hydraulic circuit with a gas compressor, you’d run into a host of issues: the fluid would compress, pressure would fluctuate wildly, and precision would suffer. So, the compressor doesn’t belong in a typical hydraulic power system.

Why the distinction matters in real life

• System behavior: Hydraulic fluids are much less compressible than air. That means the system’s response is more predictable, and precision control becomes feasible. Valves and actuators can deliver steady, repeatable motion without the “lag” you’d get if the fluid compressed appreciably.

• Safety and design: The presence of a dedicated hydraulic pump (not a gas compressor) aligns with design goals around heat generation, fluid cleanliness, and pressure stabilization. A compressor would introduce gas-driven dynamics that complicate filtration, cooling, and circuit protection.

• Maintenance realities: Oil-based hydraulics rely on filters and coolers to maintain clean, stable fluid. Introducing a gas compressor into that loop would require a totally different maintenance mindset and could invite air entrainment and contamination.

What the other options do contribute

To anchor the idea, here’s why the other components are part of hydraulic systems:

• Actuators: These are where hydraulic power becomes motion. They’re the visible “work” in the system—moving a crane arm, pushing a loader bucket, or opening a valve for a press. Without actuators, all the pressure and flow would be a fancy pump with nothing to move.

• Reservoirs: Fluid storage isn’t just about having extra oil on hand. Reservoirs help with thermal management, supply continuity, and air separation. They’re your quiet guardians ensuring the system doesn’t run dry or overheat.

• Valves: Control is king in hydraulics. Valves decide where fluid goes, how fast, and under what pressure. They’re the logic layer in a fluid world, making fine adjustments possible and enabling complex motion patterns.

A practical way to remember

If someone asks you to categorize the following as hydraulic components, you can use this quick memory trick:

• If it moves liquid, directs flow, stores it, or converts it to mechanical action, it’s a hydraulic component.

• If it’s about squeezing gas for air tools and pneumatics, it belongs to the pneumatic side.

Let me explain with a simple real-world vignette

Picture a construction site where a hydraulic excavator is digging. The operator moves a joystick, which sends signals to a directional valve. The valve routes high-pressure oil to a hydraulic cylinder, and the piston extends. Oil is drawn back into the reservoir through another path, so the arm can retract. Throughout this sequence, the pump keeps the oil moving, the valve choreographs the path, the actuator performs the work, and the reservoir buffers the whole performance. No compressor in sight—unless you’re talking about a separate air system powering something else on the machine, like an air-driven tool in a different circuit.

That said, not every hydraulic-themed device is as glamorous as a big excavator. In smaller systems—like those on a factory line—the same logic applies. You’ll see a compact pump, a compact reservoir, a few control valves, and some handy actuators. The overall rhythm is simple: energy in, controlled flow, motion out, with safety valves ready to short-circuit pressure spikes.

A quick checklist for a healthy hydraulic loop

If you ever find yourself staring at a schematic or a parts list, here’s a compact way to verify you’re looking at a hydraulic system rather than a pneumatic one:

• Look for a pump symbol (not a compressor) feeding into the system.

• Expect to see an actuator symbol—a cylinder or a motor.

• The presence of a reservoir often shows up as a tank symbol connected to the main line.

• Check for valves that control direction, flow, and pressure.

• Keep an eye out for filters and heat exchangers—these keep the fluid clean and the system cool.

A few notes on terminology and clarity

In the field, people sometimes mix terms casually. It’s worth staying precise: “pump” and “compressor” aren’t interchangeable in most hydraulic contexts. Pneumatic circuits use compressors for air supply; hydraulic circuits use pumps for fluid power. Also, while “accumulator” isn’t the star in every diagram, it’s a helpful device in some systems. It stores energy to smooth out bursts of demand or to maintain pressure when the pump isn’t running.

Why this kind of clarity matters for learners

Understanding which components belong to hydraulics helps you read diagrams faster, troubleshoot smarter, and talk shop with technicians without getting tangled in jargon. It’s not just about knowing which letter corresponds to the right answer; it’s about seeing the logic of how a whole system hangs together. When you know the role of each piece, you can spot mismatches quickly—like spotting an out-of-place compressor in a hydraulic schematic.

Digression: a couple of quick, practical tips

• If you’re ever unsure whether a component is hydraulic or pneumatic, check the fluid symbol and the type of energy conversion. Liquids imply hydraulics; gases imply pneumatics.

• In many training materials, you’ll see common symbols. A good habit is to memorize the few essentials: pump, valve, actuator, reservoir, filter, and accumulator. The rest is details that drift into more advanced topics.

• Real-world machines love to surprise you with hybrids and mixed systems. Don’t be surprised if you encounter a hydraulic system that also makes use of pneumatics somewhere else on the same machine. The key is to keep their functions straight and not conflate their power sources.

Bringing it home

So, when a question asks which item is NOT a component of hydraulic systems, the answer spotlights a fundamental distinction: compressors belong to the pneumatic family, while hydraulics lean on pumps, actuators, reservoirs, and valves to get the job done. It’s a clean line, even if the workshop floor often blurs the edges a bit with hybrid setups.

If you’re curious to explore more, you’ll find there’s a rich ecosystem of topics around hydraulic and pneumatic power systems—cooling strategies, filtration choices, safety relief practices, and even the subtle art of schematic reading. Each piece adds a little more confidence when you’re standing in front of a machine or poring over a schematic, trying to trace flow from the pump to the actuator.

In the end, the beauty of hydraulic systems isn’t just in what they do; it’s in how they do it with a purposeful simplicity. A pump sends the lifeblood streaming; valves choreograph the dance; actuators do the heavy lifting; and the reservoir keeps everything well-fed and cool. And yes, compressor-free—that’s the crisp distinction that helps keep the hydraulic world orderly and efficient.

If you’d like, we can explore more examples of hydraulic circuits, look at common failure modes, or break down another set of concepts with a practical, grounded approach. After all, a solid grasp comes from connecting the theory to the machines you’ll see in the wild—and a few good stories from the shop floor never hurt either.